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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 NTP Working Group D. Franke 3 Internet-Draft Akamai 4 Intended status: Standards Track D. Sibold 5 Expires: August 2, 2020 K. Teichel 6 PTB 7 M. Dansarie 9 R. Sundblad 10 Netnod 11 January 30, 2020 13 Network Time Security for the Network Time Protocol 14 draft-ietf-ntp-using-nts-for-ntp-21 16 Abstract 18 This memo specifies Network Time Security (NTS), a mechanism for 19 using Transport Layer Security (TLS) and Authenticated Encryption 20 with Associated Data (AEAD) to provide cryptographic security for the 21 client-server mode of the Network Time Protocol (NTP). 23 NTS is structured as a suite of two loosely coupled sub-protocols. 24 The first (NTS-KE) handles initial authentication and key 25 establishment over TLS. The second handles encryption and 26 authentication during NTP time synchronization via extension fields 27 in the NTP packets, and holds all required state only on the client 28 via opaque cookies. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at https://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on August 2, 2020. 47 Copyright Notice 49 Copyright (c) 2020 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (https://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 65 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4 66 1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5 67 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 7 68 3. TLS profile for Network Time Security . . . . . . . . . . . . 7 69 4. The NTS Key Establishment Protocol . . . . . . . . . . . . . 8 70 4.1. NTS-KE Record Types . . . . . . . . . . . . . . . . . . . 10 71 4.1.1. End of Message . . . . . . . . . . . . . . . . . . . 10 72 4.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 11 73 4.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 11 74 4.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 12 75 4.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 12 76 4.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 13 77 4.1.7. NTPv4 Server Negotiation . . . . . . . . . . . . . . 13 78 4.1.8. NTPv4 Port Negotiation . . . . . . . . . . . . . . . 13 79 4.2. Key Extraction (generally) . . . . . . . . . . . . . . . 14 80 5. NTS Extension Fields for NTPv4 . . . . . . . . . . . . . . . 14 81 5.1. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 14 82 5.2. Packet Structure Overview . . . . . . . . . . . . . . . . 15 83 5.3. The Unique Identifier Extension Field . . . . . . . . . . 15 84 5.4. The NTS Cookie Extension Field . . . . . . . . . . . . . 16 85 5.5. The NTS Cookie Placeholder Extension Field . . . . . . . 16 86 5.6. The NTS Authenticator and Encrypted Extension Fields 87 Extension Field . . . . . . . . . . . . . . . . . . . . . 17 88 5.7. Protocol Details . . . . . . . . . . . . . . . . . . . . 19 89 6. Suggested Format for NTS Cookies . . . . . . . . . . . . . . 24 90 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 91 7.1. Service Name and Transport Protocol Port Number Registry 25 92 7.2. TLS Application-Layer Protocol Negotiation (ALPN) 93 Protocol IDs Registry . . . . . . . . . . . . . . . . . . 25 94 7.3. TLS Exporter Labels Registry . . . . . . . . . . . . . . 26 95 7.4. NTP Kiss-o'-Death Codes Registry . . . . . . . . . . . . 26 96 7.5. NTP Extension Field Types Registry . . . . . . . . . . . 26 97 7.6. Network Time Security Key Establishment Record Types 98 Registry . . . . . . . . . . . . . . . . . . . . . . . . 27 99 7.7. Network Time Security Next Protocols Registry . . . . . . 28 100 7.8. Network Time Security Error and Warning Codes Registries 29 101 8. Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION 30 102 8.1. Implementation 1 . . . . . . . . . . . . . . . . . . . . 30 103 8.1.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 30 104 8.1.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 31 105 8.1.3. Contact Information . . . . . . . . . . . . . . . . . 31 106 8.1.4. Last Update . . . . . . . . . . . . . . . . . . . . . 31 107 8.2. Implementation 2 . . . . . . . . . . . . . . . . . . . . 31 108 8.2.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 31 109 8.2.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 31 110 8.2.3. Contact Information . . . . . . . . . . . . . . . . . 31 111 8.2.4. Last Update . . . . . . . . . . . . . . . . . . . . . 31 112 8.3. Implementation 3 . . . . . . . . . . . . . . . . . . . . 32 113 8.3.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 32 114 8.3.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 32 115 8.3.3. Contact Information . . . . . . . . . . . . . . . . . 32 116 8.3.4. Last Update . . . . . . . . . . . . . . . . . . . . . 32 117 8.4. Implementation 4 . . . . . . . . . . . . . . . . . . . . 32 118 8.4.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 32 119 8.4.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 33 120 8.4.3. Contact Information . . . . . . . . . . . . . . . . . 33 121 8.4.4. Last Update . . . . . . . . . . . . . . . . . . . . . 33 122 8.5. Implementation 5 . . . . . . . . . . . . . . . . . . . . 33 123 8.5.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 33 124 8.5.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 33 125 8.5.3. Contact Information . . . . . . . . . . . . . . . . . 33 126 8.5.4. Last Update . . . . . . . . . . . . . . . . . . . . . 33 127 8.6. Implementation 6 . . . . . . . . . . . . . . . . . . . . 33 128 8.6.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 34 129 8.6.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 34 130 8.6.3. Contact Information . . . . . . . . . . . . . . . . . 34 131 8.6.4. Last Update . . . . . . . . . . . . . . . . . . . . . 34 132 8.7. Interoperability . . . . . . . . . . . . . . . . . . . . 34 133 9. Security Considerations . . . . . . . . . . . . . . . . . . . 34 134 9.1. Sensitivity to DDoS attacks . . . . . . . . . . . . . . . 34 135 9.2. Avoiding DDoS Amplification . . . . . . . . . . . . . . . 35 136 9.3. Initial Verification of Server Certificates . . . . . . . 35 137 9.4. Delay Attacks . . . . . . . . . . . . . . . . . . . . . . 37 138 9.5. Random Number Generation . . . . . . . . . . . . . . . . 37 139 9.6. NTS Stripping . . . . . . . . . . . . . . . . . . . . . . 37 140 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 38 141 10.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . 38 142 10.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 38 144 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 145 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 146 12.1. Normative References . . . . . . . . . . . . . . . . . . 39 147 12.2. Informative References . . . . . . . . . . . . . . . . . 41 148 Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 42 149 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 151 1. Introduction 153 This memo specifies Network Time Security (NTS), a cryptographic 154 security mechanism for network time synchronization. A complete 155 specification is provided for application of NTS to the client-server 156 mode of the Network Time Protocol (NTP) [RFC5905]. 158 1.1. Objectives 160 The objectives of NTS are as follows: 162 o Identity: Through the use of the X.509 public key infrastructure, 163 implementations may cryptographically establish the identity of 164 the parties they are communicating with. 166 o Authentication: Implementations may cryptographically verify that 167 any time synchronization packets are authentic, i.e., that they 168 were produced by an identified party and have not been modified in 169 transit. 171 o Confidentiality: Although basic time synchronization data is 172 considered non-confidential and sent in the clear, NTS includes 173 support for encrypting NTP extension fields. 175 o Replay prevention: Client implementations may detect when a 176 received time synchronization packet is a replay of a previous 177 packet. 179 o Request-response consistency: Client implementations may verify 180 that a time synchronization packet received from a server was sent 181 in response to a particular request from the client. 183 o Unlinkability: For mobile clients, NTS will not leak any 184 information additional to NTP which would permit a passive 185 adversary to determine that two packets sent over different 186 networks came from the same client. 188 o Non-amplification: Implementations (especially server 189 implementations) may avoid acting as distributed denial-of-service 190 (DDoS) amplifiers by never responding to a request with a packet 191 larger than the request packet. 193 o Scalability: Server implementations may serve large numbers of 194 clients without having to retain any client-specific state. 196 1.2. Protocol Overview 198 The Network Time Protocol includes many different operating modes to 199 support various network topologies. In addition to its best-known 200 and most-widely-used client-server mode, it also includes modes for 201 synchronization between symmetric peers, a control mode for server 202 monitoring and administration, and a broadcast mode. These various 203 modes have differing and partly contradictory requirements for 204 security and performance. Symmetric and control modes demand mutual 205 authentication and mutual replay protection. Additionally, for 206 certain message types control mode may require confidentiality as 207 well as authentication. Client-server mode places more stringent 208 requirements on resource utilization than other modes, because 209 servers may have vast number of clients and be unable to afford to 210 maintain per-client state. However, client-server mode also has more 211 relaxed security needs, because only the client requires replay 212 protection: it is harmless for stateless servers to process replayed 213 packets. The security demands of symmetric and control modes, on the 214 other hand, are in conflict with the resource-utilization demands of 215 client-server mode: any scheme which provides replay protection 216 inherently involves maintaining some state to keep track of what 217 messages have already been seen. 219 This memo specifies NTS exclusively for the client-server mode of 220 NTP. To this end, NTS is structured as a suite of two protocols: 222 The "NTS Extensions for NTPv4" define a collection of NTP 223 extension fields for cryptographically securing NTPv4 using 224 previously-established key material. They are suitable for 225 securing client-server mode because the server can implement them 226 without retaining per-client state. All state is kept by the 227 client and provided to the server in the form of an encrypted 228 cookie supplied with each request. On the other hand, the NTS 229 Extension Fields are suitable *only* for client-server mode 230 because only the client, and not the server, is protected from 231 replay. 