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'I-D.ietf-httpbis-semantics' ** Obsolete normative reference: RFC 4492 (Obsoleted by RFC 8422) ** Downref: Normative reference to an Informational RFC: RFC 4949 ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) ** Obsolete normative reference: RFC 6347 (Obsoleted by RFC 9147) ** Obsolete normative reference: RFC 8740 (Obsoleted by RFC 9113) == Outdated reference: A later version (-18) exists of draft-ietf-tls-esni-13 == Outdated reference: A later version (-08) exists of draft-irtf-cfrg-aead-limits-03 -- Obsolete informational reference (is this intentional?): RFC 2246 (Obsoleted by RFC 4346) -- Obsolete informational reference (is this intentional?): RFC 4346 (Obsoleted by RFC 5246) -- Obsolete informational reference (is this intentional?): RFC 4347 (Obsoleted by RFC 6347) -- Obsolete informational reference (is this intentional?): RFC 5077 (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 6961 (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 7507 (Obsoleted by RFC 8996) -- Obsolete informational reference (is this intentional?): RFC 7525 (Obsoleted by RFC 9325) Summary: 7 errors (**), 0 flaws (~~), 4 warnings (==), 13 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 UTA Working Group Y. Sheffer 3 Internet-Draft Intuit 4 Obsoletes: 7525 (if approved) R. Holz 5 Updates: 5288, 6066 (if approved) University of Twente 6 Intended status: Best Current Practice P. Saint-Andre 7 Expires: 27 May 2022 Mozilla 8 T. Fossati 9 arm 10 23 November 2021 12 Recommendations for Secure Use of Transport Layer Security (TLS) and 13 Datagram Transport Layer Security (DTLS) 14 draft-ietf-uta-rfc7525bis-04 16 Abstract 18 Transport Layer Security (TLS) and Datagram Transport Layer Security 19 (DTLS) are widely used to protect data exchanged over application 20 protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the 21 last few years, several serious attacks on TLS have emerged, 22 including attacks on its most commonly used cipher suites and their 23 modes of operation. This document provides recommendations for 24 improving the security of deployed services that use TLS and DTLS. 25 The recommendations are applicable to the majority of use cases. 27 This document was published as RFC 7525 when the industry was in the 28 midst of its transition to TLS 1.2. Years later this transition is 29 largely complete and TLS 1.3 is widely available. Given the new 30 environment, we believe new guidance is needed. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on 27 May 2022. 49 Copyright Notice 51 Copyright (c) 2021 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 56 license-info) in effect on the date of publication of this document. 57 Please review these documents carefully, as they describe your rights 58 and restrictions with respect to this document. Code Components 59 extracted from this document must include Revised BSD License text as 60 described in Section 4.e of the Trust Legal Provisions and are 61 provided without warranty as described in the Revised BSD License. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 66 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 67 3. General Recommendations . . . . . . . . . . . . . . . . . . . 5 68 3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5 69 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 5 70 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6 71 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7 72 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7 73 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8 74 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8 75 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9 76 3.6. Post-Handshake Authentication . . . . . . . . . . . . . . 9 77 3.7. Server Name Indication . . . . . . . . . . . . . . . . . 10 78 3.8. Application-Layer Protocol Negotiation . . . . . . . . . 10 79 3.9. Zero Round Trip Time (0-RTT) Data in TLS 1.3 . . . . . . 11 80 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 11 81 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 11 82 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 13 83 4.2.1. Implementation Details . . . . . . . . . . . . . . . 13 84 4.3. Cipher Suites for TLS 1.3 . . . . . . . . . . . . . . . . 14 85 4.4. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 14 86 4.5. Public Key Length . . . . . . . . . . . . . . . . . . . . 15 87 4.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 16 88 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 16 89 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 17 90 5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 18 91 6. Security Considerations . . . . . . . . . . . . . . . . . . . 18 92 6.1. Host Name Validation . . . . . . . . . . . . . . . . . . 18 93 6.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 19 94 6.2.1. Nonce Reuse in TLS 1.2 . . . . . . . . . . . . . . . 19 95 6.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 20 96 6.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 21 97 6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 22 98 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 99 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 100 8.1. Normative References . . . . . . . . . . . . . . . . . . 23 101 8.2. Informative References . . . . . . . . . . . . . . . . . 26 102 Appendix A. Differences from RFC 7525 . . . . . . . . . . . . . 32 103 Appendix B. Document History . . . . . . . . . . . . . . . . . . 33 104 B.1. draft-ietf-uta-rfc7525bis-04 . . . . . . . . . . . . . . 33 105 B.2. draft-ietf-uta-rfc7525bis-03 . . . . . . . . . . . . . . 33 106 B.3. draft-ietf-uta-rfc7525bis-02 . . . . . . . . . . . . . . 33 107 B.4. draft-ietf-uta-rfc7525bis-01 . . . . . . . . . . . . . . 33 108 B.5. draft-ietf-uta-rfc7525bis-00 . . . . . . . . . . . . . . 34 109 B.6. draft-sheffer-uta-rfc7525bis-00 . . . . . . . . . . . . . 34 110 B.7. draft-sheffer-uta-bcp195bis-00 . . . . . . . . . . . . . 34 111 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 113 1. Introduction 115 Transport Layer Security (TLS) [RFC5246] and Datagram Transport 116 Security Layer (DTLS) [RFC6347] are widely used to protect data 117 exchanged over application protocols such as HTTP, SMTP, IMAP, POP, 118 SIP, and XMPP. Over the years leading to 2015, several serious 119 attacks on TLS have emerged, including attacks on its most commonly 120 used cipher suites and their modes of operation. For instance, both 121 the AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which 122 together have been the most widely deployed ciphers, have been 123 attacked in the context of TLS. A companion document [RFC7457] 124 provides detailed information about these attacks and will help the 125 reader understand the rationale behind the recommendations provided 126 here. 128 The TLS community reacted to these attacks in two ways: 130 * Detailed guidance was published on the use of TLS 1.2 and earlier 131 protocol versions. This guidance is included in the original 132 [RFC7525] and mostly retained in this revised version. 134 * A new protocol version was released, TLS 1.3 [RFC8446], which 135 largely mitigates or resolves these attacks. 137 Those who implement and deploy TLS and DTLS, in particular versions 138 1.2 or earlier of these protocols, need guidance on how TLS can be 139 used securely. This document provides guidance for deployed services 140 as well as for software implementations, assuming the implementer 141 expects his or her code to be deployed in environments defined in 142 Section 5. Concerning deployment, this document targets a wide 143 audience -- namely, all deployers who wish to add authentication (be 144 it one-way only or mutual), confidentiality, and data integrity 145 protection to their communications. 147 The recommendations herein take into consideration the security of 148 various mechanisms, their technical maturity and interoperability, 149 and their prevalence in implementations at the time of writing. 150 Unless it is explicitly called out that a recommendation applies to 151 TLS alone or to DTLS alone, each recommendation applies to both TLS 152 and DTLS. 154 This document attempts to minimize new guidance to TLS 1.2 155 implementations, and the overall approach is to encourage systems to 156 move to TLS 1.3. However this is not always practical. Newly 157 discovered attacks, as well as ecosystem changes, necessitated some 158 new requirements that apply to TLS 1.2 environments. Those are 159 summarized in Appendix A. 161 As noted, the TLS 1.3 specification resolves many of the 162 vulnerabilities listed in this document. A system that deploys TLS 163 1.3 should have fewer vulnerabilities than TLS 1.2 or below. This 164 document is being republished with this in mind, and with an explicit 165 goal to migrate most uses of TLS 1.2 into TLS 1.3. 