--- 1/draft-ietf-uta-tls-bcp-07.txt 2014-12-07 16:14:52.530403027 -0800 +++ 2/draft-ietf-uta-tls-bcp-08.txt 2014-12-07 16:14:52.578404204 -0800 @@ -1,21 +1,21 @@ UTA Y. Sheffer Internet-Draft Porticor Intended status: Best Current Practice R. Holz -Expires: May 15, 2015 TUM +Expires: June 10, 2015 TUM P. Saint-Andre &yet - November 11, 2014 + December 7, 2014 Recommendations for Secure Use of TLS and DTLS - draft-ietf-uta-tls-bcp-07 + draft-ietf-uta-tls-bcp-08 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 @@ -29,21 +29,21 @@ 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 May 15, 2015. + This Internet-Draft will expire on June 10, 2015. Copyright Notice Copyright (c) 2014 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 @@ -54,60 +54,61 @@ described in the Simplified BSD License. Table of Contents 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 . . . . . . . . . . . . . 5 + 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6 - 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 6 + 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9 4.2.1. Implementation Details . . . . . . . . . . . . . . . 10 - 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 10 + 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11 4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11 4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 12 - 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 12 - 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 12 - 5.2. Unauthenticated TLS and Opportunistic Encryption . . . . 13 - 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 - 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 14 - 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 15 - 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 15 - 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 16 + 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 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 17 - 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 18 - 9.2. Informative References . . . . . . . . . . . . . . . . . 19 - Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21 - A.1. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 21 - A.2. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 21 - A.3. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 21 - A.4. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 22 - A.5. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 22 - A.6. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 22 - A.7. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 22 - A.8. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 23 - A.9. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 23 - A.10. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 23 - A.11. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 23 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 + 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 19 + 9.2. Informative References . . . . . . . . . . . . . . . . . 20 + Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23 + A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 23 + A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 23 + A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 23 + A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 23 + A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 23 + A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 23 + A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 24 + A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 24 + A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 24 + A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 25 + A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 25 + A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 25 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 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 @@ -141,21 +142,24 @@ 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. + advised to adhere to those stricter requirements. Furthermore, this + document provides a floor, not a ceiling, so stronger options are + always allowed (e.g., depending on differing evaluations of the + importance of cryptographic strength vs. computational load). Community knowledge about the strength of various algorithms and feasible attacks can change quickly, and experience shows that a security BCP is a point-in-time statement. Readers are advised to seek out any errata or updates that apply to this document. 2. Terminology A number of security-related terms in this document are used in the sense defined in [RFC4949]. @@ -188,30 +192,32 @@ 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. o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246]. Rationale: TLS 1.0 (published in 1999) does not support many - modern, strong cipher suites. + 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 MAY negotiate TLS version 1.1 [RFC4346]. + o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346]. 