Network Time Security for the Network Time
ProtocolAkamai Technologies, Inc.150 BroadwayCambridgeMA02142United Statesdafranke@akamai.comhttps://www.dfranke.usPhysikalisch-Technische
BundesanstaltBundesallee 100BraunschweigD-38116Germany+49-(0)531-592-8420+49-531-592-698420dieter.sibold@ptb.dePhysikalisch-Technische
BundesanstaltBundesallee 100BraunschweigD-38116Germany+49-(0)531-592-8421kristof.teichel@ptb.de
Internet Area
NTP Working GroupIntegrityAuthenticationNTPSecurityDTLS
This memo specifies Network Time Security (NTS), a mechanism
for using Transport Layer Security (TLS) and Authenticated
Encryption with Associated Data (AEAD) to provide
cryptographic security for the Network Time Protocol.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.This document specifies measures to protect time synchronization
between NTP participants. In particular, it describes two main
techniques. The first is a mechanism that uses TLS (over a connection on
TCP port 123) to exchange key data and that afterwards allows to secure
NTP mode 3 and 4 packets using Authenticated Encryption with Associated
Data objects embedded in extension fields of those packets. The second
is a mechanism for using Datagram Transport Layer
Security (DTLS) to provide cryptographic security for NTP mode 1,
2 and 6 packets.While the detailed application described in this document is
inherently NTP-specific, the overall approach is not. Therefore, it
could be taken as guidance on how future work may apply the described
techniques to other time synchronization protocols (such as the Precision Time Protocol).Message Authentication CodeNetwork Time Protocol (RFC 5905)Network Time SecurityTransport Layer SecurityDatagram Transport Layer SecurityAuthenticated Encryption with Associated Data
(RFC 5116)The specific objectives for the measures described this document are
as follows:Protection for NTP time synchronization messages:Integrity: NTS protects the integrity of NTP time
synchronization protocol packets.Confidentiality: NTS does not generally provide
confidentiality protection of the time synchronization data. It
does so only in the case of NTP's symmetric/peer mode.Privacy: Once an NTS session has been established, NTS
supports unlinkability for devices that (1) use NTS as clients
and (2) minimize the information they expose in client query
(mode 3) packets per .
Unlinkability ensures that NTS does not leak data that allows an
attacker to track mobile NTP clients when they move between
networks. See for details.Request-Response-Consistency: NTS enables a client to match
an incoming response to a request it has sent. NTS also enables
the client to deduce from the response whether its request to
the server has arrived without alteration. This is to prevent
attacks employing replays of valid server responses.Additional protection for key exchange messages:Authenticity: NTS enables an NTP client to authenticate its
time server(s) during key exchange procedures.Authorization: NTS optionally enables the server to verify
the client's authorization.Modes of operation: Both the client-server mode and the symmetric
peer mode of NTP are supported. The broadcast mode of NTP can NOT be
secured with measures within this document.Hybrid mode: For all supported modes, both secure and insecure
communication modes can be used at the same time, for both NTP
servers and clients.Compatibility:NTS-secured communication does not affect NTP associations
which are not secured by NTS.NTS-secured authentication requests do not affect any NTP
servers that do not support NTS.The Network Time Protocol includes many different operating modes to
support various network topologies. In addition to its best-known and
most-widely-used client-server mode, it also includes modes for
synchronization between symmetric peers, a control mode for server
monitoring and administration and a broadcast mode. These various modes
have differing and contradictory requirements for security and
performance. Symmetric and control modes demand mutual authentication
and mutual replay protection, and for certain message types control mode
may require confidentiality as well as authentication. Client-server
mode places more stringent requirements on resource utilization than
other modes, because servers may have vast number of clients and be
unable to afford to maintain per-client state. However, client-server
mode also has more relaxed security needs, because only the client
requires replay protection: it is harmless for servers to process
replayed packets. The security demands of symmetric and control modes,
on the other hand, are in conflict with the resource-utilization demands
of client-server mode: any scheme which provides replay protection
inherently involves maintaining some state to keep track of what
messages have already been seen.This document does not discuss how to add security to NTP's broadcast
mode.The server does not keep a long-term state of the client. NTS
initially verifies the authenticity of the time server and exchanges
one or more symmetric keys. The TLS-based key exchange procedure
described in MUST be used for this
exchange.After the keys have been exchanged, the participants then use them
to protect the authenticity and the integrity of subsequent
unicast-type time synchronization packets. In order to do this,
participants attach AEAD objects to their time synchronization
packets, included in NTP extension fields and calculated over the
whole time synchronization packet. Therefore, the client can perform a
validity check on reception of a time synchronization packet.The symmetric ("peer") mode as well as the control modes, are
secured via the DTLS-encapsulated NTPv4 protocol described in . This 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.Since (as discussed in ) no single approach
can simultaneously satisfy the needs of all modes, this specification
consists of not one protocol but a suite of them: The "NTS-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 is
suitable for symmetric and control modes, and is also secure for
client/server mode but relatively wasteful of server resources.The "NTS Key Establishment" protocol (NTS-KE) uses TLS to
establish key material and negotiate some additional protocol
options, but then quickly closes the DTLS channel and does not use
it for the exchange of time packets. NTS-KE is designed to be
extensible, and might be extended to support key establishment for
other protocols such as PTP.The "NTS extensions for NTPv4" are a collection of NTP extension
fields for cryptographically securing NTPv4 using key material
previously negotiated using NTS-KE. They are suitable for securing
client/server mode because the server can implement them without
retaining per-client state, but on the other hand are suitable
*only* for client/server mode because only the client, and not the
server, is protected from replay.
