--- 1/draft-ietf-uta-tls-bcp-09.txt 2015-02-20 07:14:59.870071226 -0800 +++ 2/draft-ietf-uta-tls-bcp-10.txt 2015-02-20 07:14:59.922072487 -0800 @@ -1,49 +1,49 @@ UTA Y. Sheffer -Internet-Draft Porticor +Internet-Draft Intuit Intended status: Best Current Practice R. Holz -Expires: August 15, 2015 TUM +Expires: August 24, 2015 TUM P. Saint-Andre &yet - February 11, 2015 + February 20, 2015 Recommendations for Secure Use of TLS and DTLS - draft-ietf-uta-tls-bcp-09 + draft-ietf-uta-tls-bcp-10 Abstract Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, - including attacks on its most commonly used cipher suites and modes - of operation. This document provides recommendations for improving - the security of deployed services that use TLS and DTLS. The - recommendations are applicable to the majority of use cases. + including attacks on its most commonly used cipher suites and their + modes of operation. This document provides recommendations for + improving the security of deployed services that use TLS and DTLS. + The recommendations are applicable to the majority of use cases. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on August 15, 2015. + This Internet-Draft will expire on August 24, 2015. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -58,94 +58,94 @@ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. General Recommendations . . . . . . . . . . . . . . . . . . . 4 3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 4 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 5 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7 - 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7 + 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 8 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 10 - 4.2.1. Implementation Details . . . . . . . . . . . . . . . 10 + 4.2.1. Implementation Details . . . . . . . . . . . . . . . 11 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11 4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 12 4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 13 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 13 - 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 13 - 5.2. Unauthenticated TLS and Opportunistic Security . . . . . 14 - 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 - 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 15 - 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 16 - 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 16 - 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 17 + 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 14 + 5.2. Unauthenticated TLS and Opportunistic Security . . . . . 15 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 + 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 16 + 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 17 + 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 17 + 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 18 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 18 - 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 19 - 9.2. Informative References . . . . . . . . . . . . . . . . . 20 - Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 24 - A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 24 - A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 24 - A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 24 - A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 24 - A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 24 - A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 24 - A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 25 - A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 25 - A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 25 - A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 25 - A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 26 - A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 26 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 + 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 20 + 9.2. Informative References . . . . . . . . . . . . . . . . . 21 + Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25 + A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 25 + A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 25 + A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 25 + A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 25 + A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 26 + A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 26 + A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 26 + A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 26 + A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 27 + A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 27 + A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 27 + A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 27 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 1. Introduction Transport Layer Security (TLS) [RFC5246] and Datagram Transport Security Layer (DTLS) [RFC6347] are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher - suites and modes of operation. For instance, both the AES-CBC - [RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption - algorithms, which together are the most widely deployed ciphers, have - been attacked in the context of TLS. A companion document [RFC7457] - provides detailed information about these attacks. + suites and their modes of operation. For instance, both the AES-CBC + [RFC3602] and RC4 [RFC7465] encryption algorithms, which together + have been the most widely deployed ciphers, have been attacked in the + context of