draft-ietf-ntp-using-nts-for-ntp-09.txt   draft-ietf-ntp-using-nts-for-ntp-10.txt 
NTP Working Group D. Franke NTP Working Group D. Franke
Internet-Draft Akamai Internet-Draft Akamai
Intended status: Standards Track D. Sibold Intended status: Standards Track D. Sibold
Expires: December 28, 2017 K. Teichel Expires: May 3, 2018 K. Teichel
PTB PTB
June 26, 2017 October 30, 2017
Network Time Security for the Network Time Protocol Network Time Security for the Network Time Protocol
draft-ietf-ntp-using-nts-for-ntp-09 draft-ietf-ntp-using-nts-for-ntp-10
Abstract Abstract
This memo specifies Network Time Security (NTS), a mechanism for This memo specifies Network Time Security (NTS), a mechanism for
using Transport Layer Security (TLS) and Authenticated Encryption using Transport Layer Security (TLS) and Authenticated Encryption
with Associated Data (AEAD) to provide cryptographic security for the with Associated Data (AEAD) to provide cryptographic security for the
Network Time Protocol. Network Time Protocol.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 28, 2017. This Internet-Draft will expire on May 3, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Protocol overview . . . . . . . . . . . . . . . . . . . . 4 1.2. Protocol overview . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. TLS profile for Network Time Security . . . . . . . . . . . . 5 3. TLS profile for Network Time Security . . . . . . . . . . . . 5
4. The NTS-encapsulated NTPv4 protocol . . . . . . . . . . . . . 6 4. The NTS Key Establishment protocol . . . . . . . . . . . . . 6
5. The NTS Key Establishment protocol . . . . . . . . . . . . . 8 4.1. NTS-KE record types . . . . . . . . . . . . . . . . . . . 7
5.1. NTS-KE record types . . . . . . . . . . . . . . . . . . . 9 4.1.1. End of Message . . . . . . . . . . . . . . . . . . . 7
5.1.1. End of Message . . . . . . . . . . . . . . . . . . . 9 4.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 7
5.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 9 4.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 10 4.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 8
5.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 10 4.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 9
5.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 11 4.2. Key Extraction (generally) . . . . . . . . . . . . . . . 9
5.2. Key Extraction (generally) . . . . . . . . . . . . . . . 11 5. NTS Extension Fields for NTPv4 . . . . . . . . . . . . . . . 10
6. NTS Extensions for NTPv4 . . . . . . . . . . . . . . . . . . 11 5.1. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 10
6.1. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 11 5.2. Packet structure overview . . . . . . . . . . . . . . . . 10
6.2. Packet structure overview . . . . . . . . . . . . . . . . 12 5.3. The Unique Identifier extension field . . . . . . . . . . 11
6.3. The Unique Identifier extension . . . . . . . . . . . . . 13 5.4. The NTS Cookie extension field . . . . . . . . . . . . . 11
6.4. The NTS Cookie extension . . . . . . . . . . . . . . . . 13 5.5. The NTS Cookie Placeholder extension field . . . . . . . 11
6.5. The NTS Cookie Placeholder extension . . . . . . . . . . 13 5.6. The NTS Authenticator and Encrypted Extension Fields
6.6. The NTS Authenticator and Encrypted Extensions extension 14 extension field . . . . . . . . . . . . . . . . . . . . . 12
6.7. Protocol details . . . . . . . . . . . . . . . . . . . . 15 5.7. Protocol details . . . . . . . . . . . . . . . . . . . . 13
7. Recommended format for NTS cookies . . . . . . . . . . . . . 17 6. Suggested format for NTS cookies . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security considerations . . . . . . . . . . . . . . . . . . . 22 8. Security considerations . . . . . . . . . . . . . . . . . . . 20
9.1. Avoiding DDoS amplification . . . . . . . . . . . . . . . 22 8.1. Avoiding DDoS amplification . . . . . . . . . . . . . . . 20
9.2. Initial verification of server certificates . . . . . . . 22 8.2. Initial verification of server certificates . . . . . . . 21
9.3. Usage of NTP pools . . . . . . . . . . . . . . . . . . . 23 8.3. Usage of NTP pools . . . . . . . . . . . . . . . . . . . 22
9.4. Delay attacks . . . . . . . . . . . . . . . . . . . . . . 24 8.4. Delay attacks . . . . . . . . . . . . . . . . . . . . . . 22
9.5. Random number generation . . . . . . . . . . . . . . . . 24 8.5. Random number generation . . . . . . . . . . . . . . . . 23
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23
10.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . 24 9.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . . 23
10.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 25 9.2. Confidentiality . . . . . . . . . . . . . . . . . . . . . 23
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . 26 11.1. Normative References . . . . . . . . . . . . . . . . . . 24
12.2. Informative References . . . . . . . . . . . . . . . . . 27 11.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 28 Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
This memo specifies Network Time Security (NTS), a cryptographic This memo specifies Network Time Security (NTS), a cryptographic
security mechanism for network time synchronization. A complete security mechanism for network time synchronization. A complete
specification is provided for application of NTS to the Network Time specification is provided for application of NTS to the client-server
Protocol (NTP) [RFC5905]. However, certain sections of this memo are mode of the Network Time Protocol (NTP) [RFC5905].
not inherently NTP-specific, and enable future work to apply them to
other time synchronization protocols such as the Precision Time
Protocol (PTP) [IEC.61588_2009].
1.1. Objectives 1.1. Objectives
The objectives of NTS are as follows: The objectives of NTS are as follows:
o Identity: Through the use of the X.509 PKI, implementations may o Identity: Through the use of the X.509 PKI, implementations may
cryptographically establish the identity of the parties they are cryptographically establish the identity of the parties they are
communicating with communicating with
o Authentication: Implementations may cryptographically verify that o Authentication: Implementations may cryptographically verify that
any time synchronization packets are authentic, i.e., that they any time synchronization packets are authentic, i.e., that they
were produced by an identified party and have not been modified in were produced by an identified party and have not been modified in
transit. transit.
o Confidentiality: Although basic time synchronization data is o Confidentiality: Although basic time synchronization data is
considered non-confidential and sent in the clear, NTS includes considered non-confidential and sent in the clear, NTS includes
support for encrypting NTP extension fields. support for encrypting NTP extension fields.
o Replay prevention: Implementations may detect when a received time o Replay prevention: Client implementations may detect when a
synchronization packet is a replay of a previous packet. received time synchronization packet is a replay of a previous
packet.
o Request-response consistency: Client implementations may verify o Request-response consistency: Client implementations may verify
that a time synchronization packet received from a server was sent that a time synchronization packet received from a server was sent
in response to a particular request from the client. in response to a particular request from the client.
o Unlinkability: For mobile clients, NTS will not leak any o Unlinkability: For mobile clients, NTS will not leak any
information which would permit a passive adversary to determine information which would permit a passive adversary to determine
that two packets sent over different networks came from the same that two packets sent over different networks came from the same
client. client.
o Non-amplification: implementations may avoid acting as DDoS o Non-amplification: implementations (especially server
amplifiers by never responding to a request with a packet larger implementations) may avoid acting as DDoS amplifiers by never
than the request packet. responding to a request with a packet larger than the request
packet.
o Scalability: Servers implementations may serve large numbers of o Scalability: Server implementations may serve large numbers of
clients without having to retain any client-specific state. clients without having to retain any client-specific state.
1.2. Protocol overview 1.2. Protocol overview
The Network Time Protocol includes many different operating modes to The Network Time Protocol includes many different operating modes to
support various network topologies. In addition to its best-known support various network topologies. In addition to its best-known
and most-widely-used client-server mode, it also includes modes for and most-widely-used client-server mode, it also includes modes for
synchronization between symmetric peers, a control mode for server synchronization between symmetric peers, a control mode for server
monitoring and administration and a broadcast mode. These various monitoring and administration and a broadcast mode. These various
modes have differing and contradictory requirements for security and modes have differing and partly contradictory requirements for
performance. Symmetric and control modes demand mutual security and performance. Symmetric and control modes demand mutual
authentication and mutual replay protection, and for certain message authentication and mutual replay protection, and for certain message
types control mode may require confidentiality as well as types control mode may require confidentiality as well as
authentication. Client-server mode places more stringent authentication. Client-server mode places more stringent
requirements on resource utilization than other modes, because requirements on resource utilization than other modes, because
servers may have vast number of clients and be unable to afford to servers may have vast number of clients and be unable to afford to
maintain per-client state. However, client-server mode also has more maintain per-client state. However, client-server mode also has more
relaxed security needs, because only the client requires replay relaxed security needs, because only the client requires replay
protection: it is harmless for servers to process replayed packets. protection: it is harmless for servers to process replayed packets.
The security demands of symmetric and control modes, on the other The security demands of symmetric and control modes, on the other
hand, are in conflict with the resource-utilization demands of hand, are in conflict with the resource-utilization demands of
client-server mode: any scheme which provides replay protection client-server mode: any scheme which provides replay protection
inherently involves maintaining some state to keep track of what inherently involves maintaining some state to keep track of what
messages have already been seen. messages have already been seen.
In order to simulatenously serve these conflicting requirements, NTS This memo specifies NTS exclusively for the client-server mode of
is structured as a suite of three protocols: NTP. To this end, NTS is structured as a suite of two protocols:
The "DTLS-encapsulated NTPv4" protocol is little more than "NTP
over DTLS": the two endpoints perform a DTLS handshake and then
exchange NTP packets encapsulated as DTLS Application Data. It
provides mutual replay protection and is suitable for symmetric
and control modes, and is also secure for client/server mode but
relatively wasteful of server resources.
