Practical Mobile Digital Signatures

There are important details that give legal validity to handwritten signatures: First, the document to be signed is under control of the signatory and it is not possible to substitute or alter it, and second, the tools to produce the signature (the pen and the signatory itself) are also under control of the signatory. These details make possible that handwritten signatures are used in a law court to prove the willingness of the signatory to be bound by the content of the document. Digital signatures require complex calculations that can not be done using mental arithmetic by the signatory. In this case neither document nor tools are under direct control of the signatory but under control of a computer. Consequently, the willingness of the signatory can not be sufficiently demonstrated. Furthermore, to be able to perform digital signatures, we must assume that the user trusts the computer to perform exactly what is intended. This yields digital signatures unusable in scenarios that require mobility. In this paper we present a system to perform digital signatures in environments that require mobility. The system is based on the use of personal digital assistants and smart cards and fulfils the common requirements established in different national laws regarding digital signatures.