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Network Working Group M. Baugher
Request for Comments: 4383 Cisco
Category: Standards Track E. Carrara
Royal Institute of Technology
February 2006
The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA)
in the Secure Real-time Transport Protocol (SRTP)
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 (2006).
Abstract
This memo describes the use of the Timed Efficient Stream Loss-
tolerant Authentication (RFC 4082) transform within the Secure Real-
time Transport Protocol (SRTP), to provide data origin authentication
for multicast and broadcast data streams.
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RFC 4383 TESLA-SRTP February 2006
Table of Contents
1. Introduction ....................................................2
1.1. Notational Conventions .....................................3
2. SRTP ............................................................3
3. TESLA ...........................................................4
4. Usage of TESLA within SRTP ......................................5
4.1. The TESLA Extension ........................................5
4.2. SRTP Packet Format .........................................6
4.3. Extension of the SRTP Cryptographic Context ................7
4.4. SRTP Processing ............................................8
4.4.1. Sender Processing ...................................9
4.4.2. Receiver Processing .................................9
4.5. SRTCP Packet Format .......................................11
4.6. TESLA MAC .................................................13
4.7. PRFs ......................................................13
5. TESLA Bootstrapping and Cleanup ................................14
6. SRTP TESLA Default Parameters ..................................14
7. Security Considerations ........................................15
8. Acknowledgements ...............................................16
9. References .....................................................17
9.1. Normative References ......................................17
9.2. Informative References ....................................17
1. Introduction
Multicast and broadcast communications introduce some new security
challenges compared to unicast communication. Many multicast and
broadcast applications need "data origin authentication" (DOA), or
"source authentication", in order to guarantee that a received
message had originated from a given source, and was not manipulated
during the transmission. In unicast communication, a pairwise
security association between one sender and one receiver can provide
data origin authentication using symmetric-key cryptography (such as
a message authentication code, MAC). When the communication is
strictly pairwise, the sender and receiver agree upon a key that is
known only to them.
In groups, however, a key is shared among more than two members, and
this symmetric-key approach does not guarantee data origin
authentication. When there is a group security association [RFC4046]
instead of a pairwise security association, any of the members can
alter the packet and impersonate any other member. The MAC in this
case only guarantees that the packet was not manipulated by an
attacker outside the group (and hence not in possession of the group
key), and that the packet was sent by a source within the group.
Baugher & Carrara Standards Track [Page 2]
RFC 4383 TESLA-SRTP February 2006
Some applications cannot tolerate source ambiguity and need to
identify the true sender from any other group member. A common way
to solve the problem is by use of asymmetric cryptography, such as
digital signatures. This method, unfortunately, suffers from high
overhead in terms of time (to sign and verify) and bandwidth (to
convey the signature in the packet).
Several schemes have been proposed to provide efficient data origin
authentication in multicast and broadcast scenarios. The Timed
Efficient Stream Loss-tolerant Authentication (TESLA) is one such
scheme.
This memo specifies TESLA authentication for SRTP. SRTP TESLA can
provide data origin authentication to RTP applications that use group
security associations (such as multicast RTP applications) so long as
receivers abide by the TESLA security invariants [RFC4082].
1.1. Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification assumes that the reader is familiar with both SRTP
and TESLA. Few of their details are explained in this document, and
the reader can find them in their respective specifications,
[RFC3711] and [RFC4082]. This specification uses the same
definitions as TESLA for common terms and assumes that the reader is
familiar with the TESLA algorithms and protocols [RFC4082].
2. SRTP
The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile
of RTP, which can provide confidentiality, message authentication,
and replay protection to the RTP traffic and to the RTP control
protocol, the Real-time Transport Control Protocol (RTCP). Note that
the term "SRTP" may often be used to indicate SRTCP as well.
SRTP is a framework that allows new security functions and new
transforms to be added. SRTP currently does not define any mechanism
to provide data origin authentication for group security
associations. Fortunately, it is straightforward to add TESLA to the
SRTP cryptographic framework.
The TESLA extension to SRTP is defined in this specification, which
assumes that the reader is familiar with the SRTP specification
[RFC3711], its packet structure, and its processing rules. TESLA is
Baugher & Carrara Standards Track [Page 3]
RFC 4383 TESLA-SRTP February 2006
an alternative message-authentication algorithm that authenticates
messages from the source when a key is shared among two or more
receivers.
