Paper 2006/136

A Challenging but Feasible Blockwise-Adaptive Chosen-Plaintext Attack on SSL

Gregory V. Bard


This paper introduces a chosen-plaintext vulnerability in the Secure Sockets Layer (SSL) and Trasport Layer Security (TLS) protocols which enables recovery of low entropy strings such as can be guessed from a likely set of 2--1000 options. SSL and TLS are widely used for securing communication over the Internet. When utilizing block ciphers for encryption, the SSL and TLS standards mandate the use of the cipher block chaining (CBC) mode of encryption which requires an initialization vector (IV) in order to encrypt. Although the first IV used by SSL is a (pseudo)random string which is generated and shared during the initial handshake phase, subsequent IVs used by SSL are chosen in a deterministic, predictable pattern; in particular, the IV of a message is taken to be the final ciphertext block of the immediately-preceding message, and is therefore known to the adversary. The one-channel nature of web proxies, anonymizers or Virtual Private Networks (VPNs), results in all Internet traffic from one machine traveling over the same SSL channel. We show this provides a feasible ``point of entry'' for this attack. Moreover, we show that the location of target data among block boundaries can have a profound impact on the number of guesses required to recover that data, especially in the low-entropy case. The attack in this paper is an application of the blockwise-adaptive chosen-plaintext attack paradigm, and is the only feasible attack to use this paradigm with a reasonable probability of success. The attack will work for all versions of SSL, and TLS version 1.0. This vulnerability and others are closed in TLS 1.1 (which is still in draft status) and OpenSSL after 0.9.6d. It is hoped this paper will encourage the deprecation of SSL and speed the adoption of OpenSSL or TLS 1.1/1.2 when they are finially released.

Note: Changes relating to the directions of traffic of the adversary and target. Also some formatting changes.

Available format(s)
Publication info
Published elsewhere. Not yet published. See E-print 2004/111 which is similar in theory but very different in the actual attack.
Blockwise AdaptiveChosen Plaintext Attack (CPA)Secure Sockets Layer (SSL)Transport Layer Security (TLS)CryptanalysisHTTP-proxyInitialization Vectors (IV)Cipher Block Chaining (CBC)Virtual Private Networks (VPN).
Contact author(s)
gregory bard @ ieee org
2006-04-18: last of 2 revisions
2006-04-09: received
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Creative Commons Attribution


      author = {Gregory V.  Bard},
      title = {A Challenging but Feasible Blockwise-Adaptive Chosen-Plaintext Attack on {SSL}},
      howpublished = {Cryptology ePrint Archive, Paper 2006/136},
      year = {2006},
      note = {\url{}},
      url = {}
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