TLS. A companion document [RFC7457] provides detailed + information about these attacks and will help the reader understand + the rationale behind the recommendations provided here. Because of these attacks, those who implement and deploy TLS and DTLS need updated guidance on how TLS can be used securely. This document provides guidance for deployed services as well as for software implementations, assuming the implementer expects his or her code to - be deployed in environments defined in the following section. In - fact, this document calls for the deployment of algorithms that are - widely implemented but not yet widely deployed. Concerning - deployment, this document targets a wide audience, namely all - deployers who wish to add authentication (be it one-way only or - mutual), confidentiality, and data integrity protection to their - communications. + be deployed in environments defined in Section 5. In fact, this + document calls for the deployment of algorithms that are widely + implemented but not yet widely deployed. Concerning deployment, this + document targets a wide audience, namely all deployers who wish to + add authentication (be it one-way only or mutual), confidentiality, + and data integrity protection to their communications. The recommendations herein take into consideration the security of various mechanisms, their technical maturity and interoperability, and their prevalence in implementations at the time of writing. Unless it is explicitly called out that a recommendation applies to TLS alone or to DTLS alone, each recommendation applies to both TLS and DTLS. It is expected that the TLS 1.3 specification will resolve many of the vulnerabilities listed in this document. A system that deploys - TLS 1.3 will have fewer vulnerabilities than TLS 1.2 or below. This - document is likely to be updated after TLS 1.3 gets noticeable + TLS 1.3 should have fewer vulnerabilities than TLS 1.2 or below. + This document is likely to be updated after TLS 1.3 gets noticeable deployment. These are minimum recommendations for the use of TLS in the vast majority of implementation and deployment scenarios, with the exception of unauthenticated TLS (see Section 5). Other specifications that reference this document can have stricter requirements related to one or more aspects of the protocol, based on their particular circumstances (e.g., for use with a particular application protocol); when that is the case, implementers are advised to adhere to those stricter requirements. Furthermore, this @@ -186,30 +186,33 @@ Rationale: Today, SSLv2 is considered insecure [RFC6176]. o Implementations MUST NOT negotiate SSL version 3. Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and plugged some significant security holes, but did not support strong cipher suites. SSLv3 does not support TLS extensions, some of which (e.g., renegotiation_info) are security-critical. In addition, with the emergence of the POODLE attack [POODLE], SSLv3 - is now widely recognized as fundamentally insecure. + is now widely recognized as fundamentally insecure. See + [I-D.ietf-tls-sslv3-diediedie] for further details. - o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246]. + o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246] + unless no higher version is available in the negotiation. Rationale: TLS 1.0 (published in 1999) does not support many modern, strong cipher suites. In addition, TLS 1.0 lacks a per- record IV for CBC-based cipher suites and does not warn against common padding errors. - o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346]. + o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346] + unless no higher version is available in the negotiation. Rationale: TLS 1.1 (published in 2006) is a security improvement over TLS 1.0, but still does not support certain stronger cipher suites. o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to negotiate TLS version 1.2 over earlier versions of TLS. Rationale: Several stronger cipher suites are available only with TLS 1.2 (published in 2008). In fact, the cipher suites @@ -226,75 +229,82 @@ 1.1 was published. The following are the recommendations with respect to DTLS: o Implementations SHOULD NOT negotiate DTLS version 1.0 [RFC4347]. Version 1.0 of DTLS correlates to version 1.1 of TLS (see above). o Implementations MUST support, and prefer to negotiate, DTLS version 1.2 [RFC6347]. - Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (see - above). (There is no Version 1.1 of DTLS.) + Version 1.2 of DTLS correlates to Version 1.2 of TLS (see above). + (There is no Version 1.1 of DTLS.) 3.1.3. Fallback to Lower Versions Clients that "fall back" to lower versions of the protocol after the server rejects higher versions of the protocol MUST NOT fall back to SSLv3 or earlier. Rationale: Some client implementations revert to lower versions of TLS or even to SSLv3 if the server rejected higher versions of the protocol. This fallback can be forced by a man in the middle (MITM) attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS 1.2, the version recommended by this document. While TLS 1.0-only servers are still quite common, IP scans show that SSLv3-only servers amount to only about 3% of the current Web server population. (At the time of this writing, an explicit method for preventing downgrade attacks is being defined in [I-D.ietf-tls-downgrade-scsv].) 