Paper 2006/360

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

Marc Stevens, Arjen Lenstra, and Benne de Weger

Abstract

We have shown how, at a cost of about $2^{52}$ calls to the MD5 compression function, for any two target messages $m_1$ and $m_2$, values $b_1$ and $b_2$ can be constructed such that the concatenated values $m_1\|b_1$ and $m_2\|b_2$ collide under MD5. Although the practical attack potential of this construction of \emph{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 \emph{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.

Note: Version 1.1, contains an appendix about differential path construction. The EuroCrypt 2007 proceedings version will have the title "Chosen-prefix Collisions for MD5 and Colliding X.509 Certificates for Different Identities". It contains a lot more details on the method of finding chosen-prefix collisions for MD-5, and somewhat less on the certificates. See www.win.tue.nl/hashclash.