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We present a novel, automated way to find differential paths for MD5. As an application we have shown how, at an approximate expected cost of 2 50 calls to the MD5 compression function, for any two chosen message prefixes P and P′, suffixes S and S′ can be constructed such that the concatenated values P||S and P′||S′ collide under MD5. Although the practical attack potential of this construction of chosen-prefix collisions is limited, it is of greater concern than random collisions for MD5. To illustrate the practicality of our method, we constructed two MD5 based X.509 certificates with identical signatures but different public keys and different Distinguished Name fields, whereas our previous construction of colliding X.509 certificates required identical name fields. We speculate on other possibilities for abusing chosenprefix collisions. More details than can be included here can be found on © International Association for Cryptology Research 2007.