UTA Y. Sheffer
Internet-Draft Porticor
Intended status: Best Current Practice R. Holz
Expires: March 25, 2015 TUM
P. Saint-Andre
September 21, 2014

Recommendations for Secure Use of TLS and DTLS


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.

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.

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

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.

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.

These are minimum recommendations for the use of TLS for the 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

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.

2.1. Security Services

This document provides recommendations for an audience that wishes to secure their communication with TLS to achieve the following:

Deployers MUST verify that they do not need one of these three properties 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:

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 TLS. Recommendations related to cipher suites are discussed in the following section.

4.1. Protocol Versions

It is important both to stop using old, less secure versions of SSL/TLS and to start using modern, more secure versions. Therefore:

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.

4.2. Applicability to DTLS

DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP datagrams.

With respect to the recommendations in the current document, DTLS 1.0 is equivalent to TLS 1.1. The only exception is RC4 which is disallowed in DTLS. DTLS 1.2 is equivalent to TLS 1.2.

4.3. Fallback to SSL

Some client implementations revert to lower versions of TLS or even to SSLv3 if the server rejected higher versions of the protocol. This fall back can be forced by a man in the middle (MITM) attacker. By default, such clients MUST NOT fall back to SSLv3.

Rationale: 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.

4.4. Strict TLS

Combining unprotected and TLS-protected communication opens the way to SSL Stripping and similar attacks. Therefore:

4.5. Compression

Implementations and deployments SHOULD disable TLS-level compression ([RFC5246], Sec. 6.2.2), because it has been subject to security attacks.

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 Sec. 2.5 of [I-D.ietf-uta-tls-attacks] for further details.

4.6. 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:

4.7. 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.

4.8. 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.

5. Recommendations: Cipher Suites

TLS and its implementations provide considerable flexibility in the selection of cipher suites. Unfortunately many available cipher suites are insecure, and so misconfiguration can easily result in reduced security. This section includes recommendations on the selection and negotiation of cipher suites.

5.1. General Guidelines

It is important both to stop using old, insecure cipher suites and to start using modern, more secure cipher suites. Therefore:

5.2. Recommended Cipher Suites

Given the foregoing considerations, implementation of the following cipher suites is RECOMMENDED:

We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in general. See [RFC5289] for additional implementation details.

It is noted that those cipher suites are supported only in TLS 1.2 since they are authenticated encryption (AEAD) algorithms [RFC5116].

[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 [RFC4492]. In addition, clients SHOULD send an ec_point_formats extension with a single element, "uncompressed".

5.3. Cipher Suite Negotiation 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.

Note that other profiles of TLS 1.2 exist that 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.

This document is not an application profile standard, in the sense of Sec. 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.

5.4. Public Key Length

With a key exchange based on modular Diffie-Hellman ("DHE" cipher suites), key lengths of at least 2048 bits are RECOMMENDED.

Rationale: because Diffie-Hellman keys of 1024 bits are estimated to be roughly equivalent to 80-bit symmetric keys, it is better to use longer keys for the "DHE" family of cipher suites. Unfortunately, some existing software cannot handle (or cannot easily handle) key lengths greater than 1024 bits. The most common workaround for these systems is to prefer the "ECDHE" family of cipher suites instead of the "DHE" family. For modular groups, key lengths of at least 2048 bits are estimated to be roughly equivalent to 112-bit symmetric keys and might be sufficient for at least the next 10 years.

Servers SHOULD authenticate using 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.

5.5. Modular vs. Elliptic Curve DH Cipher Suites

Not all TLS implementations support both modular and EC Diffie-Hellman groups, as required by Section 5.2. 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. The same cipher suite, with 1024-bit parameters.

Rationale: Elliptic Curve Cryptography is not universally deployed for several reasons, including its complexity compared to modular arithmetic and longstanding IPR concerns. On the other hand, there are two related issues hindering effective use of modular Diffie-Hellman cipher suites in TLS:

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.

5.6. Truncated HMAC

The truncated HMAC extension, defined in Sec. 7 of [RFC6066] 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], and implementations MUST NOT use it.

6. IANA Considerations

This document requests no actions of IANA. [Note to RFC Editor: please remove this whole section before publication.]

7. Security Considerations

This entire document discusses the security practices directly affecting applications using the TLS protocol. This section contains broader security considerations related to technologies used in conjunction with or by TLS.

7.1. Host Name Validation

Application authors should take note that TLS implementations frequently do not validate host names, and must therefore determine if the TLS implementation they are using does, and if not write their own validation code or consider changing the TLS implementation.

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.

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

Forward secrecy (also often called Perfect Forward Secrecy or "PFS", and defined in [RFC4949]) is a defense against an attacker who records encrypted conversations where the session keys are only encrypted with the communicating parties' long-term keys. Should the attacker be able to obtain these long-term keys at some point later in time, he will be able to decrypt the session keys and thus the entire conversation. In the context of TLS and DTLS, such compromise of long-term keys is not entirely implausible. It can happen, for example, due to:

PFS 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.

PFS 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 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 PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate strict use of PFS-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:

7.5. Certificate Revocation

Unfortunately there is currently no effective, Internet-scale mechanism to affect certificate revocation:

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.

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.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C. and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5288] Salowey, J., Choudhury, A. and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, August 2008.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289, August 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S. and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, February 2010.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012.

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.
[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-tls-prohibiting-rc4] Popov, A., "Prohibiting RC4 Cipher Suites", Internet-Draft draft-ietf-tls-prohibiting-rc4-00, July 2014.
[I-D.ietf-uta-tls-attacks] Sheffer, Y., Holz, R. and P. Saint-Andre, "Summarizing Current Attacks on TLS and DTLS", Internet-Draft draft-ietf-uta-tls-attacks-03, 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.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007.
[RFC5077] Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, January 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, January 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011.
[RFC6101] Freier, A., Karlton, P. and P. Kocher, "The Secure Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, August 2011.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport Layer Security (TLS)", RFC 6460, January 2012.
[RFC6797] Hodges, J., Jackson, C. and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, November 2012.
[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.
[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-03

A.2. draft-ietf-uta-tls-bcp-02

A.3. draft-ietf-tls-bcp-01

A.4. draft-ietf-tls-bcp-00

A.5. draft-sheffer-tls-bcp-02

A.6. draft-sheffer-tls-bcp-01

A.7. draft-sheffer-tls-bcp-00

Authors' Addresses

Yaron Sheffer Porticor 29 HaHarash St. Hod HaSharon, 4501303 Israel EMail: yaronf.ietf@gmail.com
Ralph Holz Technische Universitaet Muenchen Boltzmannstr. 3 Garching, 85748 Germany EMail: holz@net.in.tum.de
Peter Saint-Andre &yet P.O. Box 787 Parker, CO 80134 USA EMail: peter@andyet.com