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, and prefer to negotiate, TLS version - 1.2 [RFC5246]. + 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 recommended by this document (Section 4.2 below) are only available in TLS 1.2. This BCP applies to TLS 1.2. It is not safe for readers to assume that the recommendations in this BCP apply to any future version of TLS. @@ -236,21 +242,23 @@ Clients that "fallback" to lower versions of the protocol after the server rejects higher versions of the protocol MUST NOT fallback to SSLv3. 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. + 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: 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. @@ -281,61 +289,62 @@ Implementations and deployments SHOULD disable TLS-level compression ([RFC5246], Section 6.2.2). 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 2.5 of [I-D.ietf-uta-tls-attacks] for + current document. See Section 2.6 of [I-D.ietf-uta-tls-attacks] for further details. 3.4. TLS Session Resumption If TLS session resumption is used, care ought to be taken to do so safely. In particular, when using session tickets [RFC5077], the resumption information MUST be authenticated and encrypted to prevent modification or eavesdropping by an attacker. Further recommendations apply to session tickets: o A strong cipher suite MUST be used when encrypting the ticket (as least as strong as the main TLS cipher suite). o Ticket keys MUST be changed regularly, e.g., once every week, so as not to negate the benefits of forward secrecy (see Section 7.3 for details on forward secrecy). - o Session ticket validity SHOULD be limited to a reasonable duration - (e.g., 1 day), for similar reasons. + o For similar reasons, session ticket validity SHOULD be limited to + a reasonable duration (e.g., half as long as ticket key validity). Rationale: session resumption is another kind of TLS handshake, and therefore must be as secure as the initial handshake. This document (Section 4) recommends the use of cipher suites that provide forward secrecy, i.e. that prevent an attacker who gains momentary access to 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, we adopt the recommendation - from [triple-handshake]: TLS clients SHOULD ensure that all - certificates received over a connection are valid for the current - server endpoint, and abort the handshake if they are not. In some - usages, it may be simplest to refuse any change of certificates - during renegotiation. + To counter the Triple Handshake attack, we adopt the recommended + countermeasures from [triple-handshake]: 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, it may be simplest to refuse any change + of certificates during renegotiation. 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. Rationale: SNI supports deployment of multiple TLS-protected virtual servers on a single address, and therefore enables fine-grained @@ -410,50 +419,58 @@ 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 the following cipher suites is RECOMMENDED: o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 - 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 since they are + + 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. + Some devices have hardware support for AES-CCM but not AES-GCM. + There are even devices that do not support public key cryptography at + all. This BCP does not cover such devices. + 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 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 is not an application profile standard, in the sense of - Section 9 of [RFC5246]. As a result, clients and servers are still - REQUIRED to support the mandatory TLS cipher suite, - TLS_RSA_WITH_AES_128_CBC_SHA. + 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. 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). 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 @@ -479,53 +497,69 @@ equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048 bits might be sufficient for at least the next 10 years. See Section 4.4 for additional information on the use of modular Diffie- Hellman in TLS. As noted in [RFC3766], correcting for the emergence of a TWIRL machine would imply that 1024-bit DH keys yield about 65 bits of equivalent strength and that a 2048-bit DH key would yield about 92 bits of equivalent strength. - Servers SHOULD authenticate using at least 2048-bit certificates. In - addition, the use of SHA-256 fingerprints is RECOMMENDED (see - [CAB-Baseline] for more details). Clients SHOULD indicate to servers - that they request SHA-256, by using the "Signature Algorithms" - extension defined in TLS 1.2. + With regard to ECDH keys, the IANA named curve registry contains + 160-bit elliptic curves which are considered to be roughly equivalent + to only an 80-bit symmetric key [ECRYPT-II]. The use of curves of + less than 192-bits is NOT RECOMMENDED. + + When using RSA servers SHOULD authenticate using certificates with at + least a 2048-bit modulus for the public key. In addition, the use of + the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for + more details). Clients SHOULD indicate to servers that they request + SHA-256, by using the "Signature Algorithms" extension defined in + TLS 1.2. 