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Network Working Group J. Arkko
Request for Comments: 3830 E. Carrara
Category: Standards Track F. Lindholm
M. Naslund
K. Norrman
Ericsson Research
August 2004
MIKEY: Multimedia Internet KEYing
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a key management scheme that can be used for
real-time applications (both for peer-to-peer communication and group
communication). In particular, its use to support the Secure Real-
time Transport Protocol is described in detail.
Security protocols for real-time multimedia applications have started
to appear. This has brought forward the need for a key management
solution to support these protocols.
Arkko, et al. Standards Track [Page 1]
RFC 3830 MIKEY August 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Existing Solutions . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
1.3. Definitions. . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Abbreviations. . . . . . . . . . . . . . . . . . . . . . 6
1.5. Outline. . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Basic Overview . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Scenarios. . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Design Goals . . . . . . . . . . . . . . . . . . . . . . 8
2.3. System Overview. . . . . . . . . . . . . . . . . . . . . 8
2.4. Relation to GKMARCH. . . . . . . . . . . . . . . . . . . 10
3. Basic Key Transport and Exchange Methods . . . . . . . . . . . 10
3.1. Pre-shared Key . . . . . . . . . . . . . . . . . . . . . 12
3.2. Public-Key Encryption. . . . . . . . . . . . . . . . . . 13
3.3. Diffie-Hellman Key Exchange. . . . . . . . . . . . . . . 14
4. Selected Key Management Functions. . . . . . . . . . . . . . . 15
4.1. Key Calculation. . . . . . . . . . . . . . . . . . . . . 16
4.1.1. Assumptions. . . . . . . . . . . . . . . . . . . 16
4.1.2. Default PRF Description. . . . . . . . . . . . . 17
4.1.3. Generating keys from TGK . . . . . . . . . . . . 18
4.1.4. Generating keys for MIKEY Messages from
an Envelope/Pre-Shared Key . . . . . . . . . . . 19
4.2 Pre-defined Transforms and Timestamp Formats . . . . . . . 19
4.2.1. Hash Functions . . . . . . . . . . . . . . . . . 19
4.2.2. Pseudo-Random Number Generator and PRF . . . . . 20
4.2.3. Key Data Transport Encryption. . . . . . . . . . 20
4.2.4. MAC and Verification Message Function. . . . . . 21
4.2.5. Envelope Key Encryption. . . . . . . . . . . . . 21
4.2.6. Digital Signatures . . . . . . . . . . . . . . . 21
4.2.7. Diffie-Hellman Groups. . . . . . . . . . . . . . 21
4.2.8. Timestamps . . . . . . . . . . . . . . . . . . . 21
4.2.9. Adding New Parameters to MIKEY . . . . . . . . . 22
4.3. Certificates, Policies and Authorization . . . . . . . . 22
4.3.1. Certificate Handling . . . . . . . . . . . . . . 22
4.3.2. Authorization. . . . . . . . . . . . . . . . . . 23
4.3.3. Data Policies. . . . . . . . . . . . . . . . . . 24
4.4. Retrieving the Data SA . . . . . . . . . . . . . . . . . 24
4.5. TGK Re-Keying and CSB Updating . . . . . . . . . . . . . 25
5. Behavior and Message Handling. . . . . . . . . . . . . . . . . 26
5.1. General. . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.1. Capability Discovery . . . . . . . . . . . . . . 26
5.1.2. Error Handling . . . . . . . . . . . . . . . . . 27
5.2. Creating a Message . . . . . . . . . . . . . . . . . . . 28
5.3. Parsing a Message. . . . . . . . . . . . . . . . . . . . 29
5.4. Replay Handling and Timestamp Usage. . . . . . . . . . . 30
6. Payload Encoding . . . . . . . . . . . . . . . . . . . . . . . 32
Arkko, et al. Standards Track [Page 2]
RFC 3830 MIKEY August 2004
6.1. Common Header Payload (HDR). . . . . . . . . . . . . . . 32
6.1.1. SRTP ID. . . . . . . . . . . . . . . . . . . . . 35
6.2. Key Data Transport Payload (KEMAC) . . . . . . . . . . . 36
6.3. Envelope Data Payload (PKE). . . . . . . . . . . . . . . 37
6.4. DH Data Payload (DH) . . . . . . . . . . . . . . . . . . 38
6.5. Signature Payload (SIGN) . . . . . . . . . . . . . . . . 39
6.6. Timestamp Payload (T). . . . . . . . . . . . . . . . . . 39
6.7. ID Payload (ID) / Certificate Payload (CERT) . . . . . . 40
6.8. Cert Hash Payload (CHASH). . . . . . . . . . . . . . . . 41
6.9. Ver msg payload (V). . . . . . . . . . . . . . . . . . . 42
6.10. Security Policy Payload (SP) . . . . . . . . . . . . . . 42
6.10.1. SRTP Policy. . . . . . . . . . . . . . . . . . . 44
6.11. RAND Payload (RAND). . . . . . . . . . . . . . . . . . . 45
6.12. Error Payload (ERR). . . . . . . . . . . . . . . . . . . 46
6.13. Key Data Sub-Payload . . . . . . . . . . . . . . . . . . 46
6.14. Key Validity Data. . . . . . . . . . . . . . . . . . . . 48
6.15. General Extension Payload. . . . . . . . . . . . . . . . 50
7. Transport Protocols. . . . . . . . . . . . . . . . . . . . . . 50
8. Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.1. Simple One-to-Many . . . . . . . . . . . . . . . . . . . 51
8.2. Small-Size Interactive Group . . . . . . . . . . . . . . 51
9. Security Considerations. . . . . . . . . . . . . . . . . . . . 52
9.1. General. . . . . . . . . . . . . . . . . . . . . . . . . 52
9.2. Key Lifetime . . . . . . . . . . . . . . . . . . . . . . 54
9.3. Timestamps . . . . . . . . . . . . . . . . . . . . . . . 55
9.4. Identity Protection. . . . . . . . . . . . . . . . . . . 55
9.5. Denial of Service. . . . . . . . . . . . . . . . . . . . 56
9.6. Session Establishment. . . . . . . . . . . . . . . . . . 56
10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 57
10.1. MIME Registration. . . . . . . . . . . . . . . . . . . . 59
11. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 59
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 60
12.1. Normative References . . . . . . . . . . . . . . . . . . 60
12.2. Informative References . . . . . . . . . . . . . . . . . 61
Appendix A. - MIKEY - SRTP Relation. . . . . . . . . . . . . . . . 63
Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 65
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 66
1. Introduction
There has recently been work to define a security protocol for the
protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management solution to
exchange keys and related security parameters. There are some
fundamental properties that such a key management scheme has to
fulfill to serve streaming and real-time applications (such as
unicast and multicast), particularly in heterogeneous (mix of wired
and wireless) networks.
Arkko, et al. Standards Track [Page 3]
RFC 3830 MIKEY August 2004
This document describes a key management solution that addresses
multimedia scenarios (e.g., SIP [SIP] calls and RTSP [RTSP]
sessions). The focus is on how to set up key management for secure
multimedia sessions such that requirements in a heterogeneous
environment are fulfilled.
1.1. Existing Solutions
There is work done in the IETF to develop key management schemes.
For example, IKE [IKE] is a widely accepted unicast scheme for IPsec,
and the MSEC WG is developing other schemes to address group
communication [GDOI, GSAKMP]. However, for reasons discussed below,
there is a need for a scheme with lower latency, suitable for
demanding cases such as real-time data over heterogeneous networks
and small interactive groups.
An option in some cases might be to use [SDP], as SDP defines one
field to transport keys, the "k=" field. However, this field cannot
be used for more general key management purposes, as it cannot be
extended from the current definition.
1.2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
1.3. Definitions
(Data) Security Protocol: the security protocol used to protect the
actual data traffic. Examples of security protocols are IPsec and
SRTP.
Data Security Association (Data SA): information for the security
protocol, including a TEK and a set of parameters/policies.
Crypto Session (CS): uni- or bi-directional data stream(s), protected
by a single instance of a security protocol. For example, when SRTP
is used, the Crypto Session will often contain two streams, an RTP
stream and the corresponding RTCP, which are both protected by a
single SRTP Cryptographic Context, i.e., they share key data and the
bulk of security parameters in the SRTP Cryptographic Context
(default behavior in [SRTP]). In the case of IPsec, a Crypto Session
would represent an instantiation of an IPsec SA. A Crypto Session
can be viewed as a Data SA (as defined in [GKMARCH]) and could
therefore be mapped to other security protocols if necessary.
Arkko, et al. Standards Track [Page 4]
RFC 3830 MIKEY August 2004
Crypto Session Bundle (CSB): collection of one or more Crypto
Sessions, which can have common TGKs (see below) and security
parameters.
Crypto Session ID: unique identifier for the CS within a CSB.
Crypto Session Bundle ID (CSB ID): unique identifier for the CSB.
TEK Generation Key (TGK): a bit-string agreed upon by two or more
parties, associated with CSB. From the TGK, Traffic-encrypting Keys
can then be generated without needing further communication.
Traffic-Encrypting Key (TEK): the key used by the security protocol
to protect the CS (this key may be used directly by the security
protocol or may be used to derive further keys depending on the
security protocol). The TEKs are derived from the CSB's TGK.
TGK re-keying: the process of re-negotiating/updating the TGK (and
consequently future TEK(s)).
Initiator: the initiator of the key management protocol, not
necessarily the initiator of the communication.
Responder: the responder in the key management protocol.
Salting key: a random or pseudo-random (see [RAND, HAC]) string used
to protect against some off-line pre-computation attacks on the
underlying security protocol.
PRF(k,x): a keyed pseudo-random function (see [HAC]).
E(k,m): encryption of m with the key k.
PKx: the public key of x
[] an optional piece of information
{} denotes zero or more occurrences
|| concatenation
| OR (selection operator)
^ exponentiation
XOR exclusive or
Bit and byte ordering: throughout the document bits and bytes are
indexed, as usual, from left to right, with the leftmost bits/bytes
being the most significant.
Arkko, et al. Standards Track [Page 5]
RFC 3830 MIKEY August 2004
1.4. Abbreviations
AES Advanced Encryption Standard
CM Counter Mode (as defined in [SRTP])
CS Crypto Session
CSB Crypto Session Bundle
DH Diffie-Hellman
DoS Denial of Service
MAC Message Authentication Code
MIKEY Multimedia Internet KEYing
PK Public-Key
PSK Pre-Shared key
RTP Real-time Transport Protocol
RTSP Real Time Streaming Protocol
SDP Session Description Protocol
SIP Session Initiation Protocol
SRTP Secure RTP
TEK Traffic-encrypting key
TGK TEK Generation Key
1.5. Outline
Section 2 describes the basic scenarios and the design goals for
which MIKEY is intended. It also gives a brief overview of the
entire solution and its relation to the group key management
architecture [GKMARCH].
The basic key transport/exchange mechanisms are explained in detail
in Section 3. The key derivation, and other general key management
procedures are described in Section 4.
Section 5 describes the expected behavior of the involved parties.
This also includes message creation and parsing.
All definitions of the payloads in MIKEY are described in Section 6.
Section 7 deals with transport considerations, while Section 8
focuses on how MIKEY is used in group scenarios.
The Security Considerations section (Section 9), gives a deeper
explanation of important security related topics.
Arkko, et al. Standards Track [Page 6]
RFC 3830 MIKEY August 2004
2. Basic Overview
2.1. Scenarios
MIKEY is mainly intended to be used for peer-to-peer, simple one-to-
many, and small-size (interactive) groups. One of the main
multimedia scenarios considered when designing MIKEY has been the
conversational multimedia scenario, where users may interact and
communicate in real-time. In these scenarios it can be expected that
peers set up multimedia sessions between each other, where a
multimedia session may consist of one or more secured multimedia
streams (e.g., SRTP streams).
peer-to-peer/ many-to-many many-to-many
simple one-to-many (distributed) (centralized)
++++ ++++ ++++ ++++ ++++
|. | |A | |B | |A |---- ----|B |
--| ++++ | |----------| | | | \ / | |
++++ / ++|. | ++++ ++++ ++++ (S) ++++
|A |---------| ++++ \ / |
| | \ ++|B | \ / |
++++ \-----| | \ ++++ / ++++
++++ \|C |/ |C |
| | | |
++++ ++++
Figure 2.1: Examples of the four scenarios: peer-to-peer, simple
one-to-many, many-to-many without a centralized server (also denoted
as small interactive group), and many-to-many with a centralized
server.