233 The "NTS Key Establishment" protocol (NTS-KE) is a mechanism for 234 establishing key material for use with the NTS Extension Fields 235 for NTPv4. It uses TLS to exchange keys, provide the client with 236 an initial supply of cookies, and negotiate some additional 237 protocol options. After this exchange, the TLS channel is closed 238 with no per-client state remaining on the server side. 240 The typical protocol flow is as follows: The client connects to an 241 NTS-KE server on the NTS TCP port and the two parties perform a TLS 242 handshake. Via the TLS channel, the parties negotiate some 243 additional protocol parameters and the server sends the client a 244 supply of cookies along with an IP address to the NTP server for 245 which the cookies are valid. The parties use TLS key export 246 [RFC5705] to extract key material which will be used in the next 247 phase of the protocol. This negotiation takes only a single round 248 trip, after which the server closes the connection and discards all 249 associated state. At this point the NTS-KE phase of the protocol is 250 complete. Ideally, the client never needs to connect to the NTS-KE 251 server again. 253 Time synchronization proceeds with one of the indicated NTP servers 254 over the NTP UDP port. The client sends the server an NTP client 255 packet which includes several extension fields. Included among these 256 fields are a cookie (previously provided by the key exchange server) 257 and an authentication tag, computed using key material extracted from 258 the NTS-KE handshake. The NTP server uses the cookie to recover this 259 key material and send back an authenticated response. The response 260 includes a fresh, encrypted cookie which the client then sends back 261 in the clear in a subsequent request. (This constant refreshing of 262 cookies is necessary in order to achieve NTS's unlinkability goal.) 264 Figure 1 provides an overview of the high-level interaction between 265 the client, the NTS-KE server, and the NTP server. Note that the 266 cookies' data format and the exchange of secrets between NTS-KE and 267 NTP servers are not part of this specification and are implementation 268 dependent. However, a suggested format for NTS cookies is provided 269 in Section 6. 271 +--------------+ 272 | | 273 +-> | NTP Server 1 | 274 | | | 275 Shared cookie | +--------------+ 276 +---------------+ encryption parameters | +--------------+ 277 | | (Implementation dependent) | | | 278 | NTS-KE Server | <------------------------------+-> | NTP Server 2 | 279 | | | | | 280 +---------------+ | +--------------+ 281 ^ | . 282 | | . 283 | 1. Negotiate parameters, | . 284 | receive initial cookie | +--------------+ 285 | supply, generate AEAD keys, | | | 286 | and receive NTP server IP +-> | NTP Server N | 287 | addresses using "NTS Key | | 288 | Establishment" protocol. +--------------+ 289 | ^ 290 | | 291 | +----------+ | 292 | | | | 293 +-----------> | Client | <-------------------------+ 294 | | 2. Perform authenticated 295 +----------+ time synchronization 296 and generate new 297 cookies using "NTS 298 Extension Fields for 299 NTPv4". 301 Figure 1: Overview of High-Level Interactions in NTS 303 2. Requirements Language 305 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 306 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 307 "OPTIONAL" in this document are to be interpreted as described in 308 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 309 capitals, as shown here. 311 3. TLS profile for Network Time Security 313 Network Time Security makes use of TLS for NTS key establishment. 315 Since the NTS protocol is new as of this publication, no backward- 316 compatibility concerns exist to justify using obsolete, insecure, or 317 otherwise broken TLS features or versions. Implementations MUST 318 conform with [RFC7525] or with a later revision of BCP 195. In 319 particular, failure to use cipher suites that provide forward secrecy 320 will make all negotiated NTS keys recoverable by anyone that gains 321 access to the NTS-KE server's private certificate. Furthermore: 323 Implementations MUST NOT negotiate TLS versions earlier than 1.2, 324 SHOULD negotiate TLS 1.3 [RFC8446] or later when possible, and MAY 325 refuse to negotiate any TLS version which has been superseded by a 326 later supported version. 328 Use of the Application-Layer Protocol Negotiation Extension [RFC7301] 329 is integral to NTS and support for it is REQUIRED for 330 interoperability. 332 4. The NTS Key Establishment Protocol 334 The NTS key establishment protocol is conducted via TCP port 335 [[TBD1]]. The two endpoints carry out a TLS handshake in conformance 336 with Section 3, with the client offering (via an ALPN [RFC7301] 337 extension), and the server accepting, an application-layer protocol 338 of "ntske/1". Immediately following a successful handshake, the 339 client SHALL send a single request as Application Data encapsulated 340 in the TLS-protected channel. Then, the server SHALL send a single 341 response followed by a TLS "Close notify" alert and then discard the 342 channel state. 344 The client's request and the server's response each SHALL consist of 345 a sequence of records formatted according to Figure 2. Requests and 346 non-error responses each SHALL include exactly one NTS Next Protocol 347 Negotiation record. The sequence SHALL be terminated by a "End of 348 Message" record. The requirement that all NTS-KE messages be 349 terminated by an End of Message record makes them self-delimiting. 351 Clients and servers MAY enforce length limits on requests and 352 responses, however, servers MUST accept requests of at least 1024 353 octets and clients SHOULD accept responses of at least 65536 octets. 355 0 1 2 3 356 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 1 357 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 358 |C| Record Type | Body Length | 359 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 360 | | 361 . . 362 . Record Body . 363 . . 364 | | 365 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 367 Figure 2: NTS-KE Record Format 369 The fields of an NTS-KE record are defined as follows: 371 C (Critical Bit): Determines the disposition of unrecognized 372 Record Types. Implementations which receive a record with an 373 unrecognized Record Type MUST ignore the record if the Critical 374 Bit is 0 and MUST treat it as an error if the Critical Bit is 1. 376 Record Type Number: A 15-bit integer in network byte order. The 377 semantics of record types 0-7 are specified in this memo. 378 Additional type numbers SHALL be tracked through the IANA Network 379 Time Security Key Establishment Record Types registry. 381 Body Length: The length of the Record Body field, in octets, as a 382 16-bit integer in network byte order. Record bodies MAY have any 383 representable length and need not be aligned to a word boundary. 385 Record Body: The syntax and semantics of this field SHALL be 386 determined by the Record Type. 388 For clarity regarding bit-endianness: the Critical Bit is the most- 389 significant bit of the first octet. In C, given a network buffer 390 `unsigned char b[]` containing an NTS-KE record, the critical bit is 391 `b[0] >> 7` while the record type is `((b[0] & 0x7f) << 8) + b[1]`. 393 Note that, although the Type-Length-Body format of an NTS-KE record 394 is similar to that of an NTP extension field, the semantics of the 395 length field differ. While the length subfield of an NTP extension 396 field gives the length of the entire extension field including the 397 type and length subfields, the length field of an NTS-KE record gives 398 just the length of the body. 400 Figure 3 provides a schematic overview of the key exchange. It 401 displays the protocol steps to be performed by the NTS client and 402 server and record types to be exchanged. 404 +---------------------------------------+ 405 | - Verify client request message. | 406 | - Extract TLS key material. | 407 | - Generate KE response message. | 408 | - Include Record Types: | 409 | o NTS Next Protocol Negotiation | 410 | o AEAD Algorithm Negotiation | 411 | o NTP Server Negotiation | 412 | o New Cookie for NTPv4 | 413 | o | 414 | o End of Message | 415 +-----------------+---------------------+ 416 | 417 | 418 Server -----------+---------------+-----+-----------------------> 419 ^ \ 420 / \ 421 / TLS application \ 422 / data \ 423 / \ 424 / V 425 Client -----+---------------------------------+-----------------> 426 | | 427 | | 428 | | 429 +-----------+----------------------+ +------+-----------------+ 430 |- Generate KE request message. | |- Verify server response| 431 | - Include Record Types: | | message. | 432 | o NTS Next Protocol Negotiation | |- Extract cookie(s). | 433 | o AEAD Algorithm Negotiation | | | 434 | o | | | 435 | o End of Message | | | 436 +----------------------------------+ +------------------------+ 438 Figure 3: NTS Key Exchange Messages 440 4.1. NTS-KE Record Types 442 The following NTS-KE Record Types are defined: 444 4.1.1. End of Message 446 The End of Message record has a Record Type number of 0 and a zero- 447 length body. It MUST occur exactly once as the final record of every 448 NTS-KE request and response. The Critical Bit MUST be set. 450 4.1.2. NTS Next Protocol Negotiation 452 The NTS Next Protocol Negotiation record has a Record Type number of 453 1. It MUST occur exactly once in every NTS-KE request and response. 454 Its body consists of a sequence of 16-bit unsigned integers in 455 network byte order. Each integer represents a Protocol ID from the 456 IANA Network Time Security Next Protocols registry. The Critical Bit 457 MUST be set. 459 The Protocol IDs listed in the client's NTS Next Protocol Negotiation 460 record denote those protocols which the client wishes to speak using 461 the key material established through this NTS-KE session. The 462 Protocol IDs listed in the server's response MUST comprise a subset 463 of those listed in the request and denote those protocols which the 464 server is willing and able to speak using the key material 465 established through this NTS-KE session. The client MAY proceed with 466 one or more of them. The request MUST list at least one protocol, 467 but the response MAY be empty. 