Network Working Group Y. Sheffer
Internet-Draft Porticor
Intended status: BCP September 8, 2013
Expires: March 12, 2014
Recommendations for Secure Use of TLS and DTLS
draft-sheffer-tls-bcp-00
Abstract
Over the last few years there have been several serious attacks on
TLS, including attacks on its most commonly used ciphers and modes of
operation. This document offers recommendations on securely using
the TLS and DTLS protocols, given existing standards and
implementations.
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 12, 2014.
Copyright Notice
Copyright (c) 2013 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.
Sheffer Expires March 12, 2014 [Page 1]
Internet-Draft TLS Recommendations September 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . 3
2. Attacks on TLS . . . . . . . . . . . . . . . . . . . . 3
2.1. BEAST . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Lucky Thirteen . . . . . . . . . . . . . . . . . . . . 4
2.3. Attacks on RC4 . . . . . . . . . . . . . . . . . . . . 4
2.4. Compression Attacks: CRIME and BREACH . . . . . . . . . 4
3. Selection Criteria . . . . . . . . . . . . . . . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . . 5
4.1. Details . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation Status . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . 6
6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . 7
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 8
A.1. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . 8
Sheffer Expires March 12, 2014 [Page 2]
Internet-Draft TLS Recommendations September 2013
1. Introduction
Over the last few years there have been several major attacks on TLS
[RFC5246], including attacks on its most commonly used ciphers and
modes of operation. Details are given in Section 2, but suffice it
to say that both AES-CBC and RC4, which together make up for most
current usage, have been seriously attacked in the context of TLS.
Given these issues, there is need for IETF guidance on how TLS can be
used securely. Unlike most IETF documents, this is guidance for
deployers rather than for implementers. In fact the recommendations
below call for the use of widely implemented algorithms, which are
not seeing widespread use today.
This recommendation applies 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 the current document.
1.1. Conventions used in this document
[[Are we normative? This section might go away.]]
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].
2. Attacks on TLS
This section lists the attacks that motivated the current
recommendation. This is not intended to be an extensive survey of
TLS's security.
While there are widely deployed mitigations for some of the attacks
listed below, we believe that their root causes necessitate a more
systemic solution.
2.1. BEAST
The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
of CBC (that is, predictable IV) to decrypt parts of a packet, and
specifically shows how this can be used to decrypt HTTP cookies when
run over TLS.
Sheffer Expires March 12, 2014 [Page 3]
Internet-Draft TLS Recommendations September 2013
2.2. Lucky Thirteen
A consequence of the MAC-then-encrypt design is the existence of
padding oracle attacks [Padding-Oracle]. A recent incarnation of
these attacks is the Lucky Thirteen attack [CBC-Attack], a timing
side-channel attack that allows the attacker to decrypt arbitrary
ciphertext.
2.3. Attacks on RC4
The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)
for many years. Attacks have also been known for a long time, e.g.
[RC4-Attack-FMS]. But recent attacks [RC4-Attack] have weakened this
algorithm even more. See [I-D.popov-tls-prohibiting-rc4] for more
details.
2.4. Compression Attacks: CRIME and BREACH
The CRIME attack [CRIME] allows an active attacker to decrypt
cyphertext (specifically, cookies) when TLS is used with protocol-
level compression. The attack is a consequence of the TLS MAC-then-
encrypt approach.
The BREACH attack [BREACH] makes similar use of HTTP-level
compression which is much more prevalent than compression at the TLS
level, to decrypt secret data passed in the HTTP response.
While the former attack can be mitigated by disabling TLS
compression, we are not aware of mitigations at the protocol level to
the latter attack, and so application-level mitigations are needed.
For example, implementations of HTTP that use CSRF tokens will need
to randomize them even when the recommendations of the current
document are adopted.
[[Is it possible to affect some length hiding using TLS 1.2 as
specified today, i.e. without draft-pironti-tls-length-hiding-01, and
using available APIs?]]
3. Selection Criteria
Given the above attacks, we are proposing that deployers opt for a
specific ciphersuite when negotiating TLS. We have used the
following criteria when framing our recommendations:
o The ciphersuite must be secure in default use, and should not
require any additional security measures beyond those defined in
the standard.