The "NTS Extensions for NTPv4" are a collection of NTP extension The "NTS Extension Fields for NTPv4" are a collection of NTP
fields for cryptographically securing NTPv4 using prevoiously- extension fields for cryptographically securing NTPv4 using
established key material. They are suitable for securing client/ previously-established key material. They are suitable for
server mode because the server can implement them without securing client/server mode because the server can implement them
retaining per-client state, but on the other hand are suitable without retaining per-client state, but on the other hand are
*only* for client/server mode because only the client, and not the suitable *only* for client/server mode because only the client,
server, is protected from replay. and not the server, is protected from replay.
The "NTS Key Establishment" protocol (NTS-KE) is mechanism for The "NTS Key Establishment" protocol (NTS-KE) is mechanism for
establishing key material for use with the NTS extensions for establishing key material for use with the NTS Extension Fields
NTPv4. It uses TLS to exchange keys and negotiate some additional for NTPv4. It uses TLS to exchange keys and negotiate some
protocol options, but then quickly closes the TLS channel and additional protocol options, but then quickly closes the TLS
permits the server to discard all associated state. NTS-KE is not channel and permits the server to discard all associated state.
NTP-specific; it is designed to be extensible, and might be
extended to support key establishment for other protocols such as
PTP.
It is intended that NTP implementations will use DTLS-encapsulated
NTPv4 to secure symmetric mode and control mode, and use NTS-KE
followed by NTS Extensions for NTPv4 to secure client/server mode.
NTS does not support NTP's broadcast mode.
As previously stated, DTLS-encapsulated NTPv4 is trivial. The
communicating parties establish a DTLS session and then exchange
arbitrary NTP packets as DTLS Application Data.
The typical protocol flow for client/server mode is as follows. The The typical protocol flow is as follows. The client connects to the
client connects to the server on the NTS TCP port and the two parties server on the NTS TCP port and the two parties perform a TLS
perform a TLS handshake. Via the TLS channel, the parties negotiate handshake. Via the TLS channel, the parties negotiate some
some additional protocol parameters and the server sends the client a additional protocol parameters and the server sends the client a
supply of cookies. The parties use TLS key export [RFC5705] to supply of cookies. The parties use TLS key export [RFC5705] to
extract key material which will be used in the next phase of the extract key material which will be used in the next phase of the
protocol. This negotiation takes only a single round trip, after protocol. This negotiation takes only a single round trip, after
which the server closes the connection and discards all associated which the server closes the connection and discards all associated
state. At this point the NTS-KE phase of the protocol is complete. state. At this point the NTS-KE phase of the protocol is complete.
Time synchronization proceeds over the NTP UDP port. The client Time synchronization proceeds over the NTP UDP port. The client
sends the server an NTP client packet which includes several sends the server an NTP client packet which includes several
extension fields. Included among these fields are a cookie extension fields. Included among these fields are a cookie
(previously provided by the server), and an authentication tag, (previously provided by the server), and an authentication tag,
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unlinkability goal.) unlinkability goal.)
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. TLS profile for Network Time Security 3. TLS profile for Network Time Security
Network Time Security makes use of both TLS (for NTS Key Network Time Security makes use of TLS for NTS key establishment.
Establishment) and DTLS (for NTS-encapsulated NTPv4). In either
case, the requirements and recommendations of this section are
similar. The notation "(D)TLS" refers to both TLS and DTLS.
Since securing time protocols is (as of 2017) a novel application of Since securing time protocols is (as of 2017) a novel application of
(D)TLS, no backward-compatibility concerns exist to justify using TLS, no backward-compatibility concerns exist to justify using
obsolete, insecure, or otherwise broken TLS features or versions. We obsolete, insecure, or otherwise broken TLS features or versions. We
therefore put forward the following requirements and guidelines, therefore put forward the following requirements and guidelines,
roughly representing 2017's best practices. roughly representing 2017's best practices.
Implementations MUST NOT negotiate (D)TLS versions earlier than 1.2. Implementations MUST NOT negotiate TLS versions earlier than 1.2.
Implementations willing to negotiate more than one possible version Implementations willing to negotiate more than one possible version
of (D)TLS SHOULD NOT respond to handshake failures by retrying with a of TLS SHOULD NOT respond to handshake failures by retrying with a
downgraded protocol version. If they do, they MUST implement downgraded protocol version. If they do, they MUST implement
[RFC7507]. [RFC7507].
(D)TLS clients MUST NOT offer, and (D)TLS servers MUST not select, TLS clients MUST NOT offer, and TLS servers MUST NOT select, RC4
RC4 cipher suites. [RFC7465] cipher suites. [RFC7465]
(D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the
TLS Renegotiation Indication Extension [RFC5746]. Regardless, they
MUST NOT initiate or permit insecure renegotiation. (*)
(D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the TLS clients SHOULD offer, and TLS servers SHOULD accept, the TLS
TLS Session Hash and Extended Master Secret Extension [RFC7627]. (*) Renegotiation Indication Extension [RFC5746]. Regardless, they MUST
NOT initiate or permit insecure renegotiation. (*)
Use of the Application-Layer Protocol Negotation Extension [RFC7301] TLS clients SHOULD offer, and TLS servers SHOULD accept, the TLS
Session Hash and Extended Master Secret Extension [RFC7627]. (*)
Use of the Application-Layer Protocol Negotiation Extension [RFC7301]
is integral to NTS and support for it is REQUIRED for is integral to NTS and support for it is REQUIRED for
interoperability. interoperability.
(*): Note that (D)TLS 1.3 or beyond may render the indicated (*): Note that TLS 1.3 or beyond may render the indicated
recommendations inapplicable. recommendations inapplicable.
4. The NTS-encapsulated NTPv4 protocol 4. The NTS Key Establishment protocol
The NTS-encapsulated NTPv4 protocol proceeds in two parts. The two
endpoints carry out a DTLS handshake in conformance with Section 3,
with the client offering (via an ALPN [RFC7301] extension), and the
server accepting, an application-layer protocol of "ntp/4". Second,
once the handshake is successfully completed, the two endpoints use
the established channel to exchange arbitrary NTPv4 packets as DTLS-
protected Application Data.
In addition to the requirements specified in Section 3,
implementations MUST enforce the anti-replay mechanism specified in
Section 4.1.2.6 of RFC 6347 [RFC6347] (or an equivalent mechanism
specified in a subsequent revision of DTLS). Servers wishing to
enforce access control SHOULD either demand a client certificate or
use a PSK-based handshake in order to establish the client's
identity.
The NTS-encapsulated NTPv4 protocol is the RECOMMENDED mechanism for
cryptographically securing mode 1 (symmetric active), 2 (symmetric
passive), and 6 (control) NTPv4 traffic. It is equally safe for mode
3/4 (client/server) traffic, but is NOT RECOMMENDED for this purpose
because it scales poorly compared to using NTS Extensions for NTPv4
(Section 6).
Since DTLS-encapsulated NTPv4 sessions may carry arbitrary NTP
packets, there is no prescribed implication from an implementation's
role as a DTLS client vs. DTLS server, to its role in the
application-level Network Time Protocol. For example, it is entirely
permissible for an implementation to initiate a DTLS handshake (thus
acting in the role of DTLS client), and then once the handshake is
completed, act as an NTP server with the DTLS server acting as an NTP
client. The following guidelines are offered as sensible default
behavior. Implementations may depart from this guidance if the user
configures them to do so.
Implementations typically should not use DTLS-encapsulated NTPv4 for
client/server mode, instead preferring to use NTS-KE and NTS
Extensions for NTPv4. If DTLS-encapsulated NTPv4 is used for client/
server mode, then the NTP client (mode 3) should be the DTLS client
and the NTP server (mode 4) should be the DTLS server.
For control mode (6), the party sending queries should be the DTLS
client and the party responding to the queries should be the DTLS
server.
For symmetric operation between an active (mode 1) and passive (mode
2) peer, the active peer should be the DTLS client and the passive
peer should be the DTLS server.
For symmetric operation between two active (mode 1) peers, both
parties should attempt to initiate a DTLS session with their peer.
If one handshake fails and the other succeeds, the successfully-
established session should be used for traffic in both directions.
If both handshakes succeed, either session may be used and packets
should receive identical dispositon regardless of which of the two
sessions they arrived over. Inactive sessions may be timed out but
the redundant session should not be proactively closed.
If, likely as a result of user error, party A is configured as a
symmetry active peer of party B, but party B is neither accepting
DTLS handshakes from party A nor initiating one with it, then after a
suitable number of failed attempts, party A may fall back to acting
as an NTP client (mode 3) of party B using NTS-KE and NTS Extensions
for NTPv4.
5. The NTS Key Establishment protocol
The NTS key establishment protocol is conducted via TCP port The NTS key establishment protocol is conducted via TCP port
[[TBD1]]. The two endpoints carry out a TLS handshake in conformance [[TBD1]]. The two endpoints carry out a TLS handshake in conformance
with Section 3, with the client offering (via an ALPN [RFC7301] with Section 3, with the client offering (via an ALPN [RFC7301]
extension), and the server accepting, an application-layer protocol extension), and the server accepting, an application-layer protocol
of "ntske/1". Immediately following a successful handshake, the of "ntske/1". Immediately following a successful handshake, the
client SHALL send a single request (as Application Data encapsulated client SHALL send a single request (as Application Data encapsulated
in the TLS-protected channel), then the server SHALL send a single in the TLS-protected channel), then the server SHALL send a single
response followed by a TLS "Close notify" alert and then discard the response followed by a TLS "Close notify" alert and then discard the
channel state. channel state.
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The requirement that all NTS-KE messages be terminated by an End of The requirement that all NTS-KE messages be terminated by an End of
Message record makes them self-delimiting. Message record makes them self-delimiting.