469 4.1.3. Error 471 The Error record has a Record Type number of 2. Its body is exactly 472 two octets long, consisting of an unsigned 16-bit integer in network 473 byte order, denoting an error code. The Critical Bit MUST be set. 475 Clients MUST NOT include Error records in their request. If clients 476 receive a server response which includes an Error record, they MUST 477 discard any negotiated key material and MUST NOT proceed to the Next 478 Protocol. 480 The following error codes are defined: 482 Error code 0 means "Unrecognized Critical Record". The server 483 MUST respond with this error code if the request included a record 484 which the server did not understand and which had its Critical Bit 485 set. The client SHOULD NOT retry its request without 486 modification. 488 Error code 1 means "Bad Request". The server MUST respond with 489 this error if, upon the expiration of an implementation-defined 490 timeout, it has not yet received a complete and syntactically 491 well-formed request from the client. 493 Error code 2 means "Internal Server Error". The server MUST 494 respond with this error if it is unable to respond properly due to 495 an internal condition. 497 4.1.4. Warning 499 The Warning record has a Record Type number of 3. Its body is 500 exactly two octets long, consisting of an unsigned 16-bit integer in 501 network byte order, denoting a warning code. The Critical Bit MUST 502 be set. 504 Clients MUST NOT include Warning records in their request. If 505 clients receive a server response which includes a Warning record, 506 they MAY discard any negotiated key material and abort without 507 proceeding to the Next Protocol. Unrecognized warning codes MUST be 508 treated as errors. 510 This memo defines no warning codes. 512 4.1.5. AEAD Algorithm Negotiation 514 The AEAD Algorithm Negotiation record has a Record Type number of 4. 515 Its body consists of a sequence of unsigned 16-bit integers in 516 network byte order, denoting Numeric Identifiers from the IANA AEAD 517 registry [RFC5116]. The Critical Bit MAY be set. 519 If the NTS Next Protocol Negotiation record offers Protocol ID 0 (for 520 NTPv4), then this record MUST be included exactly once. Other 521 protocols MAY require it as well. 523 When included in a request, this record denotes which AEAD algorithms 524 the client is willing to use to secure the Next Protocol, in 525 decreasing preference order. When included in a response, this 526 record denotes which algorithm the server chooses to use. It is 527 empty if the server supports none of the algorithms offered. In 528 requests, the list MUST include at least one algorithm. In 529 responses, it MUST include at most one. Honoring the client's 530 preference order is OPTIONAL: servers may select among any of the 531 client's offered choices, even if they are able to support some other 532 algorithm which the client prefers more. 534 Server implementations of NTS extension fields for NTPv4 (Section 5) 535 MUST support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15). 536 That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD 537 Algorithm Negotiation record and the server accepts Protocol ID 0 538 (NTPv4) in its NTS Next Protocol Negotiation record, then the 539 server's AEAD Algorithm Negotiation record MUST NOT be empty. 541 4.1.6. New Cookie for NTPv4 543 The New Cookie for NTPv4 record has a Record Type number of 5. The 544 contents of its body SHALL be implementation-defined and clients MUST 545 NOT attempt to interpret them. See Section 6 for a suggested 546 construction. 548 Clients MUST NOT send records of this type. Servers MUST send at 549 least one record of this type, and SHOULD send eight of them, if the 550 Next Protocol Negotiation response record contains Protocol ID 0 551 (NTPv4) and the AEAD Algorithm Negotiation response record is not 552 empty. The Critical Bit SHOULD NOT be set. 554 4.1.7. NTPv4 Server Negotiation 556 The NTPv4 Server Negotiation record has a Record Type number of 6. 557 Its body consists of an ASCII-encoded [ANSI.X3-4.1986] string. The 558 contents of the string SHALL be either an IPv4 address in dotted 559 decimal notation, an IPv6 address, or a fully qualified domain name 560 (FQDN). IPv6 addresses MUST conform to the "Text Representation of 561 Addresses" as specified in [RFC4291] and MUST NOT include zone 562 identifiers [RFC6874]. If internationalized labels are needed in the 563 domain name, the A-LABEL syntax specified in [RFC5891] MUST be used. 565 When NTPv4 is negotiated as a Next Protocol and this record is sent 566 by the server, the body specifies the hostname or IP address of the 567 NTPv4 server with which the client should associate and which will 568 accept the supplied cookies. If no record of this type is sent, the 569 client SHALL interpret this as a directive to associate with an NTPv4 570 server at the same IP address as the NTS-KE server. Servers MUST NOT 571 send more than one record of this type. 573 When this record is sent by the client, it indicates that the client 574 wishes to associate with the specified NTP server. The NTS-KE server 575 MAY incorporate this request when deciding what NTPv4 Server 576 Negotiation records to respond with, but honoring the client's 577 preference is OPTIONAL. The client MUST NOT send more than one 578 record of this type. 580 Servers MAY set the Critical Bit on records of this type; clients 581 SHOULD NOT. 583 4.1.8. NTPv4 Port Negotiation 585 The NTPv4 Port Negotiation record has a Record Type number of 7. Its 586 body consists of a 16-bit unsigned integer in network byte order, 587 denoting a UDP port number. 589 When NTPv4 is negotiated as a Next Protocol and this record is sent 590 by the server, the body specifies the port number of the NTPv4 server 591 with which the client should associate and which will accept the 592 supplied cookies. If no record of this type is sent, the client 593 SHALL assume a default of 123 (the registered port number for NTP). 595 When this record is sent by the client in conjunction with a NTPv4 596 Server Negotiation record, it indicates that the client wishes to 597 associate with the NTP server at the specified port. The NTS-KE 598 server MAY incorporate this request when deciding what NTPv4 Server 599 Negotiation and NTPv4 Port Negotiation records to respond with, but 600 honoring the client's preference is OPTIONAL. 602 Servers MAY set the Critical Bit on records of this type; clients 603 SHOULD NOT. 605 4.2. Key Extraction (generally) 607 Following a successful run of the NTS-KE protocol, key material SHALL 608 be extracted according to RFC 5705 [RFC5705]. Inputs to the exporter 609 function are to be constructed in a manner specific to the negotiated 610 Next Protocol. However, all protocols which utilize NTS-KE MUST 611 conform to the following two rules: 613 The disambiguating label string MUST be "EXPORTER-network-time- 614 security/1". 616 The per-association context value MUST be provided and MUST begin 617 with the two-octet Protocol ID which was negotiated as a Next 618 Protocol. 620 5. NTS Extension Fields for NTPv4 622 5.1. Key Extraction (for NTPv4) 624 Following a successful run of the NTS-KE protocol wherein Protocol ID 625 0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be 626 extracted: a client-to-server (C2S) key and a server-to-client (S2C) 627 key. These keys SHALL be computed according to RFC 5705 [RFC5705], 628 using the following inputs. 630 The disambiguating label string SHALL be "EXPORTER-network-time- 631 security/1". 633 The per-association context value SHALL consist of the following 634 five octets: 636 The first two octets SHALL be zero (the Protocol ID for NTPv4). 638 The next two octets SHALL be the Numeric Identifier of the 639 negotiated AEAD Algorithm in network byte order. 641 The final octet SHALL be 0x00 for the C2S key and 0x01 for the 642 S2C key. 644 Implementations wishing to derive additional keys for private or 645 experimental use MUST NOT do so by extending the above-specified 646 syntax for per-association context values. Instead, they SHOULD use 647 their own disambiguating label string. Note that RFC 5705 [RFC5705] 648 provides that disambiguating label strings beginning with 649 "EXPERIMENTAL" MAY be used without IANA registration. 651 5.2. Packet Structure Overview 653 In general, an NTS-protected NTPv4 packet consists of: 655 The usual 48-octet NTP header which is authenticated but not 656 encrypted. 658 Some extension fields which are authenticated but not encrypted. 660 An extension field which contains AEAD output (i.e., an 661 authentication tag and possible ciphertext). The corresponding 662 plaintext, if non-empty, consists of some extension fields which 663 benefit from both encryption and authentication. 665 Possibly, some additional extension fields which are neither 666 encrypted nor authenticated. In general, these are discarded by 667 the receiver. 669 Always included among the authenticated or authenticated-and- 670 encrypted extension fields are a cookie extension field and a unique 671 identifier extension field. The purpose of the cookie extension 672 field is to enable the server to offload storage of session state 673 onto the client. The purpose of the unique identifier extension 674 field is to protect the client from replay attacks. 676 5.3. The Unique Identifier Extension Field 678 The Unique Identifier extension field provides the client with a 679 cryptographically strong means of detecting replayed packets. It has 680 a Field Type of [[TBD2]]. When the extension field is included in a 681 client packet (mode 3), its body SHALL consist of a string of octets 682 generated uniformly at random. The string MUST be at least 32 octets 683 long. When the extension field is included in a server packet (mode 684 4), its body SHALL contain the same octet string as was provided in 685 the client packet to which the server is responding. All server 686 packets generated by NTS-implementing servers in response to client 687 packets containing this extension field MUST also contain this field 688 with the same content as in the client's request. The field's use in 689 modes other than client-server is not defined. 