Network Time Security makes use of both TLS (for NTS Key
Establishment) and DTLS (for DTLS-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 (D)TLS, no backward-compatibility concerns exist
to justify using obsolete, insecure, or otherwise broken TLS
features or versions. We therefore put forward the following
requirements and guidelines, roughly representing 2017's best
practices.
Implementations MUST NOT negotiate (D)TLS versions
earlier than 1.2.
Implementations willing to negotiate more than one possible
version of (D)TLS SHOULD NOT respond to handshake failures by
retrying with a downgraded protocol version. If they do, they
MUST implement .
(D)TLS clients MUST NOT offer, and DTLS servers MUST not select,
RC4 cipher suites.
(D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the
TLS Renegotiation Indication
Extension. Regardless, they MUST NOT initiate or permit
insecure renegotiation. (*)
(D)TLS clients SHOULD offer, and (D)TLS servers SHOULD accept, the
TLS Session Hash and Extended Master
Secret Extension. (*)
Use of the Application-Layer Protocol
Negotation Extension is integral to NTS and support for
it is REQUIRED for interoperability.
(*): Note that (D)TLS 1.3 or beyond may render the indicated
recommendations inapplicable.
The NTS-encapsulated NTPv4 protocol proceeds in two parts. The two
endpoints carry out a DTLS handshake in conformance with , with the client offering (via an ALPN 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 , implementations MUST enforce the anti-replay
mechanism specified in Section 4.1.2.6 of RFC
6347 (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.
The NTS key establishment protocol is conducted via TCP port
[TBD]. The two endpoints carry out a TLS handshake in
conformance with , with the client
offering (via an ALPN
extension), and the server accepting, an application-layer
protocol of "ntske/1". Immediately following a
successful handshake, the client SHALL send a single request
(as Application Data encapsulated in the TLS-protected
channel), then the server SHALL send a single response
followed by a TLS "Close notify" alert and then discard the
channel state.
The client's request and the server's response each SHALL
consist of a sequence of records formatted according to . The sequence SHALL be terminated by a
"End of Message" record, which has a Record Type of
zero and a zero-length body. Furthermore, requests and
non-error responses each SHALL include exactly one NTS Next
Protocol Negotiation record.
[[Ed. Note: this ad-hoc binary format should be fine as long
as we continue to keep things very simple. However, if we
think there's any reasonable probability of wanting to include
more complex data structures, we should consider using some
semi-structured data format such as JSON, Protocol Buffers, or
(ugh) ASN.1]]
The requirement that all NTS-KE messages be terminated by an
End of Message record makes them self-delimiting.
The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of
unrecognized Record Types. Implementations which receive a
record with an unrecognized Record Type MUST ignore the
record if the Critical Bit is 0, and MUST treat it as an
error if the Critical Bit is 1.
Record Type: A 15-bit integer in network byte order (from
most-to-least significant, its bits are record bits 7–1
and then 15–8). The semantics of record types 0–5 are
specified in this memo; additional type numbers SHALL be
tracked through the IANA Network Time Security Key
Establishment Record Types registry.
