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)531-592-4471kristof.teichel@ptb.de
Internet Area
NTP Working GroupIntegrityAuthenticationNTPSecurity
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.
This memo specifies Network Time Security (NTS), a
cryptographic security mechanism for network time
synchronization. A complete specification is provided for
application of NTS to the client-server mode of the
Network Time Protocol (NTP).
The objectives of NTS are as follows:
Identity: Through the use of the X.509 PKI,
implementations may cryptographically establish the
identity of the parties they are communicating with
Authentication: Implementations may cryptographically
verify that any time synchronization packets are
authentic, i.e., that they were produced by an
identified party and have not been modified in transit.
Confidentiality: Although basic time synchronization
data is considered non-confidential and sent in the
clear, NTS includes support for encrypting NTP extension
fields.
Replay prevention: Client implementations may detect when
a received time synchronization packet is a replay of
a previous packet.
Request-response consistency: Client implementations may
verify that a time synchronization packet received from
a server was sent in response to a particular request from
the client.
Unlinkability: For mobile clients, NTS will not leak any
information which would permit a passive adversary to
determine that two packets sent over different networks
came from the same client.
Non-amplification: implementations (especially server implementations) may avoid acting as
DDoS amplifiers by never responding to a request with a
packet larger than the request packet.
Scalability: Server implementations may serve large
numbers of clients without having to retain any
client-specific state.
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
partly 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 memo specifies NTS exclusively for the client-server mode
of NTP. To this end, NTS is structured as a suite of two protocols:
The "NTS Extension Fields for NTPv4" are a collection of
NTP extension fields for cryptographically securing
NTPv4 using previously-established key material. They
are suitable for securing client/server mode because the
server can implement them without retaining per-client
state, but on the other hand are suitable *only* for
client/server mode because only the client, and not the
server, is protected from replay.
The "NTS Key Establishment" protocol (NTS-KE)
is mechanism for establishing key material for use with
the NTS Extension Fields for NTPv4. It uses TLS to
exchange keys and negotiate some additional protocol
options, but then quickly closes the TLS channel and
permits the server to discard all associated state.
The typical protocol flow is as follows.
The client connects to the server on the NTS TCP
port and the two parties perform a TLS handshake. Via the TLS
channel, the parties negotiate some additional protocol
parameters and the server sends the client a supply of
cookies. The parties use TLS key
export to extract key material which will be used in
the next phase of the protocol. This negotiation takes only a
single round trip, after which the server closes the
connection and discards all associated state. At this point
the NTS-KE phase of the protocol is complete.
Time synchronization proceeds over the NTP UDP port. The
client sends the server an NTP client packet which includes
several extension fields. Included among these fields are a
cookie (previously provided by the server), and an
authentication tag, computed using key material extracted from
the NTS-KE handshake. The server uses the cookie to recover
this key material (previously discarded to avoid maintaining
state) and send back an authenticated response. The response
includes a fresh, encrypted cookie which the client then sends
back in the clear with its next request. (This constant
refreshing of cookies is necessary in order to achieve NTS's
unlinkability goal.)
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.
Network Time Security makes use of TLS for NTS key establishment.
Since securing time protocols is (as of 2017) a novel
application of 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 TLS versions
earlier than 1.2.
Implementations willing to negotiate more than one possible
version of TLS SHOULD NOT respond to handshake failures by
retrying with a downgraded protocol version. If they do, they
MUST implement .
TLS clients MUST NOT offer, and TLS servers MUST NOT select,
RC4 cipher suites.
TLS clients SHOULD offer, and TLS servers SHOULD accept, the
TLS Renegotiation Indication
Extension. Regardless, they MUST NOT initiate or permit
insecure renegotiation. (*)
TLS clients SHOULD offer, and TLS servers SHOULD accept, the
TLS Session Hash and Extended Master
Secret Extension. (*)
Use of the Application-Layer Protocol
Negotiation Extension is integral to NTS and support for
it is REQUIRED for interoperability.