691 This extension field MAY also be used standalone, without NTS, in 692 which case it provides the client with a means of detecting spoofed 693 packets from off-path attackers. Historically, NTP's origin 694 timestamp field has played both these roles, but for cryptographic 695 purposes this is suboptimal because it is only 64 bits long and, 696 depending on implementation details, most of those bits may be 697 predictable. In contrast, the Unique Identifier extension field 698 enables a degree of unpredictability and collision resistance more 699 consistent with cryptographic best practice. 701 5.4. The NTS Cookie Extension Field 703 The NTS Cookie extension field has a Field Type of [[TBD3]]. Its 704 purpose is to carry information which enables the server to recompute 705 keys and other session state without having to store any per-client 706 state. The contents of its body SHALL be implementation-defined and 707 clients MUST NOT attempt to interpret them. See Section 6 for a 708 suggested construction. The NTS Cookie extension field MUST NOT be 709 included in NTP packets whose mode is other than 3 (client) or 4 710 (server). 712 5.5. The NTS Cookie Placeholder Extension Field 714 The NTS Cookie Placeholder extension field has a Field Type of 715 [[TBD4]]. When this extension field is included in a client packet 716 (mode 3), it communicates to the server that the client wishes it to 717 send additional cookies in its response. This extension field MUST 718 NOT be included in NTP packets whose mode is other than 3. 720 Whenever an NTS Cookie Placeholder extension field is present, it 721 MUST be accompanied by an NTS Cookie extension field. The body 722 length of the NTS Cookie Placeholder extension field MUST be the same 723 as the body length of the NTS Cookie extension field. This length 724 requirement serves to ensure that the response will not be larger 725 than the request, in order to improve timekeeping precision and 726 prevent DDoS amplification. The contents of the NTS Cookie 727 Placeholder extension field's body are undefined and, aside from 728 checking its length, MUST be ignored by the server. 730 5.6. The NTS Authenticator and Encrypted Extension Fields Extension 731 Field 733 The NTS Authenticator and Encrypted Extension Fields extension field 734 is the central cryptographic element of an NTS-protected NTP packet. 735 Its Field Type is [[TBD5]]. It SHALL be formatted according to 736 Figure 4 and include the following fields: 738 Nonce Length: Two octets in network byte order, giving the length 739 of the Nonce field, excluding any padding, interpreted as an 740 unsigned integer. 742 Ciphertext Length: Two octets in network byte order, giving the 743 length of the Ciphertext field, excluding any padding, interpreted 744 as an unsigned integer. 746 Nonce: A nonce as required by the negotiated AEAD Algorithm. The 747 field is zero-padded to a word (four octets) boundary. 749 Ciphertext: The output of the negotiated AEAD Algorithm. The 750 structure of this field is determined by the negotiated algorithm, 751 but it typically contains an authentication tag in addition to the 752 actual ciphertext. The field is zero-padded to a word (four 753 octets) boundary. 755 Additional Padding: Clients which use a nonce length shorter than 756 the maximum allowed by the negotiated AEAD algorithm may be 757 required to include additional zero-padding. The necessary length 758 of this field is specified below. 760 0 1 2 3 761 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 1 762 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 763 | Nonce Length | Ciphertext Length | 764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 765 | | 766 . . 767 . Nonce, including up to 3 octets padding . 768 . . 769 | | 770 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 771 | | 772 . . 773 . Ciphertext, including up to 3 octets padding . 774 . . 775 | | 776 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 777 | | 778 . . 779 . Additional Padding . 780 . . 781 | | 782 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 784 Figure 4: NTS Authenticator and Encrypted Extension Fields Extension 785 Field Format 787 The Ciphertext field SHALL be formed by providing the following 788 inputs to the negotiated AEAD Algorithm: 790 K: For packets sent from the client to the server, the C2S key 791 SHALL be used. For packets sent from the server to the client, 792 the S2C key SHALL be used. 794 A: The associated data SHALL consist of the portion of the NTP 795 packet beginning from the start of the NTP header and ending at 796 the end of the last extension field which precedes the NTS 797 Authenticator and Encrypted Extension Fields extension field. 799 P: The plaintext SHALL consist of all (if any) NTP extension 800 fields to be encrypted; if multiple extension fields are present 801 they SHALL be joined by concatenation. Each such field SHALL be 802 formatted in accordance with RFC 7822 [RFC7822], except that, 803 contrary to the RFC 7822 requirement that fields have a minimum 804 length of 16 or 28 octets, encrypted extension fields MAY be 805 arbitrarily short (but still MUST be a multiple of 4 octets in 806 length). 808 N: The nonce SHALL be formed however required by the negotiated 809 AEAD algorithm. 811 The purpose of the Additional Padding field is to ensure that servers 812 can always choose a nonce whose length is adequate to ensure its 813 uniqueness, even if the client chooses a shorter one, and still 814 ensure that the overall length of the server's response packet does 815 not exceed the length of the request. For mode 4 (server) packets, 816 no Additional Padding field is ever required. For mode 3 (client) 817 packets, the length of the Additional Padding field SHALL be computed 818 as follows. Let `N_LEN` be the padded length of the Nonce field. 819 Let `N_MAX` be, as specified by RFC 5116 [RFC5116], the maximum 820 permitted nonce length for the negotiated AEAD algorithm. Let 821 `N_REQ` be the lesser of 16 and N_MAX, rounded up to the nearest 822 multiple of 4. If N_LEN is greater than or equal to N_REQ, then no 823 Additional Padding field is required. Otherwise, the Additional 824 Padding field SHALL be at least N_REQ - N_LEN octets in length. 825 Servers MUST enforce this requirement by discarding any packet which 826 does not conform to it. 828 Senders are always free to include more Additional Padding than 829 mandated by the above paragraph. Theoretically, it could be 830 necessary to do so in order to bring the extension field to the 831 minimum length required by [RFC7822]. This should never happen in 832 practice because any reasonable AEAD algorithm will have a nonce and 833 an authenticator long enough to bring the extension field to its 834 required length already. Nonetheless, implementers are advised to 835 explicitly handle this case and ensure that the extension field they 836 emit is of legal length. 838 The NTS Authenticator and Encrypted Extension Fields extension field 839 MUST NOT be included in NTP packets whose mode is other than 3 840 (client) or 4 (server). 842 5.7. Protocol Details 844 A client sending an NTS-protected request SHALL include the following 845 extension fields as displayed in Figure 5: 847 Exactly one Unique Identifier extension field which MUST be 848 authenticated, MUST NOT be encrypted, and whose contents MUST NOT 849 duplicate those of any previous request. 851 Exactly one NTS Cookie extension field which MUST be authenticated 852 and MUST NOT be encrypted. The cookie MUST be one which has been 853 previously provided to the client; either from the key exchange 854 server during the NTS-KE handshake or from the NTP server in 855 response to a previous NTS-protected NTP request. 857 Exactly one NTS Authenticator and Encrypted Extension Fields 858 extension field, generated using an AEAD Algorithm and C2S key 859 established through NTS-KE. 861 To protect the client's privacy, the client SHOULD avoid reusing a 862 cookie. If the client does not have any cookies that it has not 863 already sent, it SHOULD initiate a re-run the NTS-KE protocol. The 864 client MAY reuse cookies in order to prioritize resilience over 865 unlinkability. Which of the two that should be prioritized in any 866 particular case is dependent on the application and the user's 867 preference. Section 10.1 describes the privacy considerations of 868 this in further detail. 870 The client MAY include one or more NTS Cookie Placeholder extension 871 fields which MUST be authenticated and MAY be encrypted. The number 872 of NTS Cookie Placeholder extension fields that the client includes 873 SHOULD be such that if the client includes N placeholders and the 874 server sends back N+1 cookies, the number of unused cookies stored by 875 the client will come to eight. The client SHOULD NOT include more 876 than seven NTS Cookie Placeholder extension fields in a request. 877 When both the client and server adhere to all cookie-management 878 guidance provided in this memo, the number of placeholder extension 879 fields will equal the number of dropped packets since the last 880 successful volley. 882 In rare circumstances, it may be necessary to include fewer NTS 883 Cookie Placeholder extensions than recommended above in order to 884 prevent datagram fragmentation. When cookies adhere the format 885 recommended in Section 6 and the AEAD in use is the mandatory-to- 886 implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and 887 seven placeholders and still have packet size fall comfortably below 888 1280 octets if no non-NTS-related extensions are used; 1280 octets is 889 the minimum prescribed MTU for IPv6 and is in practice also safe for 890 avoiding IPv4 fragmentation. Nonetheless, senders SHOULD include 891 fewer cookies and placeholders than otherwise indicated if doing so 892 is necessary to prevent fragmentation. 