--- 1/draft-ietf-uta-tls-bcp-03.txt 2014-09-30 10:14:44.594720027 -0700 +++ 2/draft-ietf-uta-tls-bcp-04.txt 2014-09-30 10:14:44.634721019 -0700 @@ -1,131 +1,134 @@ UTA Y. Sheffer Internet-Draft Porticor Intended status: Best Current Practice R. Holz -Expires: March 25, 2015 TUM +Expires: April 3, 2015 TUM P. Saint-Andre &yet - September 21, 2014 + September 30, 2014 Recommendations for Secure Use of TLS and DTLS - draft-ietf-uta-tls-bcp-03 + draft-ietf-uta-tls-bcp-04 Abstract Transport Layer Security (TLS) and Datagram Transport Security Layer (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 both software implementations and deployed services - that use TLS and DTLS. + 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 March 25, 2015. + This Internet-Draft will expire on April 3, 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 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 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 - 2. Intended Audience . . . . . . . . . . . . . . . . . . . . . . 4 + 2. Intended Audience and Applicability Statement . . . . . . . . 4 2.1. Security Services . . . . . . . . . . . . . . . . . . . . 4 2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4 - 3. Conventions used in this document . . . . . . . . . . . . . . 4 + 3. Conventions used in this document . . . . . . . . . . . . . . 5 4. General Recommendations . . . . . . . . . . . . . . . . . . . 5 4.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5 - 4.2. Applicability to DTLS . . . . . . . . . . . . . . . . . . 5 + 4.2. Applicability to DTLS . . . . . . . . . . . . . . . . . . 6 4.3. Fallback to SSL . . . . . . . . . . . . . . . . . . . . . 6 4.4. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6 - 4.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 6 + 4.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 4.6. TLS Session Resumption . . . . . . . . . . . . . . . . . 7 4.7. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7 - 4.8. Server Name Indication . . . . . . . . . . . . . . . . . 7 - 5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 7 + 4.8. Server Name Indication . . . . . . . . . . . . . . . . . 8 + 5. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 5.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 5.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9 - 5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 9 + 5.3. Cipher Suite Negotiation Details . . . . . . . . . . . . 10 5.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 10 - 5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 10 + 5.5. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11 5.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 - 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 11 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 + 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 12 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 12 7.4. Diffie Hellman Exponent Reuse . . . . . . . . . . . . . . 13 - 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 13 + 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 14 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 15 9.2. Informative References . . . . . . . . . . . . . . . . . 15 - Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 17 - A.1. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 17 - A.2. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 17 - A.3. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 18 - A.4. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 18 - A.5. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 18 - A.6. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 18 - A.7. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 19 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 + Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 18 + A.1. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 18 + A.2. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 18 + A.3. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 18 + A.4. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 18 + A.5. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 19 + A.6. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 19 + A.7. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 19 + A.8. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 20 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction Transport Layer Security (TLS) and Datagram Transport Security Layer (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. For instance, both AES-CBC and RC4, which together comprise most current usage, have been attacked in the context of TLS. A companion document [I-D.ietf-uta-tls-attacks] 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. Note that this document provides guidance for deployed services, as well as - software implementations. 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, - confidentiality and data integrity to their communications. This - document does not address the rare deployment scenarios where one of - these three properties is not desired. + 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 confidentiality and data integrity + protection to their communications. In many (but not all) cases + authentication is also desired. This document does not address the + rare deployment scenarios where no confidentiality is desired. 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 noted otherwise, these recommendations apply to both TLS and DTLS. TLS 1.3, when it is standardized and deployed in the field, should resolve the current vulnerabilities while providing significantly better functionality, and will very likely obsolete this document. @@ -133,58 +136,80 @@ specified audience. Individual specifications may have stricter requirements related to one or more aspects of the protocol, based on their particular circumstances. When that is the case, implementers MUST adhere to those stricter requirements. 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. Intended Audience +2. Intended Audience and Applicability Statement In the following, we specify which audience this document addresses - concerning deployment. Most deployers are very likely part of this - audience, but very specialized use cases of TLS that are outside of - the intended audience can exist. + concerning deployment. This document applies only to environments + where confidentiality is required. It recommends algorithms and + configuration options that make secrecy of the data-in-transit + mandatory. While this includes the majority of the TLS use cases, + there are some notable exceptions. + + This document assumes that data integrity protection is always one of + the goals of a deployment. In cases when 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 e.g. [DegabrieleP07] + in the context of IPsec). Thus, even when using opportunistic + encryption, it is essential to provide cryptographic data integrity + protection 2.1. Security Services This document provides recommendations for an audience that wishes to secure their communication with TLS to achieve the following: - o Authentication: this means that an end-point of the TLS - communication is authenticated as the intended entity to - communicate with. TLS allows to authenticate one or both end- - points in the communication. - o Confidentiality: all (payload) communication is encrypted with the goal that no party should be able to decrypt it except the intended receiver. o Data integrity: any changes made to the communication are detectable by the receiver. - Deployers MUST verify that they do not need one of these three - properties if they deviate from the recommendations given in this + o Optionally, authentication: this means that an end-point of the + TLS communication is authenticated as the intended entity to + communicate with. TLS allows to authenticate one or both end- + points in the communication. + + Deployers MUST verify that they do not need one of the above security + services if they deviate from the recommendations given in this document. 