3.2. Strict TLS - To prevent SSL Stripping: + The following recommendations are provided to help prevent SSL + Stripping (the attack is summarized in Section 2.1 of [RFC7457]): o In cases where an application protocol allows implementations or deployments a choice between strict TLS configuration and dynamic upgrade from unencrypted to TLS-protected traffic (such as STARTTLS), clients and servers SHOULD prefer strict TLS configuration. o Application protocols typically provide a way for the server to offer TLS during an initial protocol exchange, and sometimes also provide a way for the server to advertise support for TLS (e.g., through a flag indicating that TLS is required); unfortunately, these indications are sent before the communication channel is encrypted. A client SHOULD attempt to negotiate TLS even if these indications are not communicated by the server. o HTTP client and server implementations MUST support the HTTP Strict Transport Security (HSTS) header [RFC6797], in order to allow Web servers to advertise that they are willing to accept TLS-only clients. - o When applicable, Web servers SHOULD use HSTS to indicate that they - are willing to accept TLS-only clients. + o Web servers SHOULD use HSTS to indicate that they are willing to + accept TLS-only clients, unless they are deployed in such a way + that using HSTS would in fact weaken overall security (e.g., it + can be problematic to use HSTS with self-signed certificates, as + described in Section 11.3 of [RFC6797]). Rationale: Combining unprotected and TLS-protected communication opens the way to SSL Stripping and similar attacks, since an initial part of the communication is not integrity protected and therefore can be manipulated by an attacker whose goal is to keep the communication in the clear. 3.3. Compression - Implementations and deployments SHOULD disable TLS-level compression - ([RFC5246], Section 6.2.2). + In order to help prevent compression-related attacks (summarized in + Section 2.6 of [RFC7457]), implementations and deployments SHOULD + disable TLS-level compression ([RFC5246], Section 6.2.2), unless the + application protocol in question has been shown not to be open to + such attacks. Rationale: TLS compression has been subject to security attacks, such as the CRIME attack. Implementers should note that compression at higher protocol levels can allow an active attacker to extract cleartext information from the connection. The BREACH attack is one such case. These issues can only be mitigated outside of TLS and are thus out of scope of the current document. See Section 2.6 of [RFC7457] for further details. @@ -323,34 +333,40 @@ the TLS endpoint (either client or server) and its secrets from reading either past or future communication. The tickets must be managed so as not to negate this security property. 3.5. TLS Renegotiation Where handshake renegotiation is implemented, both clients and servers MUST implement the renegotiation_info extension, as defined in [RFC5746]. - To counter the Triple Handshake attack, the recommended - countermeasures from [triple-handshake] are adopted: TLS clients - SHOULD apply the same validation policy for all certificates received - over a connection, bind the master secret to the full handshake, and - bind the abbreviated session resumption handshake to the original - full handshake. In some usages, the most secure option might be to - refuse any change of certificates during renegotiation. + The most secure option for countering the Triple Handshake attack is + to refuse any change of certificates during renegotiation. In + addition, TLS clients SHOULD apply the same validation policy for all + certificates received over a connection. The [triple-handshake] + document suggests several other possible countermeasures, such as + binding the master secret to the full handshake (see + [I-D.ietf-tls-session-hash]) and binding the abbreviated session + resumption handshake to the original full handshake. Although the + latter two techniques are still under development and thus do not + qualify as current practices, those who implement and deploy TLS are + advised to watch for further development of appropriate + countermeasures. 3.6. Server Name Indication TLS implementations MUST support the Server Name Indication (SNI) - extension for those higher level protocols which would benefit from - it, including HTTPS. However, unlike implementation, the use of SNI - in particular circumstances is a matter of local policy. + extension defined in Section 3 of [RFC6066] for those higher level + protocols which would benefit from it, including HTTPS. However, + unlike implementation, the use of SNI in particular circumstances is + a matter of local policy. Rationale: SNI supports deployment of multiple TLS-protected virtual servers on a single address, and therefore enables fine-grained security for these virtual servers, by allowing each one to have its own certificate. 4. Recommendations: Cipher Suites TLS and its implementations provide considerable flexibility in the selection of cipher suites. Unfortunately, some available cipher @@ -379,51 +395,58 @@ provide no confidentiality services. Any entity in the network with access to the connection can view the plaintext of contents being exchanged by the client and server. (Nevertheless, this document does not discourage software from implementing NULL cipher suites, since they can be useful for testing and debugging.) o Implementations MUST NOT negotiate RC4 cipher suites. Rationale: The RC4 stream cipher has a variety of cryptographic - weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. Note - that DTLS specifically forbids the use of RC4 already. + weaknesses, as documented in [RFC7465]. Note that DTLS + specifically forbids the use of RC4 already. o Implementations MUST NOT negotiate cipher suites offering less - than 112 bits of security, including the so-called "export-level" + than 112 bits of security, including so-called "export-level" encryption (which provide 40 or 56 bits of security). Rationale: Based on [RFC3766], at least 112 bits of security is needed. 