Internet DRAFT - draft-sheffer-tls-bcp
draft-sheffer-tls-bcp
UTA Y. Sheffer
Internet-Draft Porticor
Intended status: Best Current Practice R. Holz
Expires: August 17, 2014 TUM
P. Saint-Andre
&yet
February 13, 2014
Recommendations for Secure Use of TLS and DTLS
draft-sheffer-tls-bcp-02
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.
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 August 17, 2014.
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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 3
3.2. Fallback to SSL . . . . . . . . . . . . . . . . . . . . . 4
3.3. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 6
3.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 6
3.6. Session Resumption . . . . . . . . . . . . . . . . . . . 6
4. Detailed Guidelines . . . . . . . . . . . . . . . . . . . . . 6
4.1. Cipher Suite Negotiation Details . . . . . . . . . . . . 7
4.2. Alternative Cipher Suites . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . 11
A.1. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.3. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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.sheffer-uta-tls-attacks] provides
detailed information about these attacks.
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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.
The recommendations herein take into consideration the security of
various mechanisms, their technical maturity and interoperability,
and their prevalence in implementatios at the time of writing. 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 the current document.
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. 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].
3. Recommendations
3.1. 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:
o Implementations MUST NOT negotiate SSL version 2.
Rationale: SSLv2 has serious security vulnerabilities [RFC6176].
o Implementations SHOULD NOT negotiate SSL version 3.
Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
plugged some significant security holes, but did not support
strong cipher suites.
o Implementations MAY negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 (published in 1999) includes a way to downgrade
the connection to SSLv3 and does not support more modern, strong
cipher suites.
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o Implementations MAY negotiate TLS version 1.1 [RFC4346].
Rationale: TLS 1.1 (published in 2006) prevents downgrade attacks
to SSL, but does not support certain stronger cipher suites.
o Implementations MUST support, and prefer to negotiate, TLS version
1.2 [RFC5246].
Rationale: Several stronger cipher suites are available only with
TLS 1.2 (published in 2008).
As of the date of this writing, the latest version of TLS is 1.2.
When TLS is updated to a newer version, this document will be updated
to recommend support for the latest version. If this document is not
updated in a timely manner, it can be assumed that support for the
latest version of TLS is recommended.
3.2. Fallback to SSL
Some client implementations revert to SSLv3 if the server rejected
higher versions of SSL/TLS. This fallback can be forced by a MITM
attacker. Moreover, IP scans [[reference?]] show that SSLv3-only
servers amount to only about 3% of the current web server population.
Therefore, by default clients SHOULD NOT fall back from TLS to SSLv3.
3.3. Cipher Suites
It is important both to stop using old, insecure cipher suites and to
start using modern, more secure cipher suites. Therefore:
o Implementations MUST NOT negotiate the NULL cipher suites.
Rationale: The NULL cipher suites offer no encryption whatsoever
and thus are completely insecure.
o Implementations MUST NOT negotiate RC4 cipher suites
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [I-D.popov-tls-prohibiting-rc4].
o Implementations MUST NOT negotiate cipher suites offering only so-
called "export-level" encryption (including algorithms with 40
bits or 56 bits of security).
Rationale: These cipher suites are deliberately "dumbed down" and
are very easy to break.
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o Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security (even if they
advertise more bits, such as the 168-bit 3DES cipher suites).
Rationale: Although these cipher suites are not actively subject
to breakage, their useful life is short enough that stronger
cipher suites are desirable.
o Implementations SHOULD prefer cipher suites that use algorithms
with at least 128 (and, if possible, 256) bits of security.
Rationale: Although the useful life of such cipher suites is
unknown, it is probably at least several years for the 128-bit
ciphers and "until the next fundamental technology breakthrough"
for 256-bit ciphers.
o Implementations MUST support, and SHOULD prefer to negotiate,
cipher suites offering forward secrecy, such as those in the
"EDH", "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.
Given the foregoing considerations, implementation of the following
cipher suites is RECOMMENDED (see [RFC5289] for details):
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in
general.
Unfortunately, those cipher suites are supported only in TLS 1.2
since they are authenticated encryption (AEAD) algorithms [RFC5116].
A future version of this document might recommend cipher suites for
earlier versions of TLS.
