In this paper we present several weaknesses in the key scheduling algorithm of RC4, and describe their cryptanalytic significance. We identify a large number of weak keys, in which knowledge of a small number of key bits suffices to determine many state and output bits with non-negligible probability. We use these weak keys to construct new distinguishers for RC4, and to mount related key attacks with practical complexities. Finally, we show that RC4 is completely insecure in a common mode of operation which is used in the widely deployed Wired Equivalent Privacy protocol (WEP, which is part of the 802.11 standard), in which a fixed secret key is concatenated with known IV modifiers in order to encrypt different messages. Our new passive ciphertext-only attack on this mode can recover an arbitrarily long key in a negligible amount of time which grows only linearly with its size, both for 24 and 128 bit IV modifiers.
The alleged RC4 keystream generator is examined, and a method of explicitly computing digraph probabilities is given. Using this method, we demonstrate a method for distinguishing 8-bit RC4 from randomness. Our method requires less keystream output than currently published attacks, requiring only 2 30.6 bytes of output. In addition, we observe that an attacker can, on occasion, determine portions of the internal state with nontrivial probability. However, we are currently unable to extend this observation to a full attack.
Abstract. Correct authenticated decryption requires the receiver to buffer the decrypted message until the authenticity check has been performed. In high-speed networks, which must handle large message frames at low latency, this behavior becomes practically infeasible. This paper proposes CCA-secure on-line ciphers as a practical alternative to AE schemes since the former provide some defense against malicious message modifications. Unfortunately, all published on-line ciphers so far are either inherently sequential, or lack a CCA-security proof. This paper introduces POE, a family of on-line ciphers that combines provable security against chosen-ciphertext attacks with pipelineability to support efficient implementations. POE combines a block cipher and an ǫ-AXU family of hash functions. Different instantiations of POE are given, based on different universal hash functions and suitable for different platforms. Moreover, this paper introduces POET, a provably secure on-line AE scheme, which inherits pipelineability and chosen-ciphertextsecurity from POE and provides additional resistance against noncemisuse attacks.
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