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Keywords: Integrity, Authentication, NTP, Security
Internet Engineering Task Force (IETF) D. Franke
Request for Comments: 8915 Akamai
Category: Standards Track D. Sibold
ISSN: 2070-1721 K. Teichel
PTB
M. Dansarie
R. Sundblad
Netnod
September 2020
Network Time Security for the Network Time Protocol
Abstract
This memo specifies Network Time Security (NTS), a mechanism for
using Transport Layer Security (TLS) and Authenticated Encryption
with Associated Data (AEAD) to provide cryptographic security for the
client-server mode of the Network Time Protocol (NTP).
NTS is structured as a suite of two loosely coupled sub-protocols.
The first (NTS Key Establishment (NTS-KE)) handles initial
authentication and key establishment over TLS. The second (NTS
Extension Fields for NTPv4) handles encryption and authentication
during NTP time synchronization via extension fields in the NTP
packets, and holds all required state only on the client via opaque
cookies.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8915.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Objectives
1.2. Terms and Abbreviations
1.3. Protocol Overview
2. Requirements Language
3. TLS Profile for Network Time Security
4. The NTS Key Establishment Protocol
4.1. NTS-KE Record Types
4.1.1. End of Message
4.1.2. NTS Next Protocol Negotiation
4.1.3. Error
4.1.4. Warning
4.1.5. AEAD Algorithm Negotiation
4.1.6. New Cookie for NTPv4
4.1.7. NTPv4 Server Negotiation
4.1.8. NTPv4 Port Negotiation
4.2. Retry Intervals
4.3. Key Extraction (Generally)
5. NTS Extension Fields for NTPv4
5.1. Key Extraction (for NTPv4)
5.2. Packet Structure Overview
5.3. The Unique Identifier Extension Field
5.4. The NTS Cookie Extension Field
5.5. The NTS Cookie Placeholder Extension Field
5.6. The NTS Authenticator and Encrypted Extension Fields
Extension Field
5.7. Protocol Details
6. Suggested Format for NTS Cookies
7. IANA Considerations
7.1. Service Name and Transport Protocol Port Number Registry
7.2. TLS Application-Layer Protocol Negotiation (ALPN) Protocol
IDs Registry
7.3. TLS Exporter Labels Registry
7.4. NTP Kiss-o'-Death Codes Registry
7.5. NTP Extension Field Types Registry
7.6. Network Time Security Key Establishment Record Types
Registry
7.7. Network Time Security Next Protocols Registry
7.8. Network Time Security Error and Warning Codes Registries
8. Security Considerations
8.1. Protected Modes
8.2. Cookie Encryption Key Compromise
8.3. Sensitivity to DDoS Attacks
8.4. Avoiding DDoS Amplification
8.5. Initial Verification of Server Certificates
8.6. Delay Attacks
8.7. NTS Stripping
9. Privacy Considerations
9.1. Unlinkability
9.2. Confidentiality
10. References
10.1. Normative References
10.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
This memo specifies Network Time Security (NTS), a cryptographic
security mechanism for network time synchronization. A complete
specification is provided for application of NTS to the client-server
mode of the Network Time Protocol (NTP) [RFC5905].
1.1. Objectives
The objectives of NTS are as follows:
* Identity: Through the use of a X.509 public key infrastructure,
implementations can cryptographically establish the identity of
the parties they are communicating with.
* Authentication: Implementations can cryptographically verify that
any time synchronization packets are authentic, i.e., that they
were produced by an identified party and have not been modified in
transit.
* Confidentiality: Although basic time synchronization data is
considered nonconfidential and sent in the clear, NTS includes
support for encrypting NTP extension fields.
* Replay prevention: Client implementations can detect when a
received time synchronization packet is a replay of a previous
packet.
* Request-response consistency: Client implementations can verify
that a time synchronization packet received from a server was sent
in response to a particular request from the client.
* Unlinkability: For mobile clients, NTS will not leak any
information additional to NTP which would permit a passive
adversary to determine that two packets sent over different
networks came from the same client.
* Non-amplification: Implementations (especially server
implementations) can avoid acting as distributed denial-of-service
(DDoS) amplifiers by never responding to a request with a packet
larger than the request packet.
* Scalability: Server implementations can serve large numbers of
clients without having to retain any client-specific state.
* Performance: NTS must not significantly degrade the quality of the
time transfer. The encryption and authentication used when
actually transferring time should be lightweight (see Section 5.7
of RFC 7384 [RFC7384]).
1.2. Terms and Abbreviations
AEAD Authenticated Encryption with Associated Data [RFC5116]
ALPN Application-Layer Protocol Negotiation [RFC7301]
C2S Client-to-server
DoS Denial-of-Service
DDoS Distributed Denial-of-Service
EF Extension Field [RFC5905]
HKDF Hashed Message Authentication Code-based Key Derivation
Function [RFC5869]
KoD Kiss-o'-Death [RFC5905]
NTP Network Time Protocol [RFC5905]
NTS Network Time Security
NTS NAK NTS negative-acknowledgment
NTS-KE Network Time Security Key Establishment
S2C Server-to-client
TLS Transport Layer Security [RFC8446]
1.3. Protocol Overview
The Network Time Protocol includes many different operating modes to
support various network topologies (see Section 3 of RFC 5905
[RFC5905]). In addition to its best-known and most-widely-used
client-server mode, it also includes modes for synchronization
between symmetric peers, a control mode for server monitoring and
administration, and a broadcast mode. These various modes have
differing and partly contradictory requirements for security and
performance. Symmetric and control modes demand mutual
authentication and mutual replay protection. Additionally, for
certain message types, the control mode may require confidentiality
as well as authentication. Client-server mode places more stringent
requirements on resource utilization than other modes because servers
may have a vast number of clients and be unable to afford to maintain
per-client state. However, client-server mode also has more relaxed
security needs because only the client requires replay protection: it
is harmless for stateless servers to process replayed packets. The
security demands of symmetric and control modes, on the other hand,
are in conflict with the resource-utilization demands of client-
server mode: any scheme that provides replay protection inherently
involves maintaining some state to keep track of which messages have
already been seen.
This memo specifies NTS exclusively for the client-server mode of
NTP. To this end, NTS is structured as a suite of two protocols:
The "NTS Extension Fields for NTPv4" define a collection of NTP
extension fields for cryptographically securing NTPv4 using
previously established key material. They are suitable for
securing client-server mode because the server can implement them
without retaining per-client state. All state is kept by the
client and provided to the server in the form of an encrypted
cookie supplied with each request. On the other hand, the NTS
Extension Fields are suitable _only_ for client-server mode
because only the client, and not the server, is protected from
replay.
The "NTS Key Establishment" protocol (NTS-KE) is a mechanism for
establishing key material for use with the NTS Extension Fields
for NTPv4. It uses TLS to establish keys, to provide the client
with an initial supply of cookies, and to negotiate some
additional protocol options. After this, the TLS channel is
closed with no per-client state remaining on the server side.
The typical protocol flow is as follows: The client connects to an
NTS-KE server on the NTS TCP port and the two parties perform a TLS
handshake. Via the TLS channel, the parties negotiate some
additional protocol parameters, and the server sends the client a
supply of cookies along with an address and port of an NTP server for
which the cookies are valid. The parties use TLS key export
[RFC5705] to extract key material, which will be used in the next
phase of the protocol. This negotiation takes only a single round
trip, after which the server closes the connection and discards all
associated state. At this point, the NTS-KE phase of the protocol is
complete. Ideally, the client never needs to connect to the NTS-KE
server again.