167 These are minimum recommendations for the use of TLS in the vast 168 majority of implementation and deployment scenarios, with the 169 exception of unauthenticated TLS (see Section 5). Other 170 specifications that reference this document can have stricter 171 requirements related to one or more aspects of the protocol, based on 172 their particular circumstances (e.g., for use with a particular 173 application protocol); when that is the case, implementers are 174 advised to adhere to those stricter requirements. Furthermore, this 175 document provides a floor, not a ceiling, so stronger options are 176 always allowed (e.g., depending on differing evaluations of the 177 importance of cryptographic strength vs. computational load). 179 Community knowledge about the strength of various algorithms and 180 feasible attacks can change quickly, and experience shows that a Best 181 Current Practice (BCP) document about security is a point-in-time 182 statement. Readers are advised to seek out any errata or updates 183 that apply to this document. 185 2. Terminology 187 A number of security-related terms in this document are used in the 188 sense defined in [RFC4949]. 190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 192 "OPTIONAL" in this document are to be interpreted as described in 193 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 194 capitals, as shown here. 196 3. General Recommendations 198 This section provides general recommendations on the secure use of 199 TLS. Recommendations related to cipher suites are discussed in the 200 following section. 202 3.1. Protocol Versions 204 3.1.1. SSL/TLS Protocol Versions 206 It is important both to stop using old, less secure versions of SSL/ 207 TLS and to start using modern, more secure versions; therefore, the 208 following are the recommendations concerning TLS/SSL protocol 209 versions: 211 * Implementations MUST NOT negotiate SSL version 2. 213 Rationale: Today, SSLv2 is considered insecure [RFC6176]. 215 * Implementations MUST NOT negotiate SSL version 3. 217 Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and 218 plugged some significant security holes but did not support strong 219 cipher suites. SSLv3 does not support TLS extensions, some of 220 which (e.g., renegotiation_info [RFC5746]) are security-critical. 221 In addition, with the emergence of the POODLE attack [POODLE], 222 SSLv3 is now widely recognized as fundamentally insecure. See 223 [DEP-SSLv3] for further details. 225 * Implementations MUST NOT negotiate TLS version 1.0 [RFC2246]. 227 Rationale: TLS 1.0 (published in 1999) does not support many 228 modern, strong cipher suites. In addition, TLS 1.0 lacks a per- 229 record Initialization Vector (IV) for CBC-based cipher suites and 230 does not warn against common padding errors. This and other 231 recommendations in this section are in line with [RFC8996]. 233 * Implementations MUST NOT negotiate TLS version 1.1 [RFC4346]. 235 Rationale: TLS 1.1 (published in 2006) is a security improvement 236 over TLS 1.0 but still does not support certain stronger cipher 237 suites. 239 NOTE: This recommendation has been changed from SHOULD NOT to MUST 240 NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate] 241 will be published as an RFC before this document. 243 * Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to 244 negotiate TLS version 1.2 over earlier versions of TLS. 246 Rationale: Several stronger cipher suites are available only with 247 TLS 1.2 (published in 2008). In fact, the cipher suites 248 recommended by this document for TLS 1.2 (Section 4.2 below) are 249 only available in this version. 251 * Implementations SHOULD support TLS 1.3 [RFC8446] and if 252 implemented, MUST prefer to negotiate TLS 1.3 over earlier 253 versions of TLS. 255 Rationale: TLS 1.3 is a major overhaul to the protocol and 256 resolves many of the security issues with TLS 1.2. We note that 257 as long as TLS 1.2 is still allowed by a particular 258 implementation, even if it defaults to TLS 1.3, implementers MUST 259 still follow all the recommendations in this document. 261 * Implementations of "greenfield" protocols or deployments, where 262 there is no need to support legacy endpoints, SHOULD support TLS 263 1.3, with no negotiation of earlier versions. Similarly, we 264 RECOMMEND that new protocol designs that embed the TLS mechanisms 265 (such as QUIC has done [RFC9001]) include TLS 1.3. 267 Rationale: secure deployment of TLS 1.3 is significantly easier 268 and less error prone than the secure deployment of TLS 1.2. 270 This BCP applies to TLS 1.2, 1.3 and to earlier versions. It is not 271 safe for readers to assume that the recommendations in this BCP apply 272 to any future version of TLS. 274 3.1.2. DTLS Protocol Versions 276 DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS 277 1.1 was published. The following are the recommendations with 278 respect to DTLS: 280 * Implementations MUST NOT negotiate DTLS version 1.0 [RFC4347]. 282 Version 1.0 of DTLS correlates to version 1.1 of TLS (see above). 284 * Implementations MUST support and (unless a higher version is 285 available) MUST prefer to negotiate DTLS version 1.2 [RFC6347] 287 Version 1.2 of DTLS correlates to version 1.2 of TLS (see above). 288 (There is no version 1.1 of DTLS.) 290 * Implementations SHOULD support and, if available, MUST prefer to 291 negotiate DTLS version 1.3 as specified in [I-D.ietf-tls-dtls13]. 293 Version 1.3 of DTLS correlates to version 1.3 of TLS (see above). 295 3.1.3. Fallback to Lower Versions 297 TLS/DTLS 1.2 clients MUST NOT fall back to earlier TLS versions, 298 since those versions have been deprecated [RFC8996]. We note that as 299 a result of that, the SCSV mechanism [RFC7507] is no longer needed 300 for clients. In addition, TLS 1.3 implements a new version 301 negotiation mechanism. 303 3.2. Strict TLS 305 The following recommendations are provided to help prevent SSL 306 Stripping (an attack that is summarized in Section 2.1 of [RFC7457]): 308 * In cases where an application protocol allows implementations or 309 deployments a choice between strict TLS configuration and dynamic 310 upgrade from unencrypted to TLS-protected traffic (such as 311 STARTTLS), clients and servers SHOULD prefer strict TLS 312 configuration. 314 * Application protocols typically provide a way for the server to 315 offer TLS during an initial protocol exchange, and sometimes also 316 provide a way for the server to advertise support for TLS (e.g., 317 through a flag indicating that TLS is required); unfortunately, 318 these indications are sent before the communication channel is 319 encrypted. A client SHOULD attempt to negotiate TLS even if these 320 indications are not communicated by the server. 322 * HTTP client and server implementations MUST support the HTTP 323 Strict Transport Security (HSTS) header [RFC6797], in order to 324 allow Web servers to advertise that they are willing to accept 325 TLS-only clients. 327 * Web servers SHOULD use HSTS to indicate that they are willing to 328 accept TLS-only clients, unless they are deployed in such a way 329 that using HSTS would in fact weaken overall security (e.g., it 330 can be problematic to use HSTS with self-signed certificates, as 331 described in Section 11.3 of [RFC6797]). 333 Rationale: Combining unprotected and TLS-protected communication 334 opens the way to SSL Stripping and similar attacks, since an initial 335 part of the communication is not integrity protected and therefore 336 can be manipulated by an attacker whose goal is to keep the 337 communication in the clear. 339 3.3. Compression 341 In order to help prevent compression-related attacks (summarized in 342 Section 2.6 of [RFC7457]), when using TLS 1.2 implementations and 343 deployments SHOULD disable TLS-level compression (Section 6.2.2 of 344 [RFC5246]), unless the application protocol in question has been 345 shown not to be open to such attacks. Note: this recommendation 346 applies to TLS 1.2 only, because compression has been removed from 347 TLS 1.3. 349 Rationale: TLS compression has been subject to security attacks, such 350 as the CRIME attack. 