On the Impact of the SHA-1 Collider on Mexican Digital Signatures with Legal Binding

Abstract

Security warranties of the RSA-type digital signatures are based on two main hypothesis: First, in the assumption that factoring gigantic integer numbers is a computationally unfeasible problem. Second, in the assumption that hash functions produce a unique digest for any digital document. With these two hypothesis in mind, in the last decades in Mexico, and also in other countries, legislation has been enacted to legalize digital signatures. In Mexico, the combination of the RSA algorithm and the SHA-1 hash function can be used to legally validate digital contracts. This selection of algorithms is known as the RSA-SHA-1 digital signature. However, recently the SHA-1 hash function has suffered a {\it falsification} attack in which, given an arbitrary document for which the SHA-1 digest was produced, it is possible to generate a second document with the same digest. In other words, this attack permits to find arbitrary pairs of documents that share the same digest. This situation has provoked that the RSA-SHA-1 algorithm used to sign legal contracts is on risk to be broken. In this article, some of the repercussions in the information security of the legal documents signed with this protocol are discussed. We also discuss some countermeasures that can mitigate this vulnerability.