Body Length: the length of the Record Body field, in
octets, as a 16-bit integer in network byte order. Record
bodies may have any representable length and need not be
aligned to a word boundary.
Record Body: the syntax and semantics of this field shall
be determined by the Record Type.
The following NTS-KE Record Types are defined.
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 NTS-KE request and response. The
Critical Bit MUST be set.
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 body consists of a sequence of
16-octet strings. Each 16-octet string represents a
Protocol Name from the IANA Network Time Security
Next Protocols registry. The Critical Bit MUST be
set.
The Protocol Names listed in the client's NTS Next
Protocol Negotiation record denote those protocols which
the client wishes to speak using the key material
established through this NTS-KE session. The Protocol
Names listed in the server's response MUST comprise a
subset of those listed in the request, and denote those
protocols which the server is willing and able to speak
using the key material established through this NTS-KE
session. The client MAY proceed with one or more of
them. The request MUST list at least one protocol, but the
response MAY be empty.
The Error record has a Record Type number of 2. Its body
is exactly two octets long, consisting of an unsigned
16-bit integer in network byte order, denoting an error
code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request.
If clients receive a server response which includes an
Error record, they MUST discard any negotiated key
material and MUST NOT proceed to the Next Protocol.
The following error code are defined.
Error code 0 means "Unrecognized Critical
Record". The server MUST respond with this error
code if the request included a record which the server
did not understand and which had its Critical Bit
set. The client SHOULD NOT retry its request without
modification.
Error code 1 means "Bad Request". The server
MUST respond with this error if, upon the expiration
of an implementation-defined timeout, it has not yet
received a complete and syntactically well-formed
request from the client. This error is likely to be
the result of a dropped packet, so the client SHOULD
start over with a new DTLS handshake and retry its
request.
The Warning record has a Record Type number of 3. Its body
is exactly two octets long, consisting of an unsigned
16-bit integer in network byte order, denoting a warning
code. The Critical Bit MUST be set.
Clients MUST NOT include Warning records in their request.
If clients receive a server response which includes an
Warning record, they MAY discard any negotiated key
material and abort without proceeding to the Next
Protocol. Unrecognized warning codes MUST be treated as
errors.
This memo defines no warning codes.
The AEAD Algorithm Negotiation record has a Record Type
number of 4. Its body consists of a sequence of unsigned
16-bit integers in network byte order, denoting Numeric
Identifiers from the IANA AEAD
registry. The Critical Bit MAY be set.
If the NTS Next Protocol Negotiation record offers
"ntp/4",this record MUST be included exactly
once. Other protocols MAY require it as well.
When included in a request, this record denotes which AEAD
algorithms the client is willing to use to secure the Next
Protocol, in decreasing preference order. When included in
a response, this record denotes which algorithm the server
chooses to use, or is empty if the server supports none of
the algorithms offered.. In requests, the list MUST
include at least one algorithm. In responses, it MUST
include at most one. Honoring the client's preference
order is OPTIONAL: servers may select among any of the
client's offered choices, even if they are able to support
some other algorithm which the client prefers more.
Server implementations of NTS extensions for
NTPv4 MUST support AEAD_AES_SIV_CMAC_256 (Numeric
Identifier 15). That is, if the client includes
AEAD_AES_SIV_CMAC_256 in its AEAD Algorithm Negotiation
record, and the server accepts the "ntp/4"
protocol in its NTS Next Protocol Negotiation record, then
the server's AEAD Algorithm Negotation record MUST NOT be
empty.
The New Cookie for NTPv4 record has a Record Type number
of 5. The contents of its body SHALL be
implementation-defined and clients MUST NOT attempt to
interpret them. See [[TODO]] for a RECOMMENDED
construction.
Clients MUST NOT send records of this type. Servers MUST
send at least one record of this type, and SHOULD send
eight of them, if they accept "ntp/4" as a Next
Protocol. The Critical Bit SHOULD NOT be set.
[[Ed. Note: the purpose of sending eight cookies is to
allow the client to recover from dropped packets without
reusing cookies or starting a new handshake. Discussion of
cookie management should probably be broken out into its
own section.]]
Following a successful run of the NTS-KE protocol, key
material SHALL be extracted according to RFC 5705. Inputs to the exporter
function are to be constructed in a manner specific to the
negotiated Next Protocol. However, all protocols which
utilize NTS-KE MUST conform to the following two
rules:
The disambiguating label string MUST be
"EXPORTER-network-time-security/1".