352 Implementers should note that compression at higher protocol levels 353 can allow an active attacker to extract cleartext information from 354 the connection. The BREACH attack is one such case. These issues 355 can only be mitigated outside of TLS and are thus outside the scope 356 of this document. See Section 2.6 of [RFC7457] for further details. 358 3.4. TLS Session Resumption 360 Session resumption drastically reduces the number of TLS handshakes 361 and thus is an essential performance feature for most deployments. 363 Stateless session resumption with session tickets is a popular 364 strategy. For TLS 1.2, it is specified in [RFC5077]. For TLS 1.3, 365 an equivalent PSK-based mechanism is described in Section 4.6.1 of 366 [RFC8446]. When it is used, the resumption information MUST be 367 authenticated and encrypted to prevent modification or eavesdropping 368 by an attacker. Further recommendations apply to session tickets: 370 * A strong cipher suite MUST be used when encrypting the ticket (as 371 least as strong as the main TLS cipher suite). 373 * Ticket keys MUST be changed regularly, e.g., once every week, so 374 as not to negate the benefits of forward secrecy (see Section 6.3 375 for details on forward secrecy). 377 * For similar reasons, session ticket validity SHOULD be limited to 378 a reasonable duration (e.g., half as long as ticket key validity). 380 Rationale: session resumption is another kind of TLS handshake, and 381 therefore must be as secure as the initial handshake. This document 382 (Section 4) recommends the use of cipher suites that provide forward 383 secrecy, i.e. that prevent an attacker who gains momentary access to 384 the TLS endpoint (either client or server) and its secrets from 385 reading either past or future communication. The tickets must be 386 managed so as not to negate this security property. 388 TLS 1.3 provides the powerful option of forward secrecy even within a 389 long-lived connection that is periodically resumed. Section 2.2 of 390 [RFC8446] recommends that clients SHOULD send a "key_share" when 391 initiating session resumption. In order to gain forward secrecy, 392 this document recommends that server implementations SHOULD respond 393 with a "key_share", to complete an ECDHE exchange on each session 394 resumption. 396 TLS session resumption introduces potential privacy issues where the 397 server is able to track the client, in some cases indefinitely. See 398 [Sy2018] for more details. 400 3.5. TLS Renegotiation 402 Where handshake renegotiation is implemented, both clients and 403 servers MUST implement the renegotiation_info extension, as defined 404 in [RFC5746]. Note: this recommendation applies to TLS 1.2 only, 405 because renegotiation has been removed from TLS 1.3. 407 A related attack resulting from TLS session parameters not properly 408 authenticated is Triple Handshake [triple-handshake]. To address 409 this attack, TLS 1.2 implementations SHOULD support the 410 extended_master_secret extension defined in [RFC7627]. 412 3.6. Post-Handshake Authentication 414 Renegotiation in TLS 1.2 was replaced in TLS 1.3 by separate post- 415 handshake authentication and key update mechanisms. In the context 416 of protocols that multiplex requests over a single connection (such 417 as HTTP/2), post-handshake authentication has the same problems as 418 TLS 1.2 renegotiation. Multiplexed protocols SHOULD follow the 419 advice provided for HTTP/2 in [RFC8740]. 421 3.7. Server Name Indication 423 TLS implementations MUST support the Server Name Indication (SNI) 424 extension defined in Section 3 of [RFC6066] for those higher-level 425 protocols that would benefit from it, including HTTPS. However, the 426 actual use of SNI in particular circumstances is a matter of local 427 policy. Implementers are strongly encouraged to support TLS 428 Encrypted Client Hello (formerly called Encrypted SNI) once 429 [I-D.ietf-tls-esni] has been standardized. 431 Rationale: SNI supports deployment of multiple TLS-protected virtual 432 servers on a single address, and therefore enables fine-grained 433 security for these virtual servers, by allowing each one to have its 434 own certificate. However, SNI also leaks the target domain for a 435 given connection; this information leak will be plugged by use of TLS 436 Encrypted Client Hello. 438 In order to prevent the attacks described in [ALPACA], a server that 439 does not recognize the presented server name SHOULD NOT continue the 440 handshake and instead fail with a fatal-level unrecognized_name(112) 441 alert. Note that this recommendation updates Section 3 of [RFC6066]: 442 "If the server understood the ClientHello extension but does not 443 recognize the server name, the server SHOULD take one of two actions: 444 either abort the handshake by sending a fatal-level 445 unrecognized_name(112) alert or continue the handshake." It is also 446 RECOMMENDED that clients abort the handshake if the server 447 acknowledges the SNI hostname with a different hostname than the one 448 sent by the client. 450 3.8. Application-Layer Protocol Negotiation 452 TLS implementations (both client- and server-side) MUST support the 453 Application-Layer Protocol Negotiation (ALPN) extension [RFC7301]. 455 In order to prevent "cross-protocol" attacks resulting from failure 456 to ensure that a message intended for use in one protocol cannot be 457 mistaken for a message for use in another protocol, servers should 458 strictly enforce the behavior prescribed in Section 3.2 of [RFC7301]: 459 "In the event that the server supports no protocols that the client 460 advertises, then the server SHALL respond with a fatal 461 no_application_protocol alert." It is also RECOMMENDED that clients 462 abort the handshake if the server acknowledges the ALPN extension, 463 but does not select a protocol from the client list. Failure to do 464 so can result in attacks such those described in [ALPACA]. 466 Protocol developers are strongly encouraged to register an ALPN 467 identifier for their protocols. This applies to new protocols, as 468 well as well-established protocols such as SMTP. 470 3.9. Zero Round Trip Time (0-RTT) Data in TLS 1.3 472 The 0-RTT early data feature is new in TLS 1.3. It provides improved 473 latency when TLS connections are resumed, at the potential cost of 474 security. As a result, it requires special attention from 475 implementers on both the server and the client side. Typically this 476 extends to both the TLS library as well as protocol layers above it. 478 For use in HTTP-over-TLS, readers are referred to [RFC8470] for 479 guidance. 481 For QUIC-on-TLS, refer to Sec. 9.2 of [RFC9001]. 483 For other protocols, generic guidance is given in Sec. 8 and 484 Appendix E.5 of [RFC8446]. Given the complexity, we RECOMMEND to 485 avoid this feature altogether unless an explicit specification exists 486 for the application protocol in question to clarify when 0-RTT is 487 appropriate and secure. This can take the form of an IETF RFC, a 488 non-IETF standard, or even documentation associated with a non- 489 standard protocol. 491 4. Recommendations: Cipher Suites 493 TLS and its implementations provide considerable flexibility in the 494 selection of cipher suites. Unfortunately, some available cipher 495 suites are insecure, some do not provide the targeted security 496 services, and some no longer provide enough security. Incorrectly 497 configuring a server leads to no or reduced security. This section 498 includes recommendations on the selection and negotiation of cipher 499 suites. 501 4.1. General Guidelines 503 Cryptographic algorithms weaken over time as cryptanalysis improves: 504 algorithms that were once considered strong become weak. Such 505 algorithms need to be phased out over time and replaced with more 506 secure cipher suites. This helps to ensure that the desired security 507 properties still hold. SSL/TLS has been in existence for almost 20 508 years and many of the cipher suites that have been recommended in 509 various versions of SSL/TLS are now considered weak or at least not 510 as strong as desired. Therefore, this section modernizes the 511 recommendations concerning cipher suite selection. 513 * Implementations MUST NOT negotiate the cipher suites with NULL 514 encryption. 516 Rationale: The NULL cipher suites do not encrypt traffic and so 517 provide no confidentiality services. Any entity in the network 518 with access to the connection can view the plaintext of contents 519 being exchanged by the client and server. 520 Nevertheless, this document does not discourage software from 521 implementing NULL cipher suites, since they can be useful for 522 testing and debugging. 524 * Implementations MUST NOT negotiate RC4 cipher suites. 