We identify in the following some typical scenarios which involve the
multimedia applications we are dealing with (see also Figure 2.1).
a) peer-to-peer (unicast), e.g., a SIP-based [SIP] call between two
parties, where it may be desirable that the security is either set
up by mutual agreement or that each party sets up the security for
its own outgoing streams.
b) simple one-to-many (multicast), e.g., real-time presentations,
where the sender is in charge of setting up the security.
c) many-to-many, without a centralized control unit, e.g., for
small-size interactive groups where each party may set up the
security for its own outgoing media. Two basic models may be used
here. In the first model, the Initiator of the group acts as the
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RFC 3830 MIKEY August 2004
group server (and is the only one authorized to include new
members). In the second model, authorization information to
include new members can be delegated to other participants.
d) many-to-many, with a centralized control unit, e.g., for larger
groups with some kind of Group Controller that sets up the
security.
The key management solutions may be different in the above scenarios.
When designing MIKEY, the main focus has been on case a, b, and c.
For scenario c, only the first model is covered by this document.
2.2. Design Goals
The key management protocol is designed to have the following
characteristics:
* End-to-end security. Only the participants involved in the
communication have access to the generated key(s).
* Simplicity.
* Efficiency. Designed to have:
- low bandwidth consumption,
- low computational workload,
- small code size, and
- minimal number of roundtrips.
* Tunneling. Possibility to "tunnel"/integrate MIKEY in session
establishment protocols (e.g., SDP and RTSP).
* Independence from any specific security functionality of the
underlying transport.
2.3. System Overview
One objective of MIKEY is to produce a Data SA for the security
protocol, including a traffic-encrypting key (TEK), which is derived
from a TEK Generation Key (TGK), and used as input for the security
protocol.
MIKEY supports the possibility of establishing keys and parameters
for more than one security protocol (or for several instances of the
same security protocol) at the same time. The concept of Crypto
Session Bundle (CSB) is used to denote a collection of one or more
Crypto Sessions that can have common TGK and security parameters, but
which obtain distinct TEKs from MIKEY.
Arkko, et al. Standards Track [Page 8]
RFC 3830 MIKEY August 2004
The procedure of setting up a CSB and creating a TEK (and Data SA),
is done in accordance with Figure 2.2:
1. A set of security parameters and TGK(s) are agreed upon for the
Crypto Session Bundle (this is done by one of the three
alternative key transport/exchange mechanisms, see Section 3).
2. The TGK(s) is used to derive (in a cryptographically secure way) a
TEK for each Crypto Session.
3. The TEK, together with the security protocol parameters, represent
the Data SA, which is used as the input to the security protocol.
+-----------------+
| CSB |
| Key transport | (see Section 3)
| /exchange |
+-----------------+
| :
| TGK :
v :
+----------+ :
CS ID ->| TEK | : Security protocol (see Section 4)
|derivation| : parameters (policies)
+----------+ :
TEK | :
v v
Data SA
|
v
+-------------------+
| Crypto Session |
|(Security Protocol)|
+-------------------+
Figure 2.2: Overview of MIKEY key management procedure.
The security protocol can then either use the TEK directly, or, if
supported, derive further session keys from the TEK (e.g., see SRTP
[SRTP]). It is however up to the security protocol to define how the
TEK is used.
MIKEY can be used to update TEKs and the Crypto Sessions in a current
Crypto Session Bundle (see Section 4.5). This is done by executing
the transport/exchange phase once again to obtain a new TGK (and
consequently derive new TEKs) or to update some other specific CS
parameters.
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RFC 3830 MIKEY August 2004
2.4. Relation to GKMARCH
The Group key management architecture (GKMARCH) [GKMARCH] describes a
general architecture for group key management protocols. MIKEY is a
part of this architecture, and can be used as a so-called
Registration protocol. The main entities involved in the
architecture are the group controller/key server (GCKS), the
receiver(s), and the sender(s).
In MIKEY, the sender could act as GCKS and push keys down to the
receiver(s).
Note that, for example, in a SIP-initiated call, the sender may also
be a receiver. As MIKEY addresses small interactive groups, a member
may dynamically change between being a sender and receiver (or being
both simultaneously).
3. Basic Key Transport and Exchange Methods
The following sub-sections define three different methods of
transporting/establishing a TGK: with the use of a pre-shared key,
public-key encryption, and Diffie-Hellman (DH) key exchange. In the
following, we assume unicast communication for simplicity. In
addition to the TGK, a random "nonce", denoted RAND, is also
transported. In all three cases, the TGK and RAND values are then
used to derive TEKs as described in Section 4.1.3. A timestamp is
also sent to avoid replay attacks (see Section 5.4).
The pre-shared key method and the public-key method are both based on
key transport mechanisms, where the actual TGK is pushed (securely)
to the recipient(s). In the Diffie-Hellman method, the actual TGK is
instead derived from the Diffie-Hellman values exchanged between the
peers.
The pre-shared case is, by far, the most efficient way to handle the
key transport due to the use of symmetric cryptography only. This
approach also has the advantage that only a small amount of data has
to be exchanged. Of course, the problematic issue is scalability as
it is not always feasible to share individual keys with a large group
of peers. Therefore, this case mainly addresses scenarios such as
server-to-client and also those cases where the public-key modes have
already been used, thus allowing for the "cache" of a symmetric key
(see below and Section 3.2).
Public-key cryptography can be used to create a scalable system. A
disadvantage with this approach is that it is more resource consuming
than the pre-shared key approach. Another disadvantage is that in
most cases, a PKI (Public Key Infrastructure) is needed to handle the
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RFC 3830 MIKEY August 2004
distribution of public keys. Of course, it is possible to use public
keys as pre-shared keys (e.g., by using self-signed certificates).
It should also be noted that, as mentioned above, this method may be
used to establish a "cached" symmetric key that later can be used to
establish subsequent TGKs by using the pre-shared key method (hence,
the subsequent request can be executed more efficiently).
In general, the Diffie-Hellman (DH) key agreement method has a higher
resource consumption (both computationally and in bandwidth) than the
previous ones, and needs certificates as in the public-key case.
However, it has the advantage of providing perfect forward secrecy
(PFS) and flexibility by allowing implementation in several different
finite groups.
Note that by using the DH method, the two involved parties will
generate a unique unpredictable random key. Therefore, it is not
possible to use this DH method to establish a group TEK (as the
different parties in the group would end up with different TEKs). It
is not the intention of the DH method to work in this scenario, but
to be a good alternative in the special peer-to-peer case.
The following general notation is used:
HDR: The general MIKEY header, which includes MIKEY CSB related data
(e.g., CSB ID) and information mapping to the specific security
protocol used. See Section 6.1 for payload definition.
T: The timestamp, used mainly to prevent replay attacks. See
Section 6.6 for payload definition and also Section 5.4 for other
timestamp related information.
IDx: The identity of entity x (IDi=Initiator, IDr=Responder). See
Section 6.7 for payload definition.
RAND: Random/pseudo-random byte-string, which is always included in
the first message from the Initiator. RAND is used as a freshness
value for the key generation. It is not included in update messages
of a CSB. See Section 6.11 for payload definition. For randomness
recommendations for security, see [RAND].
SP: The security policies for the data security protocol. See
Section 6.10 for payload definition.
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3.1. Pre-shared key
In this method, the pre-shared secret key, s, is used to derive key
material for both the encryption (encr_key) and the integrity
protection (auth_key) of the MIKEY messages, as described in Section
4.1.4. The encryption and authentication transforms are described in
Section 4.2.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi],[IDr],
{SP}, KEMAC --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
The main objective of the Initiator's message (I_MESSAGE) is to
transport one or more TGKs (carried into KEMAC) and a set of security
parameters (SPs) to the Responder in a secure manner. As the
verification message from the Responder is optional, the Initiator
indicates in the HDR whether it requires a verification message or
not from the Responder.
KEMAC = E(encr_key, {TGK}) || MAC
The KEMAC payload contains a set of encrypted sub-payloads and a MAC.
Each sub-payload includes a TGK randomly and independently chosen by
the Initiator (and other possible related parameters, e.g., the key
lifetime). The MAC is a Message Authentication Code covering the
entire MIKEY message using the authentication key, auth_key. See
Section 6.2 for payload definition and Section 5.2 for an exact
definition of the MAC calculation.
The main objective of the verification message from the Responder is
to obtain mutual authentication. The verification message, V, is a
MAC computed over the Responder's entire message, the timestamp (the
same as the one that was included in the Initiator's message), and
the two parties identities, using the authentication key. See also
Section 5.2 for the exact definition of the Verification MAC
calculation and Section 6.9 for payload definition.
The ID fields SHOULD be included, but they MAY be left out when it
can be expected that the peer already knows the other party's ID
(otherwise it cannot look up the pre-shared key). For example, this
could be the case if the ID is extracted from SIP.
It is MANDATORY to implement this method.
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RFC 3830 MIKEY August 2004
3.2. Public-key encryption
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi], [IDr], {SP},
KEMAC, [CHASH], PKE, SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
As in the previous case, the main objective of the Initiator's
message is to transport one or more TGKs and a set of security
parameters to the Responder in a secure manner. This is done using
an envelope approach where the TGKs are encrypted (and integrity
protected) with keys derived from a randomly/pseudo-randomly chosen
"envelope key". The envelope key is sent to the Responder encrypted
with the public key of the Responder.
The PKE contains the encrypted envelope key: PKE = E(PKr, env_key).
It is encrypted using the Responder's public key (PKr). If the
Responder possesses several public keys, the Initiator can indicate
the key used in the CHASH payload (see Section 6.8).
The KEMAC contains a set of encrypted sub-payloads and a MAC:
KEMAC = E(encr_key, IDi || {TGK}) || MAC
The first payload (IDi) in KEMAC is the identity of the Initiator
(not a certificate, but generally the same ID as the one specified in
the certificate). Each of the following payloads (TGK) includes a
TGK randomly and independently chosen by the Initiator (and possible
other related parameters, e.g., the key lifetime). The encrypted
part is then followed by a MAC, which is calculated over the KEMAC
payload. The encr_key and the auth_key are derived from the envelope
key, env_key, as specified in Section 4.1.4. See also Section 6.2
for payload definition.
The SIGNi is a signature covering the entire MIKEY message, using the
Initiator's signature key (see also Section 5.2 for the exact
definition).
The main objective of the verification message from the Responder is
to obtain mutual authentication. As the verification message V from
the Responder is optional, the Initiator indicates in the HDR whether
it requires a verification message or not from the Responder. V is
calculated in the same way as in the pre-shared key mode (see also
Section 5.2 for the exact definition). See Section 6.9 for payload
definition.
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Note that there will be one encrypted IDi and possibly also one
unencrypted IDi. The encrypted one is used together with the MAC as
a countermeasure for certain man-in-the-middle attacks, while the
unencrypted one is always useful for the Responder to immediately
identify the Initiator. The encrypted IDi MUST always be verified to
be equal with the expected IDi.
It is possible to cache the envelope key, so that it can be used as a
pre-shared key. It is not recommended for this key to be cached
indefinitely (however it is up to the local policy to decide this).
This function may be very convenient during the lifetime of a CSB, if
a new crypto session needs to be added (or an expired one removed).
Then, the pre-shared key can be used, instead of the public keys (see
also Section 4.5). If the Initiator indicates that the envelope key
should be cached, the key is at least to be cached during the
lifetime of the entire CSB.
The cleartext ID fields and certificate SHOULD be included, but they
MAY be left out when it can be expected that the peer already knows
the other party's ID, or can obtain the certificate in some other
manner. For example, this could be the case if the ID is extracted
from SIP.
For certificate handling, authorization, and policies, see Section
4.3.
It is MANDATORY to implement this method.