Time synchronization proceeds with the indicated NTP server. The
client sends the server an NTP client packet that includes several
extension fields. Included among these fields are a cookie
(previously provided by the key establishment server) and an
authentication tag, computed using key material extracted from the
NTS-KE handshake. The NTP server uses the cookie to recover this key
material and send back an authenticated response. The response
includes a fresh, encrypted cookie that the client then sends back in
the clear in a subsequent request. This constant refreshing of
cookies is necessary in order to achieve NTS's unlinkability goal.
Figure 1 provides an overview of the high-level interaction between
the client, the NTS-KE server, and the NTP server. Note that the
cookies' data format and the exchange of secrets between NTS-KE and
NTP servers are not part of this specification and are implementation
dependent. However, a suggested format for NTS cookies is provided
in Section 6.
+--------------+
| |
+-> | NTP Server 1 |
| | |
Shared cookie | +--------------+
+---------------+ encryption parameters | +--------------+
| | (Implementation dependent) | | |
| NTS-KE Server | <------------------------------+-> | NTP Server 2 |
| | | | |
+---------------+ | +--------------+
^ | .
| | .
| 1. Negotiate parameters, | .
| receive initial cookie | +--------------+
| supply, generate AEAD keys, | | |
| and receive NTP server IP +-> | NTP Server N |
| addresses using "NTS Key | |
| Establishment" protocol. +--------------+
| ^
| |
| +----------+ |
| | | |
+-----------> | Client | <-------------------------+
| | 2. Perform authenticated
+----------+ time synchronization
and generate new
cookies using "NTS
Extension Fields for
NTPv4".
Figure 1: Overview of High-Level Interactions in NTS
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. TLS Profile for Network Time Security
Network Time Security makes use of TLS for NTS key establishment.
Since the NTS protocol is new as of this publication, no backward-
compatibility concerns exist to justify using obsolete, insecure, or
otherwise broken TLS features or versions. Implementations MUST
conform with RFC 7525 [RFC7525] or with a later revision of BCP 195.
Implementations MUST NOT negotiate TLS versions earlier than 1.3
[RFC8446] and MAY refuse to negotiate any TLS version that has been
superseded by a later supported version.
Use of the Application-Layer Protocol Negotiation Extension [RFC7301]
is integral to NTS, and support for it is REQUIRED for
interoperability.
Implementations MUST follow the rules in RFC 5280 [RFC5280] and RFC
6125 [RFC6125] for the representation and verification of the
application's service identity. When NTS-KE service discovery (out
of scope for this document) produces one or more host names, use of
the DNS-ID identifier type [RFC6125] is RECOMMENDED; specifications
for service discovery mechanisms can provide additional guidance for
certificate validation based on the results of discovery.
Section 8.5 of this memo discusses particular considerations for
certificate verification in the context of NTS.
4. The NTS Key Establishment Protocol
The NTS key establishment protocol is conducted via TCP port 4460.
The two endpoints carry out a TLS handshake in conformance with
Section 3, with the client offering (via an ALPN extension
[RFC7301]), and the server accepting, an application-layer protocol
of "ntske/1". Immediately following a successful handshake, the
client SHALL send a single request as Application Data encapsulated
in the TLS-protected channel. Then, the server SHALL send a single
response. After sending their respective request and response, the
client and server SHALL send TLS "close_notify" alerts in accordance
with Section 6.1 of RFC 8446 [RFC8446].
The client's request and the server's response each SHALL consist of
a sequence of records formatted according to Figure 2. The request
and a non-error response each SHALL include exactly one NTS Next
Protocol Negotiation record. The sequence SHALL be terminated by a
"End of Message" record. The requirement that all NTS-KE messages be
terminated by an End of Message record makes them self-delimiting.
Clients and servers MAY enforce length limits on requests and
responses; however, servers MUST accept requests of at least 1024
octets, and clients SHOULD accept responses of at least 65536 octets.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Record Type | Body Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Record Body .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTS-KE Record Format
The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of unrecognized Record
Types. Implementations which receive a record with an
unrecognized Record Type MUST ignore the record if the Critical
Bit is 0 and MUST treat it as an error if the Critical Bit is 1
(see Section 4.1.3).
Record Type Number: A 15-bit integer in network byte order. The
semantics of Record Types 0-7 are specified in this memo.
Additional type numbers SHALL be tracked through the IANA "Network
Time Security Key Establishment Record Types" registry.
Body Length: The length of the Record Body field, in octets, as a
16-bit integer in network byte order. Record bodies MAY have any
representable length and need not be aligned to a word boundary.
Record Body: The syntax and semantics of this field SHALL be
determined by the Record Type.
For clarity regarding bit-endianness: the Critical Bit is the most
significant bit of the first octet. In the C programming language,
given a network buffer 'unsigned char b[]' containing an NTS-KE
record, the critical bit is 'b[0] >> 7' while the record type is
'((b[0] & 0x7f) << 8) + b[1]'.
Note that, although the Type-Length-Body format of an NTS-KE record
is similar to that of an NTP extension field, the semantics of the
length field differ. While the length subfield of an NTP extension
field gives the length of the entire extension field including the
type and length subfields, the length field of an NTS-KE record gives
just the length of the body.
Figure 3 provides a schematic overview of the key establishment. It
displays the protocol steps to be performed by the NTS client and
server and Record Types to be exchanged.
+---------------------------------------+
| - Verify client request message. |
| - Extract TLS key material. |
| - Generate KE response message. |
| - Include Record Types: |
| o NTS Next Protocol Negotiation |
| o AEAD Algorithm Negotiation |
| o <NTPv4 Server Negotiation> |
| o <NTPv4 Port Negotiation> |
| o New Cookie for NTPv4 |
| o <New Cookie for NTPv4> |
| o End of Message |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
/ TLS application \
/ data \
/ \
/ V
Client -----+---------------------------------+----------------->
| |
| |
| |
+-----------+----------------------+ +------+-----------------+
|- Generate KE request message. | |- Verify server response|
| - Include Record Types: | | message. |
| o NTS Next Protocol Negotiation | |- Extract cookie(s). |
| o AEAD Algorithm Negotiation | +------------------------+
| o <NTPv4 Server Negotiation> |
| o <NTPv4 Port Negotiation> |
| o End of Message |
+----------------------------------+
Figure 3: NTS Key Establishment Messages
4.1. NTS-KE Record Types
The following NTS-KE Record Types are defined:
4.1.1. End of Message
The End of Message record has a Record Type number of 0 and a zero-
length body. It MUST occur exactly once as the final record of every
NTS-KE request and response. The Critical Bit MUST be set.
4.1.2. NTS Next Protocol Negotiation
The NTS Next Protocol Negotiation record has a Record Type number of
1. It MUST occur exactly once in every NTS-KE request and response.
Its body consists of a sequence of 16-bit unsigned integers in
network byte order. Each integer represents a Protocol ID from the
IANA "Network Time Security Next Protocols" registry (Section 7.7).
The Critical Bit MUST be set.
The Protocol IDs listed in the client's NTS Next Protocol Negotiation
record denote those protocols that the client wishes to speak using
the key material established through this NTS-KE session. Protocol
IDs listed in the NTS-KE server's response MUST comprise a subset of
those listed in the request and denote those protocols that the NTP
server is willing and able to speak using the key material
established through this NTS-KE session. The client MAY proceed with
one or more of them. The request MUST list at least one protocol,
but the response MAY be empty.