40-bit and 56-bit security are considered insecure today. TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers. o Implementations SHOULD NOT negotiate cipher suites that use algorithms offering less than 128 bits of security. Rationale: Cipher suites that offer between 112-bits and 128-bits of security are not considered weak at this time, however it is expected that their useful lifespan is short enough to justify supporting stronger cipher suites at this time. 128-bit ciphers are expected to remain secure for at least several years, and - 256-bit ciphers "until the next fundamental technology - breakthrough". Note that, because of so-called "meet-in-the- + 256-bit ciphers until the next fundamental technology + breakthrough. Note that, because of so-called "meet-in-the- middle" attacks [Multiple-Encryption] some legacy cipher suites (e.g., 168-bit 3DES) have an effective key length which is smaller than their nominal key length (112 bits in the case of 3DES). Such cipher suites should be evaluated according to their effective key length. - o Implementations MUST support, and SHOULD prefer to negotiate, - cipher suites offering forward secrecy, such as those in the - Ephemeral Diffie-Hellman and Elliptic Curve Ephemeral Diffie- - Hellman ("DHE" and "ECDHE") families. + o Implementations SHOULD NOT negotiate cipher suites based on RSA + key transport, a.k.a. "static RSA". + + Rationale: These cipher suites, which have assigned values + starting with the string "TLS_RSA_WITH_*", have several drawbacks, + especially the fact that they do not support forward secrecy. + + o Implementations MUST support and prefer to negotiate, cipher + suites offering forward secrecy, such as those in the Ephemeral + Diffie-Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" + and "ECDHE") families. Rationale: Forward secrecy (sometimes called "perfect forward secrecy") prevents the recovery of information that was encrypted with older session keys, thus limiting the amount of time during which attacks can be successful. See Section 7.3 for a detailed discussion. 4.2. Recommended Cipher Suites Given the foregoing considerations, implementation and deployment of @@ -434,56 +457,57 @@ o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 These cipher suites are supported only in TLS 1.2 because they are authenticated encryption (AEAD) algorithms [RFC5116]. Typically, in order to prefer these suites, the order of suites needs - to be explicitly configured in server software. It would be ideal if - server software implementations were to prefer these suites by + to be explicitly configured in server software (see [BETTERCRYPTO] + for helpful deployment guidelines, but note that its recommendations + differ from the current document in some details). It would be ideal + if server software implementations were to prefer these suites by default. Some devices have hardware support for AES-CCM but not AES-GCM, so they are unable to follow the foregoing recommendations regarding cipher suites. There are even devices that do not support public key cryptography at all, but they are out of scope entirely. 4.2.1. Implementation Details Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the first proposal to any server, unless they have prior knowledge that the server cannot respond to a TLS 1.2 client_hello message. - Servers SHOULD prefer this cipher suite over weaker cipher suites + Servers MUST prefer this cipher suite over weaker cipher suites whenever it is proposed, even if it is not the first proposal. Clients are of course free to offer stronger cipher suites, e.g., using AES-256; when they do, the server SHOULD prefer the stronger cipher suite unless there are compelling reasons (e.g., seriously degraded performance) to choose otherwise. This document does not change the mandatory-to-implement TLS cipher - suite(s) prescribed by TLS or application protocols using TLS. To - maximize interoperability, RFC 5246 mandates implementation of the - TLS_RSA_WITH_AES_128_CBC_SHA cipher suite, which is significantly - weaker than the cipher suites recommended here (the GCM mode does not - suffer from the same weakness, caused by the order of MAC-then- - Encrypt in TLS [Krawczyk2001], since it uses an Authenticated - Encryption with Associated Data (AEAD) mode of operation). - - Implementers should consider the interoperability gain against the - loss in security when deploying that cipher suite. Other application - protocols specify other cipher suites as mandatory to implement - (MTI). + suite(s) prescribed by TLS. To maximize interoperability, RFC 5246 + mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher + suite, which is significantly weaker than the cipher suites + recommended here (the GCM mode does not suffer from the same + weakness, caused by the order of MAC-then-Encrypt in TLS + [Krawczyk2001], since it uses an Authenticated Encryption with + Associated Data (AEAD) mode of operation). Implementers should + consider the interoperability gain against the loss in security when + deploying the TLS_RSA_WITH_AES_128_CBC_SHA cipher suite. Other + application protocols specify other cipher suites as mandatory to + implement (MTI). Note that some profiles of TLS 1.2 use different cipher suites. For example, [RFC6460] defines a profile that uses the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites. [RFC4492] allows clients and servers to negotiate ECDH parameters (curves). Both clients and servers SHOULD include the "Supported Elliptic Curves" extension [RFC4492]. For interoperability, clients and servers SHOULD support the NIST P-256 (secp256r1) curve @@ -529,21 +553,23 @@ TLS 1.2. 