uta Y. Sheffer
Internet-Draft Porticor
Intended status: Informational R. Holz
Expires: February 13, 2015 TUM
P. Saint-Andre
&yet
August 12, 2014
Summarizing Current Attacks on TLS and DTLS
draft-ietf-uta-tls-attacks-02
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
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
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This Internet-Draft will expire on February 13, 2015.
Copyright Notice
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described in the Simplified BSD License.
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. Diffie-Hellman Parameters . . . . . . . . . . . . . . . . . 4
2.8. Renegotiation . . . . . . . . . . . . . . . . . . . . . . . 5
2.9. Triple Hanshake . . . . . . . . . . . . . . . . . . . . . . 5
2.10. Denial of Service . . . . . . . . . . . . . . . . . . . . . 5
2.11. Implementation Issues . . . . . . . . . . . . . . . . . . . 5
3. Applicability to DTLS . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . 6
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . 9
A.1. draft-ietf-uta-tls-attacks-02 . . . . . . . . . . . . . . . 9
A.2. draft-ietf-uta-tls-attacks-01 . . . . . . . . . . . . . . . 9
A.3. draft-ietf-uta-tls-attacks-00 . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
recommendations 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.
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For a more detailed discussion of the attacks listed here, the
interested reader is referred to [Attacks-iSec].
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 unencrypted protocols that request the use of TLS,
specifically 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.
The Lucky Thirteen attack can be mitigated by using authenticated
encryption like AES-GCM [RFC5288] and encrypt-then-mac
[I-D.ietf-tls-encrypt-then-mac] instead of the TLS default of MAC-
then-encrypt.
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],
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[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. For further details,
the reader is refered to [I-D.ietf-tls-prohibiting-rc4].
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 TLS 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. We
are not aware of mitigations at the TLS 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. Diffie-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 man in the middle (MITM) attacks. Unfortunately the
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TLS protocol does not require this verification, see [RFC6989] for
the IPsec analogy.
2.8. Renegotiation
A major attack on the TLS renegotiation mechanism applies to all
current versions of the protocol. The attack and the TLS extension
that resolves it are described in [RFC5746].
2.9. Triple Hanshake
The triple handshake attack [[TRIPLE-HS, add the reference when
published]] enables the attacker to cause two TLS connections to
share keying material. This leads to a multitude of attacks, e.g.
Man-in-the-Middle, breaking safe renegotiation and breaking channel
binding via TLS Exporter [RFC5705] or "tls-unique" [RFC5929].
2.10. 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
computational work. It is known that such attacks have in fact been
mounted.
2.11. Implementation Issues
Even when the protocol is fully specified, the are very common issues
that often plague implementations. In particular, the integration of
higher-level protocols, TLS and its PKI-based authentication is the
source of misunderstandings and implementation "shortcuts". An
extensive survey of these issues can be found in [Georgiev2012].
o Implementations may omit validation of the server certificate
altogether. For example, this is true of the default
implementation of HTTP client libraries in Python 2.
o Implementations may not validate the server identity. This
validation typically amounts to matching the protocol-level server
name with the certificate's Subject Alternative Name field.
o Implementations may be validating the certificate chain
incorrectly or not at all, or using an incorrect or outdated trust
anchor list.
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3. Applicability to DTLS
DTLS [RFC4347] [RFC6347] is an adaptation of TLS for UDP datagrams.
With respect to the attacks described 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. 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.
5. IANA Considerations
This document requires no IANA actions. [Note to RFC Editor: please
remove this whole section before publication.]
6. Acknowledgments
We would like to thank Stephen Farrell, Simon Josefsson, John
Mattsson, 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, Kohei Kasamatsu for text on Lucky13,
Ilari Liusvaara for text on attacks and on DTLS.
The document was prepared using the lyx2rfc tool, created by Nico
Williams.
7. Informative References
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 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.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, March 2010.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
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[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013.
[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-01 (work in progress), June 2014.
[I-D.ietf-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
tls-prohibiting-rc4-00 (work in progress), July 2014.
[I-D.ietf-tls-encrypt-then-mac]
Gutmann, P., "Encrypt-then-MAC for TLS and DTLS", draft-
ietf-tls-encrypt-then-mac-03 (work in progress), July
2014.
[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/>.
[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.
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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-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>.
[Georgiev2012]
Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
D., and V. Shmatikov, "The most dangerous code in the
world: validating SSL certificates in non-browser
software", 2012,
<http://doi.acm.org/10.1145/2382196.2382204>.
[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>.
[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/>.
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[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-attacks-02
o Added implementation issues ("most dangerous code"),
renegotiation, triple handshake.
o Added text re: mitigation of Lucky13.
o Added applicability to DTLS.
A.2. draft-ietf-uta-tls-attacks-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.3. draft-ietf-uta-tls-attacks-00
o Initial version, extracted from draft-sheffer-tls-bcp-01.
Authors' Addresses
Yaron Sheffer
Porticor
29 HaHarash St.
Hod HaSharon 4501303
Israel
Email: yaronf.ietf@gmail.com
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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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