The present invention provides digital signature techniques using an information terminal, such as a portable terminal, having limited calculation resources. In one embodiment, a signature demandant generates a document to be signed, and an agent receives this document. The agent generates summary text for this document, and transmits the summary text to a signatory, and the signatory displays the summary text using his or her information terminal. The signatory confirms the contents, employs a private key stored in his or her terminal to sign (encrypt) the summary text. The signatory thereafter transmits a signature value to the agent, who generates a signed document that includes the signature value. Finally, the signature demandant verifies (decrypts) the received signed document using the public key of the signatory and confirms the contents.