2.2. Examples The intended audience covers those services that are most commonly - used on the Internet, among many others: + used on the Internet. Typically, all communication between clients + and servers requires all three of the above security services. o Operators of WWW servers (HTTPS). - o Operators of email servers (SMTPS, IMAPS, POPS). + o Operators of email servers who wish to protect the application- + layer protocols with TLS (e.g., IMAP, POP3, or SMTP between client + and server). - o Operators of instant-messaging services (XMPPS, IRCS). + o Operators of instant-messaging services who wish to protect their + application-layer protocols with TLS (e.g. XMPP or IRC between + client and server). + + An example of an audience not needing confidentiality is the + following: a monitored network where the authorities in charge of + that traffic domain require full access to unencrypted (plaintext) + traffic, and where users collaborate and send their traffic in the + clear. 3. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 4. General Recommendations This section provides general recommendations on the secure use of @@ -252,24 +277,24 @@ Combining unprotected and TLS-protected communication opens the way to SSL Stripping and similar attacks. Therefore: 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 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 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. 4.5. Compression Implementations and deployments SHOULD disable TLS-level compression ([RFC5246], Sec. 6.2.2), because it has been subject to security attacks. @@ -510,24 +535,26 @@ It is noted that the requirements regarding host name validation (and in general, binding between the TLS layer and the protocol that runs above it) vary between different protocols. For HTTPS, these requirements are defined by Sec. 3 of [RFC2818]. Readers are referred to [RFC6125] for further details regarding generic host name validation in the TLS context. In addition, the RFC contains a long list of example protocols, some of which implement a policy very different from HTTPS. - With some protocols, the host name is obtained indirectly and in an - insecure manner, e.g. by an insecure DNS query for an MX record. In - these cases, the host name SHOULD NOT be used as a trusted identity - even when it matches the presented certificate. + If the host name is discovered indirectly and in an insecure manner + (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD + NOT be used as a reference identifier [RFC6125] even when it matches + the presented certificate. This proviso does not apply if the host + name is discovered securely (for further discussion, see for example + [I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp]). 7.2. AES-GCM Sec. Section 5.2 above recommends the use of the AES-GCM authenticated encryption algorithm. Please refer to [RFC5246], Sec. 11 for general security considerations when using TLS 1.2, and to [RFC5288], Sec. 6 for security considerations that apply specifically to AES-GCM when used with TLS. 7.3. Forward Secrecy @@ -622,27 +649,27 @@ browser's configuration database cannot scale beyond a small number of the most heavily used Web servers. The current consensus appears to be that OCSP stapling, combined with a "must staple" mechanism similar to HSTS, would finally resolve this problem; in particular when used together with the extension defined in [RFC6961]. But such a mechanism has not been standardized yet. 8. Acknowledgments - We would like to thank Viktor Dukhovni, Stephen Farrell, Simon - Josefsson, Watson Ladd, Orit Levin, Johannes Merkle, Bodo Moeller, - Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ritter, Rich Salz, - Aaron Zauner for their review and 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 - lists and are not mentioned here by name. + We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen + Farrell, Simon Josefsson, Watson Ladd, Orit Levin, Johannes Merkle, + Bodo Moeller, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom + Ritter, Rich Salz, Aaron Zauner for their review and 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 lists and 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. @@ -650,22 +677,22 @@ Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006. [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, + [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. [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 @@ -678,34 +705,50 @@ Security Version 1.2", RFC 6347, January 2012. 9.2. Informative References [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, + . + [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.ietf-dane-smtp] + Finch, T., "Secure SMTP using DNS-Based Authentication of + Named Entities (DANE) TLSA records.", draft-ietf-dane- + smtp-01 (work in progress), February 2013. + + [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-07 (work in + progress), July 2014. + [I-D.ietf-tls-prohibiting-rc4] Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf- tls-prohibiting-rc4-00 (work in progress), July 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-01 (work in progress), June 2014. + attacks-04 (work in progress), September 2014. [Kleinjung2010] Kleinjung, T., "Factorization of a 768-Bit RSA Modulus", CRYPTO 10, 2010, . [PatersonRS11] Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size does matter: attacks and proofs for the TLS record protocol", 2011, . @@ -759,107 +802,111 @@ [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-03 +A.1. draft-ietf-uta-tls-bcp-04 + + o Some cleanup, and input from TLS WG discussion on applicability. + +A.2. 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.2. draft-ietf-uta-tls-bcp-02 +A.3. 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.3. draft-ietf-tls-bcp-01 +A.4. 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.4. draft-ietf-tls-bcp-00 +A.5. draft-ietf-tls-bcp-00 o Initial WG version, with only updated references. -A.5. draft-sheffer-tls-bcp-02 +A.6. 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.6. draft-sheffer-tls-bcp-01 +A.7. draft-sheffer-tls-bcp-01 o Clarified our motivation in the introduction. o Added a section justifying the need for PFS. 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.7. draft-sheffer-tls-bcp-00 +A.8. draft-sheffer-tls-bcp-00 o Initial version. Authors' Addresses Yaron Sheffer Porticor 29 HaHarash St. Hod HaSharon 4501303 Israel