The per-association context value MUST be provided, and
MUST begin with the 16-octet Protocol Name which was
negotiated as a Next Protocol.
Following a successful run of the NTS-KE protocol wherein
"ntp/4" is selected as a Next Protocol, two AEAD
keys SHALL be extracted: a client-to-server (C2S) key and a
server-to-client (S2C) key. These keys SHALL be computed
according to RFC 5705, using the
following inputs.
The disambiguating label string SHALL be
"EXPORTER-network-time-security/1".
The per-association context value SHALL consist of the
following 19 octets:
The first 16 octets SHALL be (in hexadecimal):
6E 74 70 2F 34 00 00 00 00 00 00 00 00 00 00 00
The next two octets SHALL be the Numeric Identifier of
the negotiated AEAD Algorithm, in network byte order.
The final octet SHALL be 0x00 for the C2S key and 0x01
for the S2C key.
Implementations wishing to derive additional keys for private
or experimental use MUST NOT do so by extending the
above-specified syntax for per-association context values.
Instead, they SHOULD use their own disambiguating label
string. Note that RFC 5705 provides that disambiguating label
strings beginning with "EXPERIMENTAL" MAY be used
without IANA registration.
In general, an NTS-protected NTPv4 packet consists of:
The usual 48-octet NTP header, which is authenticated
but not encrypted.
Some extensions which are authenticated but not encrypted.
An NTS extension which contains AEAD output (i.e., an
authentication tag and possible ciphertext). The
corresponding plaintext, if non-empty, consists of some
extensions which benefit from both encryption and
authentication.
Possibly, some additional extensions which are neither
encrypted nor authenticated. These are discarded by the
receiver. [[Ed. Note: right now there's no good reason
for the sender to include anything here, but eventually
there might be. We've seen Checksum Complement and LAST-EF
as two examples of semantically-void extensions that are
included to satsify constraints imposed lower on the
protocol stack, and while there's no reason to use
either of these on NTS-protected packets, I think we
could see similar examples in the future. So, rejecting
packets with unauthenticated extensions could cause
interoperability problems, while accepting and
processing those extensions would of course be a
security risk. Thus, I think "allow and
discard" is the correct policy.]]
Always included among the authenticated or
authenticated-and-encrypted extensions are a cookie
extension and a unique-identifier extension. The purpose of
the cookie extension is to enable the server to offload
storage of session state onto the client. The purpose of the
unique-identifier extension is to protect the client from
replay attacks.
The Unique Identifier extension has a Field Type of
[[TBD]]. When the extension is included in a client packet
(mode 3), its body SHALL consist of a string of octets
generated uniformly at random. The string SHOULD be 32 octets
long. When the extension is included in a server packet (mode
4), its body SHALL contain the same octet string as was
provided in the client packet to which the server is
responding. Its use in modes other than client/server is not
defined.
The Unique Identifier extension provides the client with a
cryptographically strong means of detecting replayed
packets. It may also be used standalone, without NTS, in
which case it provides the client with a means of detecting
spoofed packets from off-path attackers. Historically, NTP's
origin timestamp field has played both these roles, but for
cryptographic purposes this is suboptimal because it is only
64 bits long and, depending on implementation details, most
of those bits may be predictable. In contrast, the Unique
Identifier extension enables a degree of unpredictability
and collision-resistance more consistent with cryptographic
best practice.
[[TODO: consider using separate extension types for request
and response, thus allowing for use in symmetric mode. But
proper handling in the presence of dropped packets needs to
be documented and involves a lot of subtlety.]]
The NTS Cookie extension has a Field Type of [[TBD]]. Its
purpose is to carry information which enables the server to
recompute keys and other session state without having to
store any per-client state. The contents of its body SHALL
be implementation-defined and clients MUST NOT attempt to
interpret them. See [[TODO]] for a RECOMMENDED construction.
The NTS Cookie extension MUST NOT be included in NTP packets
whose mode is other than 3 (client) or 4 (server).
The NTS Cookie Placeholder extension has a Field Type of [[TBD]].
When this extension is included in a client packet (mode 3), it
communicates to the server that the client wishes it to send
additional cookies in its response. This extension MUST NOT
be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension is present, it
MUST be accompanied by an NTS Cookie extension, and the body
length of the NTS Cookie Placeholder extension MUST be the
same as the body length of the NTS Cookie Extension. (This
length requirement serves to ensure that the response will
not be larger than the request, in order to improve
timekeeping precision and prevent DDoS amplification). The
contents of the NTS Cookie Placeholder extension's body are
undefined and, aside from checking its length, MUST be
ignored by the server.