3.3. Diffie-Hellman key exchange
For a fixed, agreed upon, cyclic group, (G,*), we let g denote a
generator for this group. Choices for the parameters are given in
Section 4.2.7. The other transforms below are described in Section
4.2.
This method creates a DH-key, which is used as the TGK. This method
cannot be used to create group keys; it can only be used to create
single peer-to-peer keys. It is OPTIONAL to implement this method.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],[IDr]
{SP}, DHi, SIGNi --->
R_MESSAGE =
<--- HDR, T, [IDr|CERTr], IDi,
DHr, DHi, SIGNr
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The main objective of the Initiator's message is to, in a secure way,
provide the Responder with its DH value (DHi) g^(xi), where xi MUST
be randomly/pseudo-randomly and secretly chosen, and a set of
security protocol parameters.
The SIGNi is a signature covering the Initiator's MIKEY message,
I_MESSAGE, using the Initiator's signature key (see Section 5.2 for
the exact definition).
The main objective of the Responder's message is to, in a secure way,
provide the Initiator with the Responder's value (DHr) g^(xr), where
xr MUST be randomly/pseudo-randomly and secretly chosen. The
timestamp that is included in the answer is the same as the one
included in the Initiator's message.
The SIGNr is a signature covering the Responder's MIKEY message,
R_MESSAGE, using the Responder's signature key (see Section 5.2 for
the exact definition).
The DH group parameters (e.g., the group G, the generator g) are
chosen by the Initiator and signaled to the Responder. Both parties
calculate the TGK, g^(xi*xr) from the exchanged DH-values.
Note that this approach does not require that the Initiator has to
possess any of the Responder's certificates before the setup.
Instead, it is sufficient that the Responder includes its signing
certificate in the response.
The ID fields and certificate SHOULD be included, but they MAY be
left out when it can be expected that the peer already knows the
other party's ID (or can obtain the certificate in some other
manner). For example, this could be the case if the ID is extracted
from SIP.
For certificate handling, authorization, and policies, see Section
4.3.
4. Selected Key Management Functions
MIKEY manages symmetric keys in two main ways. First, following key
transport or key exchange of TGK(s) (and other parameters) as defined
by any of the above three methods, MIKEY maintains a mapping between
Data SA identifiers and Data SAs, where the identifiers used depend
on the security protocol in question, see Section 4.4. Thus, when
the security protocol requests a Data SA, given such a Data SA
identifier, an up-to-date Data SA will be obtained. In particular,
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correct keying material, TEK(s), might need to be derived. The
derivation of TEK(s) (and other keying material) is done from a TGK
and is described in Section 4.1.3.
Second, for use within MIKEY itself, two key management procedures
are needed:
* in the pre-shared case, deriving encryption and authentication key
material from a single pre-shared key, and
* in the public key case, deriving similar key material from the
transported envelope key.
These two key derivation methods are specified in section 4.1.4.
All the key derivation functionality mentioned above is based on a
pseudo-random function, defined next.
4.1. Key Calculation
In the following, we define a general method (pseudo-random function)
to derive one or more keys from a "master" key. This method is used
to derive:
* TEKs from a TGK and the RAND value,
* encryption, authentication, or salting key from a pre-shared/
envelope key and the RAND value.
4.1.1. Assumptions
We assume that the following parameters are in place:
csb_id : Crypto Session Bundle ID (32-bits unsigned integer)
cs_id : the Crypto Session ID (8-bits unsigned integer)
RAND : (at least) 128-bit (pseudo-)random bit-string sent by the
Initiator in the initial exchange.
The key derivation method has the following input parameters:
inkey : the input key to the derivation function
inkey_len : the length in bits of the input key
label : a specific label, dependent on the type of the key to be
derived, the RAND, and the session IDs
outkey_len: desired length in bits of the output key.
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The key derivation method has the following output:
outkey: the output key of desired length.
4.1.2. Default PRF Description
Let HMAC be the SHA-1 based message authentication function, see
[HMAC] [SHA-1]. Similarly to [TLS], we define:
P (s, label, m) = HMAC (s, A_1 || label) ||
HMAC (s, A_2 || label) || ...
HMAC (s, A_m || label)
where
A_0 = label,
A_i = HMAC (s, A_(i-1))
s is a key (defined below)
m is a positive integer (also defined below).
Values of label depend on the case in which the PRF is invoked, and
values are specified in the following for the default PRF. Thus,
note that other PRFs later added to MIKEY MAY specify different input
parameters.
The following procedure describes a pseudo-random function, denoted
PRF(inkey,label), based on the above P-function, applied to compute
the output key, outkey:
* let n = inkey_len / 256, rounded up to the nearest integer if not
already an integer
* split the inkey into n blocks, inkey = s_1 || ... || s_n, where *
all s_i, except possibly s_n, are 256 bits each
* let m = outkey_len / 160, rounded up to the nearest integer if not
already an integer
(The values "256" and "160" equals half the input block-size and full
output hash size, respectively, of the SHA-1 hash as part of the P-
function.)
Then, the output key, outkey, is obtained as the outkey_len most
significant bits of
PRF(inkey, label) = P(s_1, label, m) XOR P(s_2, label, m) XOR ...
XOR P(s_n, label, m).
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4.1.3. Generating keys from TGK
In the following, we describe how keying material is derived from a
TGK, thus assuming that a mapping of the Data SA identifier to the
correct TGK has already been done according to Section 4.4.
The key derivation method SHALL be executed using the above PRF with
the following input parameters:
inkey : TGK
inkey_len : bit length of TGK
label : constant || cs_id || csb_id || RAND
outkey_len : bit length of the output key.
The constant part of label depends on the type of key that is to be
generated. The constant 0x2AD01C64 is used to generate a TEK from
TGK. If the security protocol itself does not support key derivation
for authentication and encryption from the TEK, separate
authentication and encryption keys MAY be created directly for the
security protocol by replacing 0x2AD01C64 with 0x1B5C7973 and
0x15798CEF respectively, and outkey_len by the desired key-length(s)
in each case.
A salt key can be derived from the TGK as well, by using the constant
0x39A2C14B. Note that the Key data sub-payload (Section 6.13) can
carry a salt. The security protocol in need of the salt key SHALL
use the salt key carried in the Key data sub-payload (in the pre-
shared and public-key case), when present. If that is not sent, then
it is possible to derive the salt key via the key derivation
function, as described above.
The table below summarizes the constant values, used to generate keys
from a TGK.
constant | derived key from the TGK
--------------------------------------
0x2AD01C64 | TEK
0x1B5C7973 | authentication key
0x15798CEF | encryption key
0x39A2C14B | salting key
Table 4.1.3: Constant values for the derivation of keys from TGK.
Note that these 32-bit constant values (listed in the table above)
are taken from the decimal digits of e (i.e., 2.7182...), where each
constant consists of nine decimal digits (e.g., the first nine
decimal digits 718281828 = 0x2AD01C64). The strings of nine
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decimal digits are not chosen at random, but as consecutive "chunks"
from the decimal digits of e.
4.1.4. Generating keys for MIKEY messages from an envelope/pre-shared
key
This derivation is to form the symmetric encryption key (and salting
key) for the encryption of the TGK in the pre-shared key and public
key methods. This is also used to derive the symmetric key used for
the message authentication code in these messages, and the
corresponding verification messages. Hence, this derivation is
needed in order to get different keys for the encryption and the MAC
(and in the case of the pre-shared key, it will result in fresh key
material for each new CSB). The parameters for the default PRF are
here:
inkey : the envelope key or the pre-shared key
inkey_len : the bit length of inkey
label : constant || 0xFF || csb_id || RAND
outkey_len : desired bit length of the output key.
The constant part of label depends on the type of key that is to be
generated from an envelope/pre-shared key, as summarized below.
constant | derived key
--------------------------------------
0x150533E1 | encryption key
0x2D22AC75 | authentication key
0x29B88916 | salt key
Table 4.1.4: Constant values for the derivation of keys from an
envelope/pre-shared key.
4.2. Pre-defined Transforms and Timestamp Formats
This section identifies default transforms for MIKEY. It is
mandatory to implement and support the following transforms in the
respective case. New transforms can be added in the future (see
Section 4.2.9 for further guidelines).
4.2.1. Hash functions
In MIKEY, it is MANDATORY to implement SHA-1 as the default hash
function.
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4.2.2. Pseudo-random number generator and PRF
A cryptographically secure random or pseudo-random number generator
MUST be used for the generation of the keying material and nonces,
e.g., [BMGL]. However, which one to use is implementation specific
(as the choice will not affect the interoperability).
For the key derivations, it is MANDATORY to implement the PRF
specified in Section 4.1. Other PRFs MAY be added by writing
standard-track RFCs specifying the PRF constructions and their exact
use within MIKEY.
4.2.3. Key data transport encryption
The default and mandatory-to-implement key transport encryption is
AES in counter mode, as defined in [SRTP], using a 128-bit key as
derived in Section 4.1.4, SRTP_PREFIX_LENGTH set to zero, and using
the initialization vector
IV = (S XOR (0x0000 || CSB ID || T)) || 0x0000,
where S is a 112-bit salting key, also derived as in Section 4.1.4,
and where T is the 64-bit timestamp sent by the Initiator.
Note: this restricts the maximum size that can be encrypted to 2^23
bits, which is still enough for all practical purposes [SRTP].
The NULL encryption algorithm (i.e., no encryption) can be used (but
implementation is OPTIONAL). Note that this MUST NOT be used unless
the underlying protocols can guarantee security. The main reason for
including this is for specific SIP scenarios, where SDP is protected
end-to-end. For this scenario, MIKEY MAY be used with the pre-shared
key method, the NULL encryption, and NULL authentication algorithm
(see Section 4.2.4) while relying on the security of SIP. Use this
option with caution!
The AES key wrap function [AESKW] is included as an OPTIONAL
implementation method. If the key wrap function is used in the
public key method, the NULL MAC is RECOMMENDED to be used, as the key
wrap itself will provide integrity of the encrypted content (note
though that the NULL MAC SHOULD NOT be used in the pre-shared key
case, as the MAC in that case covers the entire message). The 128-
bit key and a 64-bit salt, S, are derived in accordance to Section
4.1.4 and the key wrap IV is then set to S.
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4.2.4. MAC and Verification Message function
MIKEY uses a 160-bit authentication tag, generated by HMAC with SHA-1
as the MANDATORY implementation method, see [HMAC]. Authentication
keys are derived according to Section 4.1.4. Note that the
authentication key size SHOULD be equal to the size of the hash
function's output (e.g., for HMAC-SHA-1, a 160-bit authentication key
is used) [HMAC].
The NULL authentication algorithm (i.e., no MAC) can be used together
with the NULL encryption algorithm (but implementation is OPTIONAL).
Note that this MUST NOT be used unless the underlying protocols can
guarantee security. The main reason for including this is for
specific SIP scenarios, where SDP is protected end-to-end. For this
scenario, MIKEY MAY be used with the pre-shared key method and the
NULL encryption and authentication algorithm, while relying on the
security of SIP. Use this option with caution!
4.2.5. Envelope Key encryption
The public key encryption algorithm applied is defined by, and
dependent on the certificate used. It is MANDATORY to support RSA
PKCS#1, v1.5, and it is RECOMMENDED to also support RSA OAEP [PSS].
4.2.6. Digital Signatures
The signature algorithm applied is defined by, and dependent on the
certificate used. It is MANDATORY to support RSA PKCS#1, v1.5, and it
is RECOMMENDED to also support RSA PSS [PSS].
4.2.7. Diffie-Hellman Groups
The Diffie-Hellman key exchange, when supported, uses OAKLEY 5
[OAKLEY] as a mandatory implementation. Both OAKLEY 1 and OAKLEY 2
MAY be used (but these are OPTIONAL implementations).
See Section 4.2.9 for the guidelines on specifying a new DH Group to
be used within MIKEY.
4.2.8. Timestamps
The timestamp is as defined in NTP [NTP], i.e., a 64-bit number in
seconds relative to 0h on 1 January 1900. An implementation MUST be
aware of (and take into account) the fact that the counter will
overflow approximately every 136th year. It is RECOMMENDED that the
time always be specified in UTC.