3. TESLA
TESLA provides delayed per-packet data authentication and is
specified in [RFC4082].
In addition to its SRTP data-packet definition given here, TESLA
needs an initial synchronization protocol and initial bootstrapping
procedure. The synchronization protocol allows the sender and the
receiver to compare their clocks and determine an upper bound of the
difference. The synchronization protocol is outside the scope of
this document.
TESLA also requires an initial bootstrapping procedure to exchange
needed parameters and the initial commitment to the key chain
[RFC4082]. For SRTP, it is assumed that the bootstrapping is
performed out-of-band, possibly using the key management protocol
that is exchanging the security parameters for SRTP, e.g., [RFC3547,
RFC3830]. Initial bootstrapping of TESLA is outside the scope of
this document.
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RFC 4383 TESLA-SRTP February 2006
4. Usage of TESLA within SRTP
The present specification is an extension to the SRTP specification
[RFC3711] and describes the use of TESLA with only a single key chain
and delayed-authentication [RFC4082].
4.1. The TESLA Extension
TESLA is an OPTIONAL authentication transform for SRTP. When used,
TESLA adds the fields shown in Figure 1 per-packet. The fields added
by TESLA are called "TESLA authentication extensions," whereas
"authentication tag" or "integrity protection tag" indicate the
normal SRTP integrity protection tag, when the SRTP master key is
shared by more than two endpoints [RFC3711].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Disclosed Key ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TESLA MAC ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. The "TESLA authentication extension".
i: 32 bit, MANDATORY
Identifier of the time interval i, corresponding to the key K_i,
which is used to calculate the TESLA MAC of the current packet
(and other packets sent in the current time interval i).
Disclosed Key: variable length, MANDATORY
The disclosed key (K_(i-d)), which can be used to authenticate
previous packets from earlier time intervals [RFC4082]. A
Section 4.3 parameter establishes the size of this field.
TESLA MAC (Message Authentication Code): variable length, MANDATORY
The MAC computed using the key K'_i (derived from K_i)
[RFC4082], which is disclosed in a subsequent packet (in the
Disclosed Key field). The MAC coverage is defined in Section
4.6. A Section 4.3 parameter establishes the size of this
field.
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4.2. SRTP Packet Format
Figure 2 illustrates the format of the SRTP packet when TESLA is
applied. When applied to RTP, the TESLA authentication extension
SHALL be inserted before the (optional) SRTP MKI and (recommended)
authentication tag (SRTP MAC).
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+
|V=2|P|X| CC |M| PT | sequence number | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| timestamp | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| synchronization source (SSRC) identifier | | |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| contributing source (CSRC) identifiers | | |
| .... | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| RTP extension (OPTIONAL) | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | payload ... | | |
| | +-------------------------------+ | |
| | | RTP padding | RTP pad count | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| | i | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ Disclosed Key ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ TESLA MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+
| ~ MKI ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | |
+- Encrypted Portion TESLA Authenticated Portion ---+ |
|
Authenticated Portion ---+
Figure 2. The format of the SRTP packet when TESLA is applied.
As in SRTP, the "Encrypted Portion" of an SRTP packet consists of the
encryption of the RTP payload (including RTP padding when present) of
the equivalent RTP packet.
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RFC 4383 TESLA-SRTP February 2006
The "Authenticated Portion" of an SRTP packet consists of the RTP
header, the Encrypted Portion of the SRTP packet, and the TESLA
authentication extension. Note that the definition is extended from
[RFC3711] by the inclusion of the TESLA authentication extension.
The "TESLA Authenticated Portion" of an SRTP packet consists of the
RTP header and the Encrypted Portion of the SRTP packet. As shown in
Figure 2, the SRTP MAC covers up to the MKI field but does not
include the MKI. It is necessary for packet integrity that the
SRTP-TESLA MAC tag be covered by the SRTP integrity check. SRTP does
not cover the MKI field (because it does not need to be covered for
SRTP packet integrity). In order to make the two tags (SRTP-TESLA
MAC and SRTP-MAC) contiguous, we would need to redefine the SRTP
specification to include the MKI in SRTP-MAC coverage. This change
is impossible, so the MKI field separates the TESLA MAC from the SRTP
MAC in the packet layout of Figure 2. This change to the packet
format presents no problem to an implementation that supports the new
SRTP-TESLA authentication transform.
