Colliding X.509 Certificates

With this construction we show that MD5 collisions can be crafted easily in such a way that the principles underlying the trust in Public Key Infrastructure are violated. In particular we find it worrying that from one certificate alone it cannot be determined whether another, different certificate may exist with the same signature. For the second certificate the issuing Certification Authority may not have been able to verify “proof of possession” of the private key. Therefore, a relying party using a public key certificate based on MD5 can not be certain that the alleged certificate owner is indeed in possession of the corresponding private key.