uta Y. Sheffer
Internet-Draft Porticor
Intended status: Informational R. Holz
Expires: December 26, 2014 TUM
P. Saint-Andre
&yet
June 24, 2014
Summarizing Current Attacks on TLS and DTLS
draft-ietf-uta-tls-attacks-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 summarizes these attacks, with the goal of
motivating generic and protocol-specific recommendations on the usage
of TLS and DTLS.
Status of This Memo
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This Internet-Draft will expire on December 26, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Attacks on TLS . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. SSL Stripping . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. BEAST . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Lucky Thirteen . . . . . . . . . . . . . . . . . . . . . . 3
2.4. Attacks on RC4 . . . . . . . . . . . . . . . . . . . . . . 3
2.5. Compression Attacks: CRIME and BREACH . . . . . . . . . . . 4
2.6. Certificate Attacks . . . . . . . . . . . . . . . . . . . . 4
2.7. Diffe-Hellman Parameters . . . . . . . . . . . . . . . . . 4
2.8. Denial of Service . . . . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . 7
A.1. draft-ietf-uta-tls-bcp-01 . . . . . . . . . . . . . . . . . 7
A.2. draft-ietf-uta-tls-bcp-00 . . . . . . . . . . . . . . . . . 7
A.3. draft-sheffer-uta-tls-bcp-00 . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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.
This situation motivated the creation of the UTA working group, which
is tasked with the creation of generic and protocol-specific
recommendation for the use of TLS and DTLS.
"Attacks always get better; they never get worse" (ironically, this
saying is attributed to the NSA). This list of attacks describes our
knowledge as of this writing. It seems likely that new attacks will
be invented in the future.
For a more detailed discussion of the attacks listed here, the
interested reader is referred to [Attacks-iSec].
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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
listed below, we believe that their root causes necessitate a more
systemic solution.
2.1. SSL Stripping
Various attacks attempt to remove the use of SSL/TLS altogether, by
modifying HTTP traffic and HTML pages as they pass on the wire.
These attacks are known collectively as SSL Stripping, and were first
introduced by Moxie Marlinspike [SSL-Stripping]. In the context of
Web traffic, these attacks are only effective if the client accesses
a Web server using a mixture of HTTP and HTTPS.
2.2. BEAST
The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
of CBC (that is, the predictable initialization vector) to decrypt
parts of a packet, and specifically to decrypt HTTP cookies when HTTP
is run over TLS.
2.3. 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.
2.4. Attacks on RC4
The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)
for many years. RC4 has long been known to have a variety of
cryptographic weaknesses, e.g. [RC4-Attack-Pau], [RC4-Attack-Man],
[RC4-Attack-FMS]. Recent cryptanalysis results [RC4-Attack-AlF]
exploit biases in the RC4 keystream to recover repeatedly encrypted
plaintexts.
These recent results are on the verge of becoming practically
exploitable; currently they require 2^26 sessions or 13x2^30
encryptions. As a result, RC4 can no longer be seen as providing a
sufficient level of security for TLS sessions.
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2.5. Compression Attacks: CRIME and BREACH
The CRIME attack [CRIME] allows an active attacker to decrypt
ciphertext (specifically, cookies) when TLS is used with protocol-
level compression.
The TIME attack [TIME] and the later BREACH attack [BREACH] both make
similar use of HTTP-level compression to decrypt secret data passed
in the HTTP response. We note that compression of the HTTP message
body is much more prevalent than compression at the TLS level.
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 [I-D.ietf-uta-tls-bcp] are adopted.
2.6. Certificate Attacks
There have been several practical attacks on TLS when used with RSA
certificates (the most common use case). These include
[Bleichenbacher98] and [Klima03]. While the Bleichenbacher attack
has been mitigated in TLS 1.0, the Klima attack that relies on a
version-check oracle is only mitigated by TLS 1.1.
The use of RSA certificates often involves exploitable timing issues
[Brumley03], unless the implementation takes care to explicitly
eliminate them.