4.1.3. Error
The Error record has a Record Type number of 2. Its body is exactly
two octets long, consisting of an unsigned 16-bit integer in network
byte order, denoting an error code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request. If clients
receive a server response that includes an Error record, they MUST
discard any key material negotiated during the initial TLS exchange
and MUST NOT proceed to the Next Protocol. Requirements for retry
intervals are described in Section 4.2.
The following error codes are defined:
Error code 0 means "Unrecognized Critical Record". The server
MUST respond with this error code if the request included a record
that the server did not understand and that had its Critical Bit
set. The client SHOULD NOT retry its request without
modification.
Error code 1 means "Bad Request". The server MUST respond with
this error if the request is not complete and syntactically well-
formed, or, upon the expiration of an implementation-defined
timeout, it has not yet received such a request. The client
SHOULD NOT retry its request without modification.
Error code 2 means "Internal Server Error". The server MUST
respond with this error if it is unable to respond properly due to
an internal condition. The client MAY retry its request.
4.1.4. Warning
The Warning record has a Record Type number of 3. Its body is
exactly two octets long, consisting of an unsigned 16-bit integer in
network byte order, denoting a warning code. The Critical Bit MUST
be set.
Clients MUST NOT include Warning records in their request. If
clients receive a server response that includes a Warning record,
they MAY discard any negotiated key material and abort without
proceeding to the Next Protocol. Unrecognized warning codes MUST be
treated as errors.
This memo defines no warning codes.
4.1.5. AEAD Algorithm Negotiation
The AEAD Algorithm Negotiation record has a Record Type number of 4.
Its body consists of a sequence of unsigned 16-bit integers in
network byte order, denoting Numeric Identifiers from the IANA "AEAD
Algorithms" registry [IANA-AEAD]. The Critical Bit MAY be set.
If the NTS Next Protocol Negotiation record offers Protocol ID 0 (for
NTPv4), then this record MUST be included exactly once. Other
protocols MAY require it as well.
When included in a request, this record denotes which AEAD algorithms
the client is willing to use to secure the Next Protocol, in
decreasing preference order. When included in a response, this
record denotes which algorithm the server chooses to use. It is
empty if the server supports none of the algorithms offered. In
requests, the list MUST include at least one algorithm. In
responses, it MUST include at most one. Honoring the client's
preference order is OPTIONAL: servers may select among any of the
client's offered choices, even if they are able to support some other
algorithm that the client prefers more.
Server implementations of NTS Extension Fields for NTPv4 (Section 5)
MUST support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15).
That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD
Algorithm Negotiation record, and the server accepts Protocol ID 0
(NTPv4) in its NTS Next Protocol Negotiation record, then the
server's AEAD Algorithm Negotiation record MUST NOT be empty.
4.1.6. New Cookie for NTPv4
The New Cookie for NTPv4 record has a Record Type number of 5. The
contents of its body SHALL be implementation-defined, and clients
MUST NOT attempt to interpret them. See Section 6 for a suggested
construction.
Clients MUST NOT send records of this type. Servers MUST send at
least one record of this type, and SHOULD send eight of them, if the
Next Protocol Negotiation response record contains Protocol ID 0
(NTPv4) and the AEAD Algorithm Negotiation response record is not
empty. The Critical Bit SHOULD NOT be set.
4.1.7. NTPv4 Server Negotiation
The NTPv4 Server Negotiation record has a Record Type number of 6.
Its body consists of an ASCII-encoded [RFC0020] string. The contents
of the string SHALL be either an IPv4 address, an IPv6 address, or a
fully qualified domain name (FQDN). IPv4 addresses MUST be in dotted
decimal notation. IPv6 addresses MUST conform to the "Text
Representation of Addresses" as specified in RFC 4291 [RFC4291] and
MUST NOT include zone identifiers [RFC6874]. If a label contains at
least one non-ASCII character, it is an internationalized domain
name, and an A-LABEL MUST be used as defined in Section 2.3.2.1 of
RFC 5890 [RFC5890]. If the record contains a domain name, the
recipient MUST treat it as a FQDN, e.g., by making sure it ends with
a dot.
When NTPv4 is negotiated as a Next Protocol and this record is sent
by the server, the body specifies the hostname or IP address of the
NTPv4 server with which the client should associate and that will
accept the supplied cookies. If no record of this type is sent, the
client SHALL interpret this as a directive to associate with an NTPv4
server at the same IP address as the NTS-KE server. Servers MUST NOT
send more than one record of this type.
When this record is sent by the client, it indicates that the client
wishes to associate with the specified NTP server. The NTS-KE server
MAY incorporate this request when deciding which NTPv4 Server
Negotiation records to respond with, but honoring the client's
preference is OPTIONAL. The client MUST NOT send more than one
record of this type.
If the client has sent a record of this type, the NTS-KE server
SHOULD reply with the same record if it is valid and the server is
able to supply cookies for it. If the client has not sent any record
of this type, the NTS-KE server SHOULD respond with either an NTP
server address in the same family as the NTS-KE session or a FQDN
that can be resolved to an address in that family, if such
alternatives are available.
Servers MAY set the Critical Bit on records of this type; clients
SHOULD NOT.
4.1.8. NTPv4 Port Negotiation
The NTPv4 Port Negotiation record has a Record Type number of 7. Its
body consists of a 16-bit unsigned integer in network byte order,
denoting a UDP port number.
When NTPv4 is negotiated as a Next Protocol, and this record is sent
by the server, the body specifies the port number of the NTPv4 server
with which the client should associate and that will accept the
supplied cookies. If no record of this type is sent, the client
SHALL assume a default of 123 (the registered port number for NTP).
When this record is sent by the client in conjunction with a NTPv4
Server Negotiation record, it indicates that the client wishes to
associate with the NTP server at the specified port. The NTS-KE
server MAY incorporate this request when deciding what NTPv4 Server
Negotiation and NTPv4 Port Negotiation records to respond with, but
honoring the client's preference is OPTIONAL.
Servers MAY set the Critical Bit on records of this type; clients
SHOULD NOT.
4.2. Retry Intervals
A mechanism for not unnecessarily overloading the NTS-KE server is
REQUIRED when retrying the key establishment process due to protocol,
communication, or other errors. The exact workings of this will be
dependent on the application and operational experience gathered over
time. Until such experience is available, this memo provides the
following suggestion.
Clients SHOULD use exponential backoff, with an initial and minimum
retry interval of 10 seconds, a maximum retry interval of 5 days, and
a base of 1.5. Thus, the minimum interval in seconds, 't', for the
nth retry is calculated with the following:
t = min(10 * 1.5^(n-1), 432000).
Clients MUST NOT reset the retry interval until they have performed a
successful key establishment with the NTS-KE server, followed by a
successful use of the negotiated Next Protocol with the keys and data
established during that transaction.
4.3. Key Extraction (Generally)
Following a successful run of the NTS-KE protocol, key material SHALL
be extracted using the HMAC-based Extract-and-Expand Key Derivation
Function (HKDF) [RFC5869] in accordance with Section 7.5 of RFC 8446
[RFC8446]. Inputs to the exporter function are to be constructed in
a manner specific to the negotiated Next Protocol. However, all
protocols that utilize NTS-KE MUST conform to the following two
rules:
The disambiguating label string [RFC5705] MUST be "EXPORTER-
network-time-security".