(*): Note that TLS 1.3 or beyond may render the indicated
recommendations inapplicable.
The NTS key establishment protocol is conducted via TCP port
[[TBD1]]. 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.
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. 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.
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]`.
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-bit
unsigned integers in network byte order. Each integer
represents a Protocol ID from the IANA Network Time
Security Next Protocols registry. The Critical Bit MUST be
set.
The Protocol IDs 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
IDs 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 TLS 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
Protocol ID 0 (for NTPv4), then this record MUST be
included exactly once. Other protocols MAY require it as
well.
When included in a request, this record denotes which AEAD
algorithms the client is willing to use to secure the Next
Protocol, in decreasing preference order. When included in
a response, this record denotes which algorithm the server
chooses to use, 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 extension
fields 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 Protocol ID 0 (NTPv4); in
its NTS Next Protocol Negotiation record, then the
server's AEAD Algorithm Negotiation 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 for a
suggested construction.
Clients MUST NOT send records of this type. Servers MUST
send at least one record of this type, and SHOULD send
eight of them, if they accept Protocol ID 0 (NTPv4) as a
Next Protocol. The Critical Bit SHOULD NOT be set.
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 two-octet Protocol ID which was
negotiated as a Next Protocol.
Following a successful run of the NTS-KE protocol wherein
Protocol ID 0 (NTPv4) is selected as a Next Protocol, two AEAD
keys SHALL be extracted: a client-to-server (C2S) key and a
server-to-client (S2C) key. These keys SHALL be computed
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 five octets:
The first two octets SHALL be zero (the Protocol ID
for NTPv4).
The next two octets SHALL be the Numeric Identifier of
the negotiated AEAD Algorithm, in network byte order.
The final octet SHALL be 0x00 for the C2S key and 0x01
for the S2C key.
Implementations wishing to derive additional keys for private
or experimental use MUST NOT do so by extending the
above-specified syntax for per-association context values.
Instead, they SHOULD use their own disambiguating label
string. Note that RFC 5705 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 extension fields which are authenticated but not encrypted.
An extension field which contains AEAD output (i.e., an
authentication tag and possible ciphertext). The
corresponding plaintext, if non-empty, consists of some
extension fields which benefit from both encryption and
authentication.
Possibly, some additional extension fields which are
neither encrypted nor authenticated. These are discarded
by the receiver.
Always included among the authenticated or
authenticated-and-encrypted extension fields are a cookie
extension field and a unique-identifier extension field. The
purpose of the cookie extension field is to enable the
server to offload storage of session state onto the
client. The purpose of the unique-identifier extension field
is to protect the client from replay attacks.
The Unique Identifier extension field has a Field Type of
[[TBD2]]. When the extension field 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 field is included in a
server packet (mode 4), its body SHALL contain the same
octet string as was provided in the client packet to which
the server is responding. Its use in modes other than
client/server is not defined.
The Unique Identifier extension field 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 field enables a degree of
unpredictability and collision-resistance more consistent
with cryptographic best practice.
The NTS Cookie extension field has a Field Type of
[[TBD3]]. 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 for a
suggested construction. The NTS Cookie extension field
MUST NOT be included in NTP packets whose mode is other than
3 (client) or 4 (server).
The NTS Cookie Placeholder extension field has a Field Type
of [[TBD4]]. When this extension field is included in a
client packet (mode 3), it communicates to the server that
the client wishes it to send additional cookies in its
response. This extension field MUST NOT be included in NTP
packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension field is
present, it MUST be accompanied by an NTS Cookie extension
field, and the body length of the NTS Cookie Placeholder
extension field MUST be the same as the body length of the
NTS Cookie extension field. (This length requirement serves
to ensure that the response will not be larger than the
request, in order to improve timekeeping precision and
prevent DDoS amplification). The contents of the NTS Cookie
Placeholder extension field's body are undefined and, aside
from checking its length, MUST be ignored by the server.