894 +---------------------------------------+ 895 | - Verify time request message | 896 | - Generate time response message | 897 | - Included NTPv4 extension fields | 898 | o Unique Identifier EF | 899 | o NTS Authentication and | 900 | Encrypted Extension Fields EF | 901 | - NTS Cookie EF | 902 | - | 903 | - Transmit time request packet | 904 +-----------------+---------------------+ 905 | 906 | 907 Server -----------+---------------+-----+-----------------------> 908 ^ \ 909 / \ 910 Time request / \ Time response 911 (mode 3) / \ (mode 4) 912 / \ 913 / V 914 Client -----+---------------------------------+-----------------> 915 | | 916 | | 917 | | 918 +-----------+----------------------+ +------+-----------------+ 919 |- Generate time request message | |- Verify time response | 920 | - Include NTPv4 Extension fields | | message | 921 | o Unique Identifier EF | |- Extract cookie(s) | 922 | o NTS Cookie EF | |- Time synchronization | 923 | o | | processing | 924 | | +------------------------+ 925 |- Generate AEAD tag of NTP message| 926 |- Add NTS Authentication and | 927 | Encrypted Extension Fields EF | 928 |- Transmit time request packet | 929 +----------------------------------+ 931 Figure 5: NTS Time Synchronization Messages 933 The client MAY include additional (non-NTS-related) extension fields 934 which MAY appear prior to the NTS Authenticator and Encrypted 935 Extension Fields extension fields (therefore authenticated but not 936 encrypted), within it (therefore encrypted and authenticated), or 937 after it (therefore neither encrypted nor authenticated). In 938 general, however, the server MUST discard any unauthenticated 939 extension fields and process the packet as though they were not 940 present. Servers MAY implement exceptions to this requirement for 941 particular extension fields if their specification explicitly 942 provides for such. 944 Upon receiving an NTS-protected request, the server SHALL (through 945 some implementation-defined mechanism) use the cookie to recover the 946 AEAD Algorithm, C2S key, and S2C key associated with the request, and 947 then use the C2S key to authenticate the packet and decrypt the 948 ciphertext. If the cookie is valid and authentication and decryption 949 succeed, the server SHALL include the following extension fields in 950 its response: 952 Exactly one Unique Identifier extension field which MUST be 953 authenticated, MUST NOT be encrypted, and whose contents SHALL 954 echo those provided by the client. 956 Exactly one NTS Authenticator and Encrypted Extension Fields 957 extension field, generated using the AEAD algorithm and S2C key 958 recovered from the cookie provided by the client. 960 One or more NTS Cookie extension fields which MUST be 961 authenticated and encrypted. The number of NTS Cookie extension 962 fields included SHOULD be equal to, and MUST NOT exceed, one plus 963 the number of valid NTS Cookie Placeholder extension fields 964 included in the request. The cookies returned in those fields 965 MUST be valid for use with the NTP server that sent them. They 966 MAY be valid for other NTP servers as well, but there is no way 967 for the server to indicate this. 969 We emphasize the contrast that NTS Cookie extension fields MUST NOT 970 be encrypted when sent from client to server, but MUST be encrypted 971 when sent from server to client. The former is necessary in order 972 for the server to be able to recover the C2S and S2C keys, while the 973 latter is necessary to satisfy the unlinkability goals discussed in 974 Section 10.1. We emphasize also that "encrypted" means encapsulated 975 within the NTS Authenticator and Encrypted Extensions extension 976 field. While the body of an NTS Cookie extension field will 977 generally consist of some sort of AEAD output (regardless of whether 978 the recommendations of Section 6 are precisely followed), this is not 979 sufficient to make the extension field "encrypted". 981 The server MAY include additional (non-NTS-related) extension fields 982 which MAY appear prior to the NTS Authenticator and Encrypted 983 Extension Fields extension field (therefore authenticated but not 984 encrypted), within it (therefore encrypted and authenticated), or 985 after it (therefore neither encrypted nor authenticated). In 986 general, however, the client MUST discard any unauthenticated 987 extension fields and process the packet as though they were not 988 present. Clients MAY implement exceptions to this requirement for 989 particular extension fields if their specification explicitly 990 provides for such. 992 Upon receiving an NTS-protected response, the client MUST verify that 993 the Unique Identifier matches that of an outstanding request, and 994 that the packet is authentic under the S2C key associated with that 995 request. If either of these checks fails, the packet MUST be 996 discarded without further processing. 998 If the server is unable to validate the cookie or authenticate the 999 request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC 1000 5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS 1001 negative-acknowledgment (NAK)". It MUST NOT include any NTS Cookie 1002 or NTS Authenticator and Encrypted Extension Fields extension fields. 1004 If the NTP server has previously responded with authentic NTS- 1005 protected NTP packets (i.e., packets containing the NTS Authenticator 1006 and Encrypted Extension Fields extension field), the client MUST 1007 verify that any KoD packets received from the server contain the 1008 Unique Identifier extension field and that the Unique Identifier 1009 matches that of an outstanding request. If this check fails, the 1010 packet MUST be discarded without further processing. If this check 1011 passes, the client MUST comply with RFC 5905, Section 7.4 [RFC5905] 1012 where required. A client MAY automatically re-run the NTS-KE 1013 protocol upon forced disassociation from an NTP server. In that 1014 case, it MUST be able to detect and stop looping between the NTS-KE 1015 and NTP servers by rate limiting the retries using e.g. exponential 1016 retry intervals. 1018 Upon reception of the NTS NAK kiss code, the client SHOULD wait until 1019 the next poll for a valid NTS-protected response and if none is 1020 received, initiate a fresh NTS-KE handshake to try to renegotiate new 1021 cookies, AEAD keys, and parameters. If the NTS-KE handshake 1022 succeeds, the client MUST discard all old cookies and parameters and 1023 use the new ones instead. As long as the NTS-KE handshake has not 1024 succeeded, the client SHOULD continue polling the NTP server using 1025 the cookies and parameters it has. 1027 To allow for NTP session restart when the NTS-KE server is 1028 unavailable and to reduce NTS-KE server load, the client SHOULD keep 1029 at least one unused but recent cookie, AEAD keys, negotiated AEAD 1030 algorithm, and other necessary parameters on persistent storage. 1031 This way, the client is able to resume the NTP session without 1032 performing renewed NTS-KE negotiation. 1034 6. Suggested Format for NTS Cookies 1036 This section is non-normative. It gives a suggested way for servers 1037 to construct NTS cookies. All normative requirements are stated in 1038 Section 4.1.6 and Section 5.4. 1040 The role of cookies in NTS is closely analogous to that of session 1041 cookies in TLS. Accordingly, the thematic resemblance of this 1042 section to RFC 5077 [RFC5077] is deliberate and the reader should 1043 likewise take heed of its security considerations. 1045 Servers should select an AEAD algorithm which they will use to 1046 encrypt and authenticate cookies. The chosen algorithm should be one 1047 such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental 1048 nonce reuse. It need not be the same as the one that was negotiated 1049 with the client. Servers should randomly generate and store a master 1050 AEAD key `K`. Servers should additionally choose a non-secret, unique 1051 value `I` as key-identifier for `K`. 1053 Servers should periodically (e.g., once daily) generate a new pair 1054 (I,K) and immediately switch to using these values for all newly- 1055 generated cookies. Immediately following each such key rotation, 1056 servers should securely erase any keys generated two or more rotation 1057 periods prior. Servers should continue to accept any cookie 1058 generated using keys that they have not yet erased, even if those 1059 keys are no longer current. Erasing old keys provides for forward 1060 secrecy, limiting the scope of what old information can be stolen if 1061 a master key is somehow compromised. Holding on to a limited number 1062 of old keys allows clients to seamlessly transition from one 1063 generation to the next without having to perform a new NTS-KE 1064 handshake. 1066 The need to keep keys synchronized between NTS-KE and NTP servers as 1067 well as across load-balanced clusters can make automatic key rotation 1068 challenging. However, the task can be accomplished without the need 1069 for central key-management infrastructure by using a ratchet, i.e., 1070 making each new key a deterministic, cryptographically pseudo-random 1071 function of its predecessor. A recommended concrete implementation 1072 of this approach is to use HKDF [RFC5869] to derive new keys, using 1073 the key's predecessor as Input Keying Material and its key identifier 1074 as a salt. 1076 To form a cookie, servers should first form a plaintext `P` 1077 consisting of the following fields: 1079 The AEAD algorithm negotiated during NTS-KE. 1081 The S2C key. 1083 The C2S key. 1085 Servers should then generate a nonce `N` uniformly at random, and 1086 form AEAD output `C` by encrypting `P` under key `K` with nonce `N` 1087 and no associated data. 1089 The cookie should consist of the tuple `(I,N,C)`. 1091 To verify and decrypt a cookie provided by the client, first parse it 1092 into its components `I`, `N`, and `C`. Use `I` to look up its 1093 decryption key `K`. If the key whose identifier is `I` has been 1094 erased or never existed, decryption fails; reply with an NTS NAK. 1095 Otherwise, attempt to decrypt and verify ciphertext `C` using key `K` 1096 and nonce `N` with no associated data. If decryption or verification 1097 fails, reply with an NTS NAK. Otherwise, parse out the contents of 1098 the resulting plaintext `P` to obtain the negotiated AEAD algorithm, 1099 S2C key, and C2S key. 1101 7. IANA Considerations 1103 7.1. Service Name and Transport Protocol Port Number Registry 1105 IANA is requested to allocate the following entry in the Service Name 1106 and Transport Protocol Port Number Registry [RFC6335]: 1108 Service Name: ntske 1110 Transport Protocol: tcp 1112 Assignee: IESG 1114 Contact: IETF Chair 1116 Description: Network Time Security Key Exchange 1118 Reference: [[this memo]] 1120 Port Number: [[TBD1]], selected by IANA from the User Port range 1122 [[RFC EDITOR: Replace all instances of [[TBD1]] in this document with 1123 the IANA port assignment.]] 