4.4. Modular vs. Elliptic Curve DH Cipher Suites - Not all TLS implementations support both modular and EC 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, we recommend using (in - priority order): + 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, we recommend using + (in priority order): 1. Elliptic Curve DHE with negotiated parameters [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: Elliptic Curve Cryptography is not universally deployed - for several reasons, including its complexity compared to modular + 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 - the most part, have now been resolved [RFC6090]). On the other hand, - there are two related issues hindering effective use of modular - Diffie-Hellman cipher suites in TLS: + the most part, have now been resolved [RFC6090]). Note that ECDHE + cipher suites exist for both RSA and ECDSA certificates so moving to + ECDHE cipher suites does not require moving away from RSA based + certificates. On the other hand, there are two related issues + hindering effective use of modular Diffie-Hellman cipher suites in + TLS: - o There are no protocol mechanisms to negotiate the DH groups or - parameter lengths supported by client and server. + o There are no standardized, widely implemented protocol mechanisms + to negotiate the DH groups or parameter lengths supported by + client and server. + + o Many servers choose DH parameters of 1024 bits or fewer. o There are widely deployed client implementations that reject - received DH parameters if they are longer than 1024 bits. + received DH parameters if they are longer than 1024 bits. In + addition, several implementations do not perform appropriate + validation of group parameters and are vulnerable to attacks + referenced in Section 2.9 of [I-D.ietf-uta-tls-attacks] We note that with DHE and ECDHE cipher suites, the TLS master key only depends on the Diffie-Hellman parameters and not on the strength of the RSA certificate; moreover, 1024 bit modular DH parameters are generally considered insufficient at this time. With modular ephemeral DH, deployers SHOULD carefully evaluate interoperability vs. security considerations when configuring their TLS endpoints. @@ -595,39 +629,39 @@ agents like Web browsers or email software. This document does not address the rarer deployment scenarios where one of the above three properties is not desired, such as the use case described under Section 5.2 below. Another example of an audience not needing confidentiality is the following: a monitored network where the authorities in charge of the respective traffic domain require full access to unencrypted (plaintext) traffic, and where users collaborate and send their traffic in the clear. -5.2. Unauthenticated TLS and Opportunistic Encryption +5.2. Unauthenticated TLS and Opportunistic Security Several important applications use TLS to protect data between a TLS client and a TLS server, but do so without the TLS client necessarily verifying the server's certificate. This practice is often called "unauthenticated TLS". The reader is referred to [I-D.ietf-dane-smtp-with-dane] for an example and an explanation of why this less secure practice will likely remain common in the context of SMTP (especially for MTA-to-MTA communications). The practice is also encountered in similar contexts such as server-to- server traffic on the XMPP network (where multi-tenant hosting environments make it difficult for operators to obtain proper certificates for all of the domains they service). Furthermore, in some scenarios the use of TLS itself is optional, i.e. the client decides dynamically ("opportunistically") whether to use TLS with a particular server or to connect in the clear. This - practice, often called "opportunistic encryption", and is described - at length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt]. + practice, often called "opportunistic security", and is described at + length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt]. It can be argued that the recommendations provided in this document ought to apply equally to unauthenticated TLS as well as authenticated TLS. That would keep TLS implementations and deployments in sync, which is a desirable property given that servers can be used simultaneously for unauthenticated TLS and for authenticated TLS (indeed, a server cannot know whether a client might attempt authenticated or unauthenticated TLS). On the other hand, it has been argued that some of the recommendations in this document might be too strict for unauthenticated scenarios