The NTS Authenticator and Encrypted Extensions extension is
the central cryptographic element of an NTS-protected NTP
packet. Its Field Type is [[TBD]] and the format of its body
SHALL be as follows:
Nonce length: two octets in network byte order, giving
the length of the Nonce field.
Nonce: a nonce as required by the negotiated AEAD Algorithm.
Ciphertext: the output of the negotiated AEAD
Algorithm. The structure of this field is determined by
the negotiated algorithm, but it typically contains an
authentication tag in addition to the actual ciphertext.
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 "03 03
03". The number of padding bytes should be chosen
in order to comply with the RFC
7822 requirement that (in the absence of a legacy
MAC) extensions have a total length in octets (including
the four octets for the type and length fields) which is
at least 28 and divisible by 4. At least one octet of
padding MUST be included, so that implementations can
unambiguously delimit the end of the ciphertext from the
start of the padding.
The Ciphertext field SHALL be formed by providing the
following inputs to the negotiated AEAD Algorithm:
K: For packets sent from the client to the server, the
C2S key SHALL be used. For packets sent from the server
to the client, the S2C key SHALL be used.
A: The associated data SHALL consist of the portion of
the NTP packet beginning from the start of the NTP header
and ending at the end of the last extension which precedes
the NTS Authenticator and Encrypted Extensions extension.
P: The plaintext SHALL consist of all (if any)
extensions to be encrypted.
N: The nonce SHALL be formed however required by the
negotiated AEAD Algorithm.
The NTS Authenticator and Encrypted Extensions extension
MUST NOT be included in NTP packets whose mode is other than
3 (client) or 4 (server).
A client sending an NTS-protected request SHALL include the
following extensions:
Exactly one Unique Identifier extension, which MUST be
authenticated and MUST NOT be encrypted [[Ed. Note: so
that if the server can't decrypt the request, it can
still echo back the Unique Identifier in the NTS NAK it
sends]]. MUST NOT duplicate those of any previous
request.
Exactly one NTS Cookie extension, which MUST be
authenticated and MUST NOT be encrypted. The cookie MUST
be one which the server previously provided the client;
it may have been provided during the NTS-KE handshake or
in response to a previous NTS-protected NTP request. To
protect client's privacy, the same cookie SHOULD NOT be
included in multiple requests. If the client does not
have any cookies that it has not already sent, it SHOULD
re-run the NTS-KE protocol before continuing.
Exactly one NTS Authenticator and Encrypted Extensions
extension, generated using an AEAD Algorithm and C2S key
established through NTS-KE.
The client MAY include one or more NTS Cookie Placeholder
extensions, which MUST be authenticated and MAY be
encrypted. The number of NTS Cookie Placeholder extensions
that the client includes SHOULD be such that if the client
includes N placeholders and the server sends back N+1
cookies, the number of unused cookies stored by the client
will come to eight. When both the client and server adhere
to all cookie-management guidance provided in this memo, the
number of placeholder extensions will equal the number of
dropped packets since the last successful volley.
The client MAY include additional (non-NTS-related)
extensions, which MAY appear prior to the NTS Authenticator
and Encrypted Extensions extension (therefore authenticated
but not encrypted), within it (therefore encrypted and
authenticated), or after it (therefore neither encrypted nor
authenticated). In general, however, the server MUST discard
any unauthenticated extensions and process the packet as
though they were not present. Servers MAY implement
exceptions to this requirement for particular extensions
if their specification explicitly provides for such.
Upon receiving an NTS-protected request, the server SHALL
(through some implementation-defined mechanism) use the
cookie to recover the AEAD Algorithm, C2S key, and S2C key
associated with the request, and then use the C2S key to
authenticate the packet and decrypt the ciphertext. If the
cookie is valid and authentication and decryption succeed,
then the server SHALL include the following extensions in
its response:
Exactly one Unique Identifier extension, which MUST be
authenticated, MUST NOT be encrypted, and whose contents
SHALL echo those provided by the client.
Exactly one NTS Authenticator and Encrypted Extensions
extension, generated using the AEAD algorithm and S2C
key recovered from the cookie provided by the client.
