Like paper-based signatures, digital signatures intend to respect a number of security assumptions. Methods of digital signature apposed by a single user have been defined and are widely used. But is it sufficient? What if a legal document requires witnesses and notarization, or a contract needs the signatures of several officers?This paper will first show the basics to understand digital signatures and how the a security properties of integrity, authentication, and non-repudiation are respected. We will then present the purposes of multiple signature schemes and introduce a possible classification of cases that need multiple signatures. I