The lengths of the Disclosed Key and TESLA MAC fields are Section 4.3
parameters. As in SRTP, fields that follow the packet payload are
not necessarily aligned on 32-bit boundaries.
4.3. Extension of the SRTP Cryptographic Context
When TESLA is used, the definition of cryptographic context in
Section 3.2 of SRTP SHALL include the following extensions.
Transform-Dependent Parameters
1. an identifier for the PRF (TESLA PRF), implementing the one-way
function F(x) in TESLA (to derive the keys in the chain), and
the one-way function F'(x) in TESLA (to derive the keys for the
TESLA MAC, from the keys in the chain), e.g., to indicate
HMAC-SHA1. See Section 6 for the default value.
2. a non-negative integer, n_p, determining the length of the F
output; i.e., the length of the keys in the chain (that is also
the key disclosed in an SRTP packet). See Section 6 for the
default value.
3. a non-negative integer, n_f, determining the length of the
output of F', i.e., of the key for the TESLA MAC. See Section
6 for the default value.
4. an identifier for the TESLA MAC that accepts the output of
F'(x) as its key, e.g., to indicate HMAC-SHA1. See Section 6
for the default value.
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5. a non-negative integer, n_m, determining the length of the
output of the TESLA MAC. See Section 6 for the default value.
6. the beginning of the session T_0.
7. the interval duration T_int (in msec).
8. the key disclosure delay d (in number of intervals).
9. the upper bound D_t (in sec) on the lag of the receiver clock
relative to the sender clock (this quantity has to be
calculated by the peers out-of-band).
10. a non-negative integer, n_c, determining the length of the key
chain, K_0...K_n-1 of [RFC4082] (see also Section 6 of this
document), which is determined based upon the expected duration
of the stream.
11. the initial key of the chain to which the sender has committed
himself.
F(x) is used to compute a keychain of keys in SRTP TESLA, as defined
in Section 6. Also according to TESLA, F'(x) computes a TESLA MAC
key with inputs as defined in Section 6.
Section 6 of this document defines the default values for the
transform-specific TESLA parameters.
4.4. SRTP Processing
The SRTP packet processing is described in Section 3.3 of the SRTP
specification [RFC3711]. The use of TESLA slightly changes the
processing, as the SRTP MAC is checked upon packet arrival for DoS
prevention, but the current packet is not TESLA-authenticated. Each
packet is buffered until a subsequent packet discloses its TESLA key.
The TESLA verification itself consists of some steps, such as tests
of TESLA security invariants, that are described in Sections 3.5-3.7
of [RFC4082]. The words "TESLA computation" and "TESLA verification"
hereby imply all those steps, which are not all spelled out in the
following. In particular, notice that the TESLA verification implies
checking the safety condition (Section 3.5 of [RFC4082]).
As pointed out in [RFC4082], if the packet is deemed "unsafe", then
the receiver considers the packet unauthenticated. It should discard
unsafe packets, but, at its own risk, it may choose to use them
unverified. Hence, if the safe condition does not hold, it is
RECOMMENDED to discard the packet and log the event.
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RFC 4383 TESLA-SRTP February 2006
4.4.1. Sender Processing
The sender processing is as described in Section 3.3 of [RFC3711], up
to step 5, inclusive. After that, the following process is followed:
6. When TESLA is applied, identify the key in the TESLA chain to be
used in the current time interval, and the TESLA MAC key derived
from it. Execute the TESLA computation to obtain the TESLA
authentication extension for the current packet, by appending the
current interval identifier (as i field), the disclosed key of the
chain for the previous disclosure interval (i.e., the key for
interval i is disclosed in interval i+d), and the TESLA MAC under
the current key from the chain. This step uses the related TESLA
parameters from the crypto context as for Step 4.
7. If the MKI indicator in the SRTP crypto context is set to one,
append the MKI to the packet.
8. When TESLA is applied, and if the SRTP authentication (external
tag) is required (for DoS), compute the authentication tag as
described in step 7 of Section 3.3 of the SRTP specification, but
with coverage as defined in this specification (see Section 4.6).
9. If necessary, update the rollover counter (step 8 in Section 3.3
of [RFC3711]).
4.4.2. Receiver Processing
The receiver processing is as described in Section 3.3 of [RFC3711],
up to step 4, inclusive.
