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A simple and provably good code for SHA Message Expansion

Speaker: Charanjit Jutla , IBM TJ Watson
Date: December 16 2005
Time: 10:30AM to 12:00PM
Location: 32-G449 Patil/Kiva, Stata Ctr
Contact: Be Blackburn, 3-6098, imbe@mit.edu
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We develop a new computer assisted technique for lower bounding the minimum distance of codes similar to those used in SHA-1 message expansion. Using this technique, we prove that a modified SHA-1 like code has minimum distance at least 80, and that too in just the last 64 of the 80 expanded words. We propose a new compression function which is identical to SHA-1 except for the modified message expansion code.

We also show that if a collision is obtained (as in the Wang et al attack) by putting together local collisions then the disturbance vector must be an xor of a non-zero message expansion codeword and a cipher codeword. We conclude that the modified SHA-1 is resistant to recent differential attacks and their natural extensions. This is joint work with Anindya Patthak, UT Austin.

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