and that @@ -710,20 +744,24 @@ o A long-term key used on a device as a default key [Heninger2012]. o A key generated by a Trusted Third Party like a CA, and later retrieved from it either by extortion or compromise [Soghoian2011]. o A cryptographic break-through, or the use of asymmetric keys with insufficient length [Kleinjung2010]. + o Social engineering attacks against system administrators. + + o Collection of private keys from inadequately protected backups. + Forward secrecy ensures in such cases that the session keys cannot be determined even by an attacker who obtains the long-term keys some time after the conversation. It also protects against an attacker who is in possession of the long-term keys, but remains passive during the conversation. Forward secrecy is generally achieved by using the Diffie-Hellman scheme to derive session keys. The Diffie-Hellman scheme has both parties maintain private secrets and send parameters over the network as modular powers over certain cyclic groups. The properties of the @@ -756,64 +794,78 @@ IKEv2 implementations that reuse DH exponents. 7.5. Certificate Revocation Unfortunately, no mechanism exists at this time that we can recommend as a complete and efficient solution for the problem of checking the revocation status of common public key certificates (a.k.a. PKIX certificates, [RFC5280]). The current state of the art is as follows: - o Certificate Revocation Lists (CRLs) are not scalable and therefore - rarely used. + 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 The On-Line Certification Status Protocol (OCSP) presents both - scaling and privacy issues when used for heavy traffic Web - servers. In addition, clients typically "soft-fail", meaning they - do not abort the TLS connection if the OCSP server does not - respond. + 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. + + o The On-Line Certification Status Protocol (OCSP) [RFC6960] + presents both scaling and privacy issues. In addition, clients + typically "soft-fail", meaning that they do not abort the TLS + connection if the OCSP server does not respond (however, this + might be a workaround to avoid denial of service attacks if an + OSCP responder is taken offline). o OCSP stapling (Section 8 of [RFC6066]) resolves the operational issues with OCSP, but is still ineffective in the presence of a MITM attacker because the attacker can simply ignore the client's request for a stapled OCSP response. o OCSP stapling as defined in [RFC6066] does not extend to - intermediate certificates used in a certificate chain. [RFC6961] - addresses this shortcoming, but is a recent addition without much - deployment. + intermediate certificates used in a certificate chain. Although + [RFC6961] addresses this shortcoming, it is a recent addition + without much deployment. - 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. + 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 OCSP and - OCSP stapling, including the OCSP stapling extension defined in - [RFC6961], as a best practice given the current state of the art and - as a foundation for a possible future solution. + With regard to PKIX certificates, servers SHOULD support both OCSP + [RFC6960] and OCSP stapling. To enable interoperability with the + widest range of clients, servers SHOULD support both the + status_request extension defined in [RFC6066] and the + status_request_v2 extension defined in [RFC6961]. Servers also + SHOULD support the OCSP stapling extension defined in [RFC6961] as a + best practice given the current state of the art and as a foundation + for a possible future solution. The foregoing considerations 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 We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen - Farrell, Paul Hoffman, Simon Josefsson, Watson Ladd, Orit Levin, - Ilari Liusvaara, Johannes Merkle, Bodo Moeller, Yoav Nir, Kenny - Paterson, Patrick Pelletier, Tom Ritter, Rich Salz, Sean Turner, and - Aaron Zauner for their feedback and suggested improvements. Thanks - to Brian Smith, whose "browser cipher suites" page is a great - resource. Finally, thanks to all others who commented on the TLS, - UTA, and other discussion lists but who are not mentioned here by - name. + 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 + Turner, and Aaron Zauner for their feedback and suggested + improvements. Thanks 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. 