To authenticate and replay-protect the current packet, the processing
is as follows:
First, check if the packet has been replayed (as per Section 3.3
of [RFC3711]). Note, however, that the SRTP replay list contains
SRTP indices of recently received packets that have been
authenticated by TESLA (i.e., replay list updates MUST NOT be
based on SRTP MAC). If the packet is judged to be replayed, then
the packet MUST be discarded, and the event SHOULD be logged.
Next, perform verification of the SRTP integrity protection tag
(not the TESLA MAC), if present, using the rollover counter from
the current packet, the authentication algorithm indicated in the
cryptographic context, and the session authentication key. If the
verification is unsuccessful, the packet MUST be discarded from
further processing, and the event SHOULD be logged.
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RFC 4383 TESLA-SRTP February 2006
If the verification is successful, remove and store the MKI (if
present) and authentication tag fields from the packet. The
packet is buffered, awaiting disclosure of the TESLA key in a
subsequent packet.
TESLA authentication is performed on a packet when the key is
disclosed in a subsequent packet. Recall that a key for interval
i is disclosed during interval i+d, i.e., the same key is
disclosed in packets sent over d intervals of length t_int. If
the interval identifier i from the packet (Section 4.1) has
advanced more than d intervals from the highest value of i that
has been received, then packets have been lost, and one or more
keys MUST be computed as described in Section 3.2, second
paragraph, of the TESLA specification [RFC4082]. The computation
is performed recursively for all disclosed keys that have been
lost, from the newly-received interval to the last-received
interval.
When a newly-disclosed key is received or computed, perform the
TESLA verification of the packet using the rollover counter from
the packet, the TESLA security parameters from the cryptographic
context, and the disclosed key. If the verification is
unsuccessful, the packet MUST be discarded from further
processing, and the event SHOULD be logged. If the TESLA
verification is successful, remove the TESLA authentication
extension from the packet.
To decrypt the current packet, the processing is as follows:
Decrypt the Encrypted Portion of the packet, using the decryption
algorithm indicated in the cryptographic context, the session
encryption key, and salt (if used) found in Step 4 with the index
from Step 2.
(Note that the order of decryption and TESLA verification is not
mandated. It is RECOMMENDED that the TESLA verification be performed
before decryption. TESLA application designers might choose to
implement optimistic processing techniques such as notification of
TESLA verification results after decryption or even after plaintext
processing. Optimistic verification is beyond the scope of this
document.)
Update the rollover counter and highest sequence number, s_l, in the
cryptographic context, using the packet index estimated in Step 2.
If replay protection is provided, also update the Replay List (i.e.,
the Replay List is updated after the TESLA authentication is
successfully verified).
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RFC 4383 TESLA-SRTP February 2006
4.5. SRTCP Packet Format
Figure 3 illustrates the format of the SRTCP packet when TESLA is
applied. The TESLA authentication extension SHALL be inserted before
the MKI and authentication tag. Recall from [RFC3711] that in SRTCP
the MKI is OPTIONAL, while the E-bit, the SRTCP index, and the
authentication tag are MANDATORY. This means that the SRTP
(external) MAC is MANDATORY also when TESLA is used.
As in SRTP, the "Encrypted Portion" of an SRTCP packet consists of
the encryption of the RTCP payload of the equivalent compound RTCP
packet, from the first RTCP packet, i.e., from the ninth (9) byte to
the end of the compound packet.
The "Authenticated Portion" of an SRTCP packet consists of the entire
equivalent (eventually compound) RTCP packet, the E flag, the SRTCP
index (after any encryption has been applied to the payload), and the
TESLA extension. Note that the definition is extended from [RFC3711]
by the inclusion of the TESLA authentication extension.
We define the "TESLA Authenticated Portion" of an SRTCP packet as
consisting of the RTCP header (first 8 bytes) and the Encrypted
Portion of the SRTCP packet.
Processing of an SRTCP packets is similar to the SRTP processing
(Section 4.3), but there are SRTCP-specific changes described in
Section 3.4 of the SRTP specification [RFC3711] and in Section 4.6 of
this memo.
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RFC 4383 TESLA-SRTP February 2006
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+
|V=2|P| RC | PT=SR or RR | length | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| SSRC of sender | | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| ~ sender info ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ report block 1 ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ report block 2 ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ ... ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| |V=2|P| SC | PT=SDES=202 | length | | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| | SSRC/CSRC_1 | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ SDES items ~ | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| ~ ... ~ | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | |
| |E| SRTCP index | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| | i | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ Disclosed Key ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~ TESLA MAC ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+
| ~ SRTCP MKI ~ | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| : authentication tag : | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | |
+-- Encrypted Portion TESLA Authenticated Portion -----+ |
|
Authenticated Portion -------+
Figure 3. The format of the SRTCP packet when TESLA is applied.