526 Rationale: The RC4 stream cipher has a variety of cryptographic 527 weaknesses, as documented in [RFC7465]. Note that DTLS 528 specifically forbids the use of RC4 already. 530 * Implementations MUST NOT negotiate cipher suites offering less 531 than 112 bits of security, including so-called "export-level" 532 encryption (which provide 40 or 56 bits of security). 534 Rationale: Based on [RFC3766], at least 112 bits of security is 535 needed. 40-bit and 56-bit security are considered insecure today. 536 TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers. 538 * Implementations SHOULD NOT negotiate cipher suites that use 539 algorithms offering less than 128 bits of security. 541 Rationale: Cipher suites that offer between 112-bits and 128-bits 542 of security are not considered weak at this time; however, it is 543 expected that their useful lifespan is short enough to justify 544 supporting stronger cipher suites at this time. 128-bit ciphers 545 are expected to remain secure for at least several years, and 546 256-bit ciphers until the next fundamental technology 547 breakthrough. Note that, because of so-called "meet-in-the- 548 middle" attacks [Multiple-Encryption], some legacy cipher suites 549 (e.g., 168-bit 3DES) have an effective key length that is smaller 550 than their nominal key length (112 bits in the case of 3DES). 551 Such cipher suites should be evaluated according to their 552 effective key length. 554 * Implementations SHOULD NOT negotiate cipher suites based on RSA 555 key transport, a.k.a. "static RSA". 557 Rationale: These cipher suites, which have assigned values 558 starting with the string "TLS_RSA_WITH_*", have several drawbacks, 559 especially the fact that they do not support forward secrecy. 561 * Implementations MUST support and prefer to negotiate cipher suites 562 offering forward secrecy, such as those in the Ephemeral Diffie- 563 Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and 564 "ECDHE") families. 566 Rationale: Forward secrecy (sometimes called "perfect forward 567 secrecy") prevents the recovery of information that was encrypted 568 with older session keys, thus limiting the amount of time during 569 which attacks can be successful. See Section 6.3 for a detailed 570 discussion. 572 4.2. Recommended Cipher Suites 574 Given the foregoing considerations, implementation and deployment of 575 the following cipher suites is RECOMMENDED: 577 * TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 579 * TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 581 * TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 583 * TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 585 These cipher suites are supported only in TLS 1.2 and not in earlier 586 protocol versions, because they are authenticated encryption (AEAD) 587 algorithms [RFC5116]. 589 Typically, in order to prefer these suites, the order of suites needs 590 to be explicitly configured in server software. (See [BETTERCRYPTO] 591 for helpful deployment guidelines, but note that its recommendations 592 differ from the current document in some details.) It would be ideal 593 if server software implementations were to prefer these suites by 594 default. 596 Some devices have hardware support for AES-CCM but not AES-GCM, so 597 they are unable to follow the foregoing recommendations regarding 598 cipher suites. There are even devices that do not support public key 599 cryptography at all, but they are out of scope entirely. 601 4.2.1. Implementation Details 603 Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the 604 first proposal to any server, unless they have prior knowledge that 605 the server cannot respond to a TLS 1.2 client_hello message. 607 Servers MUST prefer this cipher suite over weaker cipher suites 608 whenever it is proposed, even if it is not the first proposal. 610 Clients are of course free to offer stronger cipher suites, e.g., 611 using AES-256; when they do, the server SHOULD prefer the stronger 612 cipher suite unless there are compelling reasons (e.g., seriously 613 degraded performance) to choose otherwise. 615 This document does not change the mandatory-to-implement TLS cipher 616 suite(s) prescribed by TLS. To maximize interoperability, RFC 5246 617 mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher 618 suite, which is significantly weaker than the cipher suites 619 recommended here. (The GCM mode does not suffer from the same 620 weakness, caused by the order of MAC-then-Encrypt in TLS 621 [Krawczyk2001], since it uses an AEAD mode of operation.) 622 Implementers should consider the interoperability gain against the 623 loss in security when deploying the TLS_RSA_WITH_AES_128_CBC_SHA 624 cipher suite. Other application protocols specify other cipher 625 suites as mandatory to implement (MTI). 627 Note that some profiles of TLS 1.2 use different cipher suites. For 628 example, [RFC6460] defines a profile that uses the 629 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and 630 TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites. 632 [RFC4492] allows clients and servers to negotiate ECDH parameters 633 (curves). Both clients and servers SHOULD include the "Supported 634 Elliptic Curves" extension [RFC4492]. For interoperability, clients 635 and servers SHOULD support the NIST P-256 (secp256r1) curve 636 [RFC4492]. In addition, clients SHOULD send an ec_point_formats 637 extension with a single element, "uncompressed". 639 4.3. Cipher Suites for TLS 1.3 641 This document does not specify any cipher suites for TLS 1.3. 642 Readers are referred to Sec. 9.1 of [RFC8446] for cipher suite 643 recommendations. 645 4.4. Limits on Key Usage 647 All ciphers have an upper limit on the amount of traffic that can be 648 securely protected with any given key. In the case of AEAD cipher 649 suites, two separate limits are maintained for each key: 651 1. Confidentiality limit (CL), i.e., the number of records that can 652 be encrypted. 654 2. Integrity limit (IL), i.e., the number of records that are 655 allowed to fail authentication. 657 The latter only applies to DTLS since TLS connections are torn down 658 on the first decryption failure. 660 When a sender is approaching CL, the implementation SHOULD initiate a 661 new handshake (or in TLS 1.3, a Key Update) to rotate the session 662 key. 664 When a receiver has reached IL, the implementation SHOULD close the 665 connection. 667 For all TLS 1.3 cipher suites, readers are referred to Section 5.5 of 668 [RFC8446] for the values of CL and IL. For all DTLS 1.3 cipher 669 suites, readers are referred to Section 4.5.3 of 670 [I-D.ietf-tls-dtls13]. 672 For all AES-GCM cipher suites recommended for TLS 1.2 and DTLS 1.2 in 673 this document, CL can be derived by plugging the corresponding 674 parameters into the inequalities in Section 6.1 of 675 [I-D.irtf-cfrg-aead-limits] that apply to random, partially implicit 676 nonces, i.e., the nonce construction used in TLS 1.2. Although the 677 obtained figures are slightly higher than those for TLS 1.3, it is 678 RECOMMENDED that the same limit of 2^24.5 records is used for both 679 versions. 681 For all AES-GCM cipher suites recommended for DTLS 1.2, IL (obtained 682 from the same inequalities referenced above) is 2^28. 684 4.5. Public Key Length 686 When using the cipher suites recommended in this document, two public 687 keys are normally used in the TLS handshake: one for the Diffie- 688 Hellman key agreement and one for server authentication. Where a 689 client certificate is used, a third public key is added. 691 With a key exchange based on modular exponential (MODP) Diffie- 692 Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048 693 bits are REQUIRED. 695 Rationale: For various reasons, in practice, DH keys are typically 696 generated in lengths that are powers of two (e.g., 2^10 = 1024 bits, 697 2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits 698 would be roughly equivalent to only an 80-bit symmetric key 699 [RFC3766], it is better to use keys longer than that for the "DHE" 700 family of cipher suites. A DH key of 1926 bits would be roughly 701 equivalent to a 100-bit symmetric key [RFC3766]. A DH key of 2048 702 bits (equivalent to a 112-bit symmetric key) is the minimum allowed 703 by the latest revision of [NIST.SP.800-56A], as of this writing (see 704 in particular Appendix D). 706 As noted in [RFC3766], correcting for the emergence of a TWIRL 707 machine would imply that 1024-bit DH keys yield about 65 bits of 708 equivalent strength and that a 2048-bit DH key would yield about 92 709 bits of equivalent strength. The Logjam attack [Logjam] further 710 demonstrates that 1024-bit Diffie Hellman parameters should be 711 avoided. 