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4.2.9. Adding new parameters to MIKEY
There are two different parameter sets that can be added to MIKEY.
The first is a set of MIKEY transforms (needed for the exchange
itself), and the second is the Data SAs.
New transforms and parameters (including new policies) SHALL be added
by registering with IANA (according to [RFC2434], see also Section
10) a new number for the concerned payload, and also if necessary,
documenting how the new transform/parameter is used. Sometimes it
might be enough to point to an already specified document for the
usage, e.g., when adding a new, already standardized, hash function.
In the case of adding a new DH group, the group MUST be specified in
a companion standards-track RFC (it is RECOMMENDED that the specified
group use the same format as used in [OAKLEY]). A number can then be
assigned by IANA for such a group to be used in MIKEY.
When adding support for a new data security protocol, the following
MUST be specified:
* A map sub-payload (see Section 6.1). This is used to be able to
map a crypto session to the right instance of the data security
protocol and possibly also to provide individual parameters for
each data security protocol.
* A policy payload, i.e., specification of parameters and supported
values.
* General guidelines of usage.
4.3. Certificates, Policies and Authorization
4.3.1. Certificate handling
Certificate handling may involve a number of additional tasks not
shown here, and effect the inclusion of certain parts of the message
(c.f. [X.509]). However, the following observations can be made:
* The Initiator typically has to find the certificate of the
Responder in order to send the first message. If the Initiator
does not already have the Responder's certificate, this may
involve one or more roundtrips to a central directory agent.
* It will be possible for the Initiator to omit its own certificate
and rely on the Responder getting this certificate using other
means. However, we only recommend doing this when it is
reasonable to expect that the Responder has cached the certificate
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from a previous connection. Otherwise accessing the certificate
would mean additional roundtrips for the Responder as well.
* Verification of the certificates using Certificate Revocation
Lists (CRLs) [X.509] or protocols such as OCSP [OCSP] may be
necessary. All parties in a MIKEY exchange should have a local
policy which dictates whether such checks are made, how they are
made, and how often they are made. Note that performing the
checks may imply additional messaging.
4.3.2. Authorization
In general, there are two different models for making authorization
decisions for both the Initiator and the Responder, in the context of
the applications targeted by MIKEY:
* Specific peer-to-peer configuration. The user has configured the
application to trust a specific peer.
When pre-shared secrets are used, this is pretty much the only
available scheme. Typically, the configuration/entering of the
pre-shared secret is taken to mean that authorization is implied.
In some cases, one could also use this with public keys, e.g., if
two peers exchange keys offline and configure them to be used for
the purpose of running MIKEY.
* Trusted root. The user accepts all peers that prove to have a
certificate issued by a specific CA. The granularity of
authorization decisions is not very precise in this method.
In order to make this method possible, all participants in the
MIKEY protocol need to configure one or more trusted roots. The
participants also need to be capable of performing certificate
chain validation, and possibly transfer more than a single
certificate in the MIKEY messages (see also Section 6.7).
In practice, a combination of both mentioned methods might be
advantageous. Also, the possibility for a user to explicitly exclude
a specific peer (or sub-tree) in a trust chain might be needed.
These authorization policies address the MIKEY scenarios a-c of
Section 2.1, where the Initiator acts as the group owner and is also
the only one that can invite others. This implies that for each
Responder, the distributed keys MUST NOT be re-distributed to other
parties.
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In a many-to-many situation, where the group control functions are
distributed (and/or where it is possible to delegate the group
control function to others), a means of distributing authorization
information about who may be added to the group MUST exist. However,
it is out of scope of this document to specify how this should be
done.
For any broader communication situation, an external authorization
infrastructure may be used (following the assumptions of [GKMARCH]).
4.3.3. Data Policies
Included in the message exchange, policies (i.e., security
parameters) for the Data security protocol are transmitted. The
policies are defined in a separate payload and are specific to the
security protocol (see also Section 6.10). Together with the keys,
the validity period of these can also be specified. For example,
this can be done with an SPI (or SRTP MKI) or with an Interval (e.g.,
a sequence number interval for SRTP), depending on the security
protocol.
New parameters can be added to a policy by documenting how they
should be interpreted by MIKEY and by also registering new values in
the appropriate name space in IANA. If a completely new policy is
needed, see Section 4.2.9 for guidelines.
4.4. Retrieving the Data SA
The retrieval of a Data SA will depend on the security protocol, as
different security protocols will have different characteristics.
When adding support for a security protocol to MIKEY, some interface
of how the security protocol retrieves the Data SA from MIKEY MUST be
specified (together with policies that can be negotiated).
For SRTP, the SSRC (see [SRTP]) is one of the parameters used to
retrieve the Data SA (while the MKI may be used to indicate the
TGK/TEK used for the Data SA). However, the SSRC is not sufficient.
For the retrieval of the Data SA from MIKEY, it is RECOMMENDED that
the MIKEY implementation support a lookup using destination network
address and port together with SSRC. Note that MIKEY does not send
network addresses or ports. One reason for this is that they may not
be known in advance. Also, if a NAT exists in-between, problems may
arise. When SIP or RTSP is used, the local view of the destination
address and port can be obtained from either SIP or RTSP. MIKEY can
then use these addresses as the index for the Data SA lookup.
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4.5. TGK re-keying and CSB updating
MIKEY provides a means of updating the CSB (e.g., transporting a new
TGK/TEK or adding a new Crypto Session to the CSB). The updating of
the CSB is done by executing MIKEY again, for example, before a TEK
expires, or when a new Crypto Session is added to the CSB. Note that
MIKEY does not provide re-keying in the GKMARCH sense, only updating
of the keys by normal unicast messages.
When MIKEY is executed again to update the CSB, it is not necessary
to include certificates and other information that was provided in
the first exchange, for example, all payloads that are static or
optionally included may be left out (see Figure 4.1).
The new message exchange MUST use the same CSB ID as the initial
exchange, but MUST use a new timestamp. A new RAND MUST NOT be
included in the message exchange (the RAND will only have effect in
the Initial exchange). If desired, new Crypto Sessions are added in
the update message. Note that a MIKEY update message does not need
to contain new keying material (e.g., new TGK). In this case, the
crypto session continues to use the previously established keying
material, while updating the new information.
As explained in Section 3.2, the envelope key can be "cached" as a
pre-shared key (this is indicated by the Initiator in the first
message sent). If so, the update message is a pre-shared key message
with the cached envelope key as the pre-shared key; it MUST NOT be a
public key message. If the public key message is used, but the
envelope key is not cached, the Initiator MUST provide a new
encrypted envelope key that can be used in the verification message.
However, the Initiator does not need to provide any other keys.
Figure 4.1 visualizes the update messages that can be sent, including
the optional parts. The main difference from the original message is
that it is optional to include TGKs (or DH values in the DH method).
Also see Section 3 for more details on the specific methods.
By definition, a CSB can contain several CSs. A problem that then
might occur is to synchronize the TGK re-keying if an SPI (or similar
functionality, e.g., MKI in [SRTP]) is not used. It is therefore
RECOMMENDED that an SPI or MKI be used, if more than one CS is
present.
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Initiator Responder
Pre-shared key method:
I_MESSAGE =
HDR, T, [IDi], [IDr], {SP}, KEMAC --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
Public key method:
I_MESSAGE =
HDR, T, [IDi|CERTi], [IDr], {SP},
[KEMAC], [CHASH], PKE, SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
DH method:
I_MESSAGE =
HDR, T, [IDi|CERTi], [IDr], {SP},
[DHi], SIGNi --->
R_MESSAGE =
<--- HDR, T, [IDr|CERTr], IDi,
[DHr, DHi], SIGNr
Figure 4.1: Update messages.
Note that for the DH method, if the Initiator includes the DHi
payload, then the Responder MUST include DHr and DHi. If the
Initiator does not include DHi, the Responder MUST NOT include DHr or
DHi.
5. Behavior and message handling
Each message that is sent by the Initiator or the Responder is built
by a set of payloads. This section describes how messages are
created and also when they can be used.
5.1. General
5.1.1. Capability Discovery
The Initiator indicates the security policy to be used (i.e., in
terms of security protocol algorithms). If the Responder does not
support it (for some reason), the Responder can together with an
error message (indicating that it does not support the parameters),
send back its own capabilities (negotiation) to let the Initiator
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choose a common set of parameters. This is done by including one or
more security policy payloads in the error message sent in response
(see Section 5.1.2.). Multiple attributes can be provided in
sequence in the response. This is done to reduce the number of
roundtrips as much as possible (i.e., in most cases, where the policy
is accepted the first time, one roundtrip is enough). If the
Responder does not accept the offer, the Initiator must go out with a
new MIKEY message.
If the Responder is not willing/capable of providing security or the
parties simply cannot agree, it is up to the parties' policies how to
behave, for example, accepting or rejecting an insecure
communication.
Note that it is not the intention of this protocol to have a broad
variety of options, as it is assumed that a denied offer should
rarely occur.
In the one-to-many and many-to-many scenarios using multicast
communication, one issue is of course that there MUST be a common
security policy for all the receivers. This limits the possibility
of negotiation.
5.1.2. Error Handling
Due to the key management protocol, all errors SHOULD be reported to
the peer(s) by an error message. The Initiator SHOULD therefore
always be prepared to receive such a message from the Responder.
If the Responder does not support the set of parameters suggested by
the Initiator, the error message SHOULD include the supported
parameters (see also Section 5.1.1).
The error message is formed as:
HDR, T, {ERR}, {SP}, [V|SIGNr]
Note that if failure is due to the inability to authenticate the
peer, the error message is OPTIONAL, and does not need to be
authenticated. It is up to local policy to determine how to treat
this kind of message. However, if in response to a failed
authentication a signed error message is returned, this can be used
for DoS purposes (against the Responder). Similarly, an
unauthenticated error message could be sent to the Initiator in order
to fool the Initiator into tearing down the CSB. It is highly
RECOMMENDED that the local policy take this into consideration.
Therefore, in case of authentication failure, one recommendation
would be not to authenticate such an error message, and when
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RFC 3830 MIKEY August 2004
receiving an unauthenticated error message view it only as a
recommendation of what may have gone wrong.
5.2. Creating a message
To create a MIKEY message, a Common Header payload is first created.
This payload is then followed, depending on the message type, by a
set of information payloads (e.g., DH-value payload, Signature
payload, Security Policy payload). The defined payloads and the
exact encoding of each payload are described in Section 6.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! version ! data type ! next payload ! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... +
~ Common Header... ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! next payload ! Payload 1 ... !
+-+-+-+-+-+-+-+-+ +
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: : :
: : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! next payload ! Payload x ... !
+-+-+-+-+-+-+-+-+ +
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MAC/Signature ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5.1. MIKEY payload message example. Note that the payloads
are byte aligned and not 32-bit aligned.
The process of generating a MIKEY message consists of the following
steps:
* Create an initial MIKEY message starting with the Common Header
payload.
* Concatenate necessary payloads of the MIKEY message (see the
exchange definitions for payloads that may be included, and the
recommended order).
* As a last step (for messages that must be authenticated, this also
includes the verification message), create and concatenate the
MAC/signature payload without the MAC/signature field filled in
Arkko, et al. Standards Track [Page 28]
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(if a Next payload field is included in this payload, it is set to
Last payload).
* Calculate the MAC/signature over the entire MIKEY message, except
the MAC/Signature field, and add the MAC/signature in the field.
In the case of the verification message, the Identity_i ||
Identity_r || Timestamp MUST directly follow the MIKEY message in
the Verification MAC calculation. Note that the added identities
and timestamp are identical to those transported in the ID and T
payloads.
In the public key case, the Key data transport payload is generated
by concatenating the IDi with the TGKs. This is then encrypted and
placed in the data field. The MAC is calculated over the entire Key
data transport payload except the MAC field. Before calculating the
MAC, the Next payload field is set to zero.
Note that all messages from the Initiator MUST use a unique
timestamp. The Responder does not create a new timestamp, but uses
the timestamp used by the Initiator.