The fields of an NTS-KE record are defined as follows: The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of unrecognized C (Critical Bit): Determines the disposition of unrecognized
Record Types. Implementations which receive a record with an Record Types. Implementations which receive a record with an
unrecognized Record Type MUST ignore the record if the Critical 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. Bit is 0, and MUST treat it as an error if the Critical Bit is 1.
Record Type: A 15-bit integer in network byte order (from most-to- Record Type: A 15-bit integer in network byte order. The
least significant, its bits are record bits 7-1 and then 15-8). semantics of record types 0-5 are specified in this memo;
The semantics of record types 0-5 are specified in this memo;
additional type numbers SHALL be tracked through the IANA Network additional type numbers SHALL be tracked through the IANA Network
Time Security Key Establishment Record Types registry. Time Security Key Establishment Record Types registry.
Body Length: the length of the Record Body field, in octets, as a 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 16-bit integer in network byte order. Record bodies MAY have any
representable length and need not be aligned to a word boundary. representable length and need not be aligned to a word boundary.
Record Body: the syntax and semantics of this field shall be Record Body: the syntax and semantics of this field SHALL be
determined by the Record Type. determined by the Record Type.
5.1. NTS-KE record types For clarity regarding bit-endianness: the Critical Bit is the most-
significant bit of the first octet. In C, 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]`.
4.1. NTS-KE record types
The following NTS-KE Record Types are defined. The following NTS-KE Record Types are defined.
5.1.1. End of Message 4.1.1. End of Message
The End of Message record has a Record Type number of 0 and an zero- The End of Message record has a Record Type number of 0 and an zero-
length body. It MUST occur exactly once as the final record of every length body. It MUST occur exactly once as the final record of every
NTS-KE request and response. The Critical Bit MUST be set. NTS-KE request and response. The Critical Bit MUST be set.
5.1.2. NTS Next Protocol Negotiation 4.1.2. NTS Next Protocol Negotiation
The NTS Next Protocol Negotiation record has a record type of 1. It The NTS Next Protocol Negotiation record has a record type of 1. It
MUST occur exactly once in every NTS-KE request and response. Its MUST occur exactly once in every NTS-KE request and response. Its
body consists of a sequence of 16-bit unsigned integers in network body consists of a sequence of 16-bit unsigned integers in network
byte order. Each integer represents a Protocol ID from the IANA byte order. Each integer represents a Protocol ID from the IANA
Network Time Security Next Protocols registry. The Critical Bit MUST Network Time Security Next Protocols registry. The Critical Bit MUST
be set. be set.
The Protocol IDs listed in the client's NTS Next Protocol Negotiation The Protocol IDs listed in the client's NTS Next Protocol Negotiation
record denote those protocols which the client wishes to speak using record denote those protocols which the client wishes to speak using
the key material established through this NTS-KE session. The the key material established through this NTS-KE session. The
Protocol IDs listed in the server's response MUST comprise a subset Protocol IDs listed in the server's response MUST comprise a subset
of those listed in the request, and denote those protocols which the of those listed in the request, and denote those protocols which the
server is willing and able to speak using the key material server is willing and able to speak using the key material
established through this NTS-KE session. The client MAY proceed with established through this NTS-KE session. The client MAY proceed with
one or more of them. The request MUST list at least one protocol, one or more of them. The request MUST list at least one protocol,
but the response MAY be empty. but the response MAY be empty.
5.1.3. Error 4.1.3. Error
The Error record has a Record Type number of 2. Its body is exactly 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 two octets long, consisting of an unsigned 16-bit integer in network
byte order, denoting an error code. The Critical Bit MUST be set. byte order, denoting an error code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request. If clients Clients MUST NOT include Error records in their request. If clients
receive a server response which includes an Error record, they MUST receive a server response which includes an Error record, they MUST
discard any negotiated key material and MUST NOT proceed to the Next discard any negotiated key material and MUST NOT proceed to the Next
Protocol. Protocol.
skipping to change at page 10, line 18 skipping to change at page 8, line 31
set. The client SHOULD NOT retry its request without set. The client SHOULD NOT retry its request without
modification. modification.
Error code 1 means "Bad Request". The server MUST respond with Error code 1 means "Bad Request". The server MUST respond with
this error if, upon the expiration of an implementation-defined this error if, upon the expiration of an implementation-defined
timeout, it has not yet received a complete and syntactically timeout, it has not yet received a complete and syntactically
well-formed request from the client. This error is likely to be well-formed request from the client. This error is likely to be
the result of a dropped packet, so the client SHOULD start over the result of a dropped packet, so the client SHOULD start over
with a new TLS handshake and retry its request. with a new TLS handshake and retry its request.
5.1.4. Warning 4.1.4. Warning
The Warning record has a Record Type number of 3. Its body is 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 exactly two octets long, consisting of an unsigned 16-bit integer in
network byte order, denoting a warning code. The Critical Bit MUST network byte order, denoting a warning code. The Critical Bit MUST
be set. be set.
Clients MUST NOT include Warning records in their request. If Clients MUST NOT include Warning records in their request. If
clients receive a server response which includes an Warning record, clients receive a server response which includes an Warning record,
they MAY discard any negotiated key material and abort without they MAY discard any negotiated key material and abort without
proceeding to the Next Protocol. Unrecognized warning codes MUST be proceeding to the Next Protocol. Unrecognized warning codes MUST be
treated as errors. treated as errors.
This memo defines no warning codes. This memo defines no warning codes.
5.1.5. AEAD Algorithm Negotiation 4.1.5. AEAD Algorithm Negotiation
The AEAD Algorithm Negotiation record has a Record Type number of 4. The AEAD Algorithm Negotiation record has a Record Type number of 4.
Its body consists of a sequence of unsigned 16-bit integers in Its body consists of a sequence of unsigned 16-bit integers in
network byte order, denoting Numeric Identifiers from the IANA AEAD network byte order, denoting Numeric Identifiers from the IANA AEAD
registry [RFC5116]. The Critical Bit MAY be set. registry [RFC5116]. The Critical Bit MAY be set.
If the NTS Next Protocol Negotiation record offers "ntp/4",this If the NTS Next Protocol Negotiation record offers Protocol ID 0 (for
record MUST be included exactly once. Other protocols MAY require it NTPv4), then this record MUST be included exactly once. Other
as well. protocols MAY require it as well.
When included in a request, this record denotes which AEAD algorithms When included in a request, this record denotes which AEAD algorithms
the client is willing to use to secure the Next Protocol, in the client is willing to use to secure the Next Protocol, in
decreasing preference order. When included in a response, this decreasing preference order. When included in a response, this
record denotes which algorithm the server chooses to use, or is empty record denotes which algorithm the server chooses to use, or is empty
if the server supports none of the algorithms offered. In requests, if the server supports none of the algorithms offered. In requests,
the list MUST include at least one algorithm. In responses, it MUST the list MUST include at least one algorithm. In responses, it MUST
include at most one. Honoring the client's preference order is include at most one. Honoring the client's preference order is
OPTIONAL: servers may select among any of the client's offered OPTIONAL: servers may select among any of the client's offered
choices, even if they are able to support some other algorithm which choices, even if they are able to support some other algorithm which
the client prefers more. the client prefers more.
Server implementations of NTS extensions for NTPv4 (Section 6) MUST Server implementations of NTS extension fields for NTPv4 (Section 5)
support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15). 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 That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD
Algorithm Negotiation record, and the server accepts the "ntp/4" Algorithm Negotiation record, and the server accepts Protocol ID 0
protocol in its NTS Next Protocol Negotiation record, then the (NTPv4); in its NTS Next Protocol Negotiation record, then the
server's AEAD Algorithm Negotation record MUST NOT be empty. server's AEAD Algorithm Negotiation record MUST NOT be empty.
5.1.6. New Cookie for NTPv4 4.1.6. New Cookie for NTPv4
The New Cookie for NTPv4 record has a Record Type number of 5. The 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 contents of its body SHALL be implementation-defined and clients MUST
NOT attempt to interpret them. See Section 7 for a RECOMMENDED NOT attempt to interpret them. See Section 6 for a suggested
construction. construction.
Clients MUST NOT send records of this type. Servers MUST send at 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 they least one record of this type, and SHOULD send eight of them, if they
accept "ntp/4" as a Next Protocol. The Critical Bit SHOULD NOT be accept Protocol ID 0 (NTPv4) as a Next Protocol. The Critical Bit
set. SHOULD NOT be set.
5.2. Key Extraction (generally) 4.2. Key Extraction (generally)
Following a successful run of the NTS-KE protocol, key material SHALL Following a successful run of the NTS-KE protocol, key material SHALL
be extracted according to RFC 5705 [RFC5705]. Inputs to the exporter be extracted according to RFC 5705 [RFC5705]. Inputs to the exporter
function are to be constructed in a manner specific to the negotiated function are to be constructed in a manner specific to the negotiated
Next Protocol. However, all protocols which utilize NTS-KE MUST Next Protocol. However, all protocols which utilize NTS-KE MUST
conform to the following two rules: conform to the following two rules:
The disambiguating label string MUST be "EXPORTER-network-time- The disambiguating label string MUST be "EXPORTER-network-time-
security/1". security/1".
The per-association context value MUST be provided, and MUST begin The per-association context value MUST be provided, and MUST begin
with the two-octet Protocol ID which was negotiated as a Next with the two-octet Protocol ID which was negotiated as a Next
Protocol. Protocol.