[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Clients and servers SHOULD prefer verifiably random curves
(specifically Brainpool P-256, brainpoolp256r1 [RFC7027]), and fall
back to the commonly used NIST P-256 (secp256r1) curve [RFC4492]. In
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addition, clients SHOULD send an ec_point_formats extension with a
single element, "uncompressed".
3.4. Public Key Length
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 "DH" 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 "DH"
family, then use longer keys. Key lengths of at least 2048 bits are
RECOMMENDED, since they are estimated to be roughly equivalent to
112-bit symmetric keys and might be sufficient for at least the next
10 years. In addition to 2048-bit server certificates, the use of
SHA-256 fingerprints is RECOMMENDED (see [CAB-Baseline] for more
details).
Note: The foregoing recommendations are preliminary and will likely
be corrected and enhanced in a future version of this document.
3.5. Compression
Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Sec. 6.2.2).
3.6. Session Resumption
If TLS session resumption is used, care ought to be taken to do so
safely. In particular, the resumption information (either session
IDs [RFC5246] or session tickets [RFC5077]) needs to be authenticated
and encrypted to prevent modification or eavesdropping by an
attacker. For session tickets, a strong cipher suite SHOULD be used
when encrypting the ticket (as least as strong as the main TLS cipher
suite); ticket keys MUST be changed regularly, e.g. once every week,
so as not to negate the effect of forward secrecy. Session ticket
validity SHOULD be limited to a reasonable duration (e.g. 1 day), so
as not to negate the benefits of forward secrecy.
4. Detailed Guidelines
The following sections provide more detailed information about the
recommendations listed above.
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4.1. 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 (or a similar but stronger
one) whenever it is proposed, even if it is not the first proposal.
Both clients and servers SHOULD include the "Supported Elliptic
Curves" extension [RFC4492].
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 TLS mandatory cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA.
4.2. Alternative Cipher Suites
Elliptic Curves 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:
o There are no protocol mechanisms to negotiate the DH groups or
parameter lengths supported by client and server.
o There are widely deployed client implementations that reject
received DH parameters, if they are longer than 1024 bits.
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
the the RSA certificate; moreover, 1024 bits DH parameters are
generally considered insufficient at this time.
Because of the above, we recommend using (in priority order):
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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.
With modular ephemeral DH, deployers SHOULD carefully evaluate
interoperability vs. security considerations when configuring their
TLS endpoints.
5. IANA Considerations
This document requests no actions of IANA.
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. Forward Secrecy
Forward secrecy (also often called Perfect Forward Secrecy or "PFS")
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 the future, 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:
o A client or server being attacked by some other attack vector, and
the private key retrieved.
o A long-term key retrieved from a device that has been sold or
otherwise decommissioned without prior wiping.
o A long-term key used on a device as a default key [Heninger2012].
o A key generated by a Trusted Third Party like a CA, and later
retrieved from it either by extortion or compromise
[Soghoian2011].
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o A cryptographic break-through, or the use of asymmetric keys with
insufficient length [Kleinjung2010].
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.
Unfortunately, many TLS/DTLS cipher suites were defined that do not
enable PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate
strict use of PFS-only ciphers.
7. Acknowledgements
We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,
Kenny Paterson, Patrick Pelletier, and Rich Salz for their review.
Thanks to Brian Smith whose "browser cipher suites" page is a great
resource. Finally, thanks to all others who commented on the TLS and
other lists and are not mentioned here by name.
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.
[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.
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[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, March 2011.
[RFC7027] Merkle, J. and M. Lochter, "Elliptic Curve Cryptography
(ECC) Brainpool Curves for Transport Layer Security
(TLS)", RFC 7027, October 2013.
8.2. Informative References
[CAB-Baseline]
"Baseline Requirements for the Issuance and Management of
Publicly-Trusted Certificates Version 1.1.6", 2013,
<https://www.cabforum.org/documents.html>.
[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.popov-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-popov-
tls-prohibiting-rc4-01 (work in progress), October 2013.
[I-D.sheffer-uta-tls-attacks]
Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Current Attacks on TLS and DTLS", draft-sheffer-uta-tls-
attacks-00 (work in progress), February 2014.
[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
CRYPTO 10, 2010.
[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.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
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[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[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.
[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.
Appendix A. Appendix: Change Log
Note to RFC Editor: please remove this section before publication.
A.1. -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.2. -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.
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A.3. -00
o Initial version.
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
Email: ietf@stpeter.im
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