The NTS Authenticator and Encrypted Extension Fields extension field is
the central cryptographic element of an NTS-protected NTP
packet. Its Field Type is [[TBD5]] 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 and interpreted as an
unsigned integer.
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) extension fields 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 by examining
the last octet to determine the padding length.
The Ciphertext field SHALL be formed by providing the
following inputs to the negotiated AEAD Algorithm:
K: For packets sent from the client to the server, the
C2S key SHALL be used. For packets sent from the server
to the client, the S2C key SHALL be used.
A: The associated data SHALL consist of the portion of
the NTP packet beginning from the start of the NTP
header and ending at the end of the last extension field
which precedes the NTS Authenticator and Encrypted
Extension Fields extension field.
P: The plaintext SHALL consist of all (if any) NTP
extension fields to be encrypted. The format of any such
fields SHALL be in accordance with RFC 7822, and if multiple
extension fields are present they SHALL be joined by
concatenation.
N: The nonce SHALL be formed however required by the
negotiated AEAD Algorithm.
The NTS Authenticator and Encrypted Extension Fields
extension field MUST NOT be included in NTP packets whose
mode is other than 3 (client) or 4 (server).
A client sending an NTS-protected request SHALL include the
following extension fields:
Exactly one Unique Identifier extension field, which
MUST be authenticated, MUST NOT be encrypted, and whose
contents MUST NOT duplicate those of any previous
request.
Exactly one NTS Cookie extension field, 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 Extension
Fields extension field, generated using an AEAD
Algorithm and C2S key established through NTS-KE.
The client MAY include one or more NTS Cookie Placeholder
extension field, which MUST be authenticated and MAY be
encrypted. The number of NTS Cookie Placeholder extension
fields that the client includes SHOULD be such that if the
client includes N placeholders and the server sends back N+1
cookies, the number of unused cookies stored by the client
will come to eight. When both the client and server adhere
to all cookie-management guidance provided in this memo, the
number of placeholder extension fields will equal the number
of dropped packets since the last successful volley.
The client MAY include additional (non-NTS-related)
extension fields, which MAY appear prior to the NTS
Authenticator and Encrypted Extension Fields extension
fields (therefore authenticated but not encrypted), within
it (therefore encrypted and authenticated), or after it
(therefore neither encrypted nor authenticated). In general,
however, the server MUST discard any unauthenticated
extension fields and process the packet as though they were
not present. Servers MAY implement exceptions to this
requirement for particular extension fields 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 extension fields
in its response:
Exactly one Unique Identifier extension field, which
MUST be authenticated, MUST NOT be encrypted, and whose
contents SHALL echo those provided by the client.
Exactly one NTS Authenticator and Encrypted Extension
Fields extension field, generated using the AEAD
algorithm and S2C key recovered from the cookie provided
by the client.
One or more NTS Cookie extension fields, which MUST be
authenticated and encrypted. The number of NTS Cookie
extension fields included SHOULD be equal to, and MUST
NOT exceed, one plus the number of valid NTS Cookie
Placeholder extension fields included in the request.
The server MAY include additional (non-NTS-related)
extension fields, which MAY appear prior to the NTS
Authenticator and Encrypted Extension Fields extension field
(therefore authenticated but not encrypted), within it
(therefore encrypted and authenticated), or after it
(therefore neither encrypted nor authenticated). In general,
however, the client MUST discard any unauthenticated
extension fields and process the packet as though they were
not present. Clients MAY implement exceptions to this
requirement for particular extension fields 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 field whose contents
SHALL echo those provided by the client. It MUST NOT
include any NTS Cookie or NTS Authenticator and Encrypted
Extension Fields extension fields.
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
wait until the next poll for a valid NTS-protected response
and if none is received, discard all cookies and AEAD keys
associated with the server which sent the NAK and initiate a
fresh NTS-KE handshake.