The per-association context value [RFC5705] MUST be provided and
MUST begin with the two-octet Protocol ID that was negotiated as a
Next Protocol.
5. NTS Extension Fields for NTPv4
5.1. Key Extraction (for NTPv4)
Following a successful run of the NTS-KE protocol wherein Protocol ID
0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be
extracted: a client-to-server (C2S) key and a server-to-client (S2C)
key. These keys SHALL be computed with the HKDF defined in
Section 7.5 of RFC 8446 [RFC8446] using the following inputs:
The disambiguating label string [RFC5705] SHALL be "EXPORTER-
network-time-security".
The per-association context value [RFC5705] SHALL consist of the
following five octets:
- The first two octets SHALL be zero (the Protocol ID for NTPv4).
- The next two octets SHALL be the Numeric Identifier of the
negotiated AEAD algorithm in network byte order.
- The final octet SHALL be 0x00 for the C2S key and 0x01 for the
S2C key.
Implementations wishing to derive additional keys for private or
experimental use MUST NOT do so by extending the above-specified
syntax for per-association context values. Instead, they SHOULD use
their own disambiguating label string. Note that RFC 5705 [RFC5705]
provides that disambiguating label strings beginning with
"EXPERIMENTAL" MAY be used without IANA registration.
5.2. Packet Structure Overview
In general, an NTS-protected NTPv4 packet consists of the following:
The usual 48-octet NTP header, which is authenticated but not
encrypted.
Some extension fields, which are authenticated but not encrypted.
An extension field that contains AEAD output (i.e., an
authentication tag and possible ciphertext). The corresponding
plaintext, if non-empty, consists of some extension fields that
benefit from both encryption and authentication.
Possibly, some additional extension fields that are neither
encrypted nor authenticated. In general, these are discarded by
the receiver.
Always included among the authenticated or authenticated-and-
encrypted extension fields are a cookie extension field and a unique
identifier extension field, as described in Section 5.7. The purpose
of the cookie extension field is to enable the server to offload
storage of session state onto the client. The purpose of the unique
identifier extension field is to protect the client from replay
attacks.
5.3. The Unique Identifier Extension Field
The Unique Identifier extension field provides the client with a
cryptographically strong means of detecting replayed packets. It has
a Field Type of 0x0104. When the extension field is included in a
client packet (mode 3), its body SHALL consist of a string of octets
generated by a cryptographically secure random number generator
[RFC4086]. The string MUST be at least 32 octets long. When the
extension field is included in a server packet (mode 4), its body
SHALL contain the same octet string as was provided in the client
packet to which the server is responding. All server packets
generated by NTS-implementing servers in response to client packets
containing this extension field MUST also contain this field with the
same content as in the client's request. The field's use in modes
other than client-server is not defined.
This extension field MAY also be used standalone, without NTS, in
which case it provides the client with a means of detecting spoofed
packets from off-path attackers. Historically, NTP's origin
timestamp field has played both these roles, but this is suboptimal
for cryptographic purposes because it is only 64 bits long, and
depending on implementation details, most of those bits may be
predictable. In contrast, the Unique Identifier extension field
enables a degree of unpredictability and collision resistance more
consistent with cryptographic best practice.
5.4. The NTS Cookie Extension Field
The NTS Cookie extension field has a Field Type of 0x0204. Its
purpose is to carry information that enables the server to recompute
keys and other session state without having to store any per-client
state. The contents of its body SHALL be implementation-defined, and
clients MUST NOT attempt to interpret them. See Section 6 for a
suggested construction. The NTS Cookie extension field MUST NOT be
included in NTP packets whose mode is other than 3 (client) or 4
(server).
5.5. The NTS Cookie Placeholder Extension Field
The NTS Cookie Placeholder extension field has a Field Type of
0x0304. When this extension field is included in a client packet
(mode 3), it communicates to the server that the client wishes it to
send additional cookies in its response. This extension field MUST
NOT be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension field is present, it
MUST be accompanied by an NTS Cookie extension field. The body
length of the NTS Cookie Placeholder extension field MUST be the same
as the body length of the NTS Cookie extension field. This length
requirement serves to ensure that the response will not be larger
than the request, in order to improve timekeeping precision and
prevent DDoS amplification. The contents of the NTS Cookie
Placeholder extension field's body SHOULD be all zeros and, aside
from checking its length, MUST be ignored by the server.
5.6. The NTS Authenticator and Encrypted Extension Fields Extension
Field
The NTS Authenticator and Encrypted Extension Fields extension field
is the central cryptographic element of an NTS-protected NTP packet.
Its Field Type is 0x0404. It SHALL be formatted according to
Figure 4 and include the following fields:
Nonce Length: Two octets in network byte order, giving the length of
the Nonce field, excluding any padding, interpreted as an unsigned
integer.
Ciphertext Length: Two octets in network byte order, giving the
length of the Ciphertext field, excluding any padding, interpreted
as an unsigned integer.
Nonce: A nonce as required by the negotiated AEAD algorithm. The
end of the field is zero-padded to a word (four octets) boundary.
Ciphertext: The output of the negotiated AEAD algorithm. The
structure of this field is determined by the negotiated algorithm,
but it typically contains an authentication tag in addition to the
actual ciphertext. The end of the field is zero-padded to a word
(four octets) boundary.
Additional Padding: Clients that use a nonce length shorter than the
maximum allowed by the negotiated AEAD algorithm may be required
to include additional zero-padding. The necessary length of this
field is specified below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Length | Ciphertext Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Nonce, including up to 3 octets padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Ciphertext, including up to 3 octets padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Additional Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NTS Authenticator and Encrypted Extension Fields
Extension Field Format
The Ciphertext field SHALL be formed by providing the following
inputs to the negotiated AEAD algorithm:
K: For packets sent from the client to the server, the C2S key SHALL
be used. For packets sent from the server to the client, the S2C
key SHALL be used.
A: The associated data SHALL consist of the portion of the NTP
packet beginning from the start of the NTP header and ending at
the end of the last extension field that precedes the NTS
Authenticator and Encrypted Extension Fields extension field.
P: The plaintext SHALL consist of all (if any) NTP extension fields
to be encrypted; if multiple extension fields are present, they
SHALL be joined by concatenation. Each such field SHALL be
formatted in accordance with RFC 7822 [RFC7822], except that,
contrary to the RFC 7822 requirement that fields have a minimum
length of 16 or 28 octets, encrypted extension fields MAY be
arbitrarily short (but still MUST be a multiple of 4 octets in
length).
N: The nonce SHALL be formed however required by the negotiated AEAD
algorithm.
The purpose of the Additional Padding field is to ensure that servers
can always choose a nonce whose length is adequate to ensure its
uniqueness, even if the client chooses a shorter one, and still
ensure that the overall length of the server's response packet does
not exceed the length of the request. For mode 4 (server) packets,
no Additional Padding field is ever required. For mode 3 (client)
packets, the length of the Additional Padding field SHALL be computed
as follows. Let 'N_LEN' be the padded length of the Nonce field.
Let 'N_MAX' be, as specified by RFC 5116 [RFC5116], the maximum
permitted nonce length for the negotiated AEAD algorithm. Let
'N_REQ' be the lesser of 16 and N_MAX, rounded up to the nearest
multiple of 4. If N_LEN is greater than or equal to N_REQ, then no
Additional Padding field is required. Otherwise, the Additional
Padding field SHALL be at least N_REQ - N_LEN octets in length.
