| NTP Working Group | D. Franke |
| Internet-Draft | |
| Intended status: Standards Track | D. Sibold |
| Expires: January 3, 2019 | K. Teichel |
| PTB | |
| M. Dansarie | |
| R. Sundblad | |
| Netnod | |
| July 02, 2018 |
Network Time Security for the Network Time Protocol
draft-dansarie-nts-00
This memo specifies Network Time Security (NTS), a mechanism for using Transport Layer Security (TLS) and Authenticated Encryption with Associated Data (AEAD) to provide cryptographic security for the client-server mode of the Network Time Protocol (NTP).
NTS is structured as a suite of two loosely coupled sub-protocols: the NTS Key Establishment Protocol (NTS-KE) and the NTS Extension Fields for NTPv4. NTS-KE handles NTS service authentication, initial handshaking, and key extraction over TLS. Encryption and authentication during NTP time synchronization is performed through the NTS Extension Fields in otherwise standard NTP packets. Except for during the initial NTS-KE process, all state required by the protocol is held by the client in opaque cookies.
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Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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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:
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. Additionally, 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 stateless servers to process replayed packets. The security demands of symmetric and control modes, on the other hand, are in conflict with the resource-utilization demands of client-server mode: any scheme 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 typical protocol flow is as follows: The client connects to an NTS-KE server on the NTS TCP port and the two parties perform a TLS handshake. Via the TLS channel, the parties negotiate some additional protocol parameters and the server sends the client a supply of cookies along with a list of one or more IP addresses to NTP servers for which the cookies are valid. 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. Ideally, the client never needs to connect to the NTS-KE server again.
Time synchronization proceeds with one of the indicated NTP servers 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 key exchange server) and an authentication tag, computed using key material extracted from the NTS-KE handshake. The NTP server uses the cookie to recover this key material and send back an authenticated response. The response includes a fresh, encrypted cookie which the client then sends back in the clear in a subsequent request. (This constant refreshing of cookies is necessary in order to achieve NTS's unlinkability goal.)
Figure 1 provides an overview of the high-level interaction between the client, the NTS-KE server, and the NTP server. Note that the cookies' data format and the exchange of secrets between NTS-KE and NTP servers are not part of this specification and are implementation dependent. However, a suggested format for NTS cookies is provided in Section 7.
+--------------+
| |
+-> | NTP Server 1 |
| | |
Shared cookie | +--------------+
+---------------+ encryption parameters | +--------------+
| | (Implementation dependent) | | |
| NTS-KE Server | <------------------------------+-> | NTP Server 2 |
| | | | |
+---------------+ | +--------------+
^ | .
| | .
| 1. Negotiate parameters, | .
| receive initial cookie | +--------------+
| supply, generate AEAD keys, | | |
| and receive NTP server IP +-> | NTP Server N |
| addresses using "NTS Key | |
| Establishment" protocol. +--------------+
| ^
| |
| +----------+ |
| | | |
+-----------> | Client | <-------------------------+
| | 2. Perform authenticated
+----------+ time synchronization
and generate new
cookies using "NTS
Extension Fields for
NTPv4".
Figure 1: Overview of High-Level Interactions in NTS
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 2018) 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 2018's best practices:
Implementations MUST NOT negotiate TLS versions earlier than 1.3.
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 Fallback SCSV.
Use of the Application-Layer Protocol Negotiation Extension is integral to NTS and support for it is REQUIRED for interoperability.
The NTS key establishment protocol is conducted via TCP port [[TBD1]]. The two endpoints carry out a TLS handshake in conformance with Section 3, 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 Figure 2. Requests and non-error responses each SHALL include exactly one NTS Next Protocol Negotiation record. The sequence SHALL be terminated by a "End of Message" record. The requirement that all NTS-KE messages be terminated by an End of Message record makes them self-delimiting.
Clients and servers MAY enforce length limits on requests and responses, however, servers MUST accept requests of at least 1024 octets and clients SHOULD accept responses of at least 65536 octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Record Type | Body Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Record Body .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTS-KE Record Format
The fields of an NTS-KE record are defined as follows:
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]`.
Figure 3 provides a schematic overview of the key exchange. It displays the protocol steps to be performed by the NTS client and server and record types to be exchanged.