5.3. Parsing a message
In general, parsing of a MIKEY message is done by extracting payload
by payload and checking that no errors occur. The exact procedure is
implementation specific; however, for the Responder, it is
RECOMMENDED that the following procedure be followed:
* Extract the Timestamp and check that it is within the allowable
clock skew (if not, discard the message). Also check the replay
cache (Section 5.4) so that the message is not replayed (see
Section 5.4). If the message is replayed, discard it.
* Extract the ID and authentication algorithm (if not included,
assume the default).
* Verify the MAC/signature.
* If the authentication is not successful, an Auth failure Error
message MAY be sent to the Initiator. The message is then
discarded from further processing. See also Section 5.1.2 for
treatment of errors.
* If the authentication is successful, the message is processed and
also added to the replay cache; processing is implementation
specific. Note also that only successfully authenticated messages
are stored in the replay cache.
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* If any unsupported parameters or errors occur during the
processing, these MAY be reported to the Initiator by sending an
error message. The processing is then aborted. The error message
can also include payloads to describe the supported parameters.
* If the processing was successful and in case the Initiator
requested it, a verification/response message MAY be created and
sent to the Initiator.
5.4. Replay handling and timestamp usage
MIKEY does not use a challenge-response mechanism for replay
handling; instead, timestamps are used. This requires that the
clocks are synchronized. The required synchronization is dependent
on the number of messages that can be cached (note though, that the
replay cache only contains messages that have been successfully
authenticated). If we could assume an unlimited cache, the terminals
would not need to be synchronized at all (as the cache could then
contain all previous messages). However, if there are restrictions
on the size of the replay cache, the clocks will need to be
synchronized to some extent. In short, one can in general say that
it is a tradeoff between the size of the replay cache and the
required synchronization.
Timestamp usage prevents replay attacks under the following
assumptions:
* Each host has a clock which is at least "loosely synchronized"
with the clocks of the other hosts.
* If the clocks are to be synchronized over the network, a secure
network clock synchronization protocol SHOULD be used, e.g.,
[ISO3].
* Each Responder utilizes a replay cache in order to remember the
successfully authenticated messages presented within an allowable
clock skew (which is set by the local policy).
* Replayed and outdated messages, for example, messages that can be
found in the replay cache or which have an outdated timestamp are
discarded and not processed.
* If the host loses track of the incoming requests (e.g., due to
overload), it rejects all incoming requests until the clock skew
interval has passed.
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In a client-server scenario, servers may encounter a high workload,
especially if a replay cache is necessary. However, servers that
assume the role of MIKEY Initiators will not need to manage any
significant replay cache as they will refuse all incoming messages
that are not a response to a message previously sent by the server.
In general, a client may not expect a very high load of incoming
messages and may therefore allow the degree of looseness to be on the
order of several minutes to hours. If a (D)DoS attack is launched
and the replay cache grows too large, MIKEY MAY dynamically decrease
the looseness so that the replay cache becomes manageable. However,
note that such (D)DoS attacks can only be performed by peers that can
authenticate themselves. Hence, such an attack is very easy to trace
and mitigate.
The maximum number of messages that a client will need to cache may
vary depending on the capacity of the client itself and the network.
The number of expected messages should be taken into account.
For example, assume that we can at most spend 6kB on a replay cache.
Assume further that we need to store 30 bytes for each incoming
authenticated message (the hash of the message is 20 bytes). This
implies that it is possible to cache approximately 204 messages. If
the expected number of messages per minute can be estimated, the
clock skew can easily be calculated. For example, in a SIP scenario
where the client is expected, in the most extreme case, to receive 10
calls per minute, the clock skew needed is then approximately 20
minutes. In a not so extreme setting, where one could expect an
incoming call every 5th minute, this would result in a clock skew on
the order of 16.5 hours (approx 1000 minutes).
Consider a very extreme case, where the maximum number of incoming
messages are assumed to be on the order of 120 messages per minute,
and a requirement that the clock skew is on the order of 10 minutes,
a 48kB replay cache would be required.
Hence, one can note that the required clock skew will depend largely
on the setting in which MIKEY is used. One recommendation is to fix
a size for the replay cache, allowing the clock skew to be large (the
initial clock skew can be set depending on the application in which
it is used). As the replay cache grows, the clock skew is decreased
depending on the percentage of the used replay cache. Note that this
is locally handled, which will not require interaction with the peer
(even though it may indirectly effect the peer). However, exactly
how to implement such functionality is out of the scope of this
document and considered implementation specific.
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In case of a DoS attack, the client will most likely be able to
handle the replay cache. A more likely (and serious) DoS attack is a
CPU DoS attack where the attacker sends messages to the peer, which
then needs to expend resources on verifying the MACs/signatures of
the incoming messages.
6. Payload Encoding
This section describes, in detail, all the payloads. For all
encoding, network byte order is always used. While defining
supported types (e.g., which hash functions are supported) the
mandatory-to-implement types are indicated (as Mandatory), as well as
the default types (note, default also implies mandatory
implementation). Support for the other types are implicitly assumed
to be optional.
In the following, note that the support for SRTP [SRTP] as a security
protocol is defined. This will help us better understand the purpose
of the different payloads and fields. Other security protocols MAY
be specified for use within MIKEY, see Section 10.
In the following, the sign ~ indicates variable length field.
6.1. Common Header payload (HDR)
The Common Header payload MUST always be present as the first payload
in each message. The Common Header includes a general description of
the exchange message.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! version ! data type ! next payload !V! PRF func !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! CSB ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! #CS ! CS ID map type! CS ID map info ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* version (8 bits): the version number of MIKEY.
version = 0x01 refers to MIKEY as defined in this document.
* data type (8 bits): describes the type of message (e.g., public-
key transport message, verification message, error message).
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Data type | Value | Comment
--------------------------------------
Pre-shared | 0 | Initiator's pre-shared key message
PSK ver msg | 1 | Verification message of a Pre-shared
| | key message
Public key | 2 | Initiator's public-key transport message
PK ver msg | 3 | Verification message of a public-key
| | message
D-H init | 4 | Initiator's DH exchange message
D-H resp | 5 | Responder's DH exchange message
Error | 6 | Error message
Table 6.1.a
* next payload (8 bits): identifies the payload that is added after
this payload.
Next payload | Value | Section
------------------------------
Last payload | 0 | -
KEMAC | 1 | 6.2
PKE | 2 | 6.3
DH | 3 | 6.4
SIGN | 4 | 6.5
T | 5 | 6.6
ID | 6 | 6.7
CERT | 7 | 6.7
CHASH | 8 | 6.8
V | 9 | 6.9
SP | 10 | 6.10
RAND | 11 | 6.11
ERR | 12 | 6.12
Key data | 20 | 6.13
General Ext. | 21 | 6.15
Table 6.1.b
Note that some of the payloads cannot directly follow the header
(such as "Last payload", "Signature"). However, the Next payload
field is generic for all payloads. Therefore, a value is
allocated for each payload. The Next payload field is set to zero
(Last payload) if the current payload is the last payload.
* V (1 bit): flag to indicate whether a verification message is
expected or not (this only has meaning when it is set by the
Initiator). The V flag SHALL be ignored by the receiver in the DH
method (as the response is MANDATORY).
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V = 0 ==> no response expected
V = 1 ==> response expected
* PRF func (7 bits): indicates the PRF function that has been/will
be used for key derivation.
PRF func | Value | Comments
--------------------------------------------------------
MIKEY-1 | 0 | Mandatory (see Section 4.1.2)
Table 6.1.c
* CSB ID (32 bits): identifies the CSB. It is RECOMMENDED that the
CSB ID be chosen at random by the Initiator. This ID MUST be
unique between each Initiator-Responder pair, i.e., not globally
unique. An Initiator MUST check for collisions when choosing the
ID (if the Initiator already has one or more established CSBs with
the Responder). The Responder uses the same CSB ID in the
response.
* #CS (8 bits): indicates the number of Crypto Sessions that will be
handled within the CBS. Note that even though it is possible to
use 255 CSs, it is not likely that a CSB will include this many
CSs. The integer 0 is interpreted as no CS included. This may be
the case in an initial setup message.
* CS ID map type (8 bits): specifies the method of uniquely mapping
Crypto Sessions to the security protocol sessions.
CS ID map type | Value
-----------------------
SRTP-ID | 0
Table 6.1.d
* CS ID map info (16 bits): identifies the crypto session(s) for
which the SA should be created. The currently defined map type is
the SRTP-ID (defined in Section 6.1.1).
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6.1.1. SRTP ID
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Policy_no_1 ! SSRC_1 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SSRC_1 (cont) ! ROC_1 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ROC_1 (cont) ! Policy_no_2 ! SSRC_2 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SSRC_2 (cont) ! ROC_2 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ROC_2 (cont) ! :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
: : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Policy_no_#CS ! SSRC_#CS !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!SSRC_#CS (cont)! ROC_#CS !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ROC_#CS (cont)!
+-+-+-+-+-+-+-+-+
* Policy_no_i (8 bits): The security policy applied for the stream
with SSRC_i. The same security policy may apply for all CSs.
* SSRC_i (32 bits): specifies the SSRC that MUST be used for the
i-th SRTP stream. Note that it is the sender of the streams that
chooses the SSRC. Therefore, it is possible that the Initiator of
MIKEY cannot fill in all fields. In this case, SSRCs that are not
chosen by the Initiator are set to zero and the Responder fills in
these fields in the response message. Note that SRTP specifies
requirements on the uniqueness of the SSRCs (to avoid two-time pad
problems if the same TEK is used for more than one stream) [SRTP].
* ROC_i (32 bits): Current rollover counter used in SRTP. If the
SRTP session has not started, this field is set to 0. This field
is used to enable a member to join and synchronize with an already
started stream.
NOTE: The stream using SSRC_i will also have Crypto Session ID equal
to no i (NOT to the SSRC).
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6.2. Key data transport payload (KEMAC)
The Key data transport payload contains encrypted Key data sub-
payloads (see Section 6.13 for the definition of the Key data sub-
payload). It may contain one or more Key data payloads, each
including, for example, a TGK. The last Key data payload has its
Next payload field set to Last payload. For an update message (see
also Section 4.5), it is allowed to skip the Key data sub-payloads
(which will result in the Encr data len being equal to 0).
Note that the MAC coverage depends on the method used, i.e., pre-
shared vs public key, see below.
If the transport method used is the pre-shared key method, this Key
data transport payload is the last payload in the message (note that
the Next payload field is set to Last payload). The MAC is then
calculated over the entire MIKEY message following the directives in
Section 5.2.
If the transport method used is the public-key method, the
Initiator's identity is added in the encrypted data. This is done by
adding the ID payload as the first payload, which is then followed by
the Key data sub-payloads. Note that for an update message, the ID
is still sent encrypted to the Responder (this is to avoid certain
re-direction attacks) even though no Key data sub-payload is added
after.
In the public-key case, the coverage of the MAC field is over the Key
data transport payload only, instead of the complete MIKEY message,
as in the pre-shared case. The MAC is therefore calculated over the
Key data transport payload, except for the MAC field and where the
Next payload field has been set to zero (see also Section 5.2).
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload ! Encr alg ! Encr data len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Encr data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Mac alg ! MAC ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for defined values.
* Encr alg (8 bits): the encryption algorithm used to encrypt the
Encr data field.
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Encr alg | Value | Comment
-------------------------------------------
NULL | 0 | Very restricted usage, see Section 4.2.3!
AES-CM-128 | 1 | Mandatory; AES-CM using a 128-bit key, see
Section 4.2.3)
AES-KW-128 | 2 | AES Key Wrap using a 128-bit key, see
Section 4.2.3
Table 6.2.a
* Encr data len (16 bits): length of Encr data (in bytes).
* Encr data (variable length): the encrypted key sub-payloads (see
Section 6.13).
* MAC alg (8 bits): specifies the authentication algorithm used.
MAC alg | Value | Comments | Length (bits)
----------------------------------------------------------
NULL | 0 | restricted usage | 0
| | Section 4.2.4 |
HMAC-SHA-1-160 | 1 | Mandatory, | 160
| | Section 4.2.4 |
Table 6.2.b
* MAC (variable length): the message authentication code of the
entire message.