Servers MUST enforce this requirement by discarding any packet that
does not conform to it.
Senders are always free to include more Additional Padding than
mandated by the above paragraph. Theoretically, it could be
necessary to do so in order to bring the extension field to the
minimum length required by RFC 7822 [RFC7822]. This should never
happen in practice because any reasonable AEAD algorithm will have a
nonce and an authenticator long enough to bring the extension field
to its required length already. Nonetheless, implementers are
advised to explicitly handle this case and ensure that the extension
field they emit is of legal length.
The NTS Authenticator and Encrypted Extension Fields extension field
MUST NOT be included in NTP packets whose mode is other than 3
(client) or 4 (server).
5.7. Protocol Details
A client sending an NTS-protected request SHALL include the following
extension fields as displayed in Figure 5:
Exactly one Unique Identifier extension field that MUST be
authenticated, MUST NOT be encrypted, and whose contents MUST be
the output of a cryptographically secure random number generator
[RFC4086].
Exactly one NTS Cookie extension field that MUST be authenticated
and MUST NOT be encrypted. The cookie MUST be one which has been
previously provided to the client, either from the key
establishment server during the NTS-KE handshake or from the NTP
server in response to a previous NTS-protected NTP request.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using an AEAD algorithm and C2S key
established through NTS-KE.
To protect the client's privacy, the client SHOULD avoid reusing a
cookie. If the client does not have any cookies that it has not
already sent, it SHOULD initiate a rerun of the NTS-KE protocol. The
client MAY reuse cookies in order to prioritize resilience over
unlinkability. Which of the two that should be prioritized in any
particular case is dependent on the application and the user's
preference. Section 9.1 describes the privacy considerations of this
in further detail.
The client MAY include one or more NTS Cookie Placeholder extension
fields that MUST be authenticated and MAY be encrypted. The number
of NTS Cookie Placeholder extension fields that the client includes
SHOULD be such that if the client includes N placeholders and the
server sends back N+1 cookies, the number of unused cookies stored by
the client will come to eight. The client SHOULD NOT include more
than seven NTS Cookie Placeholder extension fields in a request.
When both the client and server adhere to all cookie-management
guidance provided in this memo, the number of placeholder extension
fields will equal the number of dropped packets since the last
successful volley.
In rare circumstances, it may be necessary to include fewer NTS
Cookie Placeholder extensions than recommended above in order to
prevent datagram fragmentation. When cookies adhere to the format
recommended in Section 6 and the AEAD in use is the mandatory-to-
implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and
seven placeholders and still have packet size fall comfortably below
1280 octets if no non-NTS-related extensions are used; 1280 octets is
the minimum prescribed MTU for IPv6 and is generally safe for
avoiding IPv4 fragmentation. Nonetheless, senders SHOULD include
fewer cookies and placeholders than otherwise indicated if doing so
is necessary to prevent fragmentation.
+---------------------------------------+
| - Verify time request message. |
| - Generate time response message. |
| - Included NTPv4 extension fields: |
| o Unique Identifier EF |
| o NTS Authentication and |
| Encrypted Extension Fields EF |
| - NTS Cookie EF |
| - <NTS Cookie EF> |
| - Transmit time request packet. |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
Time request / \ Time response
(mode 3) / \ (mode 4)
/ \
/ V
Client -----+---------------------------------+----------------->
| |
| |
| |
+-----------+-----------------------+ +-----+------------------+
|- Generate time request message. | |- Verify time response |
| - Include NTPv4 Extension fields: | | message. |
| o Unique Identifier EF | |- Extract cookie(s). |
| o NTS Cookie EF | |- Time synchronization |
| o <NTS Cookie Placeholder EF> | | processing. |
| | +------------------------+
|- Generate AEAD tag of NTP message.|
|- Add NTS Authentication and |
| Encrypted Extension Fields EF. |
|- Transmit time request packet. |
+-----------------------------------+
Figure 5: NTS-Protected NTP Time Synchronization Messages
The client MAY include additional (non-NTS-related) extension fields
that MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension fields (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). The server
MUST discard any unauthenticated extension fields. Future
specifications of extension fields MAY provide exceptions to this
rule.
Upon receiving an NTS-protected request, the server SHALL (through
some implementation-defined mechanism) use the cookie to recover the
AEAD algorithm, C2S key, and S2C key associated with the request, and
then use the C2S key to authenticate the packet and decrypt the
ciphertext. If the cookie is valid and authentication and decryption
succeed, the server SHALL include the following extension fields in
its response:
Exactly one Unique Identifier extension field that MUST be
authenticated, MUST NOT be encrypted, and whose contents SHALL
echo those provided by the client.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using the AEAD algorithm and S2C key
recovered from the cookie provided by the client.
One or more NTS Cookie extension fields that MUST be authenticated
and encrypted. The number of NTS Cookie extension fields included
SHOULD be equal to, and MUST NOT exceed, one plus the number of
valid NTS Cookie Placeholder extension fields included in the
request. The cookies returned in those fields MUST be valid for
use with the NTP server that sent them. They MAY be valid for
other NTP servers as well, but there is no way for the server to
indicate this.
We emphasize the contrast that NTS Cookie extension fields MUST NOT
be encrypted when sent from client to server but MUST be encrypted
when sent from server to client. The former is necessary in order
for the server to be able to recover the C2S and S2C keys, while the
latter is necessary to satisfy the unlinkability goals discussed in
Section 9.1. We emphasize also that "encrypted" means encapsulated
within the NTS Authenticator and Encrypted Extensions extension
field. While the body of an NTS Cookie extension field will
generally consist of some sort of AEAD output (regardless of whether
the recommendations of Section 6 are precisely followed), this is not
sufficient to make the extension field "encrypted".
The server MAY include additional (non-NTS-related) extension fields
that MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension field (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). The client
MUST discard any unauthenticated extension fields. Future
specifications of extension fields MAY provide exceptions to this
rule.
Upon receiving an NTS-protected response, the client MUST verify that
the Unique Identifier matches that of an outstanding request, and
that the packet is authentic under the S2C key associated with that
request. If either of these checks fails, the packet MUST be
discarded without further processing. In particular, the client MUST
discard unprotected responses to NTS-protected requests.
If the server is unable to validate the cookie or authenticate the
request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see
Section 7.4 of RFC 5905 [RFC5905]) with kiss code "NTSN", meaning
"NTS NAK" (NTS negative-acknowledgment). It MUST NOT include any NTS
Cookie or NTS Authenticator and Encrypted Extension Fields extension
fields.
If the NTP server has previously responded with authentic NTS-
protected NTP packets, the client MUST verify that any KoD packets
received from the server contain the Unique Identifier extension
field and that the Unique Identifier matches that of an outstanding
request. If this check fails, the packet MUST be discarded without
further processing. If this check passes, the client MUST comply
with Section 7.4 of RFC 5905 [RFC5905] where required.
A client MAY automatically rerun the NTS-KE protocol upon forced
disassociation from an NTP server. In that case, it MUST avoid
quickly looping between the NTS-KE and NTP servers by rate limiting
the retries. Requirements for retry intervals in NTS-KE are
described in Section 4.2.