+---------------------------------------+
| - Verify client request message. |
| - Extract TLS key material. |
| - Generate KE response message. |
| - Include Record Types: |
| o NTS Next Protocol Negotiation |
| o AEAD Algorithm Negotiation |
| o NTP Server Negotiation |
| o New Cookie for NTPv4 |
| o <New Cookie for NTPv4> |
| o End of Message |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
/ TLS application \
/ data \
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+----------------------+ +------+-----------------+
|- Generate KE request message. | |- Verify server response|
| - Include Record Types: | | message. |
| o NTS Next Protocol Negotiation | |- Extract cookie(s). |
| o AEAD Algorithm Negotiation | | |
| o <NTP Server Negotiation> | | |
| o End of Message | | |
+----------------------------------+ +------------------------+
Figure 3: NTS Key Exchange Messages
The following NTS-KE Record Types are defined:
The End of Message record has a Record Type number of 0 and a zero-length body. It MUST occur exactly once as the final record of every NTS-KE request and response. The Critical Bit MUST be set.
The NTS Next Protocol Negotiation record has a Record Type number of 1. It MUST occur exactly once in every NTS-KE request and response. Its body consists of a sequence of 16-bit unsigned integers in network byte order. Each integer represents a Protocol ID from the IANA Network Time Security Next Protocols registry. 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 codes are defined:
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 a Warning record, they MAY discard any negotiated key material and abort without proceeding to the Next Protocol. Unrecognized warning codes MUST be treated as errors.
This memo defines no warning codes.
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. It is empty if the server supports none of the algorithms offered. In requests, the list MUST include at least one algorithm. In responses, it MUST include at most one. Honoring the client's preference order is OPTIONAL: servers may select among any of the client's offered choices, even if they are able to support some other algorithm 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 Section 7 for a suggested construction.
Clients MUST NOT send records of this type. Servers MUST send at least one record of this type, and SHOULD send eight of them, if the Next Protocol Negotiation response record contains Protocol ID 0 (NTPv4) and the AEAD Algorithm Negotiation response record is not empty. The Critical Bit SHOULD NOT be set.
The NTP Server Negotiation record has a Record Type number of 6. The record MAY be sent by a client in a request and SHOULD be sent by a server as part of a reply. Its body consists of a sequence of IPv4 and/or IPv6 addresses. Both address types are represented by 16 octets in network byte order. To achieve this, IPv4 addresses are represented as "IPv4-mapped IPv6 addresses" in the format specified in RFC 4291, Section 2.5.5.2. For example: The IPv4 address 192.0.2.1 would be mapped to the IPv6 address space as ::ffff:192.0.2.1. The Critical Bit SHOULD be set.
When used in a request, the NTP Server Negotiation record is the client's way of indicating to the KE server which NTP servers it wishes to receive cookies for. Honoring the client's NTP server preferences is OPTIONAL. When used in a response, this record informs the client about which NTP servers the received cookies can be used with in the next phase of the protocol. The client SHOULD NOT attempt to use the received cookies with any other NTP servers than those indicated by the KE server.
If a response does not include this record, the client SHOULD assume that the received cookies are valid for use with an NTP server at the same network address as the key exchange server.
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:
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. RFC 5705 provides that disambiguating label strings beginning with "EXPERIMENTAL" MAY be used without IANA registration.
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
In general, an NTS-protected NTPv4 packet consists of:
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 provides the client with a cryptographically strong means of detecting replayed packets. It 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 MUST be at least 32 octets long. When the extension field is included in a server packet (mode 4), its body SHALL contain the same octet string as was provided in the client packet to which the server is responding. All server packets generated by NTS-implementing servers in response to client packets containing this extension field MUST also contain this field with the same content as in the client's request. The field's use in modes other than client-server is not defined.
This extension field MAY also be used standalone, without NTS, in which case it provides the client with a means of detecting spoofed packets from off-path attackers. Historically, NTP's origin timestamp field has played both these roles, but 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 Section 7 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. 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]]. It SHALL be formatted according to Figure 4 and include the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Length | Ciphertext Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Nonce, including up to 3 bytes padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Ciphertext, including up to 3 bytes padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NTS Authenticator and Encrypted Extension Fields Extension Field Format
The Ciphertext field SHALL be formed by providing the following inputs to the negotiated AEAD Algorithm:
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 as displayed in Figure 5:
The client MAY include one or more NTS Cookie Placeholder extension fields 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. The client SHOULD NOT include more than seven NTS Cookie Placeholder extension fields in a request. When both the client and server adhere to all cookie-management guidance provided in this memo, the number of placeholder extension fields will equal the number of dropped packets since the last successful volley.