9. References 9.1. Normative References [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. @@ -858,20 +910,25 @@ 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, . + [ECRYPT-II] + Smart, N., "ECRYPT II Yearly Report on Algorithms and + Keysizes (2011-2012)", 2012, + . + [Heninger2012] Heninger, N., Durumeric, Z., Wustrow, E., and J. Halderman, "Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices", Usenix Security Symposium 2012, 2012. [I-D.farrelll-mpls-opportunistic-encrypt] Farrel, A. and S. Farrell, "Opportunistic Encryption in MPLS Networks", draft-farrelll-mpls-opportunistic- encrypt-02 (work in progress), February 2014. @@ -880,20 +937,26 @@ Dukhovni, V. and W. Hardaker, "SMTP security via opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-10 (work in progress), May 2014. [I-D.ietf-dane-srv] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 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-prohibiting-rc4] Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf- tls-prohibiting-rc4-01 (work in progress), October 2014. [I-D.ietf-uta-tls-attacks] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing Current Attacks on TLS and DTLS", draft-ietf-uta-tls- attacks-04 (work in progress), September 2014. [Kleinjung2010] @@ -951,150 +1014,170 @@ [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. + [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., + Galperin, S., and C. Adams, "X.509 Internet Public Key + Infrastructure Online Certificate Status Protocol - OCSP", + RFC 6960, June 2013. + [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, June 2013. [RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman Tests for the Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 6989, July 2013. + [Smith2013] + Smith, B., "Proposal to Change the Default TLS + Ciphersuites Offered by Browsers.", 2013, . + [Soghoian2011] Soghoian, C. and S. Stamm, "Certified lies: Detecting and defeating government interception attacks against SSL.", Proc. 15th Int. Conf. Financial Cryptography and Data Security , 2011. [triple-handshake] Delignat-Lavaud, A., Bhargavan, K., and A. Pironti, "Triple Handshakes Considered Harmful: Breaking and Fixing Authentication over TLS", 2014, . Appendix A. Change Log Note to RFC Editor: please remove this section before publication. -A.1. draft-ietf-uta-tls-bcp-07 +A.1. draft-ietf-uta-tls-bcp-08 + + o More WGLC feedback. + + o TLS 1.1 is now SHOULD NOT, just like TLS 1.0. + + o SHOULD NOT use curves of less than 192 bits for ECDH. + + o Clarification regarding OCSP and OSCP stapling. + +A.2. draft-ietf-uta-tls-bcp-07 o WGLC feedback. -A.2. draft-ietf-uta-tls-bcp-06 +A.3. draft-ietf-uta-tls-bcp-06 o Undo unauthenticated TLS, following another long thread on the list. -A.3. draft-ietf-uta-tls-bcp-05 +A.4. draft-ietf-uta-tls-bcp-05 o Lots of comments by Sean Turner. o Unauthenticated TLS, following a long thread on the list. -A.4. draft-ietf-uta-tls-bcp-04 +A.5. draft-ietf-uta-tls-bcp-04 o Some cleanup, and input from TLS WG discussion on applicability. -A.5. draft-ietf-uta-tls-bcp-03 +A.6. draft-ietf-uta-tls-bcp-03 o Disallow truncated HMAC. o Applicability to DTLS. o Some more text restructuring. o Host name validation is sometimes irrelevant. o HSTS: MUST implement, SHOULD deploy. o Session identities are not protected, only tickets are. o Clarified the target audience. -A.6. draft-ietf-uta-tls-bcp-02 +A.7. draft-ietf-uta-tls-bcp-02 o Rearranged some sections for clarity and re-styled the text so that normative text is followed by rationale where possible. o Removed the recommendation to use Brainpool curves. o Triple Handshake mitigation. o MUST NOT negotiate algorithms lower than 112 bits of security. o MUST implement SNI, but use per local policy. o Changed SHOULD NOT negotiate or fall back to SSLv3 to MUST NOT. o Added hostname validation. o Non-normative discussion of DH exponent reuse. -A.7. draft-ietf-tls-bcp-01 +A.8. draft-ietf-tls-bcp-01 o Clarified that specific TLS-using protocols may have stricter requirements. o Changed TLS 1.0 from MAY to SHOULD NOT. o Added discussion of "optional TLS" and HSTS. o Recommended use of the Signature Algorithm and Renegotiation Info extensions. o Use of a strong cipher for a resumption ticket: changed SHOULD to MUST. o Added an informational discussion of certificate revocation, but no recommendations. -A.8. draft-ietf-tls-bcp-00 +A.9. draft-ietf-tls-bcp-00 o Initial WG version, with only updated references. -A.9. draft-sheffer-tls-bcp-02 +A.10. draft-sheffer-tls-bcp-02 o Reorganized the content to focus on recommendations. o Moved description of attacks to a separate document (draft- sheffer-uta-tls-attacks). o Strengthened recommendations regarding session resumption. -A.10. draft-sheffer-tls-bcp-01 +A.11. draft-sheffer-tls-bcp-01 o Clarified our motivation in the introduction. o Added a section justifying the need for forward secrecy. o Added recommendations for RSA and DH parameter lengths. Moved from DHE to ECDHE, with a discussion on whether/when DHE is appropriate. o Recommendation to avoid fallback to SSLv3. o Initial information about browser support - more still needed! o More clarity on compression. o Client can offer stronger cipher suites. o Discussion of the regular TLS mandatory cipher suite. -A.11. draft-sheffer-tls-bcp-00 +A.12. draft-sheffer-tls-bcp-00 o Initial version. Authors' Addresses Yaron Sheffer Porticor 29 HaHarash St. Hod HaSharon 4501303 Israel