Note that when additional fields are added to a packet, it will
increase the packet size and thus the RTCP average packet size.
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RFC 4383 TESLA-SRTP February 2006
4.6. TESLA MAC
Let M' denote packet data to be TESLA-authenticated. In the case of
SRTP, M' SHALL consist of the SRTP TESLA Authenticated Portion (RTP
header and SRTP Encrypted Portion; see Figure 2) of the packet
concatenated with the rollover counter (ROC) of the same packet:
M' = ROC || TESLA Authenticated Portion.
In the case of SRTCP, M' SHALL consist of the SRTCP TESLA
Authenticated Portion only (RTCP header and SRTCP Encrypted Portion).
The normal authentication tag (OPTIONAL for SRTP, MANDATORY for
SRTCP) SHALL be applied with the same coverage as specified in
[RFC3711]. That is:
- for SRTP: Authenticated Portion || ROC (with the extended
definition of SRTP Authentication Portion as in Section 4.2).
- for SRTCP: Authenticated Portion (with the extended definition of
SRTCP Authentication Portion as in Section 4.2).
The predefined authentication transform in SRTP, HMAC-SHA1 [RFC2104],
is also used to generate the TESLA MAC. For SRTP (and respectively
for SRTCP), the HMAC SHALL be applied to the key in the TESLA chain
corresponding to a particular time interval, and to M' as specified
above. The HMAC output SHALL then be truncated to the n_m left-most
bits. Default values are in Section 6.
As with SRTP, the predefined HMAC-SHA1 authentication algorithm MAY
be replaced with an alternative algorithm that is specified in a
future Internet RFC.
4.7. PRFs
TESLA requires a pseudo-random function (PRF) to implement
* one one-way function F(x) to derive the key chain, and
* one one-way function F'(x) to derive (from each key of the chain)
the key that is actually used to calculate the TESLA MAC.
When TESLA is used within SRTP, the default choice of the PRF SHALL
be HMAC-SHA1. Default values are in Section 6.
Other PRFs can be chosen, and their use SHALL follow the common
guidelines in [RFC3711] when adding new security parameters.
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RFC 4383 TESLA-SRTP February 2006
5. TESLA Bootstrapping and Cleanup
The extensions to the SRTP cryptographic context include a set of
TESLA parameters that are listed in Section 4.3 of this document.
Furthermore, TESLA MUST be bootstrapped at session setup (for the
parameter exchange and the initial key commitment) through a regular
data authentication system (a digital signature algorithm is
RECOMMENDED). Key management procedures can take care of this
bootstrapping prior to the commencement of an SRTP session where
TESLA authentication is used. The bootstrapping mechanism is out of
scope for this document (it could, for example, be part of the key
management protocol).
A critical factor for the security of TESLA is that the sender and
receiver need to be loosely synchronized. TESLA requires a bound on
clock drift to be known (D_t). Use of TESLA in SRTP assumes that the
time synchronization is guaranteed by out-of-band schemes (e.g., key
management). That is, it is not in the scope of SRTP.
It also should be noted that TESLA has some reliability requirements
in that a key is disclosed for a packet in a subsequent packet, which
can get lost. Since a key in a lost packet can be derived from a
future packet, TESLA is robust to packet loss. This key stream
stops, however, when the key-bearing data stream packets stop at the
conclusion of the RTP session. To avoid this nasty boundary
condition, send null packets with TESLA keys for one entire key-
disclosure period following the interval in which the stream ceases:
Null packets SHOULD be sent for d intervals of duration t_int (items
8 and 9 of Section 4.3). The rate of null packets SHOULD be the
average rate of the session media stream.
6. SRTP TESLA Default Parameters
Key management procedures establish SRTP TESLA operating parameters,
which are listed in Section 4.3 of this document. The operating
parameters appear in the SRTP cryptographic context and have the
default values that are described in this section. In the future, an
Internet RFC MAY define alternative settings for SRTP TESLA that are
different than those specified here. In particular, note that the
settings defined in this memo can have a large impact on bandwidth,
as they add 38 bytes to each packet (when the field length values are
the default ones). For certain applications, this overhead may
represent more than a 50% increase in packet size. Alternative
settings might seek to reduce the number and length of various TESLA
fields and outputs. No such optimizations are considered in this
memo.