+---------------------------------------+
| - Verify time request message. |
| - Generate time response message. |
| - Include NTPv4 extension fields: |
| o Unique Identifier EF |
| o NTS Cookie EF |
| o <NTS Cookie EF> |
| |
| - Generate AEAD tag of NTP message. |
| - Add NTS Authentication and |
| Encrypted Extension Fields EF. |
| - Transmit time response packet. |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
Time request / \ Time response
(mode 3) / \ (mode 4)
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+-----------------------+ +-----+------------------+
|- Generate time request message. | |- Verify time response |
| - Include NTPv4 extension fields: | | message. |
| o Unique Identifier EF | |- Extract cookie(s). |
| o NTS Cookie EF | |- Time synchronization |
| o <NTS Cookie Placeholder EF> | | processing. |
| | +------------------------+
|- Generate AEAD tag of NTP message.|
|- Add NTS Authentication and |
| Encrypted Extension Fields EF. |
|- Transmit time request packet. |
+-----------------------------------+
Figure 5: NTS Time Synchronization Messages
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, the server SHALL include the following extension fields in its response:
We emphasize the contrast that NTS Cookie extension fields MUST NOT be encrypted when sent from client to server, but MUST be encrypted from sent from server to client. The former is necessary in order for the server to be able to recover the C2S and S2C keys, while the latter is necessary to satisfy the unlinkability goals discussed in Section 11.1. We emphasize also that "encrypted" means encapsulated within the the NTS Authenticator and Encrypted Extensions extension field. While the body of an NTS Cookie extension field will generally consist of some sort of AEAD output (regardless of whether the recommendations of Section 7 are precisely followed), this is not sufficient to make the extension field "encrypted".
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.
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.
If the server is unable to validate the cookie or authenticate the request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC 5905, Section 7.4) with kiss code "NTSN", meaning "NTS negative-acknowledgment (NAK)". It MUST NOT include any NTS Cookie or NTS Authenticator and Encrypted Extension Fields extension fields.
If the NTP server has previously responded with authentic NTS-protected NTP packets (i.e., packets containing the NTS Authenticator and Encrypted Extension Fields extension field), the client MUST verify that any KoD packets received from the server contain the Unique Identifier extension field and that the Unique Identifier matches that of an outstanding request. If this check fails, the packet MUST be discarded without further processing. If this check passes, the client MUST comply with RFC 5095, Section 7.4 where required. A client MAY automatically re-run the NTS-KE protocol upon forced disassociation from an NTP server. In that case, it MUST be able to detect and stop looping between the NTS-KE and NTP servers.
Upon reception of the NTS NAK kiss code, the client SHOULD wait until the next poll for a valid NTS-protected response and if none is received, initiate a fresh NTS-KE handshake to try to renegotiate new cookies, AEAD keys, and parameters. If the NTS-KE handshake succeeds, the client MUST discard all old cookies and parameters and use the new ones instead. As long as the NTS-KE handshake has not succeeded, the client SHOULD continue polling the NTP server using the cookies and parameters it has.
The client MAY reuse cookies in order to prioritize resilience over unlinkability. Which of the two that should be prioritized in any particular case is dependent on the application and the user's preference. Section 11.1 describes the privacy considerations of this in further detail.
To allow for NTP session restart when the NTS-KE server is unavailable and to reduce NTS-KE server load, the client SHOULD keep at least one unused but recent cookie, AEAD keys, negotiated AEAD algorithm, and other necessary parameters on persistent storage. This way, the client is able to resume the NTP session without performing renewed NTS-KE negotiation.
This section is non-normative. It gives a suggested way for servers to construct NTS cookies. All normative requirements are stated in Section 4.1.6 and Section 5.3.
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. 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 between NTS-KE and NTP servers as well as across load-balanced clusters can make automatic key rotation challenging. However, the task can be accomplished without the need for central key-management infrastructure by using a ratchet, i.e., making each new key a deterministic, cryptographically 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:
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.