713 With regard to ECDH keys, implementers are referred to the IANA 714 "Supported Groups Registry" (former "EC Named Curve Registry"), 715 within the "Transport Layer Security (TLS) Parameters" registry 716 [IANA_TLS], and in particular to the "recommended" groups. Curves of 717 less than 224 bits MUST NOT be used. This recommendation is in-line 718 with the latest revision of [NIST.SP.800-56A]. 720 When using RSA, servers SHOULD authenticate using certificates with 721 at least a 2048-bit modulus for the public key. In addition, the use 722 of the SHA-256 hash algorithm is RECOMMENDED and SHA-1 or MD5 MUST 723 NOT be used (see [CAB-Baseline] for more details). Clients MUST 724 indicate to servers that they request SHA-256, by using the 725 "Signature Algorithms" extension defined in TLS 1.2. 727 4.6. Truncated HMAC 729 Implementations MUST NOT use the Truncated HMAC extension, defined in 730 Section 7 of [RFC6066]. 732 Rationale: the extension does not apply to the AEAD cipher suites 733 recommended above. However it does apply to most other TLS cipher 734 suites. Its use has been shown to be insecure in [PatersonRS11]. 736 5. Applicability Statement 738 The recommendations of this document primarily apply to the 739 implementation and deployment of application protocols that are most 740 commonly used with TLS and DTLS on the Internet today. Examples 741 include, but are not limited to: 743 * Web software and services that wish to protect HTTP traffic with 744 TLS. 746 * Email software and services that wish to protect IMAP, POP3, or 747 SMTP traffic with TLS. 749 * Instant-messaging software and services that wish to protect 750 Extensible Messaging and Presence Protocol (XMPP) or Internet 751 Relay Chat (IRC) traffic with TLS. 753 * Realtime media software and services that wish to protect Secure 754 Realtime Transport Protocol (SRTP) traffic with DTLS. 756 This document does not modify the implementation and deployment 757 recommendations (e.g., mandatory-to-implement cipher suites) 758 prescribed by existing application protocols that employ TLS or DTLS. 759 If the community that uses such an application protocol wishes to 760 modernize its usage of TLS or DTLS to be consistent with the best 761 practices recommended here, it needs to explicitly update the 762 existing application protocol definition (one example is [TLS-XMPP], 763 which updates [RFC6120]). 765 Designers of new application protocols developed through the Internet 766 Standards Process [RFC2026] are expected at minimum to conform to the 767 best practices recommended here, unless they provide documentation of 768 compelling reasons that would prevent such conformance (e.g., 769 widespread deployment on constrained devices that lack support for 770 the necessary algorithms). 772 5.1. Security Services 774 This document provides recommendations for an audience that wishes to 775 secure their communication with TLS to achieve the following: 777 * Confidentiality: all application-layer communication is encrypted 778 with the goal that no party should be able to decrypt it except 779 the intended receiver. 781 * Data integrity: any changes made to the communication in transit 782 are detectable by the receiver. 784 * Authentication: an endpoint of the TLS communication is 785 authenticated as the intended entity to communicate with. 787 With regard to authentication, TLS enables authentication of one or 788 both endpoints in the communication. In the context of opportunistic 789 security [RFC7435], TLS is sometimes used without authentication. As 790 discussed in Section 5.2, considerations for opportunistic security 791 are not in scope for this document. 793 If deployers deviate from the recommendations given in this document, 794 they need to be aware that they might lose access to one of the 795 foregoing security services. 797 This document applies only to environments where confidentiality is 798 required. It recommends algorithms and configuration options that 799 enforce secrecy of the data in transit. 801 This document also assumes that data integrity protection is always 802 one of the goals of a deployment. In cases where integrity is not 803 required, it does not make sense to employ TLS in the first place. 804 There are attacks against confidentiality-only protection that 805 utilize the lack of integrity to also break confidentiality (see, for 806 instance, [DegabrieleP07] in the context of IPsec). 808 This document addresses itself to application protocols that are most 809 commonly used on the Internet with TLS and DTLS. Typically, all 810 communication between TLS clients and TLS servers requires all three 811 of the above security services. This is particularly true where TLS 812 clients are user agents like Web browsers or email software. 814 This document does not address the rarer deployment scenarios where 815 one of the above three properties is not desired, such as the use 816 case described in Section 5.2 below. As another scenario where 817 confidentiality is not needed, consider a monitored network where the 818 authorities in charge of the respective traffic domain require full 819 access to unencrypted (plaintext) traffic, and where users 820 collaborate and send their traffic in the clear. 822 5.2. Opportunistic Security 824 There are several important scenarios in which the use of TLS is 825 optional, i.e., the client decides dynamically ("opportunistically") 826 whether to use TLS with a particular server or to connect in the 827 clear. This practice, often called "opportunistic security", is 828 described at length in [RFC7435] and is often motivated by a desire 829 for backward compatibility with legacy deployments. 831 In these scenarios, some of the recommendations in this document 832 might be too strict, since adhering to them could cause fallback to 833 cleartext, a worse outcome than using TLS with an outdated protocol 834 version or cipher suite. 836 6. Security Considerations 838 This entire document discusses the security practices directly 839 affecting applications using the TLS protocol. This section contains 840 broader security considerations related to technologies used in 841 conjunction with or by TLS. 843 6.1. Host Name Validation 845 Application authors should take note that some TLS implementations do 846 not validate host names. If the TLS implementation they are using 847 does not validate host names, authors might need to write their own 848 validation code or consider using a different TLS implementation. 850 It is noted that the requirements regarding host name validation 851 (and, in general, binding between the TLS layer and the protocol that 852 runs above it) vary between different protocols. For HTTPS, these 853 requirements are defined by Sections 4.3.3, 4.3.4 and 4.3.5 of 854 [I-D.ietf-httpbis-semantics]. 856 Readers are referred to [RFC6125] for further details regarding 857 generic host name validation in the TLS context. In addition, that 858 RFC contains a long list of example protocols, some of which 859 implement a policy very different from HTTPS. 861 If the host name is discovered indirectly and in an insecure manner 862 (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD 863 NOT be used as a reference identifier [RFC6125] even when it matches 864 the presented certificate. This proviso does not apply if the host 865 name is discovered securely (for further discussion, see [DANE-SRV] 866 and [DANE-SMTP]). 868 Host name validation typically applies only to the leaf "end entity" 869 certificate. Naturally, in order to ensure proper authentication in 870 the context of the PKI, application clients need to verify the entire 871 certification path in accordance with [RFC5280] (see also [RFC6125]). 873 6.2. AES-GCM 875 Section 4.2 above recommends the use of the AES-GCM authenticated 876 encryption algorithm. Please refer to Section 11 of [RFC5246] for 877 general security considerations when using TLS 1.2, and to Section 6 878 of [RFC5288] for security considerations that apply specifically to 879 AES-GCM when used with TLS. 881 6.2.1. Nonce Reuse in TLS 1.2 883 The existence of deployed TLS stacks that mistakenly reuse the AES- 884 GCM nonce is documented in [Boeck2016], showing there is an actual 885 risk of AES-GCM getting implemented in an insecure way and thus 886 making TLS sessions that use an AES-GCM cipher suite vulnerable to 887 attacks such as [Joux2006]. (See [CVE] records: CVE-2016-0270, CVE- 888 2016-10213, CVE-2016-10212, CVE-2017-5933.) 