--- 1/draft-ietf-uta-tls-bcp-08.txt 2015-02-11 07:14:53.787081070 -0800 +++ 2/draft-ietf-uta-tls-bcp-09.txt 2015-02-11 07:14:53.839082340 -0800 @@ -1,21 +1,21 @@ UTA Y. Sheffer Internet-Draft Porticor Intended status: Best Current Practice R. Holz -Expires: June 10, 2015 TUM +Expires: August 15, 2015 TUM P. Saint-Andre &yet - December 7, 2014 + February 11, 2015 Recommendations for Secure Use of TLS and DTLS - draft-ietf-uta-tls-bcp-08 + draft-ietf-uta-tls-bcp-09 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 @@ -29,25 +29,25 @@ 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 June 10, 2015. + This Internet-Draft will expire on August 15, 2015. Copyright Notice - Copyright (c) 2014 IETF Trust and the persons identified as the + 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as @@ -62,67 +62,66 @@ 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 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 - 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9 + 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 10 4.2.1. Implementation Details . . . . . . . . . . . . . . . 10 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11 - 4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11 - 4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 12 + 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 - 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 17 - 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . 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 + 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 . . . . . . . . . . . . . . . . 25 - A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 25 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 + A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 26 + A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 26 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 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 - [I-D.ietf-uta-tls-attacks] provides detailed information about these - attacks. + been attacked in the context of TLS. A companion document [RFC7457] + provides detailed information about these attacks. 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 @@ -210,45 +209,45 @@ 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 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. + This BCP applies to TLS 1.2, and also to earlier versions. It is not + safe for readers to assume that the recommendations in this BCP apply + to any future version of TLS. 3.1.2. DTLS Protocol Versions DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS 1.1 was published. The following are the recommendations with respect to DTLS: - o Implementations MAY negotiate DTLS version 1.0 [RFC4347]. + 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.) 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. + 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].) @@ -256,26 +255,27 @@ 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. - o In many application protocols, clients can be configured to use - TLS no matter whether the server offers TLS during a protocol - exchange or advertises support for TLS (e.g., through a flag - indicating that TLS is required). Application clients SHOULD use - TLS by default, and disable this default only through explicit - configuration by the user. + 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. Rationale: Combining unprotected and TLS-protected communication @@ -289,22 +289,21 @@ 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 2.6 of [I-D.ietf-uta-tls-attacks] for - further details. + current document. See Section 2.6 of [RFC7457] 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 @@ -324,27 +323,27 @@ 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 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. + 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. 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 @@ -356,65 +355,70 @@ TLS and its implementations provide considerable flexibility in the selection of cipher suites. Unfortunately, some available cipher suites are insecure, some do not provide the targeted security services, and some no longer provide enough security. Incorrectly configuring a server leads to no or reduced security. This section includes recommendations on the selection and negotiation of cipher suites. 4.1. General Guidelines - Cryptographic algorithms weaken over time as cryptanalysis improves. - In other words, as time progresses, algorithms that were once - considered strong but are now weak, need to be phased out over time - and replaced with more secure cipher suites to ensure that desired - security properties still hold. SSL/TLS has been in existence for - almost 20 years at this point and this section provides some much - needed recommendations concerning cipher suite selection: + Cryptographic algorithms weaken over time as cryptanalysis improves: + algorithms that were once considered strong become weak. Such + algorithms need to be phased out over time and replaced with more + secure cipher suites. This helps to ensure that the desired security + properties still hold. SSL/TLS has been in existence for almost 20 + years and many of the cipher suites that have been recommended in + various versions of SSL/TLS are now considered weak or at least not + as strong as desired. Therefore this section modernizes the + recommendations concerning cipher suite selection: o Implementations MUST NOT negotiate the cipher suites with NULL encryption. Rationale: The NULL cipher suites do not encrypt traffic and so 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. + 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]. We - note that this guideline does not apply to DTLS, which - specifically forbids the use of RC4. + weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. 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" 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 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. + 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. 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 @@ -429,48 +434,56 @@ 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. + to be explicitly configured in server software. 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. - There are even devices that do not support public key cryptography at - all. This BCP does not cover such devices. + 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 whenever it is proposed, even - if it is not the first proposal. + Servers SHOULD 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. 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). + 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). 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 @@ -488,48 +500,48 @@ With a key exchange based on modular Diffie-Hellman ("DHE" cipher suites), DH key lengths of at least 2048 bits are RECOMMENDED. Rationale: For various reasons, in practice DH keys are typically generated in lengths that are powers of two (e.g., 2^10 = 1024 bits, 2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits would be roughly equivalent to only an 80-bit symmetric key [RFC3766], it is better to use keys longer than that for the "DHE" family of cipher suites. A DH key of 1926 bits would be roughly 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. + bits might be sufficient for at least the next 10 years + [NIST.SP.800-56A]. 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. 