Many NTP server pools exist. Some of them have thousands of individual servers spread out over several continents. Due to their size and prevalence, it can be expected that a significant portion of NTP users are users of NTP pools.
The separation of the initial NTS key exchange from the authenticated NTP protocol simplifies the implementation of NTS on pool infrastructures. Since NTS key exchange over TLS is expected to be a rare occurrence in comparison with the normal authenticated NTP request and response traffic, even large pools should require a relatively small number of NTS-KE servers. This eliminates the need for complex certificate infrastructures. The lower timing and hardware requirements on NTS-KE servers also provide for load-balancing solutions that aren't suitable for NTP servers, such as virtual machine implementations that are started and stopped as needed.
The ability for NTS-KE servers to freely choose what NTP servers they will issue cookies for means that each pool can implement whatever secret-sharing system between NTS-KE and NTP servers it deems suitable. For example, in a large pool where the trust in the individual NTP server administrators is relatively low, it may be necessary to have separate shared secrets for each possible pair of NTS-KE and NTP servers. It should also be noted that not all NTS-KE servers in a pool must have the ability to issue cookies for all NTP servers in that pool.
Due to their freedom to choose what servers to issue cookies for, NTS-KE servers can perform a number of functions in addition to authenticating themselves to clients and issuing cookies. This includes load-balancing and geographic assignment of clients to NTP servers.
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:
IANA is requested to allocate the following entry in the TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry:
IANA is requested to allocate the following entry in the TLS Exporter Labels Registry:
| Value | DTLS-OK | Recommended | Reference | Note |
|---|---|---|---|---|
| EXPORTER-network- time-security/1 | Y | Y | [[this memo]], Section 4.2 |
IANA is requested to allocate the following entry in the registry of NTP Kiss-o'-Death Codes:
| Code | Meaning | Reference |
|---|---|---|
| NTSN | Network Time Security (NTS) negative-acknowledgment (NAK) | [[this memo]], Section 6 |
IANA is requested to allocate the following entries in the NTP Extension Field Types registry:
| Field Type | Meaning | Reference |
|---|---|---|
| [[TBD2]] | Unique Identifier | [[this memo]], Section 5.2 |
| [[TBD3]] | NTS Cookie | [[this memo]], Section 5.3 |
| [[TBD4]] | NTS Cookie Placeholder | [[this memo]], Section 5.4 |
| [[TBD5]] | NTS Authenticator and Encrypted Extension Fields | [[this memo]], Section 5.5 |
IANA is requested to create a new registry entitled "Network Time Security Key Establishment Record Types". Entries SHALL have the following fields:
The policy for allocation of new entries in this registry SHALL vary by the Record Type Number, as follows:
Applications for new entries SHALL specify the contents of the Description, Set Critical Bit, and Reference fields as well as which of the above ranges the Record Type Number should be allocated from. Applicants MAY request a specific Record Type Number and such requests MAY be granted at the registrar's discretion.
The initial contents of this registry SHALL be as follows:
| Record Type Number | Description | Set Critical Bit | Reference |
|---|---|---|---|
| 0 | End of Message | MUST | [[this memo]], Section 4.1.1 |
| 1 | NTS Next Protocol Negotiation | MUST | [[this memo]], Section 4.1.2 |
| 2 | Error | MUST | [[this memo]], Section 4.1.3 |
| 3 | Warning | MUST | [[this memo]], Section 4.1.4 |
| 4 | AEAD Algorithm Negotiation | MAY | [[this memo]], Section 4.1.5 |
| 5 | New Cookie for NTPv4 | SHOULD NOT | [[this memo]], Section 4.1.6 |
| 6 | NTP Server Negotiation | SHOULD | [[this memo]], Section 4.1.7 |
| 16384–32767 | Reserved for Private & Experimental Use | MAY | [[this memo]] |
IANA is requested to create a new registry entitled "Network Time Security Next Protocols". Entries SHALL have the following fields:
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 ID | Protocol Name | Reference |
|---|---|---|
| 0 | Network Time Protocol version 4 (NTPv4) | [[this memo]] |
| 32768-65535 | Reserved for Private or Experimental Use | Reserved 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:
The policy for allocation of new entries in these registries SHALL vary by their Number, as follows:
The initial contents of the Network Time Security Error Codes Registry SHALL be as follows:
| Number | Description | Reference |
|---|---|---|
| 0 | Unrecognized Critical Extension | [[this memo]], Section 4.1.3 |
| 1 | Bad Request | [[this memo]], Section 4.1.3 |
The Network Time Security Warning Codes Registry SHALL initially be empty.
