Summarizing Current Attacks on TLS and DTLS
draft-ietf-uta-tls-attacks-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 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 September 28, 2014.

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Table of Contents

• 1. Introduction
• 2. Attacks on TLS
• 2.1. BEAST
• 2.2. Lucky Thirteen
• 2.3. Attacks on RC4
• 2.4. Compression Attacks: CRIME and BREACH
• 3. Security Considerations
• 4. IANA Considerations
• 5. Acknowledgements
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Appendix: Change Log
• A.1. draft-ietf-uta-tls-bcp-00
• A.2. draft-sheffer-uta-tls-bcp-00
• Authors' Addresses

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].

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. 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 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 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.

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, and Rich Salz for their review of a previous version of this document.

The document was prepared using the lyx2rfc tool, created by Nico Williams.

6. References

6.1. Normative References

6.2. Informative References

Appendix A. Appendix: Change Log

Note to RFC Editor: please remove this section before publication.

A.1. draft-ietf-uta-tls-bcp-00

• Initial WG version, with only updated references.

A.2. draft-sheffer-uta-tls-bcp-00

• Initial version, extracted from draft-sheffer-tls-bcp-01.

Authors' Addresses