Methods, apparatus, including computer program products, and systems
implement and use techniques relating to electronic signatures. In one
implementation, a one-way hash is calculated for an electronic document
and a digital watermark representing the one-way hash is embedded in a
signature document. Based on a user input, the signature document having
the embedded digital watermark is altered, and the electronic document is
associated with the signature document. The electronic document and the
signature document together comprise a signed electronic document. The
user input can include biometric information, such as a handwritten
signature or a voice recording. For additional security, the one-way hash
can be encrypted before a representation of the hash is embedded in the
signature document.