A technique for providing message authenticity includes accepting transaction information, accepting a first data item used for authenticating an originating user, cryptographically
processing the transaction information using only a second data item, wherein the entropy of the first data item is less than the entropy of the second data item, and authenticating the originating user using the first data item. The first data item can be a sequence of digits corresponding to those displayed on an external device, such as, for example, an RSA
authorization token,
credit card, etc. In general, the first data item will be a short
alphanumeric string and the second data item will generally be much larger, e.g., a 128 bit sequence to be used principally for data
authentication. According to another aspect of the present invention, consequential evidence of the transaction may be secured to provide after-the-fact evidence of the transaction. This evidence can include a message written to a tamper-resistant log
record, the message including the transaction information, the first data item, the second item, and an identifier for the originating user, as well as other information. At a subsequent point, the transaction can be shown to have been sent by the originating user and received by the intended recipient, by consulting the log
record. Preferably, the validity of the transaction would be ascertained by an independent, mutually
trusted third party.