The Prediction of Serial Number in OpenSSL’s X.509 Certificate

Abstract

In 2007, a real faked X.509 certificate based on the chosen-prefix collision of MD5 was presented by Marc Stevens. In the method, attackers needed to predict the serial number of X.509 certificates generated by CAs besides constructing the collision pairs of MD5. After that, the randomness of the serial number is required. Then, in this case, how do we predict the random serial number? Thus, the way of generating serial number in OpenSSL was reviewed. The vulnerability was found that the value of the field “not before” of X.509 certificates generated by OpenSSL leaked the generating time of the certificates. Since the time is the seed of generating serial number in OpenSSL, we can limit the seed in a narrow range and get a series of candidate serial numbers and use these candidate serial numbers to construct faked X.509 certificates through Stevens’s method. Although MD5 algorithm has been replaced by CAs, the kind of attack will be feasible if the chosen-prefix collision of current hash functions is found in the future. Furthermore, we investigate the way of generating serial numbers of certificates in other open source libraries, such as EJBCA, CFSSL, NSS, Botan, and Fortify.