1125 7.2. TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs 1126 Registry 1128 IANA is requested to allocate the following entry in the TLS 1129 Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry 1130 [RFC7301]: 1132 Protocol: Network Time Security Key Establishment, version 1 1134 Identification Sequence: 1135 0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1") 1137 Reference: [[this memo]], Section 4 1139 7.3. TLS Exporter Labels Registry 1141 IANA is requested to allocate the following entry in the TLS Exporter 1142 Labels Registry [RFC5705]: 1144 +--------------------+---------+-------------+---------------+------+ 1145 | Value | DTLS-OK | Recommended | Reference | Note | 1146 +--------------------+---------+-------------+---------------+------+ 1147 | EXPORTER-network- | Y | Y | [[this | | 1148 | time-security/1 | | | memo]], | | 1149 | | | | Section 4.2 | | 1150 +--------------------+---------+-------------+---------------+------+ 1152 7.4. NTP Kiss-o'-Death Codes Registry 1154 IANA is requested to allocate the following entry in the registry of 1155 NTP Kiss-o'-Death Codes [RFC5905]: 1157 +------+---------------------------------------+--------------------+ 1158 | Code | Meaning | Reference | 1159 +------+---------------------------------------+--------------------+ 1160 | NTSN | Network Time Security (NTS) negative- | [[this memo]], | 1161 | | acknowledgment (NAK) | Section 5.7 | 1162 +------+---------------------------------------+--------------------+ 1164 7.5. NTP Extension Field Types Registry 1166 IANA is requested to allocate the following entries in the NTP 1167 Extension Field Types registry [RFC5905]: 1169 +----------+-----------------------------+--------------------------+ 1170 | Field | Meaning | Reference | 1171 | Type | | | 1172 +----------+-----------------------------+--------------------------+ 1173 | [[TBD2]] | Unique Identifier | [[this memo]], | 1174 | | | Section 5.3 | 1175 | [[TBD3]] | NTS Cookie | [[this memo]], Section | 1176 | | | 5.4 | 1177 | [[TBD4]] | NTS Cookie Placeholder | [[this memo]], | 1178 | | | Section 5.5 | 1179 | [[TBD5]] | NTS Authenticator and | [[this memo]], Section | 1180 | | Encrypted Extension Fields | 5.6 | 1181 +----------+-----------------------------+--------------------------+ 1183 [[RFC EDITOR: Replace all instances of [[TBD2]], [[TBD3]], [[TBD4]], 1184 and [[TBD5]] in this document with the respective IANA assignments. 1186 7.6. Network Time Security Key Establishment Record Types Registry 1188 IANA is requested to create a new registry entitled "Network Time 1189 Security Key Establishment Record Types". Entries SHALL have the 1190 following fields: 1192 Record Type Number (REQUIRED): An integer in the range 0-32767 1193 inclusive. 1195 Description (REQUIRED): A short text description of the purpose of 1196 the field. 1198 Reference (REQUIRED): A reference to a document specifying the 1199 semantics of the record. 1201 The policy for allocation of new entries in this registry SHALL vary 1202 by the Record Type Number, as follows: 1204 0-1023: IETF Review 1206 1024-16383: Specification Required 1208 16384-32767: Private and Experimental Use 1210 Applications for new entries SHALL specify the contents of the 1211 Description, Set Critical Bit, and Reference fields as well as which 1212 of the above ranges the Record Type Number should be allocated from. 1213 Applicants MAY request a specific Record Type Number and such 1214 requests MAY be granted at the registrar's discretion. 1216 The initial contents of this registry SHALL be as follows: 1218 +-------------+-------------------------+---------------------------+ 1219 | Record Type | Description | Reference | 1220 | Number | | | 1221 +-------------+-------------------------+---------------------------+ 1222 | 0 | End of Message | [[this memo]], Section | 1223 | | | 4.1.1 | 1224 | 1 | NTS Next Protocol | [[this memo]], | 1225 | | Negotiation | Section 4.1.2 | 1226 | 2 | Error | [[this memo]], Section | 1227 | | | 4.1.3 | 1228 | 3 | Warning | [[this memo]], Section | 1229 | | | 4.1.4 | 1230 | 4 | AEAD Algorithm | [[this memo]], Section | 1231 | | Negotiation | 4.1.5 | 1232 | 5 | New Cookie for NTPv4 | [[this memo]], Section | 1233 | | | 4.1.6 | 1234 | 6 | NTPv4 Server | [[this memo]], Section | 1235 | | Negotiation | 4.1.7 | 1236 | 7 | NTPv4 Port Negotiation | [[this memo]], Section | 1237 | | | 4.1.8 | 1238 | 16384-32767 | Reserved for Private & | [[this memo]] | 1239 | | Experimental Use | | 1240 +-------------+-------------------------+---------------------------+ 1242 7.7. Network Time Security Next Protocols Registry 1244 IANA is requested to create a new registry entitled "Network Time 1245 Security Next Protocols". Entries SHALL have the following fields: 1247 Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive, 1248 functioning as an identifier. 1250 Protocol Name (REQUIRED): A short text string naming the protocol 1251 being identified. 1253 Reference (REQUIRED): A reference to a relevant specification 1254 document. 1256 The policy for allocation of new entries in these registries SHALL 1257 vary by their Protocol ID, as follows: 1259 0-1023: IETF Review 1261 1024-32767: Specification Required 1263 32768-65535: Private and Experimental Use 1265 The initial contents of this registry SHALL be as follows: 1267 +-------------+-------------------------------+---------------------+ 1268 | Protocol ID | Protocol Name | Reference | 1269 +-------------+-------------------------------+---------------------+ 1270 | 0 | Network Time Protocol version | [[this memo]] | 1271 | | 4 (NTPv4) | | 1272 | 32768-65535 | Reserved for Private or | Reserved by [[this | 1273 | | Experimental Use | memo]] | 1274 +-------------+-------------------------------+---------------------+ 1276 7.8. Network Time Security Error and Warning Codes Registries 1278 IANA is requested to create two new registries entitled "Network Time 1279 Security Error Codes" and "Network Time Security Warning Codes". 1280 Entries in each SHALL have the following fields: 1282 Number (REQUIRED): An integer in the range 0-65535 inclusive 1284 Description (REQUIRED): A short text description of the condition. 1286 Reference (REQUIRED): A reference to a relevant specification 1287 document. 1289 The policy for allocation of new entries in these registries SHALL 1290 vary by their Number, as follows: 1292 0-1023: IETF Review 1294 1024-32767: Specification Required 1296 32768-65535: Private and Experimental Use 1298 The initial contents of the Network Time Security Error Codes 1299 Registry SHALL be as follows: 1301 +-------------+------------------------------+----------------------+ 1302 | Number | Description | Reference | 1303 +-------------+------------------------------+----------------------+ 1304 | 0 | Unrecognized Critical | [[this memo]], | 1305 | | Extension | Section 4.1.3 | 1306 | 1 | Bad Request | [[this memo]], | 1307 | | | Section 4.1.3 | 1308 | 2 | Internal Server Error | [[this memo]], | 1309 | | | Section 4.1.3 | 1310 | 32768-65535 | Reserved for Private or | Reserved by [[this | 1311 | | Experimental Use | memo]] | 1312 +-------------+------------------------------+----------------------+ 1313 The Network Time Security Warning Codes Registry SHALL initially be 1314 empty except for the reserved range, i.e.: 1316 +-------------+-------------------------------+---------------------+ 1317 | Number | Description | Reference | 1318 +-------------+-------------------------------+---------------------+ 1319 | 32768-65535 | Reserved for Private or | Reserved by [[this | 1320 | | Experimental Use | memo]] | 1321 +-------------+-------------------------------+---------------------+ 1323 8. Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION 1325 This section records the status of known implementations of the 1326 protocol defined by this specification at the time of posting of this 1327 Internet-Draft, and is based on a proposal described in RFC 7942. 1328 The description of implementations in this section is intended to 1329 assist the IETF in its decision processes in progressing drafts to 1330 RFCs. Please note that the listing of any individual implementation 1331 here does not imply endorsement by the IETF. Furthermore, no effort 1332 has been spent to verify the information presented here that was 1333 supplied by IETF contributors. This is not intended as, and must not 1334 be construed to be, a catalog of available implementations or their 1335 features. Readers are advised to note that other implementations may 1336 exist. 1338 According to RFC 7942, "this will allow reviewers and working groups 1339 to assign due consideration to documents that have the benefit of 1340 running code, which may serve as evidence of valuable experimentation 1341 and feedback that have made the implemented protocols more mature. 1342 It is up to the individual working groups to use this information as 1343 they see fit". 1345 8.1. Implementation 1 1347 Organization: Ostfalia University of Applied Science 1349 Implementor: Martin Langer 1351 Maturity: Proof-of-Concept Prototype 1353 This implementation was used to verify consistency and to ensure 1354 completeness of this specification. 1356 8.1.1. Coverage 1358 This implementation covers the complete specification. 1360 8.1.2. Licensing 1362 The code is released under a Apache License 2.0 license. 1364 The source code is available at: https://gitlab.com/MLanger/nts/ 1366 8.1.3. Contact Information 1368 Contact Martin Langer: mart.langer@ostfalia.de 1370 8.1.4. Last Update 1372 The implementation was updated 25. February 2019. 1374 8.2. Implementation 2 1376 Organization: Netnod 1378 Implementor: Christer Weinigel 1380 Maturity: Proof-of-Concept Prototype 1382 This implementation was used to verify consistency and to ensure 1383 completeness of this specification. 1385 8.2.1. Coverage 1387 This implementation covers the complete specification. 1389 8.2.2. Licensing 1391 The source code is available at: https://github.com/Netnod/nts-poc- 1392 python. 1394 See LICENSE file for details on licensing (BSD 2). 1396 8.2.3. Contact Information 1398 Contact Christer Weinigel: christer@weinigel.se 1400 8.2.4. Last Update 1402 The implementation was updated 31. January 2019. 1404 8.3. Implementation 3 1406 Organization: Red Hat 1408 Implementor: Miroslav Lichvar 1410 Maturity: Prototype 1412 This implementation was used to verify consistency and to ensure 1413 completeness of this specification. 1415 8.3.1. Coverage 1417 This implementation covers the complete specification. 1419 8.3.2. Licensing 1421 Licensing is GPLv2. 1423 The source code is available at: https://github.com/mlichvar/chrony- 1424 nts 1426 8.3.3. Contact Information 1428 Contact Miroslav Lichvar: mlichvar@redhat.com 1430 8.3.4. Last Update 1432 The implementation was updated 28. March 2019. 