This section is non-normative. It gives a suggested way for
servers to construct NTS cookies. All normative requirements
are stated in and .
The role of cookies in NTS is closely analogous 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
accidental 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.
The need to keep keys synchronized across load-balanced
clusters can make automatic key rotation challenging. However,
the task can be accomplished without the need for central
key-management infrastructure by using a ratchet, i.e., making
each new key a deterministic, cryptographically pseudo-random
function of its predecessor. A recommended concrete
implementation of this approach is to use HKDF to derive new keys, using the
key's predecessor as Input Keying Material and its key identifier
as a salt.
To form a cookie, servers SHOULD first form a plaintext `P`
consisting of the following fields:
The AEAD algorithm negotiated during NTS-KEThe S2C keyThe C2S key
Servers SHOULD then generate a nonce `N` uniformly at random,
and form AEAD output `C` by encrypting `P` under key `K` with
nonce `N` and no associated data.
The cookie SHOULD consist of the tuple `(I,N,C)`.
To verify and decrypt a cookie provided by the client, first
parse it into its components `I`, `N`, and `C`. Use `I` to
look up its decryption key `K`. If the key whose identifier is
`I` has been erased or never existed, decryption fails; reply
with an NTS NAK. Otherwise, attempt to decrypt and verify
ciphertext `C` using key `K` and nonce `N` with no associated
data. If decryption or verification fails, reply with an NTS
NAK. Otherwise, parse out the contents of the resulting
plaintext `P` to obtain the negotiated AEAD algorithm, S2C key,
and C2S key.
IANA is requested to allocate two entries, identical except
for the Transport Protocol, in the Service Name and Transport
Protocol Port Number Registry as follows:
Service Name: ntsTransport Protocol: tcp, udpAssignee: IESG <iesg@ietf.org>Contact: IETF Chair <chair@ietf.org>Description: Network Time SecurityReference: [[this memo]]Port Number: [[TBD1]], selected by IANA from the user port range
IANA is requested to allocate the following entry 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]]
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 entry in the registry
of NTP Kiss-o'-Death codes:
CodeMeaningNTSNNTS NAK
IANA is requested to allocate the following entries in the
NTP Extensions Field Types registry:
Field TypeMeaningReference[[TBD2]]Unique Identifier[[this memo]][[TBD3]]NTS Cookie[[this memo]][[TBD4]]NTS Cookie Placeholder[[this memo]][[TBD5]]NTS Authenticator and Encrypted Extension Fields[[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: IETF Review1024–16383: 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 negotiationMAY[[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 ID (REQUIRED): a integer in the range 0-65535
inclusive, functioning as an identifier.
Protocol Name (REQUIRED): a short text string naming the
protocol being identified.
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 upon approval by a
Designated Expert. Protocol IDs 32768-65535 SHALL be reserved
for Private or Experimental Use, and SHALL NOT be
registered.
The initial contents of this registry SHALL be as follows:
Protocol IDHuman-Readable NameReference0Network Time Protocol version 4 (NTPv4)[[this memo]]32768-65535Reserved for Private or Experimental UseReserved 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 integer in the range 0-65535 inclusiveDescription (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: IETF Review1024–32767: 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.
Certain non-standard and/or deprecated features of the Network
Time Protocol enable clients to send a request to a server
which causes the server to send a response much larger than
the request. Servers which enable these features can be abused
in order to amplify traffic volume in distributed
denial-of-service (DDoS) attacks by sending them a request
with a spoofed source IP. In recent years, attacks of this nature
have become an endemic nuisance.
NTS is designed to avoid contributing any further to this
problem by ensuring that NTS-related extension fields included in
server responses will be the same size as the NTS-related
extension fields sent by the client. In particular, this is why
the client is required to send a separate and appropriately
padded-out NTS Cookie Placeholder extension field for every cookie
it wants to get back, rather than being permitted simply to
specify a desired quantity.