4.4. Modular vs. Elliptic Curve DH Cipher Suites Not all TLS implementations support both modular and elliptic curve Diffie-Hellman groups, as required by Section 4.2. Some implementations are severely limited in the length of DH values. When such implementations need to be accommodated, the following are RECOMMENDED (in priority order): - 1. Elliptic Curve DHE with negotiated parameters [RFC5289] + 1. Elliptic Curve DHE with appropriately negotiated parameters + (e.g., the curve to be used) and a MAC algorithm stronger than + HMAC-SHA1 [RFC5289] 2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit Diffie-Hellman parameters 3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters. Rationale: Although Elliptic Curve Cryptography is widely deployed there are some communities where its uptake has been limited for several reasons, including its complexity compared to modular arithmetic and longstanding perceptions of IPR concerns (which, for @@ -579,31 +605,52 @@ Implementations MUST NOT use the Truncated HMAC extension, defined in Section 7 of [RFC6066]. Rationale: the extension does not apply to the AEAD cipher suites recommended above. However it does apply to most other TLS cipher suites. Its use has been shown to be insecure in [PatersonRS11]. 5. Applicability Statement - The deployment recommendations of this document address the operators - of application layer services that are most commonly used on the - Internet, including, but not limited to: + The recommendations of this document primarily apply to the + implementation and deployment of application protocols that are most + commonly used with TLS and DTLS on the Internet today. Examples + include, but are not limited to: - o Operators of web services that wish to protect HTTP with TLS. + o Web software and services that wish to protect HTTP traffic with + TLS. - o Operators of email services who wish to protect the application- - layer protocols with TLS (e.g., IMAP, POP3 or SMTP). + o Email software and services that wish to protect IMAP, POP3, or + SMTP traffic with TLS. - o Operators of instant-messaging services who wish to protect their - application-layer protocols with TLS (e.g., XMPP or IRC). + o Instant-messaging software and services that wish to protect XMPP + or IRC traffic with TLS. + + o Realtime media software and services that wish to protect SRTP + traffic with DTLS. + + This document does not modify the implementation and deployment + recommendations (e.g., mandatory-to-implement cipher suites) + prescribed by existing application protocols that employ TLS or DTLS. + If the community that uses such an application protocol wishes to + modernize its usage of TLS or DTLS to be consistent with the best + practices recommended here, it needs to explicitly update the + existing application protocol definition (one example is + [I-D.ietf-uta-xmpp], which updates [RFC6120]). + + Designers of new application protocols developed through the Internet + Standards Process are expected to conform to the best practices + recommended here, unless they provide documentation of compelling + reasons that would prevent such conformance (e.g., widespread + deployment on constrained devices that lack support for the necessary + algorithms). 5.1. Security Services This document provides recommendations for an audience that wishes to secure their communication with TLS to achieve the following: o Confidentiality: all application-layer communication is encrypted with the goal that no party should be able to decrypt it except the intended receiver. @@ -805,21 +852,21 @@ known attacks. These tests are not standardized in TLS at the time of writing. See [RFC6989] for recipient tests required of IKEv2 implementations that reuse DH exponents. 7.5. Certificate Revocation The following considerations and recommendations represent the current state of the art regarding certificate revocation, even though no complete and efficient solution exists for the problem of checking the revocation status of common public key certificates - (a.k.a. PKIX certificates, [RFC5280]): + [RFC5280]: o Although Certificate Revocation Lists (CRLs) are the most widely supported mechanism for distributing revocation information, they have known scaling challenges that limit their usefulness (despite workarounds such as partitioned CRLS and delta CRLs). o Proprietary mechanisms that embed revocation lists in the Web browser's configuration database cannot scale beyond a small number of the most heavily used Web servers. @@ -838,34 +885,34 @@ o OCSP stapling as defined in [RFC6066] does not extend to intermediate certificates used in a certificate chain. Although [RFC6961] addresses this shortcoming, it is a recent addition without much deployment. o Both CRLs and OSCP depend on relatively reliable connectivity to the Internet, which might not be available to certain kinds of nodes (such as newly provisioned devices that need to establish a secure connection in order to boot up for the first time). - With regard to PKIX certificates, servers SHOULD support the - following as a best practice given the current state of the art and - as a foundation for a possible future solution: + With regard to common public key certificates, servers SHOULD support + the following as a best practice given the current state of the art + and as a foundation for a possible future solution: 1. OCSP [RFC6960] 2. Both the status_request extension defined in [RFC6066] and the status_request_v2 extension defined in [RFC6961] (this might enable interoperability with the widest range of clients) 3. The OCSP stapling extension defined in [RFC6961] - The foregoing considerations do not apply to scenarios where the - DANE-TLSA resource record [RFC6698] is used to signal to a client + The considerations in this section do not apply to scenarios where + the DANE-TLSA resource record [RFC6698] is used to signal to a client which certificate a server considers valid and good to use for TLS connections. 