1434 8.4. Implementation 4 1436 Organization: NTPsec 1438 Implementor: Hal Murray and NTPsec team 1440 Maturity:Looking for testers. Servers running at 1441 ntp1.glypnod.com:123 and ntp2.glypnod.com:123 1443 This implementation was used to verify consistency and to ensure 1444 completeness of this specification. 1446 8.4.1. Coverage 1448 This implementation covers the complete specification. 1450 8.4.2. Licensing 1452 The source code is available at: https://gitlab.com/NTPsec/ntpsec. 1453 Licensing details in LICENSE. 1455 8.4.3. Contact Information 1457 Contact Hal Murray: hmurray@megapathdsl.net, devel@ntpsec.org 1459 8.4.4. Last Update 1461 The implementation was updated 2019-Apr-10. 1463 8.5. Implementation 5 1465 Organization: Cloudflare 1467 Implementor: Watson Ladd 1469 Maturity: 1471 This implementation was used to verify consistency and to ensure 1472 completeness of this specification. 1474 8.5.1. Coverage 1476 This implementation covers the server side of the NTS specification. 1478 8.5.2. Licensing 1480 The source code is available at: https://github.com/wbl/nts-rust 1482 Licensing is ISC (details see LICENSE.txt file). 1484 8.5.3. Contact Information 1486 Contact Watson Ladd: watson@cloudflare.com 1488 8.5.4. Last Update 1490 The implementation was updated 21. March 2019. 1492 8.6. Implementation 6 1494 Organization: Netnod 1496 Implementor: Michael Cardell Widerkrantz et. al. 1498 Maturity: Early proof of concept 1500 8.6.1. Coverage 1502 NTS-KE client and server. 1504 8.6.2. Licensing 1506 ???? 1508 The source code is available at: https://github.com/mchackorg/gonts 1510 8.6.3. Contact Information 1512 Contact Michael Cardell Widerkrantz: mc@netnod.se 1514 8.6.4. Last Update 1516 The implementation was updated 24. March 2019. 1518 8.7. Interoperability 1520 The Interoperability tests distinguished between NTS key 1521 establishment protocol and NTS time exchange messages. For the 1522 implementations 1, 2, 3, and 4 pairwise interoperability of the NTS 1523 key establishment protocol and exchange of NTS protected NTP messages 1524 have been verified successfully. The implementation 2 was able to 1525 successfully perform the key establishment protocol against the 1526 server side of the implementation 5. 1528 These tests successfully demonstrate that there are at least four 1529 running implementations of this draft which are able to interoperate. 1531 9. Security Considerations 1533 9.1. Sensitivity to DDoS attacks 1535 The introduction of NTS brings with it the introduction of asymmetric 1536 cryptography to NTP. Asymmetric cryptography is necessary for 1537 initial server authentication and AEAD key extraction. Asymmetric 1538 cryptosystems are generally orders of magnitude slower than their 1539 symmetric counterparts. This makes it much harder to build systems 1540 that can serve requests at a rate corresponding to the full line 1541 speed of the network connection. This, in turn, opens up a new 1542 possibility for DDoS attacks on NTP services. 1544 The main protection against these attacks in NTS lies in that the use 1545 of asymmetric cryptosystems is only necessary in the initial NTS-KE 1546 phase of the protocol. Since the protocol design enables separation 1547 of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE 1548 server separated from the NTP service it supports will not affect NTP 1549 users that have already performed initial authentication, AEAD key 1550 extraction, and cookie exchange. 1552 NTS users should also consider that they are not fully protected 1553 against DDoS attacks by on-path adversaries. In addition to dropping 1554 packets and attacks such as those described in Section 9.4, an on- 1555 path attacker can send spoofed kiss-o'-death replies, which are not 1556 authenticated, in response to NTP requests. This could result in 1557 significantly increased load on the NTS-KE server. Implementers have 1558 to weigh the user's need for unlinkability against the added 1559 resilience that comes with cookie reuse in cases of NTS-KE server 1560 unavailability. 1562 9.2. Avoiding DDoS Amplification 1564 Certain non-standard and/or deprecated features of the Network Time 1565 Protocol enable clients to send a request to a server which causes 1566 the server to send a response much larger than the request. Servers 1567 which enable these features can be abused in order to amplify traffic 1568 volume in DDoS attacks by sending them a request with a spoofed 1569 source IP. In recent years, attacks of this nature have become an 1570 endemic nuisance. 1572 NTS is designed to avoid contributing any further to this problem by 1573 ensuring that NTS-related extension fields included in server 1574 responses will be the same size as the NTS-related extension fields 1575 sent by the client. In particular, this is why the client is 1576 required to send a separate and appropriately padded-out NTS Cookie 1577 Placeholder extension field for every cookie it wants to get back, 1578 rather than being permitted simply to specify a desired quantity. 1580 Due to the RFC 7822 [RFC7822] requirement that extensions be padded 1581 and aligned to four-octet boundaries, response size may still in some 1582 cases exceed request size by up to three octets. This is 1583 sufficiently inconsequential that we have declined to address it. 1585 9.3. Initial Verification of Server Certificates 1587 NTS's security goals are undermined if the client fails to verify 1588 that the X.509 certificate chain presented by the NTS-KE server is 1589 valid and rooted in a trusted certificate authority. RFC 5280 1590 [RFC5280] and RFC 6125 [RFC6125] specify how such verification is to 1591 be performed in general. However, the expectation that the client 1592 does not yet have a correctly-set system clock at the time of 1593 certificate verification presents difficulties with verifying that 1594 the certificate is within its validity period, i.e., that the current 1595 time lies between the times specified in the certificate's notBefore 1596 and notAfter fields. It may be operationally necessary in some cases 1597 for a client to accept a certificate which appears to be expired or 1598 not yet valid. While there is no perfect solution to this problem, 1599 there are several mitigations the client can implement to make it 1600 more difficult for an adversary to successfully present an expired 1601 certificate: 1603 Check whether the system time is in fact unreliable. If the 1604 system clock has previously been synchronized since last boot, 1605 then on operating systems which implement a kernel-based phase- 1606 locked-loop API, a call to ntp_gettime() should show a maximum 1607 error less than NTP_PHASE_MAX. In this case, the clock SHOULD be 1608 considered reliable and certificates can be strictly validated. 1610 Allow the system administrator to specify that certificates should 1611 *always* be strictly validated. Such a configuration is 1612 appropriate on systems which have a battery-backed clock and which 1613 can reasonably prompt the user to manually set an approximately- 1614 correct time if it appears to be needed. 1616 Once the clock has been synchronized, periodically write the 1617 current system time to persistent storage. Do not accept any 1618 certificate whose notAfter field is earlier than the last recorded 1619 time. 1621 NTP time replies are expected to be consistent with the NTS-KE TLS 1622 certificate validity period, i.e. time replies received 1623 immediately after an NTS-KE handshake are expected to lie within 1624 the certificate validity period. Implementations are recommended 1625 to check that this is the case. Performing a new NTS-KE handshake 1626 based solely on the fact that the certificate used by the NTS-KE 1627 server in a previous handshake has expired is normally not 1628 necessary. Clients that still wish to do this must take care not 1629 to cause an inadvertent denial-of-service attack on the NTS-KE 1630 server, for example by picking a random time in the week preceding 1631 certificate expiry to perform the new handshake. 1633 Use multiple time sources. The ability to pass off an expired 1634 certificate is only useful to an adversary who has compromised the 1635 corresponding private key. If the adversary has compromised only 1636 a minority of servers, NTP's selection algorithm (RFC 5905 section 1637 11.2.1 [RFC5905]) will protect the client from accepting bad time 1638 from the adversary-controlled servers. 1640 9.4. Delay Attacks 1642 In a packet delay attack, an adversary with the ability to act as a 1643 man-in-the-middle delays time synchronization packets between client 1644 and server asymmetrically [RFC7384]. Since NTP's formula for 1645 computing time offset relies on the assumption that network latency 1646 is roughly symmetrical, this leads to the client to compute an 1647 inaccurate value [Mizrahi]. The delay attack does not reorder or 1648 modify the content of the exchanged synchronization packets. 1649 Therefore, cryptographic means do not provide a feasible way to 1650 mitigate this attack. However, the maximum error that an adversary 1651 can introduce is bounded by half of the round trip delay. 1653 RFC 5905 [RFC5905] specifies a parameter called MAXDIST which denotes 1654 the maximum round-trip latency (including not only the immediate 1655 round trip between client and server, but the whole distance back to 1656 the reference clock as reported in the Root Delay field) that a 1657 client will tolerate before concluding that the server is unsuitable 1658 for synchronization. The standard value for MAXDIST is one second, 1659 although some implementations use larger values. Whatever value a 1660 client chooses, the maximum error which can be introduced by a delay 1661 attack is MAXDIST/2. 1663 Usage of multiple time sources, or multiple network paths to a given 1664 time source [Shpiner], may also serve to mitigate delay attacks if 1665 the adversary is in control of only some of the paths. 1667 9.5. Random Number Generation 1669 At various points in NTS, the generation of cryptographically secure 1670 random numbers is required. Whenever this draft specifies the use of 1671 random numbers, cryptographically secure random number generation 1672 MUST be used. RFC 4086 [RFC4086] contains guidelines concerning this 1673 topic. 