Upon reception of the NTS NAK kiss code, the client SHOULD wait until
the next poll for a valid NTS-protected response, and if none is
received, initiate a fresh NTS-KE handshake to try to renegotiate new
cookies, AEAD keys, and parameters. If the NTS-KE handshake
succeeds, the client MUST discard all old cookies and parameters and
use the new ones instead. As long as the NTS-KE handshake has not
succeeded, the client SHOULD continue polling the NTP server using
the cookies and parameters it has.
To allow for NTP session restart when the NTS-KE server is
unavailable and to reduce NTS-KE server load, the client SHOULD keep
at least one unused but recent cookie, AEAD keys, negotiated AEAD
algorithm, and other necessary parameters in persistent storage.
This way, the client is able to resume the NTP session without
performing renewed NTS-KE negotiation.
6. Suggested Format for NTS Cookies
This section is non-normative. It gives a suggested way for servers
to construct NTS cookies. All normative requirements are stated in
Section 4.1.6 and Section 5.4.
The role of cookies in NTS is closely analogous to that of session
tickets in TLS. Accordingly, the thematic resemblance of this
section to RFC 5077 [RFC5077] is deliberate, and the reader should
likewise take heed of its security considerations.
Servers should select an AEAD algorithm that they will use to encrypt
and authenticate cookies. The chosen algorithm should be one such as
AEAD_AES_SIV_CMAC_256 [RFC5297], which resists accidental nonce
reuse. It need not be the same as the one that was negotiated with
the client. Servers should randomly generate and store a secret
master AEAD key 'K'. Servers should additionally choose a non-
secret, unique value 'I' as key identifier for 'K'.
Servers should periodically (e.g., once daily) generate a new pair
'(I,K)' and immediately switch to using these values for all newly-
generated cookies. Following each such key rotation, servers should
securely erase any previously generated keys that should now be
expired. Servers should continue to accept any cookie generated
using keys that they have not yet erased, even if those keys are no
longer current. Erasing old keys provides for forward secrecy,
limiting the scope of what old information can be stolen if a master
key is somehow compromised. Holding on to a limited number of old
keys allows clients to seamlessly transition from one generation to
the next without having to perform a new NTS-KE handshake.
The need to keep keys synchronized between NTS-KE and NTP servers as
well as across load-balanced clusters can make automatic key rotation
challenging. However, the task can be accomplished without the need
for central key-management infrastructure by using a ratchet, i.e.,
making each new key a deterministic, cryptographically pseudorandom
function of its predecessor. A recommended concrete implementation
of this approach is to use HKDF [RFC5869] to derive new keys, using
the key's predecessor as Input Keying Material and its key identifier
as a salt.
To form a cookie, servers should first form a plaintext 'P'
consisting of the following fields:
The AEAD algorithm negotiated during NTS-KE.
The S2C key.
The C2S key.
Servers should then generate a nonce 'N' uniformly at random, and
form AEAD output 'C' by encrypting 'P' under key 'K' with nonce 'N'
and no associated data.
The cookie should consist of the tuple '(I,N,C)'.
To verify and decrypt a cookie provided by the client, first parse it
into its components 'I', 'N', and 'C'. Use 'I' to look up its
decryption key 'K'. If the key whose identifier is 'I' has been
erased or never existed, decryption fails; reply with an NTS NAK.
Otherwise, attempt to decrypt and verify ciphertext 'C' using key 'K'
and nonce 'N' with no associated data. If decryption or verification
fails, reply with an NTS NAK. Otherwise, parse out the contents of
the resulting plaintext 'P' to obtain the negotiated AEAD algorithm,
S2C key, and C2S key.
7. IANA Considerations
7.1. Service Name and Transport Protocol Port Number Registry
IANA has allocated the following entry in the "Service Name and
Transport Protocol Port Number Registry" [RFC6335]:
Service Name: ntske
Port Number: 4460
Transport Protocol: tcp
Description: Network Time Security Key Establishment
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Registration Date: 2020-04-07
Reference: RFC 8915
7.2. TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs
Registry
IANA has allocated the following entry in the "TLS Application-Layer
Protocol Negotiation (ALPN) Protocol IDs" registry [RFC7301]:
Protocol: Network Time Security Key Establishment, version 1
Identification Sequence: 0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31
("ntske/1")
Reference: RFC 8915, Section 4
7.3. TLS Exporter Labels Registry
IANA has allocated the following entry in the TLS Exporter Labels
registry [RFC5705]:
+================================+=======+===========+=========+====+
| Value |DTLS-OK|Recommended|Reference|Note|
+================================+=======+===========+=========+====+
| EXPORTER-network-time-security |Y |Y |RFC 8915,| |
| | | |Section | |
| | | |4.3 | |
+--------------------------------+-------+-----------+---------+----+
Table 1
7.4. NTP Kiss-o'-Death Codes Registry
IANA has allocated the following entry in the "NTP Kiss-o'-Death
Codes" registry [RFC5905]:
+======+===============================+=============+
| Code | Meaning | Reference |
+======+===============================+=============+
| NTSN | Network Time Security (NTS) | RFC 8915, |
| | negative-acknowledgment (NAK) | Section 5.7 |
+------+-------------------------------+-------------+
Table 2
7.5. NTP Extension Field Types Registry
IANA has allocated the following entries in the "NTP Extension Field
Types" registry [RFC5905]:
+============+============================+=======================+
| Field Type | Meaning | Reference |
+============+============================+=======================+
| 0x0104 | Unique Identifier | RFC 8915, Section 5.3 |
+------------+----------------------------+-----------------------+
| 0x0204 | NTS Cookie | RFC 8915, Section 5.4 |
+------------+----------------------------+-----------------------+
| 0x0304 | NTS Cookie Placeholder | RFC 8915, Section 5.5 |
+------------+----------------------------+-----------------------+
| 0x0404 | NTS Authenticator and | RFC 8915, Section 5.6 |
| | Encrypted Extension Fields | |
+------------+----------------------------+-----------------------+
Table 3
7.6. Network Time Security Key Establishment Record Types Registry
IANA has created a new registry entitled "Network Time Security Key
Establishment Record Types". Entries have the following fields:
Record Type Number (REQUIRED): An integer in the range 0-32767
inclusive.
Description (REQUIRED): A short text description of the purpose of
the field.
Reference (REQUIRED): A reference to a document specifying the
semantics of the record.
The registration policy varies by Record Type Number, as follows:
0-1023: IETF Review
1024-16383: Specification Required
16384-32767: Private or Experimental Use
The initial contents of this registry are as follows:
+====================+======================+===============+
| Record Type Number | Description | Reference |
+====================+======================+===============+
| 0 | End of Message | RFC 8915, |
| | | Section 4.1.1 |
+--------------------+----------------------+---------------+
| 1 | NTS Next Protocol | RFC 8915, |
| | Negotiation | Section 4.1.2 |
+--------------------+----------------------+---------------+
| 2 | Error | RFC 8915, |
| | | Section 4.1.3 |
+--------------------+----------------------+---------------+
| 3 | Warning | RFC 8915, |
| | | Section 4.1.4 |
+--------------------+----------------------+---------------+
| 4 | AEAD Algorithm | RFC 8915, |
| | Negotiation | Section 4.1.5 |
+--------------------+----------------------+---------------+
| 5 | New Cookie for NTPv4 | RFC 8915, |
| | | Section 4.1.6 |
+--------------------+----------------------+---------------+
| 6 | NTPv4 Server | RFC 8915, |
| | Negotiation | Section 4.1.7 |
+--------------------+----------------------+---------------+
| 7 | NTPv4 Port | RFC 8915, |
| | Negotiation | Section 4.1.8 |
+--------------------+----------------------+---------------+
| 8-16383 | Unassigned | |
+--------------------+----------------------+---------------+
| 16384-32767 | Reserved for Private | RFC 8915 |
| | or Experimental Use | |
+--------------------+----------------------+---------------+
Table 4
7.7. Network Time Security Next Protocols Registry
IANA has created a new registry entitled "Network Time Security Next
Protocols". Entries have the following fields:
Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive,
functioning as an identifier.
