Target Collisions for MD5 and Colliding X.509 Certificates for Different Identities

We have shown how, at a cost of about 2 calls to the MD5 compression function, for any two target messages m1 and m2, values b1 and b2 can be constructed such that the concatenated values m1‖b1 and m2‖b2 collide under MD5. Although the practical attack potential of this construction of target collisions is limited, it is of greater concern than random collisions for MD5. In this note we sketch our construction. To illustrate its practicality, we present 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 target collisions.