The introduction of NTS brings with it the introduction of asymmetric cryptography to NTP. Asymmetric cryptography is necessary for initial server authentication and AEAD key extraction. Asymmetric cryptosystems are generally orders of magnitude slower than their symmetric counterparts. This makes it much harder to build systems that can serve requests at a rate corresponding to the full line speed of the network connection. This, in turn, opens up a new possibility for DDoS attacks on NTP services.
The main protection against these attacks in NTS lies in that the use of asymmetric cryptosystems is only necessary in the initial NTS-KE phase of the protocol. Since the protocol design enables separation of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE server separated from the NTP service it supports will not affect NTP users that have already performed initial authentication, AEAD key extraction, and cookie exchange. Furthermore, as noted in Section 8, NTP-KE capacity is easier to scale up and down than NTP server capacity.
NTS users should also consider that they are not fully protected against DDoS attacks by on-path adversaries. In addition to dropping packets and attacks such as those described in Section 10.4, an on-path attacker can send spoofed kiss-o'-death replies, which are not authenticated, in response to NTP requests. This could result in significantly increased load on the NTS-KE server. Implementers have to weigh the user's need for unlinkability against the added resilience that comes with cookie reuse in cases of NTS-KE server unavailability.
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 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.
Due to the RFC 7822 requirement that extensions be padded and aligned to four-octet boundaries, response size may still in some cases exceed request size by up to three octets. This is sufficiently inconsequential that we have declined to address it.
NTS's security goals are undermined if the client fails to verify that the X.509 certificate chain presented by the NTS-KE server is valid and rooted in a trusted certificate authority. RFC 5280 and RFC 6125 specify how such verification is to be performed in general. However, the expectation that the client does not yet have a correctly-set system clock at the time of certificate verification presents difficulties with verifying that the certificate is within its validity period, i.e., that the current time lies between the times specified in the certificate's notBefore and notAfter fields. It may be operationally necessary in some cases for a client to accept a certificate 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:
In a packet delay attack, an adversary with the ability to act as a man-in-the-middle delays time synchronization packets between client and server asymmetrically [RFC7384]. Since NTP's formula for computing time offset relies on the assumption that network latency is roughly symmetrical, this leads to the client to compute an inaccurate value [Mizrahi]. The delay attack does not reorder or modify the content of the exchanged synchronization packets. Therefore, cryptographic means do not provide a feasible way to mitigate this attack. However, the maximum error that an adversary can introduce is bounded by half of the round trip delay.
RFC 5905 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 [Shpiner], 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. Whenever this draft specifies the use of random numbers, cryptographically secure random number generation MUST be used. RFC 4086 contains 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 through 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 NTP Client Data Minimization Internet-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 NTP Client Data Minimization, 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.
The idea of separation of the NTS-KE server from the NTP server was added by Marcus Dansarie and Ragnar Sundblad. Thanks for this work goes to Patrik Fältström (Faltstrom) and Joachim Strömbergsson (Strombergsson) for review and ideas.
| [I-D.ietf-ntp-data-minimization] | Franke, D. and A. Malhotra, "NTP Client Data Minimization", Internet-Draft draft-ietf-ntp-data-minimization-02, July 2018. |
| [Mizrahi] | Mizrahi, T., "A game theoretic analysis of delay attacks against time synchronization protocols", in Proceedings of Precision Clock Synchronization for Measurement Control and Communication, ISPCS 2012, pp. 1-6, September 2012. |
| [RFC4086] | Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005. |
| [RFC5077] | Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008. |
| [RFC5280] | Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008. |
| [RFC5297] | Harkins, D., "Synthetic Initialization Vector (SIV) Authenticated Encryption Using the Advanced Encryption Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October 2008. |
| [RFC5869] | Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010. |
| [RFC7384] | Mizrahi, T., "Security Requirements of Time Protocols in Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, October 2014. |
| [Shpiner] | "Multi-path Time Protocols", in Proceedings of IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication (ISPCS), September 2013. |