6. NTS Extensions for NTPv4 5. NTS Extension Fields for NTPv4
6.1. Key Extraction (for NTPv4) 5.1. Key Extraction (for NTPv4)
Following a successful run of the NTS-KE protocol wherein "ntp/4" is Following a successful run of the NTS-KE protocol wherein Protocol ID
selected as a Next Protocol, two AEAD keys SHALL be extracted: a 0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be
client-to-server (C2S) key and a server-to-client (S2C) key. These extracted: a client-to-server (C2S) key and a server-to-client (S2C)
keys SHALL be computed according to RFC 5705 [RFC5705], using the key. These keys SHALL be computed according to RFC 5705 [RFC5705],
following inputs. using the following inputs.
The disambiguating label string SHALL be "EXPORTER-network-time- The disambiguating label string SHALL be "EXPORTER-network-time-
security/1". security/1".
The per-association context value SHALL consist of the following The per-association context value SHALL consist of the following
five octets: five octets:
The first two octets SHALL be zero. The first two octets SHALL be zero (the Protocol ID for NTPv4).
The next two octets SHALL be the Numeric Identifier of the The next two octets SHALL be the Numeric Identifier of the
negotiated AEAD Algorithm, in network byte order. negotiated AEAD Algorithm, in network byte order.
The final octet SHALL be 0x00 for the C2S key and 0x01 for the The final octet SHALL be 0x00 for the C2S key and 0x01 for the
S2C key. S2C key.
Implementations wishing to derive additional keys for private or Implementations wishing to derive additional keys for private or
experimental use MUST NOT do so by extending the above-specified experimental use MUST NOT do so by extending the above-specified
syntax for per-association context values. Instead, they SHOULD use syntax for per-association context values. Instead, they SHOULD use
their own disambiguating label string. Note that RFC 5705 provides their own disambiguating label string. Note that RFC 5705 provides
that disambiguating label strings beginning with "EXPERIMENTAL" MAY that disambiguating label strings beginning with "EXPERIMENTAL" MAY
be used without IANA registration. be used without IANA registration.
6.2. Packet structure overview 5.2. Packet structure overview
In general, an NTS-protected NTPv4 packet consists of: In general, an NTS-protected NTPv4 packet consists of:
The usual 48-octet NTP header, which is authenticated but not The usual 48-octet NTP header, which is authenticated but not
encrypted. encrypted.
Some extensions which are authenticated but not encrypted. Some extension fields which are authenticated but not encrypted.
An NTS extension which contains AEAD output (i.e., an An extension field which contains AEAD output (i.e., an
authentication tag and possible ciphertext). The corresponding authentication tag and possible ciphertext). The corresponding
plaintext, if non-empty, consists of some extensions which benefit plaintext, if non-empty, consists of some extension fields which
from both encryption and authentication. benefit from both encryption and authentication.
Possibly, some additional extensions which are neither encrypted Possibly, some additional extension fields which are neither
nor authenticated. These are discarded by the receiver. encrypted nor authenticated. These are discarded by the receiver.
Always included among the authenticated or authenticated-and- Always included among the authenticated or authenticated-and-
encrypted extensions are a cookie extension and a unique-identifier encrypted extension fields are a cookie extension field and a unique-
extension. The purpose of the cookie extension is to enable the identifier extension field. The purpose of the cookie extension
server to offload storage of session state onto the client. The field is to enable the server to offload storage of session state
purpose of the unique-identifier extension is to protect the client onto the client. The purpose of the unique-identifier extension
from replay attacks. field is to protect the client from replay attacks.
6.3. The Unique Identifier extension 5.3. The Unique Identifier extension field
The Unique Identifier extension has a Field Type of [[TBD2]]. When The Unique Identifier extension field has a Field Type of [[TBD2]].
the extension is included in a client packet (mode 3), its body SHALL When the extension field is included in a client packet (mode 3), its
consist of a string of octets generated uniformly at random. The body SHALL consist of a string of octets generated uniformly at
string SHOULD be 32 octets long. When the extension is included in a random. The string SHOULD be 32 octets long. When the extension
server packet (mode 4), its body SHALL contain the same octet string field is included in a server packet (mode 4), its body SHALL contain
as was provided in the client packet to which the server is the same octet string as was provided in the client packet to which
responding. Its use in modes other than client/server is not the server is responding. Its use in modes other than client/server
defined. is not defined.
The Unique Identifier extension provides the client with a The Unique Identifier extension field provides the client with a
cryptographically strong means of detecting replayed packets. It may cryptographically strong means of detecting replayed packets. It MAY
also be used standalone, without NTS, in which case it provides the also be used standalone, without NTS, in which case it provides the
client with a means of detecting spoofed packets from off-path client with a means of detecting spoofed packets from off-path
attackers. Historically, NTP's origin timestamp field has played attackers. Historically, NTP's origin timestamp field has played
both these roles, but for cryptographic purposes this is suboptimal both these roles, but for cryptographic purposes this is suboptimal
because it is only 64 bits long and, depending on implementation because it is only 64 bits long and, depending on implementation
details, most of those bits may be predictable. In contrast, the details, most of those bits may be predictable. In contrast, the
Unique Identifier extension enables a degree of unpredictability and Unique Identifier extension field enables a degree of
collision-resistance more consistent with cryptographic best unpredictability and collision-resistance more consistent with
practice. cryptographic best practice.
6.4. The NTS Cookie extension 5.4. The NTS Cookie extension field
The NTS Cookie extension has a Field Type of [[TBD3]]. Its purpose The NTS Cookie extension field has a Field Type of [[TBD3]]. Its
is to carry information which enables the server to recompute keys purpose is to carry information which enables the server to recompute
and other session state without having to store any per-client state. keys and other session state without having to store any per-client
The contents of its body SHALL be implementation-defined and clients state. The contents of its body SHALL be implementation-defined and
MUST NOT attempt to interpret them. See Section 7 for a RECOMMENDED clients MUST NOT attempt to interpret them. See Section 6 for a
construction. The NTS Cookie extension MUST NOT be included in NTP suggested construction. The NTS Cookie extension field MUST NOT be
packets whose mode is other than 3 (client) or 4 (server). included in NTP packets whose mode is other than 3 (client) or 4
(server).
6.5. The NTS Cookie Placeholder extension 5.5. The NTS Cookie Placeholder extension field
The NTS Cookie Placeholder extension has a Field Type of [[TBD4]]. The NTS Cookie Placeholder extension field has a Field Type of
When this extension is included in a client packet (mode 3), it [[TBD4]]. When this extension field is included in a client packet
communicates to the server that the client wishes it to send (mode 3), it communicates to the server that the client wishes it to
additional cookies in its response. This extension MUST NOT be send additional cookies in its response. This extension field MUST
included in NTP packets whose mode is other than 3. NOT be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension is present, it MUST be Whenever an NTS Cookie Placeholder extension field is present, it
accompanied by an NTS Cookie extension, and the body length of the MUST be accompanied by an NTS Cookie extension field, and the body
NTS Cookie Placeholder extension MUST be the same as the body length length of the NTS Cookie Placeholder extension field MUST be the same
of the NTS Cookie Extension. (This length requirement serves to as the body length of the NTS Cookie extension field. (This length
ensure that the response will not be larger than the request, in requirement serves to ensure that the response will not be larger
order to improve timekeeping precision and prevent DDoS than the request, in order to improve timekeeping precision and
amplification). The contents of the NTS Cookie Placeholder prevent DDoS amplification). The contents of the NTS Cookie
extension's body are undefined and, aside from checking its length, Placeholder extension field's body are undefined and, aside from
MUST be ignored by the server. checking its length, MUST be ignored by the server.
6.6. The NTS Authenticator and Encrypted Extensions extension 5.6. The NTS Authenticator and Encrypted Extension Fields extension
field
The NTS Authenticator and Encrypted Extensions extension is the The NTS Authenticator and Encrypted Extension Fields extension field
central cryptographic element of an NTS-protected NTP packet. Its is the central cryptographic element of an NTS-protected NTP packet.
Field Type is [[TBD5]] and the format of its body SHALL be as Its Field Type is [[TBD5]] and the format of its body SHALL be as
follows: follows:
Nonce length: two octets in network byte order, giving the length Nonce length: two octets in network byte order, giving the length
of the Nonce field. of the Nonce field and interpreted as an unsigned integer.
Nonce: a nonce as required by the negotiated AEAD Algorithm. Nonce: a nonce as required by the negotiated AEAD Algorithm.
Ciphertext: the output of the negotiated AEAD Algorithm. The Ciphertext: the output of the negotiated AEAD Algorithm. The
structure of this field is determined by the negotiated algorithm, structure of this field is determined by the negotiated algorithm,
but it typically contains an authentication tag in addition to the but it typically contains an authentication tag in addition to the
actual ciphertext. actual ciphertext.
Padding: between 1 and 24 octets of padding, with every octet set Padding: between 1 and 24 octets of padding, with every octet set
to the number of padding octets included, e.g., "01", "02 02", or to the number of padding octets included, e.g., "01", "02 02", or
"03 03 03". The number of padding bytes should be chosen in order "03 03 03". The number of padding bytes SHOULD be chosen in order
to comply with the RFC 7822 [RFC7822] requirement that (in the to comply with the RFC 7822 [RFC7822] requirement that (in the
absence of a legacy MAC) extensions have a total length in octets absence of a legacy MAC) extension fields have a total length in
(including the four octets for the type and length fields) which octets (including the four octets for the type and length fields)
is at least 28 and divisible by 4. At least one octet of padding which is at least 28 and divisible by 4. At least one octet of
MUST be included, so that implementations can unambiguously padding MUST be included, so that implementations can
delimit the end of the ciphertext from the start of the padding. unambiguously delimit the end of the ciphertext from the start of
the padding by examining the last octet to determine the padding
length.