Protocol Name (REQUIRED): A short text string naming the protocol
being identified.
Reference (REQUIRED): A reference to a relevant specification
document.
The registration policy varies by Protocol ID, as follows:
0-1023: IETF Review
1024-32767: Specification Required
32768-65535: Private or Experimental Use
The initial contents of this registry are as follows:
+=============+=========================================+===========+
| Protocol ID | Protocol Name | Reference |
+=============+=========================================+===========+
| 0 | Network Time Protocol | RFC 8915 |
| | version 4 (NTPv4) | |
+-------------+-----------------------------------------+-----------+
| 1-32767 | Unassigned | |
+-------------+-----------------------------------------+-----------+
| 32768-65535 | Reserved for Private | RFC 8915 |
| | or Experimental Use | |
+-------------+-----------------------------------------+-----------+
Table 5
7.8. Network Time Security Error and Warning Codes Registries
IANA has created two new registries entitled "Network Time Security
Error Codes" and "Network Time Security Warning Codes". Entries in
each have the following fields:
Number (REQUIRED): An integer in the range 0-65535 inclusive
Description (REQUIRED): A short text description of the condition.
Reference (REQUIRED): A reference to a relevant specification
document.
The registration policy varies by Number, as follows:
0-1023: IETF Review
1024-32767: Specification Required
32768-65535: Private or Experimental Use
The initial contents of the "Network Time Security Error Codes"
registry are as follows:
+=============+==============================+===============+
| Number | Description | Reference |
+=============+==============================+===============+
| 0 | Unrecognized Critical Record | RFC 8915, |
| | | Section 4.1.3 |
+-------------+------------------------------+---------------+
| 1 | Bad Request | RFC 8915, |
| | | Section 4.1.3 |
+-------------+------------------------------+---------------+
| 2 | Internal Server Error | RFC 8915, |
| | | Section 4.1.3 |
+-------------+------------------------------+---------------+
| 3-32767 | Unassigned | |
+-------------+------------------------------+---------------+
| 32768-65535 | Reserved for Private or | RFC 8915 |
| | Experimental Use | |
+-------------+------------------------------+---------------+
Table 6
The "Network Time Security Warning Codes" registry is initially empty
except for the reserved range, i.e.:
+=============+======================+===========+
| Number | Description | Reference |
+=============+======================+===========+
| 0-32767 | Unassigned | |
+-------------+----------------------+-----------+
| 32768-65535 | Reserved for Private | RFC 8915 |
| | or Experimental Use | |
+-------------+----------------------+-----------+
Table 7
8. Security Considerations
8.1. Protected Modes
NTP provides many different operating modes in order to support
different network topologies and to adapt to various requirements.
This memo only specifies NTS for NTP modes 3 (client) and 4 (server)
(see Section 1.3). The best current practice for authenticating the
other NTP modes is using the symmetric message authentication code
feature as described in RFC 5905 [RFC5905] and RFC 8573 [RFC8573].
8.2. Cookie Encryption Key Compromise
If the suggested format for NTS cookies in Section 6 of this document
is used, an attacker who has gained access to the secret cookie
encryption key 'K' can impersonate the NTP server, including
generating new cookies. NTP and NTS-KE server operators SHOULD
remove compromised keys as soon as the compromise is discovered.
This will cause the NTP servers to respond with NTS NAK, thus forcing
key renegotiation. Note that this measure does not protect against
MITM attacks where the attacker has access to a compromised cookie
encryption key. If another cookie scheme is used, there are likely
similar considerations for that particular scheme.
8.3. Sensitivity to DDoS Attacks
The introduction of NTS brings with it the introduction of asymmetric
cryptography to NTP. Asymmetric cryptography is necessary for
initial server authentication and AEAD key extraction. Asymmetric
cryptosystems are generally orders of magnitude slower than their
symmetric counterparts. This makes it much harder to build systems
that can serve requests at a rate corresponding to the full line
speed of the network connection. This, in turn, opens up a new
possibility for DDoS attacks on NTP services.
The main protection against these attacks in NTS lies in that the use
of asymmetric cryptosystems is only necessary in the initial NTS-KE
phase of the protocol. Since the protocol design enables separation
of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
server separated from the NTP service it supports will not affect NTP
users that have already performed initial authentication, AEAD key
extraction, and cookie exchange.
NTS users should also consider that they are not fully protected
against DoS attacks by on-path adversaries. In addition to dropping
packets and attacks such as those described in Section 8.6, an on-
path attacker can send spoofed Kiss-o'-Death replies, which are not
authenticated, in response to NTP requests. This could result in
significantly increased load on the NTS-KE server. Implementers have
to weigh the user's need for unlinkability against the added
resilience that comes with cookie reuse in cases of NTS-KE server
unavailability.
8.4. Avoiding DDoS Amplification
Certain nonstandard and/or deprecated features of the Network Time
Protocol enable clients to send a request to a server that causes the
server to send a response much larger than the request. Servers that
enable these features can be abused in order to amplify traffic
volume in DDoS attacks by sending them a request with a spoofed
source IP address. In recent years, attacks of this nature have
become an endemic nuisance.
NTS is designed to avoid contributing any further to this problem by
ensuring that NTS-related extension fields included in server
responses will be the same size as the NTS-related extension fields
sent by the client. In particular, this is why the client is
required to send a separate and appropriately padded-out NTS Cookie
Placeholder extension field for every cookie it wants to get back,
rather than being permitted simply to specify a desired quantity.
Due to the RFC 7822 [RFC7822] requirement that extensions be padded
and aligned to four-octet boundaries, response size may still in some
cases exceed request size by up to three octets. This is
sufficiently inconsequential that we have declined to address it.
8.5. Initial Verification of Server Certificates
NTS's security goals are undermined if the client fails to verify
that the X.509 certificate chain presented by the NTS-KE server is
valid and rooted in a trusted certificate authority. RFC 5280
[RFC5280] and RFC 6125 [RFC6125] specify how such verification is to
be performed in general. However, the expectation that the client
does not yet have a correctly-set system clock at the time of
certificate verification presents difficulties with verifying that
the certificate is within its validity period, i.e., that the current
time lies between the times specified in the certificate's notBefore
and notAfter fields. It may be operationally necessary in some cases
for a client to accept a certificate that appears to be expired or
not yet valid. While there is no perfect solution to this problem,
there are several mitigations the client can implement to make it
more difficult for an adversary to successfully present an expired
certificate:
Check whether the system time is in fact unreliable. On systems
with the ntp_adjtime() system call, a return code other than
TIME_ERROR indicates that some trusted software has already set
the time and certificates can be strictly validated.
Allow the system administrator to specify that certificates should
_always_ be strictly validated. Such a configuration is
appropriate on systems that have a battery-backed clock or that
can reasonably prompt the user to manually set an approximately
correct time if it appears to be needed.
