Preimage Attacks on 3-Pass HAVAL and Step-Reduced MD5

This paper presents preimage attacks on the hash functions 3-pass HAVAL and step-reduced MD5. Introduced in 1992 and 1991 respectively, these functions underwent severe collision attacks, but no preimage attack. We describe two preimage attacks on the compression function of 3-pass HAVAL. The attacks have a complexity of about 2 compression function evaluations instead of 2. We present several preimage attacks on the MD5 compression function that invert up to 47 steps (out of 64) within 2 trials instead of 2. Although our attacks are not practical, they show that the security margin of 3-pass HAVAL and step-reduced MD5 with respect to preimage attacks is not as high as expected.