The Ciphertext field SHALL be formed by providing the following The Ciphertext field SHALL be formed by providing the following
inputs to the negotiated AEAD Algorithm: inputs to the negotiated AEAD Algorithm:
K: For packets sent from the client to the server, the C2S key 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, SHALL be used. For packets sent from the server to the client,
the S2C key SHALL be used. the S2C key SHALL be used.
A: The associated data SHALL consist of the portion of the NTP A: The associated data SHALL consist of the portion of the NTP
packet beginning from the start of the NTP header and ending at packet beginning from the start of the NTP header and ending at
the end of the last extension which precedes the NTS Authenticator the end of the last extension field which precedes the NTS
and Encrypted Extensions extension. Authenticator and Encrypted Extension Fields extension field.
P: The plaintext SHALL consist of all (if any) extensions to be P: The plaintext SHALL consist of all (if any) NTP extension
encrypted. fields to be encrypted. The format of any such fields SHALL be in
accordance with RFC 7822 [RFC7822], and if multiple extension
fields are present they SHALL be joined by concatenation.
N: The nonce SHALL be formed however required by the negotiated N: The nonce SHALL be formed however required by the negotiated
AEAD Algorithm. AEAD Algorithm.
The NTS Authenticator and Encrypted Extensions extension MUST NOT be The NTS Authenticator and Encrypted Extension Fields extension field
included in NTP packets whose mode is other than 3 (client) or 4 MUST NOT be included in NTP packets whose mode is other than 3
(server). (client) or 4 (server).
6.7. Protocol details 5.7. Protocol details
A client sending an NTS-protected request SHALL include the following A client sending an NTS-protected request SHALL include the following
extensions: extension fields:
Exactly one Unique Identifier extension, which MUST be Exactly one Unique Identifier extension field, which MUST be
authenticated, MUST NOT be encrypted, and whose contents MUST NOT authenticated, MUST NOT be encrypted, and whose contents MUST NOT
duplicate those of any previous request. duplicate those of any previous request.
Exactly one NTS Cookie extension, which MUST be authenticated and Exactly one NTS Cookie extension field, which MUST be
MUST NOT be encrypted. The cookie MUST be one which the server authenticated and MUST NOT be encrypted. The cookie MUST be one
previously provided the client; it may have been provided during which the server previously provided the client; it may have been
the NTS-KE handshake or in response to a previous NTS-protected provided during the NTS-KE handshake or in response to a previous
NTP request. To protect client's privacy, the same cookie SHOULD NTS-protected NTP request. To protect client's privacy, the same
NOT be included in multiple requests. If the client does not have cookie SHOULD NOT be included in multiple requests. If the client
any cookies that it has not already sent, it SHOULD re-run the does not have any cookies that it has not already sent, it SHOULD
NTS-KE protocol before continuing. re-run the NTS-KE protocol before continuing.
Exactly one NTS Authenticator and Encrypted Extensions extension, Exactly one NTS Authenticator and Encrypted Extension Fields
generated using an AEAD Algorithm and C2S key established through extension field, generated using an AEAD Algorithm and C2S key
NTS-KE. established through NTS-KE.
The client MAY include one or more NTS Cookie Placeholder extensions, The client MAY include one or more NTS Cookie Placeholder extension
which MUST be authenticated and MAY be encrypted. The number of NTS field, which MUST be authenticated and MAY be encrypted. The number
Cookie Placeholder extensions that the client includes SHOULD be such of NTS Cookie Placeholder extension fields that the client includes
that if the client includes N placeholders and the server sends back SHOULD be such that if the client includes N placeholders and the
N+1 cookies, the number of unused cookies stored by the client will server sends back N+1 cookies, the number of unused cookies stored by
come to eight. When both the client and server adhere to all cookie- the client will come to eight. When both the client and server
management guidance provided in this memo, the number of placeholder adhere to all cookie-management guidance provided in this memo, the
extensions will equal the number of dropped packets since the last number of placeholder extension fields will equal the number of
successful volley. dropped packets since the last successful volley.
The client MAY include additional (non-NTS-related) extensions, which The client MAY include additional (non-NTS-related) extension fields,
MAY appear prior to the NTS Authenticator and Encrypted Extensions which MAY appear prior to the NTS Authenticator and Encrypted
extension (therefore authenticated but not encrypted), within it Extension Fields extension fields (therefore authenticated but not
(therefore encrypted and authenticated), or after it (therefore encrypted), within it (therefore encrypted and authenticated), or
neither encrypted nor authenticated). In general, however, the after it (therefore neither encrypted nor authenticated). In
server MUST discard any unauthenticated extensions and process the general, however, the server MUST discard any unauthenticated
packet as though they were not present. Servers MAY implement extension fields and process the packet as though they were not
exceptions to this requirement for particular extensions if their present. Servers MAY implement exceptions to this requirement for
specification explicitly provides for such. particular extension fields if their specification explicitly
provides for such.
Upon receiving an NTS-protected request, the server SHALL (through Upon receiving an NTS-protected request, the server SHALL (through
some implementation-defined mechanism) use the cookie to recover the some implementation-defined mechanism) use the cookie to recover the
AEAD Algorithm, C2S key, and S2C key associated with the request, and AEAD Algorithm, C2S key, and S2C key associated with the request, and
then use the C2S key to authenticate the packet and decrypt the then use the C2S key to authenticate the packet and decrypt the
ciphertext. If the cookie is valid and authentication and decryption ciphertext. If the cookie is valid and authentication and decryption
succeed, then the server SHALL include the following extensions in succeed, then the server SHALL include the following extension fields
its response: in its response:
Exactly one Unique Identifier extension, which MUST be Exactly one Unique Identifier extension field, which MUST be
authenticated, MUST NOT be encrypted, and whose contents SHALL authenticated, MUST NOT be encrypted, and whose contents SHALL
echo those provided by the client. echo those provided by the client.
Exactly one NTS Authenticator and Encrypted Extensions extension, Exactly one NTS Authenticator and Encrypted Extension Fields
generated using the AEAD algorithm and S2C key recovered from the extension field, generated using the AEAD algorithm and S2C key
cookie provided by the client. recovered from the cookie provided by the client.
One or more NTS Cookie extensions, which MUST be authenticated and One or more NTS Cookie extension fields, which MUST be
encrypted. The number of NTS Cookie extensions included SHOULD be authenticated and encrypted. The number of NTS Cookie extension
equal to, and MUST NOT exceed, one plus the number of valid NTS fields included SHOULD be equal to, and MUST NOT exceed, one plus
Cookie Placeholder extensions included in the request. the number of valid NTS Cookie Placeholder extension fields
included in the request.
The server MAY include additional (non-NTS-related) extensions, which The server MAY include additional (non-NTS-related) extension fields,
MAY appear prior to the NTS Authenticator and Encrypted Extensions which MAY appear prior to the NTS Authenticator and Encrypted
extension (therefore authenticated but not encrypted), within it Extension Fields extension field (therefore authenticated but not
(therefore encrypted and authenticated), or after it (therefore encrypted), within it (therefore encrypted and authenticated), or
neither encrypted nor authenticated). In general, however, the after it (therefore neither encrypted nor authenticated). In
client MUST discard any unauthenticated extensions and process the general, however, the client MUST discard any unauthenticated
packet as though they were not present. Clients MAY implement extension fields and process the packet as though they were not
exceptions to this requirement for particular extensions if their present. Clients MAY implement exceptions to this requirement for
specification explicitly provides for such. particular extension fields if their specification explicitly
provides for such.
If the server is unable to validate the cookie or authenticate the If the server is unable to validate the cookie or authenticate the
request, it SHOULD respond with a Kiss-o'-Death packet (see RFC 5905, request, it SHOULD respond with a Kiss-o'-Death packet (see RFC 5905,
Section 7.4) [RFC5905]) with kiss code "NTSN" (meaning "NTS NAK"). Section 7.4) [RFC5905]) with kiss code "NTSN" (meaning "NTS NAK").
Such a response MUST include exactly one Unique Identifier extension Such a response MUST include exactly one Unique Identifier extension
whose contents SHALL echo those provided by the client. It MUST NOT field whose contents SHALL echo those provided by the client. It
include any NTS Cookie or NTS Authenticator and Encrypted Extensions MUST NOT include any NTS Cookie or NTS Authenticator and Encrypted
extension. Extension Fields extension fields.
Upon receiving an NTS-protected response, the client MUST verify that Upon receiving an NTS-protected response, the client MUST verify that
the Unique Identifier matches that of an outstanding request, and the Unique Identifier matches that of an outstanding request, and
that the packet is authentic under the S2C key associated with that that the packet is authentic under the S2C key associated with that
request. If either of these checks fails, the packet MUST be request. If either of these checks fails, the packet MUST be
discarded without further processing. discarded without further processing.
Upon receiving an NTS NAK, the client MUST verify that the Unique Upon receiving an NTS NAK, the client MUST verify that the Unique
Identifier matches that of an outstanding request. If this check Identifier matches that of an outstanding request. If this check
fails, the packet MUST be discarded without further processing. If fails, the packet MUST be discarded without further processing. If
this check passes, the client SHOULD wait until the next poll for a this check passes, the client SHOULD wait until the next poll for a
valid NTS-protected response and if none is received, discard all valid NTS-protected response and if none is received, discard all
cookies and AEAD keys associated with the server which sent the NAK cookies and AEAD keys associated with the server which sent the NAK
and initiate a fresh NTS-KE handshake. and initiate a fresh NTS-KE handshake.
7. Recommended format for NTS cookies 6. Suggested format for NTS cookies
This section provides a RECOMMENDED way for servers to construct NTS This section is non-normative. It gives a suggested way for servers
cookies. Clients MUST NOT examine the cookie under the assumption to construct NTS cookies. All normative requirements are stated in
that it is constructed according to this section. Section 4.1.6 and Section 5.4.