6.3. Envelope data payload (PKE)
The Envelope data payload contains the encrypted envelope key that is
used in the public-key transport to protect the data in the Key data
transport payload. The encryption algorithm used is implicit from
the certificate/public key used.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! C ! Data len ! Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* C (2 bits): envelope key cache indicator (Section 3.2).
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RFC 3830 MIKEY August 2004
Cache type | Value | Comments
--------------------------------------
No cache | 0 | The envelope key MUST NOT be cached
Cache | 1 | The envelope key MUST be cached
Cache for CSB | 2 | The envelope key MUST be cached, but only
| | to be used for the specific CSB.
Table 6.3
* Data len (14 bits): the length of the data field (in bytes).
* Data (variable length): the encrypted envelope key.
6.4. DH data payload (DH)
The DH data payload carries the DH-value and indicates the DH-group
used. Notice that in this sub-section, "MANDATORY" is conditioned
upon DH being supported.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! DH-Group ! DH-value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Reserv! KV ! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* DH-Group (8 bits): identifies the DH group used.
DH-Group | Value | Comment | DH Value length (bits)
--------------------------------------|---------------------
OAKLEY 5 | 0 | Mandatory | 1536
OAKLEY 1 | 1 | | 768
OAKLEY 2 | 2 | | 1024
Table 6.4
* DH-value (variable length): the public DH-value (the length is
implicit from the group used).
* KV (4 bits): indicates the type of key validity period specified.
This may be done by using an SPI (alternatively an MKI in SRTP) or
by providing an interval in which the key is valid (e.g., in the
latter case, for SRTP this will be the index range where the key
is valid). See Section 6.13 for pre-defined values.
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* KV data (variable length): This includes either the SPI/MKI or an
interval (see Section 6.14). If KV is NULL, this field is not
included.
6.5. Signature payload (SIGN)
The Signature payload carries the signature and its related data.
The signature payload is always the last payload in the PK transport
and DH exchange messages. The signature algorithm used is implicit
from the certificate/public key used.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! S type| Signature len ! Signature ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* S type (4 bits): indicates the signature algorithm applied by the
signer.
S type | Value | Comments
-------------------------------------
RSA/PKCS#1/1.5| 0 | Mandatory, PKCS #1 version 1.5 signature
[PSS]
RSA/PSS | 1 | RSASSA-PSS signature [PSS]
Table 6.5
* Signature len (12 bits): the length of the signature field (in
bytes).
* Signature (variable length): the signature (its formatting and
padding depend on the type of signature).
6.6. Timestamp payload (T)
The timestamp payload carries the timestamp information.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! TS type ! TS value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* TS type (8 bits): specifies the timestamp type used.
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RFC 3830 MIKEY August 2004
TS type | Value | Comments | length of TS value
-------------------------------------|-------------------
NTP-UTC | 0 | Mandatory | 64-bits
NTP | 1 | Mandatory | 64-bits
COUNTER | 2 | Optional | 32-bits
Table 6.6
Note: COUNTER SHALL be padded (with leading zeros) to a 64-bit
value when used as input for the default PRF.
* TS-value (variable length): The timestamp value of the specified
TS type.
6.7. ID payload (ID) / Certificate Payload (CERT)
Note that the ID payload and the Certificate payload are two
completely different payloads (having different payload identifiers).
However, as they share the same payload structure, they are described
in the same section.
The ID payload carries a uniquely defined identifier.
The certificate payload contains an indicator of the certificate
provided as well as the certificate data. If a certificate chain is
to be provided, each certificate in the chain should be included in a
separate CERT payload.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! ID/Cert Type ! ID/Cert len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID/Certificate Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
If the payload is an ID payload, the following values apply for the
ID type field:
* ID Type (8 bits): specifies the identifier type used.
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ID Type | Value | Comments
----------------------------------------------
NAI | 0 | Mandatory (see [NAI])
URI | 1 | Mandatory (see [URI])
Table 6.7.a
If the payload is a Certificate payload, the following values applies
for the Cert type field:
* Cert Type (8 bits): specifies the certificate type used.
Cert Type | Value | Comments
----------------------------------------------
X.509v3 | 0 | Mandatory
X.509v3 URL | 1 | plain ASCII URL to the location of the Cert
X.509v3 Sign | 2 | Mandatory (used for signatures only)
X.509v3 Encr | 3 | Mandatory (used for encryption only)
Table 6.7.b
* ID/Cert len (16 bits): the length of the ID or Certificate field
(in bytes).
* ID/Certificate (variable length): The ID or Certificate data. The
X.509 [X.509] certificates are included as a bytes string using
DER encoding as specified in X.509.
6.8. Cert hash payload (CHASH)
The Cert hash payload contains the hash of the certificate used.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Hash func ! Hash ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* Hash func (8 bits): indicates the hash function that is used (see
also Section 4.2.1).
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Hash func | Value | Comment | hash length (bits)
-------------------------------------------------
SHA-1 | 0 | Mandatory | 160
MD5 | 1 | | 128
Table 6.8
* Hash (variable length): the hash data. The hash length is
implicit from the hash function used.
6.9. Ver msg payload (V)
The Ver msg payload contains the calculated verification message in
the pre-shared key and the public-key transport methods. Note that
the MAC is calculated over the entire MIKEY message, as well as the
IDs and Timestamp (see also Section 5.2).
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Auth alg ! Ver data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* Auth alg (8 bits): specifies the MAC algorithm used for the
verification message. See Section 6.2 for defined values.
* Ver data (variable length): the verification message data. The
length is implicit from the authentication algorithm used.
6.10. Security Policy payload (SP)
The Security Policy payload defines a set of policies that apply to a
specific security protocol.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload ! Policy no ! Prot type ! Policy param ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ length (cont) ! Policy param ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
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RFC 3830 MIKEY August 2004
* Policy no (8 bits): each security policy payload must be given a
distinct number for the current MIKEY session by the local peer.
This number is used to map a crypto session to a specific policy
(see also Section 6.1.1).
* Prot type (8 bits): defines the security protocol.
Prot type | Value |
---------------------------
SRTP | 0 |
Table 6.10
* Policy param length (16 bits): defines the total length of the
policy parameters for the specific security protocol.
* Policy param (variable length): defines the policy for the
specific security protocol.
The Policy param part is built up by a set of Type/Length/Value
fields. For each security protocol, a set of possible
types/values that can be negotiated is defined.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Type ! Length ! Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Type (8 bits): specifies the type of the parameter.
* Length (8 bits): specifies the length of the Value field (in
bytes).
* Value (variable length): specifies the value of the parameter.
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RFC 3830 MIKEY August 2004
6.10.1. SRTP policy
This policy specifies the parameters for SRTP and SRTCP. The
types/values that can be negotiated are defined by the following
table:
Type | Meaning | Possible values
----------------------------------------------------
0 | Encryption algorithm | see below
1 | Session Encr. key length | depends on cipher used
2 | Authentication algorithm | see below
3 | Session Auth. key length | depends on MAC used
4 | Session Salt key length | see [SRTP] for recommendations
5 | SRTP Pseudo Random Function | see below
6 | Key derivation rate | see [SRTP] for recommendations
7 | SRTP encryption off/on | 0 if off, 1 if on
8 | SRTCP encryption off/on | 0 if off, 1 if on
9 | sender's FEC order | see below
10 | SRTP authentication off/on | 0 if off, 1 if on
11 | Authentication tag length | in bytes
12 | SRTP prefix length | in bytes
Table 6.10.1.a
Note that if a Type/Value is not set, the default is used (according
to SRTP's own criteria). Note also that, if "Session Encr. key
length" is set, this should also be seen as the Master key length
(otherwise, the SRTP default Master key length is used).
For the Encryption algorithm, a one byte length is enough. The
currently defined possible Values are:
SRTP encr alg | Value
---------------------
NULL | 0
AES-CM | 1
AES-F8 | 2
Table 6.10.1.b
where AES-CM is AES in CM, and AES-F8 is AES in f8 mode [SRTP].
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For the Authentication algorithm, a one byte length is enough. The
currently defined possible Values are:
SRTP auth alg | Value
---------------------
NULL | 0
HMAC-SHA-1 | 1
Table 6.10.1.c
For the SRTP pseudo-random function, a one byte length is also
enough. The currently defined possible Values are:
SRTP PRF | Value
---------------------
AES-CM | 0
Table 6.10.1.d
If FEC is used at the same time SRTP is used, MIKEY can negotiate the
order in which these should be applied at the sender side.
FEC order | Value | Comments
--------------------------------
FEC-SRTP | 0 | First FEC, then SRTP
Table 6.10.1.e
6.11. RAND payload (RAND)
The RAND payload consists of a (pseudo-)random bit-string. The RAND
MUST be independently generated per CSB (note that if the CSB has
several members, the Initiator MUST use the same RAND for all the
members). For randomness recommendations for security, see [RAND].
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload ! RAND len ! RAND ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* RAND len (8 bits): length of the RAND (in bytes). It SHOULD be at
least 16.
* RAND (variable length): a (pseudo-)randomly chosen bit-string.
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6.12. Error payload (ERR)
The Error payload is used to specify the error(s) that may have
occurred.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Error no ! Reserved !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* Error no (8 bits): indicates the type of error that was
encountered.
Error no | Value | Comment
-------------------------------------------------------
Auth failure | 0 | Authentication failure
Invalid TS | 1 | Invalid timestamp
Invalid PRF | 2 | PRF function not supported
Invalid MAC | 3 | MAC algorithm not supported
Invalid EA | 4 | Encryption algorithm not supported
Invalid HA | 5 | Hash function not supported
Invalid DH | 6 | DH group not supported
Invalid ID | 7 | ID not supported
Invalid Cert | 8 | Certificate not supported
Invalid SP | 9 | SP type not supported
Invalid SPpar | 10 | SP parameters not supported
Invalid DT | 11 | not supported Data type
Unspecified error | 12 | an unspecified error occurred
Table 6.12
6.13. Key data sub-payload
The Key data payload contains key material, e.g., TGKs. The Key data
payloads are never included in clear, but as an encrypted part of the
Key data transport payload.
Note that a Key data transport payload can contain multiple Key data
sub-payloads.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Type ! KV ! Key data len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Key data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Salt len (optional) ! Salt data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload. See Section 6.1 for values.
* Type (4 bits): indicates the type of key included in the payload.
Type | Value
-----------------
TGK | 0
TGK+SALT | 1
TEK | 2
TEK+SALT | 3
Table 6.13.a
Note that the possibility of including a TEK (instead of using the
TGK) is provided. When sent directly, the TEK can generally not
be shared between more than one Crypto Session (unless the
Security protocol allows for this, e.g., [SRTP]). The recommended
use of sending a TEK, instead of a TGK, is when pre-encrypted
material exists and therefore, the TEK must be known in advance.
* KV (4 bits): indicates the type of key validity period specified.
This may be done by using an SPI (or MKI in the case of [SRTP]) or
by providing an interval in which the key is valid (e.g., in the
latter case, for SRTP this will be the index range where the key
is valid).
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KV | Value | Comments
-------------------------------------------
Null | 0 | No specific usage rule (e.g., a TEK
| | that has no specific lifetime)
SPI | 1 | The key is associated with the SPI/MKI
Interval | 2 | The key has a start and expiration time
| | (e.g., an SRTP TEK)
Table 6.13.b
Note that when NULL is specified, any SPI or Interval is valid.
For an Interval, this means that the key is valid from the first
observed sequence number until the key is replaced (or the
security protocol is shutdown).
* Key data len (16 bits): the length of the Key data field (in
bytes). Note that the sum of the overall length of all the Key
data payloads contained in a single Key data transport payload
(KEMAC) MUST be such that the KEMAC payload does not exceed a
length of 2^16 bytes (total length of KEMAC, see Section 6.2).
* Key data (variable length): The TGK or TEK data.
* Salt len (16 bits): The salt key length in bytes. Note that this
field is only included if the salt is specified in the Type-field.
* Salt data (variable length): The salt key data. Note that this
field is only included if the salt is specified in the Type-field.
(For SRTP, this is the so-called master salt.)
* KV data (variable length): This includes either the SPI or an
interval (see Section 6.14). If KV is NULL, this field is not
included.