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. + to only an 80-bit symmetric key [ECRYPT-II]. Curves of less than + 192-bits SHOULD NOT be used. 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 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): + When such implementations need to be accommodated, the following are + RECOMMENDED (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: Although Elliptic Curve Cryptography is widely deployed there are some communities where its uptake has been limited for @@ -545,49 +557,49 @@ 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. 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] + referenced in Section 2.9 of [RFC7457] - 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 + 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 + With modular ephemeral DH, deployers ought to carefully evaluate interoperability vs. security considerations when configuring their TLS endpoints. 4.5. Truncated HMAC 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: - o Operators of web servers that wish to protect HTTP with TLS. + o Operators of web services that wish to protect HTTP with TLS. - o Operators of email servers who wish to protect the application- + o Operators of email services who wish to protect the application- layer protocols with TLS (e.g., IMAP, POP3 or SMTP). o Operators of instant-messaging services who wish to protect their application-layer protocols with TLS (e.g., XMPP or IRC). 5.1. Security Services This document provides recommendations for an audience that wishes to secure their communication with TLS to achieve the following: @@ -601,67 +613,67 @@ o Authentication: an end-point of the TLS communication is authenticated as the intended entity to communicate with. With regard to authentication, TLS enables authentication of one or both end-points in the communication. Although some TLS usage scenarios do not require authentication, those scenarios are not in scope for this document (a rationale for this decision is provided under Section 5.2). If deployers deviate from the recommendations given in this document, - they MUST verify that they do not need one of the foregoing security - services. + they need to be aware that they might lose access to one of the + foregoing security services. This document applies only to environments where confidentiality is required. It recommends algorithms and configuration options that enforce secrecy of the data-in-transit. This document also assumes that data integrity protection is always one of the goals of a deployment. In cases where integrity is not required, it does not make sense to employ TLS in the first place. There are attacks against confidentiality-only protection that utilize the lack of integrity to also break confidentiality (see for instance [DegabrieleP07] in the context of IPsec). - The intended audience covers those services that are most commonly - used on the Internet. Typically, all communication between TLS - clients and TLS servers requires all three of the above security - services. This is particularly true where TLS clients are user - agents like Web browsers or email software. + This document addresses itself to application protocols that are most + commonly used on the Internet with TLS and DTLS. Typically, all + communication between TLS clients and TLS servers requires all three + of the above security services. This is particularly true where TLS + clients are user 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. + case described under Section 5.2 below. As another scenario where + confidentiality is not needed, consider 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 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 security", and is described at - length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt]. + practice, often called "opportunistic security", is described at + length in [RFC7435]. 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 @@ -758,55 +770,56 @@ 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 - so-called Discrete Logarithm Problem (DLP) allow to derive the - session keys without an eavesdropper being able to do so. There is - currently no known attack against DLP if sufficiently large + so-called Discrete Logarithm Problem (DLP) allow the parties to + derive the session keys without an eavesdropper being able to do so. + There is currently no known attack against DLP if sufficiently large parameters are chosen. A variant of the Diffie-Hellman scheme uses Elliptic Curves instead of the originally proposed modular arithmetics. Unfortunately, many TLS/DTLS cipher suites were defined that do not - feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. We - thus advocate strict use of forward-secrecy-only ciphers. + feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This + document therefore advocates strict use of forward-secrecy-only + ciphers. 7.4. Diffie-Hellman Exponent Reuse For performance reasons, many TLS implementations reuse Diffie- Hellman and Elliptic Curve Diffie-Hellman exponents across multiple connections. Such reuse can result in major security issues: o If exponents are reused for a long time (e.g., more than a few hours), an attacker who gains access to the host can decrypt previous connections. In other words, exponent reuse negates the effects of forward secrecy. o TLS implementations that reuse exponents should test the DH public key they receive for group membership, in order to avoid some 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 - 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: + 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]): 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. @@ -825,52 +838,67 @@ 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 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. + 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: + + 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 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 + 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 Turner, and Aaron Zauner for their feedback and suggested - improvements. Thanks to Brian Smith, who has provided a great + 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. + + 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. @@ -921,55 +949,58 @@ 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. - [I-D.ietf-dane-smtp-with-dane] 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] 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] + Merkle, R. and M. Hellman, "On the security of multiple + encryption", Communications of the ACM 24, 1981, + . + + [NIST.SP.800-56A] + Barker, E., Chen, L., Roginsky, A., and M. Smid, + "Recommendation for Pair-Wise Key Establishment Schemes + Using Discrete Logarithm Cryptography", NIST Special + Publication 800-56A, 2013, . + [POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE Bites: Exploiting the SSL 3.0 Fallback", 2014, . [PatersonRS11] Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size does matter: attacks and proofs for the TLS record protocol", 2011, . @@ -1027,20 +1058,27 @@ 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. + [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection + Most of the Time", RFC 7435, December 2014. + + [RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing + Known Attacks on Transport Layer Security (TLS) and + Datagram TLS (DTLS)", RFC 7457, February 2015. + [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.