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 rooted in a trusted certificate
authority. and specifies how such verification is to be
performed in general. However, the expectation that the
client does not yet have a correctly-set system clock at the
time of certificate verification presents difficulties with
verifying that the certificate is within its validity
period, i.e., that the current time lies between the times
specified in the certificate's notBefore and notAfter
fields, 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. While there is no perfect solution
to this problem, there are several mitigations the client
can implement to make it more difficult for an adversary to
successfully present an expired certificate:
Check whether the system time is in fact unreliable. If
the system clock has previously been synchronized since
last boot, then on operating systems which implement a
kernel-based phase-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
considered reliable and certificates can be strictly
validated.
Allow the system administrator to specify that
certificates should *always* be strictly validated. Such
a configuration is appropriate on systems which have a
battery-backed clock and which can reasonably prompt the
user to manually set an approximately-correct time if it
appears to be needed.
Once the clock has been synchronized, periodically write
the current system time to persistent storage. Do not accept
any certificate whose notAfter field is earlier than the last
recorded time.
Do not process time packets from servers if the time
computed from them falls outside the validity period of
the server's certificate.
Use multiple time sources. The ability to pass off an
expired certificate is only useful to an adversary who
has compromised the corresponding private key. If the
adversary has compromised only a minority of servers,
NTP's selection algorithm (
section 11.2.1) will protect the client from accepting
bad time from the adversary-controlled servers.
Additional standardization work and infrastructure
development is necessary before NTS can be used with public
NTP server pools. First, a scheme will need to be specified
for determining what constitutes an acceptable certificate
for a pool server, such as establishing a value required to
be contained in its Extended Key Usage attribute, and how to
determine, given the DNS name of a pool, what Subject
Alternative Name to expect in the certificates of its
members. Implementing any such specification will
necessitate infrastructure work: pool organizers will need
to act as certificate authorities, regularly monitor the
behavior of servers to which certificates have been issued,
and promptly revoke the certificate of any server found to
be serving incorrect time.
In a packet delay attack, an adversary with the ability to
act as a man-in-the-middle delays time synchronization
packets between client and server asymmetrically . Since NTP's formula for computing time
offset relies on the assumption that network latency is
roughly symmetrical, this leads to the client to compute an
inaccurate value . The delay attack
does not reorder or modify the content of the exchanged
synchronization packets. Therefore, cryptographic means do
not provide a feasible way to mitigate this attack. However,
the maximum error that an adversary can introduce is bounded
by half of the round trip delay.
specifies a parameter called
MAXDIST which denotes the maximum round-trip latency
(including not only the immediate round trip between client
and server but the whole distance back to the reference
clock as reported in the Root Delay field) that a client
will tolerate before concluding that the server is
unsuitable for synchronization. The standard value for
MAXDIST is one second, although some implementations use
larger values. Whatever value a client chooses, the maximum
error which can be introduced by a delay attack is
MAXDIST/2.
Usage of multiple time sources, or multiple network paths to
a given time source , may also serve
to mitigate delay attacks if the adversary is in control of
only some of the paths.
At various points in NTS, the generation of
cryptographically secure random numbers is required. See
for guidelines concerning this
topic.
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.
NTS does not protect the confidentiality of information in
NTP's header fields. When clients implement , client packet
headers do not contain any information which the client
could conceivably wish to keep secret: one field is random,
and all others are fixed. Information in server packet
headers is likewise public: the origin timestamp is copied
from the client's (random) transmit timestamp, and all other
fields are set the same regardless of the identity of the
client making the request.
Future extension fields could hypothetically contain
sensitive information, in which case NTS provides a
mechanism for encrypting them.
The authors would like to thank Richard Barnes, Steven
Bellovin, Scott Fluhrer, 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 and comments on the design of
NTS.A game theoretic analysis of delay attacks against time
synchronization protocolsMulti-path Time Protocols
Authenticated Encryption with Associated Data
Distributed Denial of Service
Network Time Protocol
Network Time Security
Transport Layer Security