One or more NTS Cookie extensions, which MUST be
authenticated and encrypted. The number of NTS Cookie
extensions included SHOULD be equal to, and MUST NOT
exceed, one plus the number of valid NTS Cookie
Placeholder extensions included in the request.
The server MAY include additional (non-NTS-related)
extensions, which MAY appear prior to the NTS Authenticator
and Encrypted Extensions extension (therefore authenticated
but not encrypted), within it (therefore encrypted and
authenticated), or after it (therefore neither encrypted nor
authenticated). In general, however, the client MUST discard
any unauthenticated extensions and process the packet as
though they were not present. Clients MAY implement
exceptions to this requirement for particular extensions
if their specification explicitly provides for such.
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,
Section 7.4)) with kiss code "NTSN"
(meaning "NTS NAK"). Such a response MUST
include exactly one Unique Identifier extension whose
contents SHALL echo those provided by the client. It MUST
NOT include any NTS Cookie or NTS Authenticator and
Encrypted Extensions extension. [[Ed. Note: RFC 5905 already
provides the kiss code "CRYP" meaning
"Cryptographic authentication or identification failed"
but I think this is meant to be Autokey-specific.]]
Upon receiving an NTS-protected response, the client MUST
verify that the Unique Identifier matches that of an
outstanding request, and that the packet is authentic under
the S2C key associated with that request. If either of these
checks fails, the packet MUST be discarded without further
processing.
Upon receiving an NTS NAK, the client MUST verify that the
Unique Identifier matches that of an outstanding request. If
this check fails, the packet MUST be discarded without
further processing. If this check passes, the client SHOULD
discard all cookies and AEAD keys associated with the server
which sent the NAK and initiate a fresh NTS-KE handshake.
This section provides a RECOMMENDED way for servers to
construct NTS cookies. Clients MUST NOT examine the cookie
under the assumption that it is constructed according to this
section.
The role of cookies in NTS is closely analagous to that of
session cookies in TLS. Accordingly, the thematic resemblance
of this section to RFC 5077 is
deliberate, and the reader should likewise take heed of its
security considerations.
Servers should select an AEAD algorithm which they will use to
encrypt and authenticate cookies. The chosen algorithm should
be one such as AEAD_AES_SIV_CMAC_256 which resists
accidential nonce reuse, and it need not be the same as the
one that was negotiated with the client. Servers should
randomly generate and store a master AEAD key `K`. Servers
should additionally choose a non-secret, unique value `I` as
key-identifier for `K`.
Servers should periodically (e.g., once daily) generate a new
pair (I,K) and immediately switch to using these values for
all newly-generated cookies. Immediately following each such
key rotation, servers should securely erase any keys generated
two or more rotation periods prior. Servers should continue to
accept any cookie generated using keys that they have not yet
erased, even if those keys are no longer current. Erasing old
keys provides for forward secrecy, limiting the scope of what
old information can be stolen if a master key is somehow
compromised. Holding on to a limited number of old keys allows
clients to seamlessly transition from one generation to the
next without having to perform a new NTS-KE handshake.
[[TODO: discuss key management considerations for load-balanced
servers]]
To form a cookie, servers should first form a plaintext `P`
consisting of the following fields:
The AEAD algorithm negotiated during NTS-KEThe S2C keyThe C2S key
Servers should the generate a nonce `N` uniformly at random,
and form AEAD output `C` by encrypting `P` under key `K` with
nonce `N` and no associated data.
The cookie should consist of the tuple `(I,N,C)`.
[[TODO: explicitly specify how to verify and decrypt a cookie,
not just how to form one]]
IANA is requested to allocate an entry in the Service Name and
Transport Protocol Port Number Registry as follows:
Service Name: ntsTransport Protocol: udpAssignee: IESG <iesg@ietf.org>Contact: IETF Chair <chair@ietf.org>Description: Network Time SecurityReference: [[this memo]]Port Number: selected by IANA from the user port range
IANA is requested to allocate the following two entries in the
Application-Layer Protocol Negotation (ALPN) Protocol IDs
registry:
Protocol: Network Time Security Key Establishment, version 1
Identification
Sequence: 0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")
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 Label Registry:
ValueDTLS-OKReferenceNoteEXPORTER-network-time-security/1Y[[this memo]]
IANA is requested to allocate the following entries in the registry
of NTP Kiss-o'-Death codes:
CodeMeaningDTLSPacket conveys a DTLS recordNTSNNTS NAK
IANA is requested to allocate the following entries in the
NTP Extensions Field Types registry:
Field TypeMeaningReference[[TBD]]DTLS Record[[this memo]][[TBD]]Unique Identifier[[this memo]][[TBD]]NTS Cookie[[this memo]][[TBD]]NTS Cookie Placeholder[[this memo]][[TBD]]NTS Authenticator and Encrypted Extensions[[this memo]]
IANA is requested to create a new registry entitled
"Network Time Security Key Establishment Record Types".