Baugher & Carrara Standards Track [Page 14]
RFC 4383 TESLA-SRTP February 2006
It is RECOMMENDED that the SRTP MAC be truncated to 32 bits, since
the SRTP MAC provides only group authentication and serves only as
protection against external DoS.
The default values for the security parameters are listed in the
following table.
Parameter Mandatory-to-support Default
--------- -------------------- -------
TESLA PRF HMAC-SHA1 HMAC-SHA1
BIT-OUTPUT LENGTH n_p 160 160
BIT-OUTPUT LENGTH n_f 160 160
TESLA MAC HMAC-SHA1 HMAC-SHA1
(TRUNCATED) BIT-OUTPUT LENGTH n_m 80 80
As shown above, TESLA implementations MUST support HMAC-SHA1
[RFC2104] for the TESLA MAC and the TESLA PRF. The TESLA keychain
generator is recursively defined as follows [RFC4082].
K_i=HMAC_SHA1(K_{i+1},0), i=0..N-1
where N-1=n_c from the cryptographic context.
The TESLA MAC key generator is defined as follows [RFC4082].
K'_i=HMAC_SHA1(K_i,1)
The TESLA MAC uses a truncated output of ten bytes [RFC2104] and is
defined as follows.
HMAC_SHA1(K'_i, M')
where M' is as specified in Section 4.6.
7. Security Considerations
Denial of Service (DoS) attacks on delayed authentication are
discussed in [PCST]. TESLA requires receiver buffering before
authentication; therefore, the receiver can suffer a denial of
service attack due to a flood of bogus packets. To address this
problem, the external SRTP MAC, based on the group key, MAY be used
in addition to the TESLA MAC. The short size of the SRTP MAC
(default 32 bits) is motivated because that MAC is purely for DoS
prevention from attackers external to the group. The shorter output
tag means that an attacker has a better chance of getting a forged
packet accepted, which is about 2^31 attempts on average. As a first
line of defense against a denial of service attack, a short tag is
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RFC 4383 TESLA-SRTP February 2006
probably adequate; a victim will likely have ample evidence that it
is under attack before accepting a forged packet, which will
subsequently fail the TESLA check. [RFC4082] describes other
mechanisms that can be used to prevent DoS, in place of the external
group-key MAC. If used, they need to be added as processing steps
(following the guidelines of [RFC4082]).
The use of TESLA in SRTP defined in this specification is subject to
the security considerations discussed in the SRTP specification
[RFC3711] and in the TESLA specification [RFC4082]. In particular,
the TESLA security is dependent on the computation of the "safety
condition" as defined in Section 3.5 of [RFC4082].
SRTP TESLA depends on the effective security of the systems that
perform bootstrapping (time synchronization) and key management.
These systems are external to SRTP and are not considered in this
specification.
The length of the TESLA MAC is by default 80 bits. RFC 2104 requires
the MAC length to be at least 80 bits and at least half the output
size of the underlying hash function. The SHA-1 output size is 160
bits, so both of these requirements are met with the 80-bit MAC
specified in this document. Note that IPsec implementations tend to
use 96 bits for their MAC values to align the header with a 64-bit
boundary. Both MAC sizes are well beyond the reach of current
cryptanalytic techniques.
8. Acknowledgements
The authors would like to thank Ran Canetti, Karl Norrman, Mats
Naslund, Fredrik Lindholm, David McGrew, and Bob Briscoe for their
valuable help.
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9. References
9.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
9.2. Informative References
[PCST] Perrig, A., Canetti, R., Song, D., Tygar, D., "Efficient
and Secure Source Authentication for Multicast", in Proc.
of Network and Distributed System Security Symposium NDSS
2001, pp. 35-46, 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, April 2005.
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Authors' Addresses
Questions and comments should be directed to the authors and
msec@ietf.org.
Mark Baugher
Cisco Systems, Inc.
5510 SW Orchid Street
Portland, OR 97219 USA
Phone: +1 408-853-4418
EMail: mbaugher@cisco.com
Elisabetta Carrara
Royal Institute of Technology
Stockholm
Sweden
EMail: carrara@kth.se
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RFC 4383 TESLA-SRTP February 2006
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