890 While this problem has been fixed in TLS 1.3, which enforces a 891 deterministic method to generate nonces from record sequence numbers 892 and shared secrets for all of its AEAD cipher suites (including AES- 893 GCM), TLS 1.2 implementations could still choose their own 894 (potentially insecure) nonce generation methods. 896 It is therefore RECOMMENDED that TLS 1.2 implementations use the 897 64-bit sequence number to populate the nonce_explicit part of the GCM 898 nonce, as described in the first two paragraphs of Section 5.3 of 899 [RFC8446]. Note that this recommendation updates Section 3 of 900 [RFC5288]: "The nonce_explicit MAY be the 64-bit sequence number." 902 We note that at the time of writing there are no cipher suites 903 defined for nonce reuse resistant algorithms such as AES-GCM-SIV 904 [RFC8452]. 906 6.3. Forward Secrecy 908 Forward secrecy (also called "perfect forward secrecy" or "PFS" and 909 defined in [RFC4949]) is a defense against an attacker who records 910 encrypted conversations where the session keys are only encrypted 911 with the communicating parties' long-term keys. 913 Should the attacker be able to obtain these long-term keys at some 914 point later in time, the session keys and thus the entire 915 conversation could be decrypted. 917 In the context of TLS and DTLS, such compromise of long-term keys is 918 not entirely implausible. It can happen, for example, due to: 920 * A client or server being attacked by some other attack vector, and 921 the private key retrieved. 923 * A long-term key retrieved from a device that has been sold or 924 otherwise decommissioned without prior wiping. 926 * A long-term key used on a device as a default key [Heninger2012]. 928 * A key generated by a trusted third party like a CA, and later 929 retrieved from it either by extortion or compromise 930 [Soghoian2011]. 932 * A cryptographic break-through, or the use of asymmetric keys with 933 insufficient length [Kleinjung2010]. 935 * Social engineering attacks against system administrators. 937 * Collection of private keys from inadequately protected backups. 939 Forward secrecy ensures in such cases that it is not feasible for an 940 attacker to determine the session keys even if the attacker has 941 obtained the long-term keys some time after the conversation. It 942 also protects against an attacker who is in possession of the long- 943 term keys but remains passive during the conversation. 945 Forward secrecy is generally achieved by using the Diffie-Hellman 946 scheme to derive session keys. The Diffie-Hellman scheme has both 947 parties maintain private secrets and send parameters over the network 948 as modular powers over certain cyclic groups. The properties of the 949 so-called Discrete Logarithm Problem (DLP) allow the parties to 950 derive the session keys without an eavesdropper being able to do so. 951 There is currently no known attack against DLP if sufficiently large 952 parameters are chosen. A variant of the Diffie-Hellman scheme uses 953 Elliptic Curves instead of the originally proposed modular 954 arithmetic. 956 Unfortunately, many TLS/DTLS cipher suites were defined that do not 957 feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This 958 document therefore advocates strict use of forward-secrecy-only 959 ciphers. 961 6.4. Diffie-Hellman Exponent Reuse 963 For performance reasons, many TLS implementations reuse Diffie- 964 Hellman and Elliptic Curve Diffie-Hellman exponents across multiple 965 connections. Such reuse can result in major security issues: 967 * If exponents are reused for too long (e.g., even more than a few 968 hours), an attacker who gains access to the host can decrypt 969 previous connections. In other words, exponent reuse negates the 970 effects of forward secrecy. 972 * TLS implementations that reuse exponents should test the DH public 973 key they receive for group membership, in order to avoid some 974 known attacks. These tests are not standardized in TLS at the 975 time of writing. See [RFC6989] for recipient tests required of 976 IKEv2 implementations that reuse DH exponents. 978 * Under certain conditions, the use of static DH keys, or of 979 ephemeral DH keys that are reused across multiple connections, can 980 lead to timing attacks (such as those described in [RACCOON]) on 981 the shared secrets used in Diffie-Hellman key exchange. 983 To address these concerns, TLS implementations SHOULD NOT use static 984 DH keys and SHOULD NOT reuse ephemeral DH keys across multiple 985 connections. 987 // TODO: revisit when draft-bartle-tls-deprecate-ffdhe becomes a TLS 988 // WG item, since it specifies MUST NOT rather than SHOULD NOT. 990 6.5. Certificate Revocation 992 The following considerations and recommendations represent the 993 current state of the art regarding certificate revocation, even 994 though no complete and efficient solution exists for the problem of 995 checking the revocation status of common public key certificates 996 [RFC5280]: 998 * Although Certificate Revocation Lists (CRLs) are the most widely 999 supported mechanism for distributing revocation information, they 1000 have known scaling challenges that limit their usefulness (despite 1001 workarounds such as partitioned CRLs and delta CRLs). 1003 * Proprietary mechanisms that embed revocation lists in the Web 1004 browser's configuration database cannot scale beyond a small 1005 number of the most heavily used Web servers. 1007 * The On-Line Certification Status Protocol (OCSP) [RFC6960] 1008 presents both scaling and privacy issues. In addition, clients 1009 typically "soft-fail", meaning that they do not abort the TLS 1010 connection if the OCSP server does not respond. (However, this 1011 might be a workaround to avoid denial-of-service attacks if an 1012 OCSP responder is taken offline.) 1014 * The TLS Certificate Status Request extension (Section 8 of 1015 [RFC6066]), commonly called "OCSP stapling", resolves the 1016 operational issues with OCSP. However, it is still ineffective in 1017 the presence of a MITM attacker because the attacker can simply 1018 ignore the client's request for a stapled OCSP response. 1020 * OCSP stapling as defined in [RFC6066] does not extend to 1021 intermediate certificates used in a certificate chain. Although 1022 the Multiple Certificate Status extension [RFC6961] addresses this 1023 shortcoming, it is a recent addition without much deployment. 1025 * Both CRLs and OCSP depend on relatively reliable connectivity to 1026 the Internet, which might not be available to certain kinds of 1027 nodes (such as newly provisioned devices that need to establish a 1028 secure connection in order to boot up for the first time). 1030 With regard to common public key certificates, servers SHOULD support 1031 the following as a best practice given the current state of the art 1032 and as a foundation for a possible future solution: 1034 1. OCSP [RFC6960] 1035 2. Both the status_request extension defined in [RFC6066] and the 1036 status_request_v2 extension defined in [RFC6961] (This might 1037 enable interoperability with the widest range of clients.) 1039 3. The OCSP stapling extension defined in [RFC6961] 1041 The considerations in this section do not apply to scenarios where 1042 the DANE-TLSA resource record [RFC6698] is used to signal to a client 1043 which certificate a server considers valid and good to use for TLS 1044 connections. 1046 7. Acknowledgments 1048 The following acknowledgments are inherited from [RFC7525]. 1050 Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen 1051 Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson 1052 Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller, 1053 Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom 1054 Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean 1055 Turner, and Aaron Zauner for their feedback and suggested 1056 improvements. Thanks also to Brian Smith, who has provided a great 1057 resource in his "Proposal to Change the Default TLS Ciphersuites 1058 Offered by Browsers" [Smith2013]. Finally, thanks to all others who 1059 commented on the TLS, UTA, and other discussion lists but who are not 1060 mentioned here by name. 1062 Robert Sparks and Dave Waltermire provided helpful reviews on behalf 1063 of the General Area Review Team and the Security Directorate, 1064 respectively. 1066 During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins, 1067 Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick 1068 provided comments that led to further improvements. 1070 Ralph Holz gratefully acknowledges the support by Technische 1071 Universitaet Muenchen. 