We propose a new mode of multiple encryption-triple-DES external feedback cipher block chaining with output feedback masking. The aim is to provide increased protection against certain attacks (dictionary attacks and matching ciphertext attacks) which exploit the short message-block size of DES. The new mode obtains this protection through the introduction of secret masking values that are exclusive-ORed with the intermediate outputs of each triple-DES encryption operation. The secret mask value is derived from a fourth encryption operation per message block, in addition to the three used in previous modes. The new mode is part of a suite of encryption modes proposed in the ANSI X9.F.1 triple-DES draft standard (X9.52). I Don Coppersmith, "A Chosen-Clphertext Attack on Triple DES CBC," revised February X, 1995, provided to ANSI XY.F.1. 2 Don Coppersmith, "A Chosen-Plaintext Attack on 2-Key Inner Triple DES CBCIEDE, Preliminary Version," April 19, 1995, provlded to ANSI X9.F.1. 3 Burt S. Kallski, Jr., "On the Security and Performance of Several Triple-DES Modes (Extended Abstract)," private communlcation provlded to ANSI X9.F.1 (January 12, 1994"second draft). 4 James L. Masey, "Analysis o f Two-Key Triple DES Encryptton," April 2, 1994, prov~ded to ANSI X9.F.1.

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With any strong cryptographic algorithm, such as the data encryption standard (DES), it is possible to devise protocols for . "authentication. One technique, which allows arbitrary, time-invar-.~I'iant quantities (such as encrypted keys and passwords) to be auj ; ..~ thenticated, is based upon a secret-cryptographic (master) key residing in a host processor. Each quantity to be authenticated has a corresponding precomputed test pattern. At any later time, the test pattern can be used together with the quantity to be authenticated to generate a nonsecret verification pattern. The verification pattern can in turn be used as the basis for accepting or rejecting the quantity to be authenticated.

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