A draft paper by Scott Fluhrer, Itsik Mantin and Adi Shamir was released on July 25, 2001 and announces new attacks on the RC4 cipher that is the basis for CipherSaber-1. Some of these attacks specifically involve the use of an IV with a secret key, the very scheme used in CipherSaber. Prof. Shamir states in an e-mail accompanying the release: "Attached you will find a new paper which describes a truly practical direct attack on WEP's cryptography. It is an extremely powerful attack which can be applied even when WEP's RC4 stream cipher uses a 2048 bit secret key (its maximal size) and 128 bit IV modifiers (as proposed in WEP2). The attacker can be a completely passive eavesdropper (i.e., he does not have to inject packets, monitor responses, or use accomplices) and thus his existence is essentially undetectable. It is a pure known-ciphertext attack (i.e., the attacker need not know or choose their corresponding plaintexts). After scanning several hundred thousand packets, the attacker can completely recover the secret key and thus decrypt all the ciphertexts. The running time of the attack grows linearly instead of exponentially with the key size, and thus it is negligible even for 2048 bit keys." The paper itself, titled "Weaknesses in the Key Scheduling Algorithm of RC4," has been posted at http://www.eyetap.org/~rguerra/toronto2001/rc4_ksaproc.pdf (in PDF format) and at http://www.crypto.com/papers/others/rc4_ksaproc.ps (in Postscript). WEP is an encryption system used with 802.11 wireless Ethernet that employs RC4, but the attack affects CipherSaber as well. Note that "several hundred thousand" separate CipherSaber messages encrypted with the same key would have to be collected for this attack to succeed. None the less, from a cryptographic standpoint, this is too close for comfort. Accordingly I recommend that CipherSaber users switch to CipherSaber-2 with a parameter N=20 or larger. The RC4 state vector will thus be mixed 20 times instead of once. This large a value for N is probably overkill, but until there is time to fully digest the implications of this paper, it is better to err on the safe side. If this is impractical for any reason, I recommend changing keys on a regular basis to limit the amount of traffic encrypted with any one CipherSaber key (even though the IVs differ). If and when a consensus develops on the best way to fix RC4, I will announce a corresponding version of CipherSaber. Visit the CipherSaber page http://ciphersaber.gurus.com periodically for updated information. Arnold Reinhold