8. Acknowledgments Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller, Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean @@ -873,74 +920,88 @@ improvements. Thanks also to Brian Smith, who has provided a great resource in his "Proposal to Change the Default TLS Ciphersuites Offered by Browsers" [Smith2013]. Finally, thanks to all others who commented on the TLS, UTA, and other discussion lists but who are not mentioned here by name. Robert Sparks and Dave Waltermire provided helpful reviews on behalf of the General Area Review Team and the Security Directorate, respectively. + During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins, + Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick + provided comments that led to further improvements. + The authors gratefully acknowledge the assistance of Leif Johansson and Orit Levin as the working group chairs and Pete Resnick as the sponsoring Area Director. 9. References 9.1. Normative References - [I-D.ietf-tls-prohibiting-rc4] - Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf- - tls-prohibiting-rc4-01 (work in progress), October 2014. - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006. + [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC + 4949, August 2007. + [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, August 2008. [RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289, August 2008. [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, February 2010. + [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: + Extension Definitions", RFC 6066, January 2011. + [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011. [RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, March 2011. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012. + [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, + February 2015. + 9.2. Informative References + [BETTERCRYPTO] + bettercrypto.org, , "Applied Crypto Hardening", 2015, + . + [CAB-Baseline] CA/Browser Forum, , "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates Version 1.1.6", 2013, . [DegabrieleP07] Degabriele, J. and K. Paterson, "Attacking the IPsec standards in encryption-only configurations", 2007, . @@ -966,20 +1027,38 @@ Based Authentication of Named Entities (DANE) TLSA Records with SRV Records", draft-ietf-dane-srv-06 (work in progress), June 2014. [I-D.ietf-tls-downgrade-scsv] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks", draft-ietf-tls-downgrade-scsv-02 (work in progress), November 2014. + [I-D.ietf-tls-session-hash] + Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley, + A., and M. Ray, "Transport Layer Security (TLS) Session + Hash and Extended Master Secret Extension", draft-ietf- + tls-session-hash-03 (work in progress), November 2014. + + [I-D.ietf-tls-sslv3-diediedie] + Barnes, R., Thomson, M., Pironti, A., and A. Langley, + "Deprecating Secure Sockets Layer Version 3.0", draft- + ietf-tls-sslv3-diediedie-00 (work in progress), December + 2014. + + [I-D.ietf-uta-xmpp] + Saint-Andre, P. and a. alkemade, "Use of Transport Layer + Security (TLS) in the Extensible Messaging and Presence + Protocol (XMPP)", draft-ietf-uta-xmpp-05 (work in + progress), January 2015. + [Kleinjung2010] Kleinjung, T., "Factorization of a 768-Bit RSA Modulus", CRYPTO 10, 2010, . [Krawczyk2001] Krawczyk, H., "The order of encryption and authentication for protecting communications (Or: how secure is SSL?)", CRYPTO 01, 2001, . [Multiple-Encryption] @@ -1010,45 +1089,42 @@ [RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher Algorithm and Its Use with IPsec", RFC 3602, September 2003. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006. - [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC - 4949, August 2007. - [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, January 2008. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, January 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. - [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: - Extension Definitions", RFC 6066, January 2011. - [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011. [RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, August 2011. + [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence + Protocol (XMPP): Core", RFC 6120, March 2011. + [RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport Layer Security (TLS)", RFC 6460, January 2012. [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August 2012. [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, November 2012. @@ -1208,23 +1284,23 @@ o Discussion of the regular TLS mandatory cipher suite. A.12. draft-sheffer-tls-bcp-00 o Initial version. Authors' Addresses Yaron Sheffer - Porticor - 29 HaHarash St. - Hod HaSharon 4501303 + Intuit + 4 HaHarash St. + Hod HaSharon 4524075 Israel Email: yaronf.ietf@gmail.com Ralph Holz Technische Universitaet Muenchen Boltzmannstr. 3 Garching 85748 Germany