Entries SHALL have the following fields:
Type Number (REQUIRED): An integer in the range 0–32767
inclusive
Description (REQUIRED): short text description of the
purpose of the field
Set Critical Bit (REQUIRED): One of "MUST",
"SHOULD", "MAY", "SHOULD NOT",
or "MUST NOT"
Reference (REQUIRED): A reference to a document specifying
the semantics of the record.
The policy for allocation of new entries in this registry SHALL vary
by the Type Number, as follows:
0–1023: Standards Action1024–16383: Specification Required16384–32767: Private and Experimental Use
Applications for new entries SHALL specify the contents of the
Description, Set Critical Bit and Reference fields and which
of the above ranges the Type Number should be allocated
from. Applicants MAY request a specific Type Number, and such
requests MAY be granted at the registrar's discretion.
The initial contents of this registry SHALL be as follows:
Field NumberDescriptionCriticalReference0End of messageMUST[[this memo]]1NTS next protocol negotiationMUST[[this memo]]2ErrorMUST[[this memo]]3WarningMUST[[this memo]]4AEAD algorithm negotationMAY[[this memo]]5New cookie for NTPv4SHOULD NOT[[this memo]]16384–32767Reserved for Private & Experimental UseMAY[[this memo]]
IANA is requested to create a new registry entitled
"Network Time Security Next Protocols".
Entries SHALL have the following fields:
Protocol Name (REQUIRED): a sequence of 16 octets. Shorter
sequences SHALL implicitly be right-padded with null
octets (0x00).
Human-Readable Name (OPTIONAL): if the sequence of octets
making up the protocol name intentionally represent a
valid UTF-8 string, this
field SHALL consist of that string.
Reference (RECOMMENDED): a reference to a relevant
specification document. If no relevant document exists, a
point-of-contact for questions regarding the entry SHOULD
be listed here in lieu.
Applications for new entries in this registry SHALL specify
all desired fields, and SHALL be granted on a First Come,
First Serve basis. Protocol Names beginning with 0x78 0x2D
("x-") SHALL be reserved for Private or Experimental
Use, and SHALL NOT be registered. The reserved entry
"ptp/2" may be updated or released by a future
Standards Action.
The initial contents of this registry SHALL be as follows:
Protocol NameHuman-Readable NameReference 0x6E 0x74 0x70 0x2F 0x34 ntp/4[[this memo]] 0x70 0x74 0x70 0x2F 0x32 ptp/2Reserved by [[this memo]]
IANA is requested to create two new registries entitled
"Network Time Security Error Codes" and
"Network Time Security Warning Codes". Entries in
each SHALL have the following fields:
Number (REQUIRED): a 16-bit unsigned integerDescription (REQUIRED): a short text description of the condition.Reference (REQUIRED): a reference to a relevant specification document.
The policy for allocation of new entries in these registries
SHALL vary by their Number, as follows:
0–1023: Standards Action1024–32767: Specification Required32768–65535: Private and Experimental Use
The initial contents of the Network Time Security Error Codes Registry SHALL be as follows:
NumberDescriptionReference0Unrecognized Critical Extension[[this memo]]1Bad Request[[this memo]]
The Network Time Security Warning Codes Registry SHALL initially be empty.