Once the clock has been synchronized, periodically write the
current system time to persistent storage. Do not accept any
certificate whose notAfter field is earlier than the last recorded
time.
NTP time replies are expected to be consistent with the NTS-KE TLS
certificate validity period, i.e. time replies received
immediately after an NTS-KE handshake are expected to lie within
the certificate validity period. Implementations are recommended
to check that this is the case. Performing a new NTS-KE handshake
based solely on the fact that the certificate used by the NTS-KE
server in a previous handshake has expired is normally not
necessary. Clients that still wish to do this must take care not
to cause an inadvertent denial-of-service attack on the NTS-KE
server, for example by picking a random time in the week preceding
certificate expiry to perform the new handshake.
Use multiple time sources. The ability to pass off an expired
certificate is only useful to an adversary who has compromised the
corresponding private key. If the adversary has compromised only
a minority of servers, NTP's selection algorithm (Section 11.2.1
of RFC 5905 [RFC5905]) will protect the client from accepting bad
time from the adversary-controlled servers.
8.6. Delay Attacks
In a packet delay attack, an adversary with the ability to act as a
man-in-the-middle delays time synchronization packets between client
and server asymmetrically [RFC7384]. Since NTP's formula for
computing time offset relies on the assumption that network latency
is roughly symmetrical, this leads to the client to compute an
inaccurate value [Mizrahi]. The delay attack does not reorder or
modify the content of the exchanged synchronization packets.
Therefore, cryptographic means do not provide a feasible way to
mitigate this attack. However, the maximum error that an adversary
can introduce is bounded by half of the round-trip delay.
RFC 5905 [RFC5905] specifies a parameter called MAXDIST, which
denotes the maximum round-trip latency (including not only the
immediate round trip between client and server, but the whole
distance back to the reference clock as reported in the Root Delay
field) that a client will tolerate before concluding that the server
is unsuitable for synchronization. The standard value for MAXDIST is
one second, although some implementations use larger values.
Whatever value a client chooses, the maximum error that can be
introduced by a delay attack is MAXDIST/2.
Usage of multiple time sources, or multiple network paths to a given
time source [Shpiner], may also serve to mitigate delay attacks if
the adversary is in control of only some of the paths.
8.7. NTS Stripping
Implementers must be aware of the possibility of "NTS stripping"
attacks, where an attacker attempts to trick clients into reverting
to plain NTP. Naive client implementations might, for example,
revert automatically to plain NTP if the NTS-KE handshake fails. A
man-in-the-middle attacker can easily cause this to happen. Even
clients that already hold valid cookies can be vulnerable, since an
attacker can force a client to repeat the NTS-KE handshake by sending
faked NTP mode 4 replies with the NTS NAK kiss code. Forcing a
client to repeat the NTS-KE handshake can also be the first step in
more advanced attacks.
For the reasons described here, implementations SHOULD NOT revert
from NTS-protected to unprotected NTP with any server without
explicit user action.
9. Privacy Considerations
9.1. Unlinkability
Unlinkability prevents a device from being tracked when it changes
network addresses (e.g., because said device moved between different
networks). In other words, unlinkability thwarts an attacker that
seeks to link a new network address used by a device with a network
address that it was formerly using because of recognizable data that
the device persistently sends as part of an NTS-secured NTP
association. This is the justification for continually supplying the
client with fresh cookies, so that a cookie never represents
recognizable data in the sense outlined above.
NTS's unlinkability objective is merely to not leak any additional
data that could be used to link a device's network address. NTS does
not rectify legacy linkability issues that are already present in
NTP. Thus, a client that requires unlinkability must also minimize
information transmitted in a client query (mode 3) packet as
described in the document NTP Client Data Minimization
[NTP-DATA-MIN].
The unlinkability objective only holds for time synchronization
traffic, as opposed to key establishment traffic. This implies that
it cannot be guaranteed for devices that function not only as time
clients, but also as time servers (because the latter can be
externally triggered to send linkable data, such as the TLS
certificate).
It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g.
reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information
revealed by control queries.
Note that the unlinkability objective does not prevent a client
device from being tracked by its time servers.
9.2. Confidentiality
NTS does not protect the confidentiality of information in NTP's
header fields. When clients implement NTP Client Data Minimization
[NTP-DATA-MIN], client packet headers do not contain any information
that the client could conceivably wish to keep secret: one field is
random, and all others are fixed. Information in server packet
headers is likewise public: the origin timestamp is copied from the
client's (random) transmit timestamp, and all other fields are set
the same regardless of the identity of the client making the request.
Future extension fields could hypothetically contain sensitive
information, in which case NTS provides a mechanism for encrypting
them.
10. References
10.1. Normative References
[IANA-AEAD]
IANA, "Authenticated Encryption with Associated Data
(AEAD) Parameters",
<https://www.iana.org/assignments/aead-parameters/>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <https://www.rfc-editor.org/info/rfc5297>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/info/rfc6874>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <https://www.rfc-editor.org/info/rfc7822>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
10.2. Informative References
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", 2012 IEEE
International Symposium on Precision Clock Synchronization
for Measurement, Control and Communication Proceedings,
pp. 1-6, DOI 10.1109/ISPCS.2012.6336612, September 2012,
<https://doi.org/10.1109/ISPCS.2012.6336612>.
[NTP-DATA-MIN]
Franke, D. F. and A. Malhotra, "NTP Client Data
Minimization", Work in Progress, Internet-Draft, draft-
ietf-ntp-data-minimization-04, 25 March 2019,
<https://tools.ietf.org/html/draft-ietf-ntp-data-
minimization-04>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[RFC8573] Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", RFC 8573,
DOI 10.17487/RFC8573, June 2019,
<https://www.rfc-editor.org/info/rfc8573>.
[Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", 2013 IEEE International Symposium on Precision
Clock Synchronization for Measurement, Control and
Communication (ISPCS) Proceedings, pp. 1-6,
DOI 10.1109/ISPCS.2013.6644754, September 2013,
<https://doi.org/10.1109/ISPCS.2013.6644754>.
Acknowledgments
The authors would like to thank Richard Barnes, Steven Bellovin,
Scott Fluhrer, Patrik Fältström, Sharon Goldberg, Russ Housley,
Benjamin Kaduk, Suresh Krishnan, Mirja Kühlewind, Martin Langer,
Barry Leiba, Miroslav Lichvar, Aanchal Malhotra, Danny Mayer, Dave
Mills, Sandra Murphy, Hal Murray, Karen O'Donoghue, Eric K. Rescorla,
Kurt Roeckx, Stephen Roettger, Dan Romascanu, Kyle Rose, Rich Salz,
Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn, Joachim
Strömbergsson, Martin Thomson, Éric Vyncke, Richard Welty, Christer
Weinigel, and Magnus Westerlund for contributions to this document
and comments on the design of NTS.
Authors' Addresses
Daniel Fox Franke
Akamai Technologies
145 Broadway
Cambridge, MA 02142
United States of America
Email: dafranke@akamai.com
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
D-38116 Braunschweig
Germany
Phone: +49-(0)531-592-8462
Email: dieter.sibold@ptb.de
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
D-38116 Braunschweig
Germany
Phone: +49-(0)531-592-4471
Email: kristof.teichel@ptb.de
Marcus Dansarie
Sweden
Email: marcus@dansarie.se
URI: https://orcid.org/0000-0001-9246-0263
Ragnar Sundblad
Netnod
Sweden
Email: ragge@netnod.se