tls Y. Sheffer
Internet-Draft Porticor
Intended status: BCP R. Holz
Expires: March 24, 2014 TUM
September 20, 2013
Recommendations for Secure Use of TLS and DTLS
draft-sheffer-tls-bcp-01
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 24, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . 3
2. Attacks on TLS . . . . . . . . . . . . . . . . . . . . 3
2.1. BEAST . . . . . . . . . . . . . . . . . . . . . . . . 4
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. Summary . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Cipher Suite Negotiation Details . . . . . . . . . . . 6
4.3. Downgrade Attacks . . . . . . . . . . . . . . . . . . 6
4.4. Alternatives . . . . . . . . . . . . . . . . . . . . . 6
5. Implementation Status . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . 8
6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Perfect Forward Secrecy (PFS) . . . . . . . . . . . . 8
6.3. Session Resumption . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . 10
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 12
A.1. -01 . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. -00 . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . 12
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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, as well as for implementers. In fact the recommendations
below call for the use of widely implemented algorithms, which are
not seeing widespread use today.
Rather than standardizing new mechanisms in TLS, our goal is to
recommend a few already-specified mechanisms and cipher suites, and
to encourage the industry to use them in order to improve the overall
security of TLS-protected network traffic. When picking these
mechanisms, we consider their security, their technical maturity and
interoperability, as well as their prevalence at the time of writing.
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.
Our knowledge about the strength of various algorithms and feasible
attacks can change quickly, and experience shows that a crypto BCP is
a point-in-time statement more than other BCPs. Readers are advised
to seek out any errata or udpates that apply to this document.
1.1. Conventions used in this document
[[Are we normative? Currently we're not and 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
recommendations. This is not intended to be an extensive survey of
TLS's security.
While there are widely deployed mitigations for some of the attacks
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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.
2.2. Lucky Thirteen
A consequence of the MAC-then-encrypt design in all current versions
of TLS 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],
[RC4-Attack-AlF]) 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 BREACH attack [BREACH] makes similar use of HAdded TTP-level
compression, which is much more prevalent than compression at the TLS
level, to decrypt secret data passed in the HTTP response.
The former attack can be mitigated by disabling TLS compression, as
recommended below. We are not aware of mitigations at the protocol
level to the latter attack, and so application-level mitigations are
needed (see [BREACH]). For example, implementations of HTTP that use
CSRF tokens will need to randomize them even when the recommendations
of the current document are adopted.
3. Selection Criteria
Given the above attacks, we are proposing that deployers opt for a
specific cipher suite when negotiating TLS. We have used the
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following criteria when framing our recommendations:
o The cipher suite must be secure in default use, and should not
require any additional security measures beyond those defined in
the standard.
o The cipher suite must be widely implemented, i.e. available in a
large percentage of popular cryptographic libraries.
o The cipher suite 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 cipher suites that provide client-side privacy and
perfect forward secrecy, i.e. those that use ephemeral Diffie-
Hellman. See Section 6.2 for more details.
o As currently specified and implemented, elliptic curve groups are
preferable over modular DH groups: they are easier and safer to
use within TLS.
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 cipher suite.
4. Recommendations
Following are recommendations for people implementing and deploying
client and server-side TLS.
4.1. Summary
Based on the criteria above, we recommend using as a preferred cipher
suite the following:
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5829]
It is noted that the above cipher suite is an authenticated
encryption (AEAD) algorithm [RFC5116], and therefore requires the use
of TLS 1.2.
We recommend using 2048-bit server certificates, with a SHA-256
fingerprint. See [CAB-Baseline] for more details.
[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). We recommend that clients and servers prefer verifiably
random curves (specifically Brainpool P-256, brainpoolp256r1
[I-D.merkle-tls-brainpool]), and fall back to the commonly used NIST
P-256 (secp256r1) [RFC4492]. In addition, clients should send an
ec_point_formats extension with a single element, "uncompressed".
We recommend to always disable TLS-level compression ([RFC5246], Sec.
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6.2.2).
Finally, we recommend that clients disable fallback to SSLv3 (see
Section 4.3).
4.2. Cipher Suite Negotiation Details
We recommend that clients include the above 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 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 reasons (e.g. 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.
4.3. Downgrade Attacks
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 about 3% of the current server population. As a
result, we recommend that by default, clients should avoid falling
back to SSLv3.
4.4. Alternatives
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
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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):
1. Elliptic Curve DHE with negotiated parameters, as described in
Section 4.1.
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. Implementation Status
Since this document does not propose a new protocol or a new cipher
suite, we do not provide a full implementation status, as per
[RFC6982]. However it is useful to list some known existing
implementations of the recommended cipher suite(s).
+----------+--------------+---------------------+-------------------+
| Category | Software | As Of Version | Comment |
+----------+--------------+---------------------+-------------------+
| Library | OpenSSL | 1.0.1 | |
| | GnuTLS | | |
| | NSS | 3.11.1 | |
| Browser | Internet | IE8 on Windows 7 | |
| | Explorer | | |
| | Firefox | TBD | |
| | Chrome | TLS 1.2 and AES-GCM | |
| | | expected in Chrome | |
| | | 30 | |
| | Safari | TBD | |
| Web | Apache | ?? | |
| server | (mod_gnutls) | | |
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| | 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. Perfect Forward Secrecy (PFS)
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].
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
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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. These are listed in Section
Section 4.1.
6.3. Session Resumption
TBD, https://www.imperialviolet.org/2013/06/27/botchingpfs.html.
7. IANA Considerations
[Note to RFC Editor: please remove this section before publication.]
This document requires no IANA actions.
8. 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.
The document was prepared using the lyx2rfc tool, created by Nico
Williams.
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.
[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.
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[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5829] Brown, A., Clemm, G., and J. Reschke, "Link Relation Types
for Simple Version Navigation between Web Resources",
RFC 5829, April 2010.
[I-D.merkle-tls-brainpool]
Merkle, J. and M. Lochter, "ECC Brainpool Curves for
Transport Layer Security (TLS)",
draft-merkle-tls-brainpool-04 (work in progress),
July 2013.
9.2. 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.
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[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]
ISOBE, T., OHIGASHI, T., WATANABE, Y., and M. MORII, "Full
Plaintext Recovery Attack on Broadcast RC4", International
Workshop on Fast Software Encryption , 2013.
[RC4-Attack-AlF]
AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
and J. Schuldt, "On the Security of RC4 in TLS", Usenix
Security Symposium 2013, 2013, <https://www.usenix.org/
conference/usenixsecurity13/security-rc4-tls>.
[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>.
[CAB-Baseline]
"Baseline Requirements for the Issuance and Management of
Publicly-Trusted Certificates Version 1.1.6", 2013,
<https://www.cabforum.org/documents.html>.
[TLS-IANA]
"Transport Layer Security (TLS) Parameters - TLS Cipher
Suite Registry", <https://www.iana.org/assignments/
tls-parameters/tls-parameters.xhtml#tls-parameters-4>.
[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.
[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
CRYPTO 10, 2010.
[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.
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Appendix A. Appendix: Change Log
Note to RFC Editor: please remove this section before publication.
A.1. -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.2. -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
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