All security considerations described in have to be taken into
account. The application of NTS to NTP requires the following additional
considerations.At various points of the protocol, the generation of random numbers
is required. The employed methods of generation need to be
cryptographically secure. See for guidelines
concerning this topic.The certification-based authentication scheme described in is not applicable to the
concept of NTP pools. Therefore, NTS is unable to provide secure usage
of NTP pools.The client may wish to verify the validity of certificates during
the initial association phase. Since it generally has no reliable time
during this initial communication phase, it is impossible to verify
the period of validity of the certificates.NTP packets which contains extension fields with key exchange
messages do not provide integrity and authenticity protection of the
included time stamps. Therefore these NTP packets MUST NOT be used for
clock synchronization. Otherwise an initial attack on the client's
clock can potentially circumvent the
employed security measures of later messages .... TBDIn a packet delay attack, an adversary with the ability to act as a
MITM delays time synchronization packets between client and server
asymmetrically . This prevents the client from
accurately measuring the network delay, and hence its time offset to
the server . The delay attack does not modify
the content of the exchanged synchronization packets. Therefore,
cryptographic means do not provide a feasible way to mitigate this
attack. However, the maximum error that an adversary can introduced is
bounded by half of the round trip delay. Also, several
non-cryptographic precautions can be taken in order to detect this
attack.Usage of multiple time servers: this enables the client to
detect the attack, provided that the adversary is unable to delay
the synchronization packets between the majority of servers. This
approach is commonly used in NTP to exclude incorrect time servers
.Multiple communication paths: The client and server utilize
different paths for packet exchange. The client can detect the
attack, provided that the adversary is unable to manipulate the
majority of the available paths . Note
that this approach is not yet available, neither for NTP nor for
PTP.Usage of an encrypted connection: the client exchanges all
packets with the time server over an encrypted connection (e.g.
IPsec). This measure does not mitigate the delay attack, but it
makes it more difficult for the adversary to identify the time
synchronization packets.Introduction of a threshold value for the delay time of the
synchronization packets. The client can discard a time server if
the packet delay time of this time server is larger than the
threshold value.The actual time synchronization data in NTP packets does not
involve any information that needs to be kept secret. There also does
not seem to be any necessity to disguise the nature of an NTP
association. This is why content confidentiality is a non-objective
for this document.Unlinkability prevents a device from being tracked when it changes
network addresses (e.g. because said device moved between different
networks). In other words, unlinkability thwarts an attacker that
seeks to link a new network address used by a device with a network
address that it was formerly using, because of recognizable data that
the device persistently sends as part of an NTS-secured NTP
association. This is the justification for continually supplying the
client with fresh cookies, so that a cookie never represents
recognizable data in the sense outlined above. NTS's unlinkability objective is merely to not leak any additional
data that could be used to link a device's network address. NTS does
not rectify legacy linkability issues that are already present in NTP.
Thus, a client that requires unlinkability MUST also minimize
information transmitted in a client query (mode 3) packet as described
in the draft .
The unlinkability objective only holds for time synchronization
traffic, as opposed to key exchange traffic. This implies that it
cannot be guaranteed for devices that function not only as time
clients, but also as time servers (because the latter can be
externally triggered to send authentication data). It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g.
reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information
revealed by control queries. The authors would like to thank Richard Barnes, Steven Bellovin,
Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar, Aanchal
Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue, Eric K. Rescorla,
Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Sniffen,
Susan Sons, Douglas Stebila, Harlan Stenn, Martin Thomson, and Richard
Welty for contributions to this document. on the design of NTS.Precision clock synchronization protocol for networked
measurement and control systemsIEEE/IECA protocol is provided in this standard that enables precise
synchronization of clocks in measurement and control systems
implemented with technologies such as network communication, local
computing, and distributed objects. The protocol is applicable to
systems communicating via packet networks. Heterogeneous systems
are enabled that include clocks of various inherent precision,
resolution, and stability to synchronize. System-wide
synchronization accuracy and precision in the sub-microsecond
range are supported with minimal network and local clock computing
resources. Simple systems are installed and operated without
requiring the management attention of users because the default
behavior of the protocol allows for it.A game theoretic analysis of delay attacks against time
synchronization protocolsBypassing HTTP Strict Transport SecurityFor the last few years, some different attacks against SSL/TLS
have been released. Some of them based on cryptography or protocol
weaknesses such as BEAST, CRIME, BREACH, etc, and some others,
such as SSLStrip, based on rewriting HTTPS links into HTTP ones
and keep user communications always in HTTP. In order to protect
users against SSLStrip attacks, a new protection called HTTP
Strict Transport Security (HSTS) has been developed and it’s
currently supported by most widely used browsers. However, under
certain circumstances, an attacker could exploit an
inter-operation vulnerability in order to bypass HTTP Strict
Transport Security protection and use other well-known attack
techniques such as SSLStrip. In this paper, we review the HSTS
strengths and weaknesses, and we go in-depth on this
inter-operation vulnerability and how it could be exploited.Multi-path Time ProtocolsAttacking the Network Time Protocol