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Prohibiting RC4 Cipher Suites in TLS

Prohibiting RC4 Cipher Suites in TLS. http ://datatracker.ietf.org/doc/draft-popov-tls-prohibiting-rc4 By: Andrei Popov andreipo@microsoft.com. RC4 is Popular. Been around since 1987. High performance (7 cycles per byte on Pentium). Stream cipher: immune to BEAST attack on TLS1.0.

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Prohibiting RC4 Cipher Suites in TLS

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  1. Prohibiting RC4 Cipher Suites in TLS http://datatracker.ietf.org/doc/draft-popov-tls-prohibiting-rc4 By: Andrei Popov andreipo@microsoft.com

  2. RC4 is Popular • Been around since 1987. • High performance (7 cycles per byte on Pentium). • Stream cipher: immune to BEAST attack on TLS1.0. • As a result, ~90% of HTTPS sites surveyed by trustworthyinternet.org support some RC4 cipher suites. • The popularity of RC4 on the server side prevents browser vendors from disabling this cipher.

  3. RC4 is Insecure • For over a decade, RC4 was known to have biases in the keystream: • Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the Key Scheduling Algorithm of RC4. Selected Areas in Cryptography, pp. 1-24", 2001. • Mantin, I. and A. Shamir, "A Practical Attack on Broadcast RC4. FSE, pp. 152-164.", 2001. • Paul, G. and S. Maitra, "Permutation after RC4 Key Scheduling Reveals the Secret Key. In Proceedings of the 14th Workshop on Selected Areas in Cryptography (SAC), pp. 360-377, vol. 4876, LNCS, Springer.", 2007. • Recent cryptanalysis results exploit biases in the RC4 keystream to recover repeatedly encrypted plaintexts: • AlFardan, N., Bernstein, D., Paterson, K., Poettering, B., and J. Schuldt, "On the security of RC4 in TLS and WPA. USENIX Security Symposium.", 2013.

  4. Applicability of Attacks to HTTPS • The latest known attacks are on the verge of becoming practically exploitable. Currently they require 2^26 sessions or 13x2^30 encryptions. • The attacker causes a browser to transmit many HTTP requests. Each request contains the same secret (cookie). Known biases in RC4 are used to recover plaintext. • The attacker does not need to be located close to the server, and no packet injection capability is required. • It suffices for the attacker to record encrypted traffic for later offline analysis.

  5. Prohibiting RC4 Cipher Suites • TLS clients MUST NOT include RC4 cipher suites in the ClientHello message. • TLS servers MUST NOT select an RC4 cipher suite when a TLS client sends such a cipher suite in the ClientHello message. • If the TLS client only offers RC4 cipher suites, the TLS server MUST terminate the handshake. The TLS server MAY send the insufficient_security fatal alert in this case.

  6. Links and Contact Information • AlFardan, N., Bernstein, D., Paterson, K., Poettering, B., and J. Schuldt, "On the security of RC4 in TLS and WPA. USENIX Security Symposium.", 2013, https://www.usenix.org/conference/usenixsecurity13/security-rc4-tls • Prohibiting RC4 Internet-Draft: http://datatracker.ietf.org/doc/draft-popov-tls-prohibiting-rc4 • Andrei Popov andreipo@microsoft.com

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