6.14. Key validity data
The Key validity data is not a standalone payload, but part of either
the Key data payload (see Section 6.13) or the DH payload (see
Section 6.4). The Key validity data gives a guideline of when the
key should be used. There are two KV types defined (see Section
6.13), SPI/MKI (SPI) or a lifetime range (interval).
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SPI/MKI
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SPI Length ! SPI ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* SPI Length (8 bits): the length of the SPI (or MKI) in bytes.
* SPI (variable length): the SPI (or MKI) value.
Interval
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! VF Length ! Valid From ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! VT Length ! Valid To (expires) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* VF Length (8 bits): length of the Valid From field in bytes.
* Valid From (variable length): sequence number, index, timestamp,
or other start value that the security protocol uses to identify
the start position of the key usage.
* VT Length (8 bits): length of the Valid To field in bytes.
* Valid To (variable length): sequence number, index, timestamp, or
other expiration value that the security protocol can use to
identify the expiration of the key usage.
Note that for SRTP usage, the key validity period for a TGK/TEK
should be specified with either an interval, where the VF/VT
Length is equal to 6 bytes (i.e., the size of the index), or with
an MKI. It is RECOMMENDED that if more than one SRTP stream is
sharing the same keys and key update/re-keying is desired, this is
handled using MKI rather than the From-To method.
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6.15. General Extension Payload
The General extensions payload is included to allow possible
extensions to MIKEY without the need for defining a completely new
payload each time. This payload can be used in any MIKEY message and
is part of the authenticated/signed data part.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload ! Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload (8 bits): identifies the payload that is added after
this payload.
* Type (8 bits): identifies the type of general payload.
Type | Value | Comments
---------------------------------------
Vendor ID | 0 | Vendor specific byte string
SDP IDs | 1 | List of SDP key mgmt IDs (allocated for use in
[KMASDP])
Table 6.15
* Length (16 bits): the length in bytes of the Data field.
* Data (variable length): the general payload data.
7. Transport protocols
MIKEY MAY be integrated within session establishment protocols.
Currently, integration of MIKEY within SIP/SDP and RTSP is defined in
[KMASDP]. MIKEY MAY use other transports, in which case how MIKEY is
transported over such a transport protocol has to be defined.
8. Groups
What has been discussed up to now is not limited to single peer-to-
peer communication (except for the DH method), but can be used to
distribute group keys for small-size interactive groups and simple
one-to-many scenarios. Section 2.1. describes the scenarios in the
focus of MIKEY. This section describes how MIKEY is used in a group
scenario (though, see also Section 4.3 for issues related to
authorization).
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8.1. Simple one-to-many
++++
|S |
| |
++++
|
--------+-------------- - -
| | |
v v v
++++ ++++ ++++
|A | |B | |C |
| | | | | |
++++ ++++ ++++
Figure 8.1. Simple one-to-many scenario.
In the simple one-to-many scenario, a server is streaming to a small
group of clients. RTSP or SIP is used for the registration and the
key management set up. The streaming server acts as the Initiator of
MIKEY. In this scenario, the pre-shared key or public key transport
mechanism will be appropriate in transporting the same TGK to all the
clients (which will result in common TEKs for the group).
Note, if the same TGK/TEK(s) should be used by all the group members,
the streaming server MUST specify the same CSB_ID and CS_ID(s) for
the session to all the group members.
As the communication may be performed using multicast, the members
need a common security policy if they want to be part of the group.
This limits the possibility of negotiation.
Furthermore, the Initiator should carefully consider whether to
request the verification message in reply from each receiver, as this
may result in a certain load for the Initiator itself as the group
size increases.
8.2. Small-size interactive group
As described in the overview section, for small-size interactive
groups, one may expect that each client will be in charge for setting
up the security for its outgoing streams. In these scenarios, the
pre-shared key or the public-key transport method is used.
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++++ ++++
|A | -------> |B |
| | <------- | |
++++ ++++
^ | | ^
| | | |
| | ++++ | |
| --->|C |<--- |
------| |------
++++
Figure 8.2. Small-size group without a centralized controller.
One scenario may then be that the client sets up a three-part call,
using SIP. Due to the small size of the group, unicast SRTP is used
between the clients. Each client sets up the security for its
outgoing stream(s) to the others.
As for the simple one-to-many case, the streaming client specifies
the same CSB_ID and CS_ID(s) for its outgoing sessions if the same
TGK/TEK(s) is used for all the group members.
9. Security Considerations
9.1. General
Key management protocols based on timestamps/counters and one-
roundtrip key transport have previously been standardized, for
example ISO [ISO1, ISO2]. The general security of these types of
protocols can be found in various articles and literature, c.f. [HAC,
AKE, LOA].
No chain is stronger than its weakest link. If a given level of
protection is wanted, then the cryptographic functions protecting the
keys during transport/exchange MUST offer a security corresponding to
at least that level.
For instance, if a security against attacks with a complexity 2^96 is
wanted, then one should choose a secure symmetric cipher supporting
at least 96 bit keys (128 bits may be a practical choice) for the
actual media protection, and a key transport mechanism that provides
equivalent protection, e.g., MIKEY's pre-shared key transport with
128 bit TGK, or RSA with 1024 bit keys (which according to [LV]
corresponds to the desired 96 bit level, with some margin).
In summary, key size for the key-exchange mechanism MUST be weighed
against the size of the exchanged TGK so that it at least offers the
required level. For efficiency reasons, one SHOULD also avoid a
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security overkill, e.g., by not using a public key transport with
public keys giving a security level that is orders of magnitude
higher than length of the transported TGK. We refer to [LV] for
concrete key size recommendations.
Moreover, if the TGKs are not random (or pseudo-random), a brute
force search may be facilitated, again lowering the effective key
size. Therefore, care MUST be taken when designing the (pseudo-)
random generators for TGK generation, see [FIPS][RAND].
For the selection of the hash function, SHA-1 with 160-bit output is
the default one. In general, hash sizes should be twice the
"security level", indicating that SHA-1-256, [SHA256], should be used
for the default 128-bit level. However, due to the real-time aspects
in the scenarios we are treating, hash sizes slightly below 256 are
acceptable, as the normal "existential" collision probabilities would
be of secondary importance.
In a Crypto Session Bundle, the Crypto Sessions can share the same
TGK as discussed earlier. From a security point of view, to satisfy
the criterion in case the TGK is shared, the encryption of the
individual Crypto Sessions are performed "independently". In MIKEY,
this is accomplished by having unique Crypto Session identifiers (see
also Section 4.1) and a TEK derivation method that provides
cryptographically independent TEKs to distinct Crypto Sessions
(within the Crypto Session Bundle), regardless of the security
protocol used.
Specifically, the key derivations, as specified in Section 4.1, are
implemented by a pseudo-random function. The one used here is a
simplified version of that used in TLS [TLS]. Here, only one single
hash function is used, whereas TLS uses two different functions.
This choice is motivated by the high confidence in the SHA-1 hash
function, and by efficiency and simplicity of design (complexity does
not imply security). Indeed, as shown in [DBJ], if one of the two
hashes is severely broken, the TLS PRF is actually less secure than
as if a single hash had been used on the whole key, as is done in
MIKEY.
In the pre-shared key and public-key schemes, the TGK is generated by
a single party (Initiator). This makes MIKEY somewhat more sensitive
if the Initiator uses a bad random number generator. It should also
be noted that neither the pre-shared nor the public-key scheme
provides perfect forward secrecy. If mutual contribution or perfect
forward secrecy is desired, the Diffie-Hellman method is to be used.
Authentication (e.g., signatures) in the Diffie-Hellman method is
required to prevent man-in-the-middle attacks.
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Forward/backward security: if the TGK is exposed, all generated TEKs
are compromised. However, under the assumption that the derivation
function is a pseudo-random function, disclosure of an individual TEK
does not compromise other (previous or later) TEKs derived from the
same TGK. The Diffie-Hellman mode can be considered by cautious
users, as it is the only one that supports so called perfect forward
secrecy (PFS). This is in contrast to a compromise of the pre-shared
key (or the secret key of the public key mode), where future sessions
and recorded sessions from the past are then also compromised.
The use of random nonces (RANDs) in the key derivation is of utmost
importance to counter off-line pre-computation attacks. Note however
that update messages re-use the old RAND. This means that the total
effective key entropy (relative to pre-computation attacks) for k
consecutive key updates, assuming the TGKs and RAND are each n bits
long, is about L = n*(k+1)/2 bits, compared to the theoretical
maximum of n*k bits. In other words, a 2^L work effort MAY enable an
attacker to get all k n-bit keys, which is better than brute force
(except when k = 1). While this might seem like a defect, first note
that for a proper choice of n, the 2^L complexity of the attack is
way out of reach. Moreover, the fact that more than one key can be
compromised in a single attack is inherent to the key exchange
problem. Consider for instance a user who, using a fixed 1024-bit
RSA key, exchanges keys and communicates during a one or two year
lifetime of the public key. Breaking this single RSA key will enable
access to all exchanged keys and consequently the entire
communication of that user over the whole period.
All the pre-defined transforms in MIKEY use state-of-the-art
algorithms that have undergone large amounts of public evaluation.
One of the reasons for using the AES-CM from SRTP [SRTP], is to have
the possibility of limiting the overall number of different
encryption modes and algorithms, while offering a high level of
security at the same time.
9.2. Key lifetime
Even if the lifetime of a TGK (or TEK) is not specified, it MUST be
taken into account that the encryption transform in the underlying
security protocol can in some way degenerate after a certain amount
of encrypted data. It is not possible to here state universally
applicable, general key lifetime bounds; each security protocol
should define such maximum amount and trigger a re-keying procedure
before the "exhaustion" of the key. For example, according to SRTP
[SRTP] the TEK, together with the corresponding TGK, MUST be changed
at least every 2^48 SRTP packet.
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Still, the following can be said as a rule of thumb. If the security
protocol uses an "ideal" b-bit block cipher (in CBC mode, counter
mode, or a feedback mode, e.g., OFB, with full b-bit feedback),
degenerate behavior in the crypto stream, possibly useful for an
attacker, is (with constant probability) expected to occur after a
total of roughly 2^(b/2) encrypted b-bit blocks (using random IVs).
For security margin, re-keying MUST be triggered well in advance
compared to the above bound. See [BDJR] for more details.
For use of a dedicated stream cipher, we refer to the analysis and
documentation of said cipher in each specific case.
9.3. Timestamps
The use of timestamps, instead of challenge-responses, requires the
systems to have synchronized clocks. Of course, if two clients are
not synchronized, they will have difficulties in setting up the
security. The current timestamp based solution has been selected to
allow a maximum of one roundtrip (i.e., two messages), but still
provide a reasonable replay protection. A (secure) challenge-
response based version would require at least three messages. For a
detailed description of the timestamp and replay handling in MIKEY,
see Section 5.4.
Practical experiences of Kerberos and other timestamp-based systems
indicate that it is not always necessary to synchronize the terminals
over the network. Manual configuration could be a feasible
alternative in many cases (especially in scenarios where the degree
of looseness is high). However, the choice must be made carefully
with respect to the usage scenario.
9.4. Identity Protection
User privacy is a complex matter that to some extent can be enforced
by cryptographic mechanisms, but also requires policy enforcement and
various other functionalities. One particular facet of privacy is
user identity protection. However, identity protection was not a
main design goal for MIKEY. Such a feature will add more complexity
to the protocol and was therefore not chosen to be included. As
MIKEY is anyway proposed to be transported over, e.g., SIP, the
identity may be exposed by this. However, if the transporting
protocol is secured and also provides identity protection, MIKEY
might inherit the same feature. How this should be done is for
future study.
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9.5. Denial of Service
This protocol is resistant to Denial of Service attacks in the sense
that a Responder does not construct any state (at the key management
protocol level) before it has authenticated the Initiator. However,
this protocol, like many others, is open to attacks that use spoofed
IP addresses to create a large number of fake requests. This may for
example, be solved by letting the protocol transporting MIKEY do an
IP address validity test. The SIP protocol can provide this using
the anonymous authentication challenge mechanism (specified in
Section 22.1 of [SIP]).