1073 The authors gratefully acknowledge the assistance of Leif Johansson 1074 and Orit Levin as the working group chairs and Pete Resnick as the 1075 sponsoring Area Director. 1077 8. References 1079 8.1. Normative References 1081 [I-D.ietf-httpbis-semantics] 1082 Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP 1083 Semantics", Work in Progress, Internet-Draft, draft-ietf- 1084 httpbis-semantics-19, 12 September 2021, 1085 . 1088 [I-D.ietf-tls-dtls13] 1089 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 1090 Datagram Transport Layer Security (DTLS) Protocol Version 1091 1.3", Work in Progress, Internet-Draft, draft-ietf-tls- 1092 dtls13-43, 30 April 2021, 1093 . 1096 [I-D.ietf-tls-oldversions-deprecate] 1097 Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1098 1.1", Work in Progress, Internet-Draft, draft-ietf-tls- 1099 oldversions-deprecate-12, 21 January 2021, 1100 . 1103 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1104 Requirement Levels", BCP 14, RFC 2119, 1105 DOI 10.17487/RFC2119, March 1997, 1106 . 1108 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 1109 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 1110 RFC 3766, DOI 10.17487/RFC3766, April 2004, 1111 . 1113 [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. 1114 Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites 1115 for Transport Layer Security (TLS)", RFC 4492, 1116 DOI 10.17487/RFC4492, May 2006, 1117 . 1119 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1120 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 1121 . 1123 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1124 (TLS) Protocol Version 1.2", RFC 5246, 1125 DOI 10.17487/RFC5246, August 2008, 1126 . 1128 [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois 1129 Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, 1130 DOI 10.17487/RFC5288, August 2008, 1131 . 1133 [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, 1134 "Transport Layer Security (TLS) Renegotiation Indication 1135 Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, 1136 . 1138 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 1139 Extensions: Extension Definitions", RFC 6066, 1140 DOI 10.17487/RFC6066, January 2011, 1141 . 1143 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 1144 Verification of Domain-Based Application Service Identity 1145 within Internet Public Key Infrastructure Using X.509 1146 (PKIX) Certificates in the Context of Transport Layer 1147 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 1148 2011, . 1150 [RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer 1151 (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March 1152 2011, . 1154 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1155 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1156 January 2012, . 1158 [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, 1159 "Transport Layer Security (TLS) Application-Layer Protocol 1160 Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, 1161 July 2014, . 1163 [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, 1164 DOI 10.17487/RFC7465, February 2015, 1165 . 1167 [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., 1168 Langley, A., and M. Ray, "Transport Layer Security (TLS) 1169 Session Hash and Extended Master Secret Extension", 1170 RFC 7627, DOI 10.17487/RFC7627, September 2015, 1171 . 1173 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1174 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1175 May 2017, . 1177 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1178 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1179 . 1181 [RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740, 1182 DOI 10.17487/RFC8740, February 2020, 1183 . 1185 [RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1186 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021, 1187 . 1189 8.2. Informative References 1191 [ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D., 1192 Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel, 1193 "ALPACA: Application Layer Protocol Confusion - Analyzing 1194 and Mitigating Cracks in TLS Authentication", 30th USENIX 1195 Security Symposium (USENIX Security 21) , 2021, 1196 . 1199 [BETTERCRYPTO] 1200 bettercrypto.org, "Applied Crypto Hardening", April 2015, 1201 . 1203 [Boeck2016] 1204 Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P. 1205 Jovanovic, "Nonce-Disrespecting Adversaries: Practical 1206 Forgery Attacks on GCM in TLS", May 2016, 1207 . 1209 [CAB-Baseline] 1210 CA/Browser Forum, "Baseline Requirements for the Issuance 1211 and Management of Publicly-Trusted Certificates Version 1212 1.1.6", 2013, . 1214 [CVE] MITRE, "Common Vulnerabilities and Exposures", 1215 . 1217 [DANE-SMTP] 1218 Dukhovni, V. and W. Hardaker, "SMTP Security via 1219 Opportunistic DNS-Based Authentication of Named Entities 1220 (DANE) Transport Layer Security (TLS)", RFC 7672, 1221 DOI 10.17487/RFC7672, October 2015, 1222 . 1224 [DANE-SRV] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1225 Based Authentication of Named Entities (DANE) TLSA Records 1226 with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 1227 2015, . 1229 [DegabrieleP07] 1230 Degabriele, J. and K. Paterson, "Attacking the IPsec 1231 Standards in Encryption-only Configurations", 2007 IEEE 1232 Symposium on Security and Privacy (SP '07), 1233 DOI 10.1109/sp.2007.8, May 2007, 1234 . 1236 [DEP-SSLv3] 1237 Barnes, R., Thomson, M., Pironti, A., and A. Langley, 1238 "Deprecating Secure Sockets Layer Version 3.0", RFC 7568, 1239 DOI 10.17487/RFC7568, June 2015, 1240 . 1242 [Heninger2012] 1243 Heninger, N., Durumeric, Z., Wustrow, E., and J.A. 1244 Halderman, "Mining Your Ps and Qs: Detection of Widespread 1245 Weak Keys in Network Devices", Usenix Security 1246 Symposium 2012, 2012. 1248 [I-D.ietf-tls-esni] 1249 Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS 1250 Encrypted Client Hello", Work in Progress, Internet-Draft, 1251 draft-ietf-tls-esni-13, 12 August 2021, 1252 . 1255 [I-D.irtf-cfrg-aead-limits] 1256 Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on 1257 AEAD Algorithms", Work in Progress, Internet-Draft, draft- 1258 irtf-cfrg-aead-limits-03, 12 July 2021, 1259 . 1262 [IANA_TLS] IANA, "Transport Layer Security (TLS) Parameters", 1263 . 1265 [Joux2006] Joux, A., "Authentication Failures in NIST version of 1266 GCM", 2006, . 1270 [Kleinjung2010] 1271 Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thomé, 1272 E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P., 1273 Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann, 1274 "Factorization of a 768-Bit RSA Modulus", Advances in 1275 Cryptology - CRYPTO 2010 pp. 333-350, 1276 DOI 10.1007/978-3-642-14623-7_18, 2010, 1277 . 1279 [Krawczyk2001] 1280 Krawczyk, H., "The Order of Encryption and Authentication 1281 for Protecting Communications (Or: How Secure is SSL?)", 1282 CRYPTO 01, 2001, 1283 . 1285 [Logjam] Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P., 1286 Green, M., Halderman, J., Heninger, N., Springall, D., 1287 Thomé, E., Valenta, L., VanderSloot, B., Wustrow, E., 1288 Zanella-Béguelin, S., and P. Zimmermann, "Imperfect 1289 Forward Secrecy: How Diffie-Hellman Fails in Practice", 1290 Proceedings of the 22nd ACM SIGSAC Conference on Computer 1291 and Communications Security, DOI 10.1145/2810103.2813707, 1292 October 2015, . 1294 [Multiple-Encryption] 1295 Merkle, R. and M. Hellman, "On the security of multiple 1296 encryption", Communications of the ACM Vol. 24, pp. 1297 465-467, DOI 10.1145/358699.358718, July 1981, 1298 . 1300 [NIST.SP.800-56A] 1301 Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. 1302 Davis, "Recommendation for pair-wise key-establishment 1303 schemes using discrete logarithm cryptography", National 1304 Institute of Standards and Technology report, 1305 DOI 10.6028/nist.sp.800-56ar3, April 2018, 1306 . 1308 [PatersonRS11] 1309 Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size 1310 Does Matter: Attacks and Proofs for the TLS Record 1311 Protocol", Lecture Notes in Computer Science pp. 372-389, 1312 DOI 10.1007/978-3-642-25385-0_20, 2011, 1313 . 1315 [POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE 1316 Attack", October 2014, 1317 . 1319 [RACCOON] Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J., 1320 Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and 1321 Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)", 1322 30th USENIX Security Symposium (USENIX Security 21) , 1323 2021, . 1326 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 1327 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, 1328 . 