The role of cookies in NTS is closely analagous to that of session The role of cookies in NTS is closely analogous to that of session
cookies in TLS. Accordingly, the thematic resemblance of this cookies in TLS. Accordingly, the thematic resemblance of this
section to RFC 5077 [RFC5077] is deliberate, and the reader should section to RFC 5077 [RFC5077] is deliberate, and the reader should
likewise take heed of its security considerations. likewise take heed of its security considerations.
Servers should select an AEAD algorithm which they will use to Servers SHOULD select an AEAD algorithm which they will use to
encrypt and authenticate cookies. The chosen algorithm should be one encrypt and authenticate cookies. The chosen algorithm SHOULD be one
such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidential such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental
nonce reuse, and it need not be the same as the one that was nonce reuse, and it need not be the same as the one that was
negotiated with the client. Servers should randomly generate and negotiated with the client. Servers SHOULD randomly generate and
store a master AEAD key `K`. Servers should additionally choose a store a master AEAD key `K`. Servers SHOULD additionally choose a
non-secret, unique value `I` as key-identifier for `K`. non-secret, unique value `I` as key-identifier for `K`.
Servers should periodically (e.g., once daily) generate a new pair Servers SHOULD periodically (e.g., once daily) generate a new pair
(I,K) and immediately switch to using these values for all newly- (I,K) and immediately switch to using these values for all newly-
generated cookies. Immediately following each such key rotation, generated cookies. Immediately following each such key rotation,
servers should securely erase any keys generated two or more rotation servers SHOULD securely erase any keys generated two or more rotation
periods prior. Servers should continue to accept any cookie periods prior. Servers SHOULD continue to accept any cookie
generated using keys that they have not yet erased, even if those generated using keys that they have not yet erased, even if those
keys are no longer current. Erasing old keys provides for forward keys are no longer current. Erasing old keys provides for forward
secrecy, limiting the scope of what old information can be stolen if secrecy, limiting the scope of what old information can be stolen if
a master key is somehow compromised. Holding on to a limited number a master key is somehow compromised. Holding on to a limited number
of old keys allows clients to seamlessly transition from one of old keys allows clients to seamlessly transition from one
generation to the next without having to perform a new NTS-KE generation to the next without having to perform a new NTS-KE
handshake. handshake.
The need to keep keys synchronized across load-balanced clusters can The need to keep keys synchronized across load-balanced clusters can
make automatic key rotation challenging. However, the task can be make automatic key rotation challenging. However, the task can be
accomplished without the need for central key-management accomplished without the need for central key-management
infrastructure by using a ratchet, i.e., making each new key a infrastructure by using a ratchet, i.e., making each new key a
deterministic, cryptographically pseudo-random function of its deterministic, cryptographically pseudo-random function of its
predecessor. A recommended concrete implementation of this approach predecessor. A recommended concrete implementation of this approach
is to use HKDF [RFC5869] to derive new keys, using the key's is to use HKDF [RFC5869] to derive new keys, using the key's
predecessor as Input Keying Material and its key identifier as a predecessor as Input Keying Material and its key identifier as a
salt. salt.
To form a cookie, servers should first form a plaintext `P` To form a cookie, servers SHOULD first form a plaintext `P`
consisting of the following fields: consisting of the following fields:
The AEAD algorithm negotiated during NTS-KE The AEAD algorithm negotiated during NTS-KE
The S2C key The S2C key
The C2S key The C2S key
Servers should the generate a nonce `N` uniformly at random, and form Servers SHOULD then generate a nonce `N` uniformly at random, and
AEAD output `C` by encrypting `P` under key `K` with nonce `N` and no form AEAD output `C` by encrypting `P` under key `K` with nonce `N`
associated data. and no associated data.
The cookie should consist of the tuple `(I,N,C)`. The cookie SHOULD consist of the tuple `(I,N,C)`.
To verify and decrypt a cookie provided by the client, first parse it 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 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 decryption key `K`. If the key whose identifier is `I` has been
erased or never existed, decryption fails; reply with an NTS NAK. erased or never existed, decryption fails; reply with an NTS NAK.
Otherwise, attempt to decrypt and verify ciphertext `C` using key `K` Otherwise, attempt to decrypt and verify ciphertext `C` using key `K`
and nonce `N` with no associated data. If decryption or verification and nonce `N` with no associated data. If decryption or verification
fails, reply with an NTS NAK. Otherwise, parse out the contents of fails, reply with an NTS NAK. Otherwise, parse out the contents of
the resulting plaintext `P` to obtain the negotiated AEAD algorithm, the resulting plaintext `P` to obtain the negotiated AEAD algorithm,
S2C key, and C2S key. S2C key, and C2S key.
8. IANA Considerations 7. IANA Considerations
IANA is requested to allocate two entries, identical except for the IANA is requested to allocate two entries, identical except for the
Transport Protocol, in the Service Name and Transport Protocol Port Transport Protocol, in the Service Name and Transport Protocol Port
Number Registry as follows: Number Registry as follows:
Service Name: nts Service Name: nts
Transport Protocol: tcp, udp Transport Protocol: tcp, udp
Assignee: IESG <iesg@ietf.org> Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org> Contact: IETF Chair <chair@ietf.org>
Description: Network Time Security Description: Network Time Security
Reference: [[this memo]] Reference: [[this memo]]
Port Number: [[TBD1]], selected by IANA from the user port range Port Number: [[TBD1]], selected by IANA from the user port range
skipping to change at page 18, line 47 skipping to change at page 17, line 14
Assignee: IESG <iesg@ietf.org> Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org> Contact: IETF Chair <chair@ietf.org>
Description: Network Time Security Description: Network Time Security
Reference: [[this memo]] Reference: [[this memo]]
Port Number: [[TBD1]], selected by IANA from the user port range Port Number: [[TBD1]], selected by IANA from the user port range
IANA is requested to allocate the following two entries in the IANA is requested to allocate the following entry in the Application-
Application-Layer Protocol Negotation (ALPN) Protocol IDs registry: Layer Protocol Negotation (ALPN) Protocol IDs registry:
Protocol: Network Time Security Key Establishment, version 1 Protocol: Network Time Security Key Establishment, version 1
Identification Sequence: Identification Sequence:
0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1") 0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")
Reference: [[this memo]] Reference: [[this memo]]
Protocol: Network Time Protocol, version 4
Identification Sequence:
0x6E 0x74 0x70 0x2F 0x34 ("ntp/4")
Reference: [[this memo]]
IANA is requested to allocate the following entry in the TLS Exporter IANA is requested to allocate the following entry in the TLS Exporter
Label Registry: Label Registry:
+----------------------------------+---------+---------------+------+ +----------------------------------+---------+---------------+------+
| Value | DTLS-OK | Reference | Note | | Value | DTLS-OK | Reference | Note |
+----------------------------------+---------+---------------+------+ +----------------------------------+---------+---------------+------+
| EXPORTER-network-time-security/1 | Y | [[this memo]] | | | EXPORTER-network-time-security/1 | Y | [[this memo]] | |
+----------------------------------+---------+---------------+------+ +----------------------------------+---------+---------------+------+
IANA is requested to allocate the following entry in the registry of IANA is requested to allocate the following entry in the registry of
skipping to change at page 19, line 37 skipping to change at page 18, line 5
+------+---------+ +------+---------+
| Code | Meaning | | Code | Meaning |
+------+---------+ +------+---------+
| NTSN | NTS NAK | | NTSN | NTS NAK |
+------+---------+ +------+---------+
IANA is requested to allocate the following entries in the NTP IANA is requested to allocate the following entries in the NTP
Extensions Field Types registry: Extensions Field Types registry:
+------------+---------------------------------------+--------------+ +-----------+-----------------------------------------+-------------+
| Field Type | Meaning | Reference | | Field | Meaning | Reference |
+------------+---------------------------------------+--------------+ | Type | | |
| [[TBD2]] | Unique Identifier | [[this | +-----------+-----------------------------------------+-------------+
| | | memo]] | | [[TBD2]] | Unique Identifier | [[this |
| [[TBD3]] | NTS Cookie | [[this | | | | memo]] |
| | | memo]] | | [[TBD3]] | NTS Cookie | [[this |
| [[TBD4]] | NTS Cookie Placeholder | [[this | | | | memo]] |
| | | memo]] | | [[TBD4]] | NTS Cookie Placeholder | [[this |
| [[TBD5]] | NTS Authenticator and Encrypted | [[this | | | | memo]] |
| | Extensions | memo]] | | [[TBD5]] | NTS Authenticator and Encrypted | [[this |
+------------+---------------------------------------+--------------+ | | Extension Fields | memo]] |
+-----------+-----------------------------------------+-------------+
IANA is requested to create a new registry entitled "Network Time IANA is requested to create a new registry entitled "Network Time
Security Key Establishment Record Types". Entries SHALL have the Security Key Establishment Record Types". Entries SHALL have the
following fields: following fields:
Type Number (REQUIRED): An integer in the range 0-32767 inclusive Type Number (REQUIRED): An integer in the range 0-32767 inclusive
Description (REQUIRED): short text description of the purpose of Description (REQUIRED): short text description of the purpose of
the field the field
skipping to change at page 21, line 4 skipping to change at page 19, line 24
| | | | memo]] | | | | | memo]] |
| 3 | Warning | MUST | [[this | | 3 | Warning | MUST | [[this |
| | | | memo]] | | | | | memo]] |
| 4 | AEAD algorithm negotiation | MAY | [[this | | 4 | AEAD algorithm negotiation | MAY | [[this |
| | | | memo]] | | | | | memo]] |
| 5 | New cookie for NTPv4 | SHOULD | [[this | | 5 | New cookie for NTPv4 | SHOULD | [[this |
| | | NOT | memo]] | | | | NOT | memo]] |
| 16384-32767 | Reserved for Private & | MAY | [[this | | 16384-32767 | Reserved for Private & | MAY | [[this |
| | Experimental Use | | memo]] | | | Experimental Use | | memo]] |
+-------------+-----------------------------+----------+------------+ +-------------+-----------------------------+----------+------------+
IANA is requested to create a new registry entitled "Network Time IANA is requested to create a new registry entitled "Network Time
Security Next Protocols". Entries SHALL have the following fields: Security Next Protocols". Entries SHALL have the following fields:
Protocol ID (REQUIRED): a 16-bit unsigned integer functioning as Protocol ID (REQUIRED): a integer in the range 0-65535 inclusive,
an identifier. functioning as an identifier.