1675 9.6. NTS Stripping 1677 Implementers must be aware of the possibility of "NTS stripping" 1678 attacks, where an attacker tricks clients into reverting to plain 1679 NTP. Naive client implementations might, for example, revert 1680 automatically to plain NTP if the NTS-KE handshake fails. A man-in- 1681 the-middle attacker can easily cause this to happen. Even clients 1682 that already hold valid cookies can be vulnerable, since an attacker 1683 can force a client to repeat the NTS-KE handshake by sending faked 1684 NTP mode 4 replies with the NTS NAK kiss code. Forcing a client to 1685 repeat the NTS-KE handshake can also be the first step in more 1686 advanced attacks. 1688 For the reasons described here, implementations SHOULD NOT revert 1689 from NTS-protected to unprotected NTP with any server without 1690 explicit user action. 1692 10. Privacy Considerations 1694 10.1. Unlinkability 1696 Unlinkability prevents a device from being tracked when it changes 1697 network addresses (e.g. because said device moved between different 1698 networks). In other words, unlinkability thwarts an attacker that 1699 seeks to link a new network address used by a device with a network 1700 address that it was formerly using, because of recognizable data that 1701 the device persistently sends as part of an NTS-secured NTP 1702 association. This is the justification for continually supplying the 1703 client with fresh cookies, so that a cookie never represents 1704 recognizable data in the sense outlined above. 1706 NTS's unlinkability objective is merely to not leak any additional 1707 data that could be used to link a device's network address. NTS does 1708 not rectify legacy linkability issues that are already present in 1709 NTP. Thus, a client that requires unlinkability must also minimize 1710 information transmitted in a client query (mode 3) packet as 1711 described in the draft [I-D.ietf-ntp-data-minimization]. 1713 The unlinkability objective only holds for time synchronization 1714 traffic, as opposed to key exchange traffic. This implies that it 1715 cannot be guaranteed for devices that function not only as time 1716 clients, but also as time servers (because the latter can be 1717 externally triggered to send authentication data). 1719 It should also be noted that it could be possible to link devices 1720 that operate as time servers from their time synchronization traffic, 1721 using information exposed in (mode 4) server response packets (e.g. 1722 reference ID, reference time, stratum, poll). Also, devices that 1723 respond to NTP control queries could be linked using the information 1724 revealed by control queries. 1726 Note that the unlinkability objective does not prevent a client 1727 device to be tracked by its time servers. 1729 10.2. Confidentiality 1731 NTS does not protect the confidentiality of information in NTP's 1732 header fields. When clients implement 1733 [I-D.ietf-ntp-data-minimization], client packet headers do not 1734 contain any information which the client could conceivably wish to 1735 keep secret: one field is random, and all others are fixed. 1737 Information in server packet headers is likewise public: the origin 1738 timestamp is copied from the client's (random) transmit timestamp, 1739 and all other fields are set the same regardless of the identity of 1740 the client making the request. 1742 Future extension fields could hypothetically contain sensitive 1743 information, in which case NTS provides a mechanism for encrypting 1744 them. 1746 11. Acknowledgements 1748 The authors would like to thank Richard Barnes, Steven Bellovin, 1749 Patrik Faeltstroem (Faltstrom), Scott Fluhrer, Sharon Goldberg, Russ 1750 Housley, Martin Langer, Miroslav Lichvar, Aanchal Malhotra, Dave 1751 Mills, Danny Mayer, Karen O'Donoghue, Eric K. Rescorla, Stephen 1752 Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Sniffen, Susan 1753 Sons, Douglas Stebila, Harlan Stenn, Joachim Stroembergsson 1754 (Strombergsson), Martin Thomson, Richard Welty, and Christer Weinigel 1755 for contributions to this document and comments on the design of NTS. 1757 12. References 1759 12.1. Normative References 1761 [ANSI.X3-4.1986] 1762 American National Standards Institute, "Coded Character 1763 Set - 7-bit American Standard Code for Information 1764 Interchange", ANSI X3.4, 1986. 1766 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1767 Requirement Levels", BCP 14, RFC 2119, 1768 DOI 10.17487/RFC2119, March 1997, 1769 . 1771 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1772 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 1773 2006, . 1775 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated 1776 Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, 1777 . 1779 [RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV) 1780 Authenticated Encryption Using the Advanced Encryption 1781 Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October 1782 2008, . 1784 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport 1785 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, 1786 March 2010, . 1788 [RFC5891] Klensin, J., "Internationalized Domain Names in 1789 Applications (IDNA): Protocol", RFC 5891, 1790 DOI 10.17487/RFC5891, August 2010, 1791 . 1793 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 1794 "Network Time Protocol Version 4: Protocol and Algorithms 1795 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 1796 . 1798 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 1799 Verification of Domain-Based Application Service Identity 1800 within Internet Public Key Infrastructure Using X.509 1801 (PKIX) Certificates in the Context of Transport Layer 1802 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 1803 2011, . 1805 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1806 Cheshire, "Internet Assigned Numbers Authority (IANA) 1807 Procedures for the Management of the Service Name and 1808 Transport Protocol Port Number Registry", BCP 165, 1809 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1810 . 1812 [RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing 1813 IPv6 Zone Identifiers in Address Literals and Uniform 1814 Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874, 1815 February 2013, . 1817 [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, 1818 "Transport Layer Security (TLS) Application-Layer Protocol 1819 Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, 1820 July 2014, . 1822 [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher 1823 Suite Value (SCSV) for Preventing Protocol Downgrade 1824 Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, 1825 . 1827 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1828 "Recommendations for Secure Use of Transport Layer 1829 Security (TLS) and Datagram Transport Layer Security 1830 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1831 2015, . 1833 [RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4 1834 (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822, 1835 March 2016, . 1837 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1838 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1839 May 2017, . 1841 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1842 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1843 . 1845 12.2. Informative References 1847 [I-D.ietf-ntp-data-minimization] 1848 Franke, D. and A. Malhotra, "NTP Client Data 1849 Minimization", draft-ietf-ntp-data-minimization-04 (work 1850 in progress), March 2019. 1852 [Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks 1853 against time synchronization protocols", in Proceedings 1854 of Precision Clock Synchronization for Measurement Control 1855 and Communication, ISPCS 2012, pp. 1-6, September 2012. 1857 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1858 DOI 10.17487/RFC0768, August 1980, 1859 . 1861 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1862 RFC 793, DOI 10.17487/RFC0793, September 1981, 1863 . 1865 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 1866 "Randomness Requirements for Security", BCP 106, RFC 4086, 1867 DOI 10.17487/RFC4086, June 2005, 1868 . 1870 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1871 "Transport Layer Security (TLS) Session Resumption without 1872 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1873 January 2008, . 1875 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1876 Housley, R., and W. Polk, "Internet X.509 Public Key 1877 Infrastructure Certificate and Certificate Revocation List 1878 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 1879 . 1881 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 1882 Key Derivation Function (HKDF)", RFC 5869, 1883 DOI 10.17487/RFC5869, May 2010, 1884 . 1886 [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in 1887 Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, 1888 October 2014, . 1890 [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time 1891 Protocols", in Proceedings of IEEE International Symposium 1892 on Precision Clock Synchronization for Measurement, 1893 Control and Communication (ISPCS), September 2013. 1895 Appendix A. Terms and Abbreviations 1897 AEAD Authenticated Encryption with Associated Data [RFC5116] 1899 ALPN Application-Layer Protocol Negotiation [RFC7301] 1901 C2S Client-to-server 1903 DDoS Distributed Denial-of-Service 1905 EF Extension Field [RFC5905] 1907 HKDF Hashed Message Authentication Code-based Key Derivation 1908 Function [RFC5869] 1910 IANA Internet Assigned Numbers Authority 1912 IP Internet Protocol 1914 KoD Kiss-o'-Death [RFC5905] 1916 NTP Network Time Protocol [RFC5905] 1918 NTS Network Time Security 1920 NTS-KE Network Time Security Key Exchange 1922 S2C Server-to-client 1924 SCSV Signaling Cipher Suite Value [RFC7507] 1926 TCP Transmission Control Protocol [RFC0793] 1928 TLS Transport Layer Security [RFC8446] 1929 UDP User Datagram Protocol [RFC0768] 1931 Authors' Addresses 1933 Daniel Fox Franke 1934 Akamai Technologies 1935 150 Broadway 1936 Cambridge, MA 02142 1937 United States 1939 Email: dafranke@akamai.com 1940 URI: https://www.dfranke.us 1942 Dieter Sibold 1943 Physikalisch-Technische 1944 Bundesanstalt 1945 Bundesallee 100 1946 Braunschweig D-38116 1947 Germany 1949 Phone: +49-(0)531-592-8420 1950 Fax: +49-531-592-698420 1951 Email: dieter.sibold@ptb.de 1953 Kristof Teichel 1954 Physikalisch-Technische 1955 Bundesanstalt 1956 Bundesallee 100 1957 Braunschweig D-38116 1958 Germany 1960 Phone: +49-(0)531-592-4471 1961 Email: kristof.teichel@ptb.de 1963 Marcus Dansarie 1965 Email: marcus@dansarie.se 1967 Ragnar Sundblad 1968 Netnod 1970 Email: ragge@netnod.se