2.7. Diffe-Hellman Parameters
TLS allows to define ephemeral Diffie-Hellman and Elliptic Curve
Diffie-Hellman parameters in its respective key exchange modes. This
results in an outstanding attack, detailed in [Cross-Protocol]. In
addition, clients that do not properly verify the received parameters
are exposed to MITM attacks. Unfortunately the TLS protocol does not
require this verification, see [RFC6989] for the IPsec analogy.
2.8. Denial of Service
Server CPU power has progressed over the years so that TLS can now be
turned on by default. However the risk of malicious clients and
coordinated groups of clients ("botnets") mounting denial of service
attacks is still very real. TLS adds another vector for
computational attacks, since a client can easily (with little
computational effort) force the server to expend relatively large
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computational work. It is known that such attacks have in fact been
mounted.
3. Security Considerations
This document describes protocol attacks in an informational manner,
and in itself does not have any security implications. Its companion
documents certainly do.
4. IANA Considerations
This document requires no IANA actions.
5. Acknowledgements
We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,
Kenny Paterson, Patrick Pelletier, Tom Ritter and Rich Salz for their
review of this document. We thank Andrei Popov for contributing text
on RC4.
The document was prepared using the lyx2rfc tool, created by Nico
Williams.
6. References
6.1. Normative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
6.2. Informative References
[I-D.ietf-uta-tls-bcp]
Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of TLS and DTLS", draft-
ietf-uta-tls-bcp-00 (work in progress), March 2014.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, 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.
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[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/>.
[TIME] Be'ery, T. and A. Shulman, "A Perfect CRIME? Only TIME
Will Tell", Black Hat Europe 2013, 2013,
<https://media.blackhat.com/eu-13/briefings/Beery/bh-
eu-13-a-perfect-crime-beery-wp.pdf>.
[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-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>.
[Attacks-iSec]
Sarkar, P. and S. Fitzgerald, "Attacks on SSL, a
comprehensive study of BEAST, CRIME, TIME, BREACH, Lucky13
and RC4 biases", 8 2013, <https://www.isecpartners.com/
media/106031/ssl_attacks_survey.pdf>.
[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>.
[Cross-Protocol]
Mavrogiannopoulos, N., Vercauteren, F., Velichkov, V., and
B. Preneel, "A cross-protocol attack on the TLS protocol",
2012, <http://doi.acm.org/10.1145/2382196.2382206>.
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[RC4-Attack-Pau]
Paul, G. and S. Maitra, "Permutation after RC4 key
scheduling reveals the secret key.", 2007,
<http://dblp.uni-trier.de/db/conf/sacrypt/
sacrypt2007.html#PaulM07>.
[RC4-Attack-Man]
Mantin, I. and A. Shamir, "A practical attack on broadcast
RC4", 2001.
[SSL-Stripping]
Marlinspike, M., "SSL Stripping", February 2009,
<http://www.thoughtcrime.org/software/sslstrip/>.
[Bleichenbacher98]
Bleichenbacher, D., "Chosen ciphertext attacks against
protocols based on the RSA encryption standard pkcs1",
1998.
[Klima03] Klima, V., Pokorny, O., and T. Rosa, "Attacking RSA-based
sessions in SSL/TLS", 2003.
[Brumley03]
Brumley, D. and D. Boneh, "Remote timing attacks are
practical", 2003.
Appendix A. Appendix: Change Log
Note to RFC Editor: please remove this section before publication.
A.1. draft-ietf-uta-tls-bcp-01
o Added SSL Stripping, attacks related to certificates, Diffie
Hellman parameters and denial of service.
o Expanded on RC4 attacks, thanks to Andrei Popov.
A.2. draft-ietf-uta-tls-bcp-00
o Initial WG version, with only updated references.
A.3. draft-sheffer-uta-tls-bcp-00
o Initial version, extracted from draft-sheffer-tls-bcp-01.
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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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