MD5 is weaker than weak: Attacks on concatenated combiners

Abstract

We consider a long standing problem in cryptanalysis: attacks on hash function combiners. In this paper, we propose the first attack that allows collision attacks on combiners with a runtime below the birthday-bound of the smaller compression function. This answers an open question by Joux posed in 2004. As a concrete example we give such an attack on combiners with the widely used hash function MD5. The cryptanalytic technique we use combines a partial birthday phase with a differential inside-out technique, and may be of independent interest. This potentially reduces the effort for a collision attack on a combiner like MD5||SHA-1 for the first time. © 2009 Springer-Verlag.