Protocol Name (REQUIRED): a short text string naming the protocol Protocol Name (REQUIRED): a short text string naming the protocol
being identified. being identified.
Reference (RECOMMENDED): a reference to a relevant specification Reference (RECOMMENDED): a reference to a relevant specification
document. If no relevant document exists, a point-of-contact for document. If no relevant document exists, a point-of-contact for
questions regarding the entry SHOULD be listed here in lieu. questions regarding the entry SHOULD be listed here in lieu.
Applications for new entries in this registry SHALL specify all Applications for new entries in this registry SHALL specify all
desired fields, and SHALL be granted upon approval by a Designated desired fields, and SHALL be granted upon approval by a Designated
Expert. Protocol IDs 32768-65535 SHALL be reserved for Private or Expert. Protocol IDs 32768-65535 SHALL be reserved for Private or
Experimental Use, and SHALL NOT be registered. Experimental Use, and SHALL NOT be registered.
The initial contents of this registry SHALL be as follows: The initial contents of this registry SHALL be as follows:
+-------------+-------------------------------+---------------------+ +-------------+-------------------------------+---------------------+
| Protocol | Human-Readable Name | Reference | | Protocol ID | Human-Readable Name | Reference |
| Name | | |
+-------------+-------------------------------+---------------------+ +-------------+-------------------------------+---------------------+
| 0 | Network Time Protocol version | [[this memo]] | | 0 | Network Time Protocol version | [[this memo]] |
| | 4 | | | | 4 (NTPv4) | |
| 1 | Precision Time Protocol | Reserved by [[this |
| | version 2 | memo]] |
| 32768-65535 | Reserved for Private or | Reserved by [[this | | 32768-65535 | Reserved for Private or | Reserved by [[this |
| | Experimental Use | memo]] | | | Experimental Use | memo]] |
+-------------+-------------------------------+---------------------+ +-------------+-------------------------------+---------------------+
IANA is requested to create two new registries entitled "Network Time IANA is requested to create two new registries entitled "Network Time
Security Error Codes" and "Network Time Security Warning Codes". Security Error Codes" and "Network Time Security Warning Codes".
Entries in each SHALL have the following fields: Entries in each SHALL have the following fields:
Number (REQUIRED): a 16-bit unsigned integer Number (REQUIRED): a integer in the range 0-65535 inclusive
Description (REQUIRED): a short text description of the condition. Description (REQUIRED): a short text description of the condition.
Reference (REQUIRED): a reference to a relevant specification Reference (REQUIRED): a reference to a relevant specification
document. document.
The policy for allocation of new entries in these registries SHALL The policy for allocation of new entries in these registries SHALL
vary by their Number, as follows: vary by their Number, as follows:
0-1023: IETF Review 0-1023: IETF Review
skipping to change at page 22, line 21 skipping to change at page 20, line 37
+--------+---------------------------------+---------------+ +--------+---------------------------------+---------------+
| Number | Description | Reference | | Number | Description | Reference |
+--------+---------------------------------+---------------+ +--------+---------------------------------+---------------+
| 0 | Unrecognized Critical Extension | [[this memo]] | | 0 | Unrecognized Critical Extension | [[this memo]] |
| 1 | Bad Request | [[this memo]] | | 1 | Bad Request | [[this memo]] |
+--------+---------------------------------+---------------+ +--------+---------------------------------+---------------+
The Network Time Security Warning Codes Registry SHALL initially be The Network Time Security Warning Codes Registry SHALL initially be
empty. empty.
9. Security considerations 8. Security considerations
9.1. Avoiding DDoS amplification 8.1. Avoiding DDoS amplification
Certain non-standard and/or deprecated features of the Network Time Certain non-standard and/or deprecated features of the Network Time
Protocol enable clients to send a request to a server which causes Protocol enable clients to send a request to a server which causes
the server to send a response much larger than the request. Servers the server to send a response much larger than the request. Servers
which enable these features can be abused in order to amplify traffic which enable these features can be abused in order to amplify traffic
volume in distributed denial-of-service (DDoS) attacks by sending volume in distributed denial-of-service (DDoS) attacks by sending
them a request with a spoofed source IP. In recent years, attacks of them a request with a spoofed source IP. In recent years, attacks of
this nature have become an endemic nuisance. this nature have become an endemic nuisance.
NTS is designed to avoid contributing any further to this problem by NTS is designed to avoid contributing any further to this problem by
ensuring that NTS-related extensions included in server responses ensuring that NTS-related extension fields included in server
will be the same size as the NTS-related extensions sent by the responses will be the same size as the NTS-related extension fields
client. In particular, this is why the client is required to send a sent by the client. In particular, this is why the client is
separate and appropriately padded-out NTS Cookie Placeholder required to send a separate and appropriately padded-out NTS Cookie
extension for every cookie it wants to get back, rather than being Placeholder extension field for every cookie it wants to get back,
permitted simply to specify a desired quantity. rather than being permitted simply to specify a desired quantity.
9.2. Initial verification of server certificates 8.2. Initial verification of server certificates
NTS's security goals are undermined if the client fails to verify NTS's security goals are undermined if the client fails to verify
that the X.509 certificate chain presented by the server is valid and that the X.509 certificate chain presented by the server is valid and
rooted in a trusted certificate authority. [RFC5280] and [RFC6125] rooted in a trusted certificate authority. [RFC5280] and [RFC6125]
specifies how such verification is to be performed in general. specifies how such verification is to be performed in general.
However, the expectation that the client does not yet have a However, the expectation that the client does not yet have a
correctly-set system clock at the time of certificate verification correctly-set system clock at the time of certificate verification
presents difficulties with verifying that the certificate is within presents difficulties with verifying that the certificate is within
its validity period, i.e., that the current time lies between the its validity period, i.e., that the current time lies between the
times specified in the certificate's notBefore and notAfter fields, times specified in the certificate's notBefore and notAfter fields,
and it may be operationally necessary in some cases for a client to and it may be operationally necessary in some cases for a client to
accept a certificate which appears to be expired or not yet valid. accept a certificate which appears to be expired or not yet valid.
While there is no perfect solution to this problem, there are several While there is no perfect solution to this problem, there are several
mitigations the client can implement to make it more difficult for an mitigations the client can implement to make it more difficult for an
adversary to successfully present an expired certificate: adversary to successfully present an expired certificate:
Check whether the system time is in fact unreliable. If the Check whether the system time is in fact unreliable. If the
system clock has previously been synchronized since last boot, system clock has previously been synchronized since last boot,
then on operating systems which implement a kernel-based phase- then on operating systems which implement a kernel-based phase-
locked-loop API, a call to ntp_gettime() should show a maximum locked-loop API, a call to ntp_gettime() should show a maximum
error less than NTP_PHASE_MAX. In this case, the clock should be error less than NTP_PHASE_MAX. In this case, the clock SHOULD be
considered reliable and certificates can be strictly validated. considered reliable and certificates can be strictly validated.
Allow the system administrator to specify that certificates should Allow the system administrator to specify that certificates should
*always* be strictly validated. Such a configuration is *always* be strictly validated. Such a configuration is
appropriate on systems which have a battery-backed clock and which appropriate on systems which have a battery-backed clock and which
can reasonably prompt the user to manually set an approximately- can reasonably prompt the user to manually set an approximately-
correct time if it appears to be needed. correct time if it appears to be needed.
Once the clock has been synchronized, periodically write the Once the clock has been synchronized, periodically write the
current system time to persistent storage. Do not accept any current system time to persistent storage. Do not accept any
skipping to change at page 23, line 39 skipping to change at page 22, line 7
them falls outside the validity period of the server's them falls outside the validity period of the server's
certificate. certificate.
Use multiple time sources. The ability to pass off an expired Use multiple time sources. The ability to pass off an expired
certificate is only useful to an adversary who has compromised the certificate is only useful to an adversary who has compromised the
corresponding private key. If the adversary has compromised only corresponding private key. If the adversary has compromised only
a minority of servers, NTP's selection algorithm ([RFC5905] a minority of servers, NTP's selection algorithm ([RFC5905]
section 11.2.1) will protect the client from accepting bad time section 11.2.1) will protect the client from accepting bad time
from the adversary-controlled servers. from the adversary-controlled servers.
9.3. Usage of NTP pools 8.3. Usage of NTP pools
Additional standardization work and infrastructure development is Additional standardization work and infrastructure development is
necessary before NTS can be used with public NTP server pools. necessary before NTS can be used with public NTP server pools.