It is highly RECOMMENDED to include IDr in the Initiator's message.
If not included, its absence can be used for DoS purposes (the
largest DoS-impact being on the public key and DH methods), where a
message intended for other entities is sent to the target. In fact,
the target may verify the signature correctly due to the fact that
the Initiator's ID is correct and the message is actually signed by
the claimed Initiator (e.g., by re-directing traffic from another
session).
However, in the public key method, the envelop key and the MAC will
ensure that the message is not accepted (still, compared to a normal
faked message, where the signature verification would detect the
problem, one extra public key decryption is needed to detect the
problem in this case).
In the DH method, a message would be accepted (without detecting the
error) and a response (and state) would be created for the malicious
request.
As also discussed in Section 5.4, the tradeoff between time
synchronization and the size of the replay cache may be affected in
case of for example, a flooding DoS attack. However, if the
recommendations of using a dynamic size of the replay cache are
followed, it is believed that the client will in most cases be able
to handle the replay cache. Of course, as the replay cache decreases
in size, the required time synchronization is more restricted.
However, a bigger problem during such an attack would probably be to
process the messages (e.g., verify signatures/MACs) due to the
computational workload this implies.
9.6. Session Establishment
It should be noted that if the session establishment protocol is
insecure, there may be attacks on this that will have indirect
security implications on the secured media streams. This however
only applies to groups (and is not specific to MIKEY). The threat is
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RFC 3830 MIKEY August 2004
that one group member may re-direct a stream from one group member to
another. This will have the same implication as when a member tries
to impersonate another member, e.g., by changing its IP address. If
this is seen as a problem, it is RECOMMENDED that a Data Origin
Authentication (DOA) scheme (e.g., digital signatures) be applied to
the security protocol.
Re-direction of streams can of course be done even if it is not a
group. However, the effect will not be the same as compared to a
group where impersonation can be done if DOA is not used. Instead,
re-direction will only deny the receiver the possibility of receiving
(or just delay) the data.
10. IANA Considerations
This document defines several new name spaces associated with the
MIKEY payloads. This section summarizes the name spaces for which
IANA is requested to manage the allocation of values. IANA is
requested to record the pre-defined values defined in the given
sections for each name space. IANA is also requested to manage the
definition of additional values in the future. Unless explicitly
stated otherwise, values in the range 0-240 for each name space
SHOULD be approved by the process of IETF consensus and values in the
range 241-255 are reserved for Private Use, according to [RFC2434].
The name spaces for the following fields in the Common header payload
(from Section 6.1) are requested to be managed by IANA (in bracket is
the reference to the table with the initially registered values):
* version
* data type (Table 6.1.a)
* Next payload (Table 6.1.b)
* PRF func (Table 6.1.c). This name space is between 0-127, where
values between 0-111 should be approved by the process of IETF
consensus and values between 112-127 are reserved for Private Use.
* CS ID map type (Table 6.1.d)
The name spaces for the following fields in the Key data transport
payload (from Section 6.2) are requested to be managed by IANA:
* Encr alg (Table 6.2.a)
* MAC alg (Table 6.2.b)
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RFC 3830 MIKEY August 2004
The name spaces for the following fields in the Envelope data payload
(from Section 6.3) are requested to be managed by IANA:
* C (Table 6.3)
The name spaces for the following fields in the DH data payload (from
Section 6.4) are requested to be managed by IANA:
* DH-Group (Table 6.4)
The name spaces for the following fields in the Signature payload
(from Section 6.5) are requested to be managed by IANA:
* S type (Table 6.5)
The name spaces for the following fields in the Timestamp payload
(from Section 6.6) are requested to be managed by IANA:
* TS type (Table 6.6)
The name spaces for the following fields in the ID payload and the
Certificate payload (from Section 6.7) are requested to be managed by
IANA:
* ID type (Table 6.7.a)
* Cert type (Table 6.7.b)
The name spaces for the following fields in the Cert hash payload
(from Section 6.8) are requested to be managed by IANA:
* Hash func (Table 6.8)
The name spaces for the following fields in the Security policy
payload (from Section 6.10) are requested to be managed by IANA:
* Prot type (Table 6.10)
For each security protocol that uses MIKEY, a set of unique
parameters MAY be registered.
From Section 6.10.1.
* SRTP Type (Table 6.10.1.a)
* SRTP encr alg (Table 6.10.1.b)
* SRTP auth alg (Table 6.10.1.c)
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RFC 3830 MIKEY August 2004
* SRTP PRF (Table 6.10.1.d)
* FEC order (Table 6.10.1.e)
The name spaces for the following fields in the Error payload (from
Section 6.12) are requested to be managed by IANA:
* Error no (Table 6.12)
The name spaces for the following fields in the Key data payload
(from Section 6.13) are requested to be managed by IANA:
* Type (Table 6.13.a). This name space is between 0-16, which
should be approved by the process of IETF consensus.
* KV (Table 6.13.b). This name space is between 0-16, which should
be approved by the process of IETF consensus.
The name spaces for the following fields in the General Extensions
payload (from Section 6.15) are requested to be managed by IANA:
* Type (Table 6.15).
10.1. MIME Registration
This section gives instructions to IANA to register the
application/mikey MIME media type. This registration is as follows:
MIME media type name : application
MIME subtype name : mikey
Required parameters : none
Optional parameters : version
version: The MIKEY version number of the enclosed message
(e.g., 1). If not present, the version defaults to 1.
Encoding Considerations : binary, base64 encoded
Security Considerations : see section 9 in this memo
Interoperability considerations : none
Published specification : this memo
11. Acknowledgments
The authors would like to thank Mark Baugher, Ran Canetti, Martin
Euchner, Steffen Fries, Peter Barany, Russ Housley, Pasi Ahonen (with
his group), Rolf Blom, Magnus Westerlund, Johan Bilien, Jon-Olov
Vatn, Erik Eliasson, and Gerhard Strangar for their valuable
feedback.
Arkko, et al. Standards Track [Page 59]
RFC 3830 MIKEY August 2004
12. References
12.1. Normative References
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[NAI] Aboba, B. and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
[PSS] PKCS #1 v2.1 - RSA Cryptography Standard, RSA Laboratories,
June 14, 2002, www.rsalabs.com
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[SHA-1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real Time Transport Protocol", RFC
3711, March 2004.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[X.509] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[AESKW] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
Arkko, et al. Standards Track [Page 60]
RFC 3830 MIKEY August 2004
12.2. Informative References
[AKE] Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange
Protocols and their use for Building Secure Channels",
Eurocrypt 2001, LNCS 2054, pp. 453-474, 2001.
[BDJR] Bellare, M., Desai, A., Jokipii, E., and P. Rogaway, "A
Concrete Analysis of Symmetric Encryption: Analysis of the
DES Modes of Operation", in Proceedings of the 38th
Symposium on Foundations of Computer Science, IEEE, 1997,
pp. 394-403.
[BMGL] Hastad, J. and M. Naslund: "Practical Construction and
Analysis of Pseduo-randomness Primitives", Proceedings of
Asiacrypt 2001, LNCS. vol 2248, pp. 442-459, 2001.
[DBJ] Johnson, D.B., "Theoretical Security Concerns with TLS use
of MD5", Contribution to ANSI X9F1 WG, 2001.
[FIPS] "Security Requirements for Cryptographic Modules", Federal
Information Processing Standard Publications (FIPS PUBS)
140-2, December 2002.
[GKMARCH] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Group Key Management Architecture", Work in Progress.
[GDOI] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[GSAKMP] Harney, H., Colegrove, A., Harder, E., Meth, U., and R.
Fleischer, "Group Secure Association Key Management
Protocol", Work in Progress.
[HAC] Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
of Applied Cryptography", CRC press, 1996.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[ISO1] ISO/IEC 9798-3: 1997, Information technology - Security
techniques - Entity authentication - Part 3: Mechanisms
using digital signature techniques.
[ISO2] ISO/IEC 11770-3: 1997, Information technology - Security
techniques - Key management - Part 3: Mechanisms using
digital signature techniques.
Arkko, et al. Standards Track [Page 61]
RFC 3830 MIKEY August 2004
[ISO3] ISO/IEC 18014 Information technology - Security techniques
- Time-stamping services, Part 1-3.
[KMASDP] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "Key Management Extensions for SDP and RTSP", Work
in Progress.
[LOA] Burrows, Abadi, and Needham, "A logic of authentication",
ACM Transactions on Computer Systems 8 No.1 (Feb. 1990),
18-36.
[LV] Lenstra, A. K. and E. R. Verheul, "Suggesting Key Sizes for
Cryptosystems", http://www.cryptosavvy.com/suggestions.htm
[NTP] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
March 1992.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RAND] Eastlake, 3rd, D., Crocker, S., and J. Schiller,
"Randomness Requirements for Security", RFC 1750, December
1994.
[RTSP] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[SDP] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[SHA256] NIST, "Description of SHA-256, SHA-384, and SHA-512",
http://csrc.nist.gov/encryption/shs/sha256-384-512.pdf
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol - Version 1.0",
RFC 2246, January 1999.
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RFC 3830 MIKEY August 2004
Appendix A. MIKEY - SRTP Relation
The terminology in MIKEY differs from the one used in SRTP as MIKEY
needs to be more general, nor is tight to SRTP only. Therefore, it
might be hard to see the relations between keys and parameters
generated in MIKEY and those used by SRTP. This section provides
some hints on their relation.
MIKEY | SRTP
-------------------------------------------------
Crypto Session | SRTP stream (typically with related SRTCP stream)
Data SA | input to SRTP's crypto context
TEK | SRTP master key
The Data SA is built up by a TEK and the security policy exchanged.
SRTP may use an MKI to index the TEK or TGK (the TEK is then derived
from the TGK that is associated with the corresponding MKI), see
below.
A.1. MIKEY-SRTP Interactions
In the following, we give a brief outline of the interface between
SRTP and MIKEY and the processing that takes place. We describe the
SRTP receiver side only, the sender side will require analogous
interfacing.
1. When an SRTP packet arrives at the receiver and is processed, the
triple <SSRC, destination address, destination port> is extracted
from the packet and used to retrieve the correct SRTP crypto
context, hence the Data SA. (The actual retrieval can, for
example, be done by an explicit request from the SRTP
implementation to MIKEY, or, by the SRTP implementation accessing
a "database", maintained by MIKEY. The application will typically
decide which implementation is preferred.)
2. If an MKI is present in the SRTP packet, it is used to point to
the correct key within the SA. Alternatively, if SRTP's <From,
To> feature is used, the ROC||SEQ of the packet is used to
determine the correct key.
3. Depending on whether the key sent in MIKEY (as obtained in step 2)
was a TEK or a TGK, there are now two cases.
- If the key obtained in step 2 is the TEK itself, it is used
directly by SRTP as a master key.
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RFC 3830 MIKEY August 2004
- If the key instead is a TGK, the mapping with the CS_ID
(internal to MIKEY, Section 6.1.1) allows MIKEY to compute the
correct TEK from the TGK as described in Section 4.1 before
SRTP uses it.
If multiple TGKs (or TEKs) are sent, it is RECOMMENDED that each TGK
(or TEK) be associated with a distinct MKI. It is RECOMMENDED that
the use of <From, To> in this scenario be limited to very simple
cases, e.g., one stream only.
Besides the actual master key, other information in the Data SA
(e.g., transform identifiers) will of course also be communicated
from MIKEY to SRTP.
Arkko, et al. Standards Track [Page 64]
RFC 3830 MIKEY August 2004
Authors' Addresses
Jari Arkko
Ericsson Research
02420 Jorvas
Finland
Phone: +358 40 5079256
EMail: jari.arkko@ericsson.com
Elisabetta Carrara
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 50877040
EMail: elisabetta.carrara@ericsson.com
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 58531705
EMail: fredrik.lindholm@ericsson.com
Mats Naslund
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 58533739
EMail: mats.naslund@ericsson.com
Karl Norrman
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 4044502
EMail: karl.norrman@ericsson.com
Arkko, et al. Standards Track [Page 65]
RFC 3830 MIKEY August 2004
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Arkko, et al. Standards Track [Page 66]