Numbers standing for cash money can be spent only one time each, otherwise the account from which they were withdrawn would be revealed. More generally, a technique for issuing and showing blind digital signatures ensures that if they are shown responsive to different challenges, then certain information their signer ensures they contain will be revealed and can be recovered efficiently. Some embodiments allow the signatures to be unconditionally untraceable if shown no more than once. Extensions allow values to be encoded in the signatures when they are shown, and for change on unshown value to be obtained in a form that is aggregated and untraceable.