1330 [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", 1331 RFC 2246, DOI 10.17487/RFC2246, January 1999, 1332 . 1334 [RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher 1335 Algorithm and Its Use with IPsec", RFC 3602, 1336 DOI 10.17487/RFC3602, September 2003, 1337 . 1339 [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security 1340 (TLS) Protocol Version 1.1", RFC 4346, 1341 DOI 10.17487/RFC4346, April 2006, 1342 . 1344 [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1345 Security", RFC 4347, DOI 10.17487/RFC4347, April 2006, 1346 . 1348 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1349 "Transport Layer Security (TLS) Session Resumption without 1350 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1351 January 2008, . 1353 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated 1354 Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, 1355 . 1357 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1358 Housley, R., and W. Polk, "Internet X.509 Public Key 1359 Infrastructure Certificate and Certificate Revocation List 1360 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 1361 . 1363 [RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure 1364 Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, 1365 DOI 10.17487/RFC6101, August 2011, 1366 . 1368 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 1369 Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, 1370 March 2011, . 1372 [RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport 1373 Layer Security (TLS)", RFC 6460, DOI 10.17487/RFC6460, 1374 January 2012, . 1376 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 1377 of Named Entities (DANE) Transport Layer Security (TLS) 1378 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 1379 2012, . 1381 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict 1382 Transport Security (HSTS)", RFC 6797, 1383 DOI 10.17487/RFC6797, November 2012, 1384 . 1386 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 1387 Galperin, S., and C. Adams, "X.509 Internet Public Key 1388 Infrastructure Online Certificate Status Protocol - OCSP", 1389 RFC 6960, DOI 10.17487/RFC6960, June 2013, 1390 . 1392 [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) 1393 Multiple Certificate Status Request Extension", RFC 6961, 1394 DOI 10.17487/RFC6961, June 2013, 1395 . 1397 [RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman 1398 Tests for the Internet Key Exchange Protocol Version 2 1399 (IKEv2)", RFC 6989, DOI 10.17487/RFC6989, July 2013, 1400 . 1402 [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection 1403 Most of the Time", RFC 7435, DOI 10.17487/RFC7435, 1404 December 2014, . 1406 [RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing 1407 Known Attacks on Transport Layer Security (TLS) and 1408 Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457, 1409 February 2015, . 1411 [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher 1412 Suite Value (SCSV) for Preventing Protocol Downgrade 1413 Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, 1414 . 1416 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1417 "Recommendations for Secure Use of Transport Layer 1418 Security (TLS) and Datagram Transport Layer Security 1419 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1420 2015, . 1422 [RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV: 1423 Nonce Misuse-Resistant Authenticated Encryption", 1424 RFC 8452, DOI 10.17487/RFC8452, April 2019, 1425 . 1427 [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early 1428 Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 1429 2018, . 1431 [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure 1432 QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, 1433 . 1435 [Smith2013] 1436 Smith, B., "Proposal to Change the Default TLS 1437 Ciphersuites Offered by Browsers.", 2013, 1438 . 1440 [Soghoian2011] 1441 Soghoian, C. and S. Stamm, "Certified Lies: Detecting and 1442 Defeating Government Interception Attacks Against SSL", 1443 SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, 2010, 1444 . 1446 [Sy2018] Sy, E., Burkert, C., Federrath, H., and M. Fischer, 1447 "Tracking Users across the Web via TLS Session 1448 Resumption", Proceedings of the 34th Annual Computer 1449 Security Applications Conference, 1450 DOI 10.1145/3274694.3274708, December 2018, 1451 . 1453 [TLS-XMPP] Saint-Andre, P. and T. Alkemade, "Use of Transport Layer 1454 Security (TLS) in the Extensible Messaging and Presence 1455 Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June 1456 2015, . 1458 [triple-handshake] 1459 Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and 1460 P. Strub, "Triple Handshakes and Cookie Cutters: Breaking 1461 and Fixing Authentication over TLS", 2014 IEEE Symposium 1462 on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014, 1463 . 1465 Appendix A. Differences from RFC 7525 1467 This revision of the Best Current Practices contains numerous 1468 changes, and this section is focused on the normative changes. 1470 * High level differences: 1472 - Clarified items (e.g. renegotiation) that only apply to TLS 1473 1.2. 1475 - Changed status of TLS 1.0 and 1.1 from SHOULD NOT to MUST NOT. 1477 - Added TLS 1.3 at a SHOULD level. 1479 - Similar changes to DTLS, pending publication of DTLS 1.3. 1481 - Specific guidance for multiplexed protocols. 1483 - MUST-level implementation requirement for ALPN, and more 1484 specific SHOULD-level guidance for ALPN and SNI. 1486 - Limits on key usage. 1488 - New attacks since [RFC7457]: ALPACA, Raccoon, Logjam, "Nonce- 1489 Disrespecting Adversaries". 1491 * Differences specific to TLS 1.2: 1493 - SHOULD-level guidance on AES-GCM nonce generation. 1495 - SHOULD NOT use static DH keys or reuse ephemeral DH keys across 1496 multiple connections. 1498 - 2048-bit DH now a MUST, ECDH minimal curve size is 224, vs. 192 1499 previously. 1501 - Support for extended_master_secret is a SHOULD. Also removed 1502 other, more complicated, related mitigations. 1504 * Differences specific to TLS 1.3: 1506 - New TLS 1.3 capabilities: 0-RTT. 1508 - Removed capabilities: renegotiation, compression. 1510 - Added mention of TLS Encrypted Client Hello, but no 1511 recommendation to use until it is finalized. 1513 - SHOULD-level requirement for forward secrecy in TLS 1.3 session 1514 resumption. 1516 - Generic SHOULD-level guidance to avoid 0-RTT unless it is 1517 documented for the particular protocol. 1519 Appendix B. Document History 1521 // Note to RFC Editor: please remove before publication. 1523 B.1. draft-ietf-uta-rfc7525bis-04 1525 * No version fallback from TLS 1.2 to earlier versions, therefore no 1526 SCSV. 1528 B.2. draft-ietf-uta-rfc7525bis-03 1530 * Cipher integrity and confidentiality limits. 1532 * Require extended_master_secret. 1534 B.3. draft-ietf-uta-rfc7525bis-02 1536 * Adjusted text about ALPN support in application protocols 1538 * Incorporated text from draft-ietf-tls-md5-sha1-deprecate 1540 B.4. draft-ietf-uta-rfc7525bis-01 1542 * Many more changes, including: 1544 - SHOULD-level requirement for forward secrecy in TLS 1.3 session 1545 resumption. 1547 - Removed TLS 1.2 capabilities: renegotiation, compression. 1549 - Specific guidance for multiplexed protocols. 1551 - MUST-level implementation requirement for ALPN, and more 1552 specific SHOULD-level guidance for ALPN and SNI. 1554 - Generic SHOULD-level guidance to avoid 0-RTT unless it is 1555 documented for the particular protocol. 1557 - SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2. 1559 - SHOULD NOT use static DH keys or reuse ephemeral DH keys across 1560 multiple connections. 1562 - 2048-bit DH now a MUST, ECDH minimal curve size is 224, up from 1563 192. 1565 B.5. draft-ietf-uta-rfc7525bis-00 1567 * Renamed: WG document. 1569 * Started populating list of changes from RFC 7525. 1571 * General rewording of abstract and intro for revised version. 1573 * Protocol versions, fallback. 1575 * Reference to ECHO. 1577 B.6. draft-sheffer-uta-rfc7525bis-00 1579 * Renamed, since the BCP number does not change. 1581 * Added an empty "Differences from RFC 7525" section. 1583 B.7. draft-sheffer-uta-bcp195bis-00 1585 * Initial release, the RFC 7525 text as-is, with some minor 1586 editorial changes to the references. 1588 Authors' Addresses 1590 Yaron Sheffer 1591 Intuit 1593 Email: yaronf.ietf@gmail.com 1595 Ralph Holz 1596 University of Twente 1598 Email: ralph.ietf@gmail.com 1600 Peter Saint-Andre 1601 Mozilla 1603 Email: stpeter@mozilla.com 1604 Thomas Fossati 1605 arm 1607 Email: thomas.fossati@arm.com