Sheffer Expires March 12, 2014 [Page 4]
Internet-Draft TLS Recommendations September 2013
o The ciphersuite must be widely implemented, i.e. available in a
large percentage of popular cryptographic libraries.
o The ciphersuite must have undergone a significant amount of
analysis, and the algorithm and mode of operation must both be
standardized by relevant organizations.
o We prefer ciphersuites that provide client-side privacy and
perfect forward secrecy, i.e. those that use ephemeral Diffie-
Hellman.
o When there are multiple key sizes available, we have chosen the
current industry standard, 128 bits of strength. Of course
deployers are free to opt for a stronger ciphersuite.
4. Recommendations
Based on the criteria above, we recommend using as a preferred
ciphersuite the following:
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288]
It is noted that the above ciphersuite is an authenticated encryption
(AEAD) algorithm [RFC5116], and therefore requires the use of TLS
1.2.
4.1. Details
We recommend that clients include this cipher suite 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.
We recommend that servers prefer this ciphersuite (or a similar but
stronger one) whenever it is proposed, even if it is not the first
proposal.
Note that other profiles of TLS 1.2 exist that use different
ciphersuites. 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 ciphersuites.
5. Implementation Status
Since this document does not propose a new protocol or a new
ciphersuite, we do not provide a full implementation status, as per
[RFC6982]. However it is useful to list some known existing
implementations of the recommended ciphersuite(s).
Sheffer Expires March 12, 2014 [Page 5]
Internet-Draft TLS Recommendations September 2013
+----------+----------------+--------------+------------------------+
| Category | Software | As Of | Comment |
| | | Version | |
+----------+----------------+--------------+------------------------+
| Library | OpenSSL | 1.0.1 | |
| | GnuTLS | | |
| | NSS | 3.11.1 | |
| Browser | Internet | IE8 on | |
| | Explorer | Windows 7 | |
| | Firefox | | TBD |
| | Chrome | | TBD |
| | Safari | | TBD |
| Web | Apache | ?? | |
| server | (mod_gnutls) | | |
| | Apache | ?? | |
| | (mod_ssl) | | |
| | Nginx | 1.0.9, 1.1.6 | With a recent version |
| | | | of OpenSSL |
+----------+----------------+--------------+------------------------+
6. Security Considerations
6.1. AES-GCM
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.
6.2. Downgrade Attacks
[[Do we need to disallow some protocol variants, e.g. SSL 3.0, so
that there are no downgrade attacks possible?]]
7. IANA Considerations
This document requires no IANA actions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Sheffer Expires March 12, 2014 [Page 6]
Internet-Draft TLS Recommendations September 2013
[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.
8.2. Informative References
[I-D.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites",
draft-popov-tls-prohibiting-rc4-00 (work in progress),
August 2013.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
July 2013.
[CBC-Attack]
AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
the TLS and DTLS Record Protocols", IEEE Symposium on
Security and Privacy , 2013.
[BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
2011, <http://packetstormsecurity.com/files/105499/
Browser-Exploit-Against-SSL-TLS.html>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
Security Conference 2012, 2012.
[BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
2013, <http://breachattack.com/>.
[RC4] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C, 2nd Ed.", 1996.
[RC4-Attack-FMS]
Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
Key Scheduling Algorithm of RC4", Selected Areas in
Cryptography , 2001.
[RC4-Attack]
Sheffer Expires March 12, 2014 [Page 7]
Internet-Draft TLS Recommendations September 2013
ISOBE, T., OHIGASHI, T., WATANABE, Y., and M. MORII, "Full
Plaintext Recovery Attack on Broadcast RC4", International
Workshop on Fast Software Encryption , 2013.
[Padding-Oracle]
Vaudenay, S., ""Security Flaws Induced by CBC Padding
Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
2002, <http://www.iacr.org/cryptodb/archive/2002/
EUROCRYPT/2850/2850.pdf>.
Appendix A. Appendix: Change Log
Note to RFC Editor: please remove this section before publication.
A.1. -00
o Initial version.
Author's Address
Yaron Sheffer
Porticor
29 HaHarash St.
Hod HaSharon 4501303
Israel
Email: yaronf.ietf@gmail.com
Sheffer Expires March 12, 2014 [Page 8]