First, a scheme needs to be specified for determining what First, a scheme will need to be specified for determining what
constitutes an acceptable certificate for a pool server, such as constitutes an acceptable certificate for a pool server, such as
establishing a value required to be contained in its Extended Key establishing a value required to be contained in its Extended Key
Usage attribute, and how to determine, given the DNS name of a pool, Usage attribute, and how to determine, given the DNS name of a pool,
what Subject Alternative Name to expect in the certificates of its what Subject Alternative Name to expect in the certificates of its
members. A more important matter, however, is that pool operators members. Implementing any such specification will necessitate
need procedures for establishing and maintaining trust in their infrastructure work: pool organizers will need to act as certificate
members. Pools in existence as of 2017 are volunteer-run, with authorities, regularly monitor the behavior of servers to which
minimal requirements for admission and no organized effort to monitor certificates have been issued, and promptly revoke the certificate of
pool servers for misbehavior. Without any sort of policing in place, any server found to be serving incorrect time.
there is nothing to prevent an adversary from going through normal
channels to obtain a valid certificate for participation in a pool
and then proceeding to serve maliciously inaccurate time.
9.4. Delay attacks 8.4. Delay attacks
In a packet delay attack, an adversary with the ability to act as a In a packet delay attack, an adversary with the ability to act as a
man-in-the-middle delays time synchronization packets between client man-in-the-middle delays time synchronization packets between client
and server asymmetrically [RFC7384]. Since NTP's formula for and server asymmetrically [RFC7384]. Since NTP's formula for
computing time offset relies on the assumption that network latency computing time offset relies on the assumption that network latency
is roughly symmetrical, this leads to the client to compute an is roughly symmetrical, this leads to the client to compute an
inaccurate value [Mizrahi]. The delay attack does not reorder or inaccurate value [Mizrahi]. The delay attack does not reorder or
modify the content of the exchanged synchronization packets. modify the content of the exchanged synchronization packets.
Therefore, cryptographic means do not provide a feasible way to Therefore, cryptographic means do not provide a feasible way to
mitigate this attack. However, the maximum error that an adversary mitigate this attack. However, the maximum error that an adversary
can introduce is bounded by half of the round trip delay. can introduce is bounded by half of the round trip delay.
[RFC5905] specifies a parameter called MAXDIST which denotes the [RFC5905] specifies a parameter called MAXDIST which denotes the
maximum round-trip latency (including not only the immediate round maximum round-trip latency (including not only the immediate round
trip between client and server but the whole distance back to the trip between client and server but the whole distance back to the
reference clock as reported in the Root Delay filed) that a client reference clock as reported in the Root Delay field) that a client
will tolerate before concluding that the server is unsuitable for will tolerate before concluding that the server is unsuitable for
synchronization. The standard value for MAXDIST is one second, synchronization. The standard value for MAXDIST is one second,
although some implementations use larger values. Whatever value a although some implementations use larger values. Whatever value a
client chooses, the maximum error which can be introduced by a delay client chooses, the maximum error which can be introduced by a delay
attack is MAXDIST/2. attack is MAXDIST/2.
Usage of multiple time sources, or multiple network paths to a given Usage of multiple time sources, or multiple network paths to a given
time source [Shpiner], may also serve to mitigate delay attacks if time source [Shpiner], may also serve to mitigate delay attacks if
the adversary is in control of only some of the paths. the adversary is in control of only some of the paths.
9.5. Random number generation 8.5. Random number generation
At various points in NTS, the generation of cryptographically secure At various points in NTS, the generation of cryptographically secure
random numbers is required. See [RFC4086] for guidelines concerning random numbers is required. See [RFC4086] for guidelines concerning
this topic. this topic.
10. Privacy Considerations 9. Privacy Considerations
10.1. Unlinkability 9.1. Unlinkability
Unlinkability prevents a device from being tracked when it changes Unlinkability prevents a device from being tracked when it changes
network addresses (e.g. because said device moved between different network addresses (e.g. because said device moved between different
networks). In other words, unlinkability thwarts an attacker that networks). In other words, unlinkability thwarts an attacker that
seeks to link a new network address used by a device with a network 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 address that it was formerly using, because of recognizable data that
the device persistently sends as part of an NTS-secured NTP the device persistently sends as part of an NTS-secured NTP
association. This is the justification for continually supplying the association. This is the justification for continually supplying the
client with fresh cookies, so that a cookie never represents client with fresh cookies, so that a cookie never represents
recognizable data in the sense outlined above. recognizable data in the sense outlined above.
skipping to change at page 25, line 28 skipping to change at page 23, line 45
clients, but also as time servers (because the latter can be clients, but also as time servers (because the latter can be
externally triggered to send authentication data). externally triggered to send authentication data).
It should also be noted that it could be possible to link devices It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic, that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g. using information exposed in (mode 4) server response packets (e.g.
reference ID, reference time, stratum, poll). Also, devices that reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information respond to NTP control queries could be linked using the information
revealed by control queries. revealed by control queries.
10.2. Confidentiality 9.2. Confidentiality
NTS does not protect the confidentiality of information in NTP's NTS does not protect the confidentiality of information in NTP's
header fields. When clients implement header fields. When clients implement
[I-D.ietf-ntp-data-minimization], client packet headers do not [I-D.ietf-ntp-data-minimization], client packet headers do not
contain any information which the client could conceivably wish to contain any information which the client could conceivably wish to
keep secret: one field is random, and all others are fixed. keep secret: one field is random, and all others are fixed.
Information in server packet headers is likewise public: the origin Information in server packet headers is likewise public: the origin
timestamp is copied from the client's (random) transmit timestamp, timestamp is copied from the client's (random) transmit timestamp,
and all other fields are set the same regardless of the identity of and all other fields are set the same regardless of the identity of
the client making the request. the client making the request.
Future extension fields could hypothetically contain sensitive Future extension fields could hypothetically contain sensitive
information, in which case NTS provides a mechanism for encrypting information, in which case NTS provides a mechanism for encrypting
them. them.
11. Acknowledgements 10. Acknowledgements
The authors would like to thank Richard Barnes, Steven Bellovin, The authors would like to thank Richard Barnes, Steven Bellovin,
Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar, Scott Fluhrer, Sharon Goldberg, Russ Housley, Martin Langer, Miroslav
Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue, Eric K. Lichvar, Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue,
Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Eric K. Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich
Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn, Martin Thomson, Salz, Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn,
and Richard Welty for contributions to this document. on the design Martin Thomson, and Richard Welty for contributions to this document
of NTS. and comments on the design of NTS.
12. References 11. References
12.1. Normative References 11.1. Normative References
[I-D.ietf-ntp-data-minimization] [I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-00 (work Minimization", draft-ietf-ntp-data-minimization-00 (work
in progress), May 2017. in progress), May 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV) [RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <http://www.rfc-editor.org/info/rfc5297>. 2008, <https://www.rfc-editor.org/info/rfc5297>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <http://www.rfc-editor.org/info/rfc5705>. March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication "Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>. <https://www.rfc-editor.org/info/rfc5746>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>. January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol "Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>. July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015, DOI 10.17487/RFC7465, February 2015,
<http://www.rfc-editor.org/info/rfc7465>. <https://www.rfc-editor.org/info/rfc7465>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<http://www.rfc-editor.org/info/rfc7507>. <https://www.rfc-editor.org/info/rfc7507>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS) Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension", Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015, RFC 7627, DOI 10.17487/RFC7627, September 2015,
<http://www.rfc-editor.org/info/rfc7627>. <https://www.rfc-editor.org/info/rfc7627>.
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4 [RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822, (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <http://www.rfc-editor.org/info/rfc7822>. March 2016, <https://www.rfc-editor.org/info/rfc7822>.
12.2. Informative References
[IEC.61588_2009] 11.2. Informative References
IEEE/IEC, "Precision clock synchronization protocol for
networked measurement and control systems",
IEEE 1588-2008(E), IEC 61588:2009(E),
DOI 10.1109/IEEESTD.2009.4839002, February 2009,
<http://ieeexplore.ieee.org/servlet/
opac?punumber=4839000>.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks [Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings against time synchronization protocols", in Proceedings
of Precision Clock Synchronization for Measurement Control of Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012. and Communication, ISPCS 2012, pp. 1-6, September 2012.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, "Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>. <https://www.rfc-editor.org/info/rfc4086>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without "Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077, Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>. January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>. <https://www.rfc-editor.org/info/rfc5869>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>. October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[Shpiner] "Multi-path Time Protocols", in Proceedings of IEEE [Shpiner] "Multi-path Time Protocols", in Proceedings of IEEE
International Symposium on Precision Clock Synchronization International Symposium on Precision Clock Synchronization
for Measurement, Control and Communication (ISPCS), for Measurement, Control and Communication (ISPCS),
September 2013. September 2013.
Appendix A. Terms and Abbreviations Appendix A. Terms and Abbreviations
AEAD Authenticated Encryption with Associated Data [RFC5116] AEAD Authenticated Encryption with Associated Data [RFC5116]
DDoS Distributed Denial of Service DDoS Distributed Denial of Service
DTLS Datagram Transport Layer Security
NTP Network Time Protocol [RFC5905] NTP Network Time Protocol [RFC5905]
NTS Network Time Security NTS Network Time Security
PTP Precision Time Protocol
TLS Transport Layer Security TLS Transport Layer Security
Authors' Addresses Authors' Addresses
Daniel Fox Franke Daniel Fox Franke
Akamai Technologies, Inc. Akamai Technologies, Inc.
150 Broadway 150 Broadway
Cambridge, MA 02142 Cambridge, MA 02142
United States United States
Email: dafranke@akamai.com Email: dafranke@akamai.com
URI: https://www.dfranke.us URI: https://www.dfranke.us
Dieter Sibold Dieter Sibold
skipping to change at page 29, line 29 skipping to change at page 27, line 32
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
Kristof Teichel Kristof Teichel
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8421 Phone: +49-(0)531-592-4471
Email: kristof.teichel@ptb.de Email: kristof.teichel@ptb.de
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