A method for time stamping a digital document is disclosed. The document
originator creates a time stamp receipt using the document and the
current time. The time stamp receipt is submitted to a time stamping
authority having a trusted clock. The time stamping authority validates
the time stamp receipt by comparing the time value specified in the time
stamp receipt to the current time. If the time value specified in the
time stamp receipt is within a predetermined time window, the time
stamping authority cryptographically binds the time value and document,
or the time value and some representation of the document, e.g., by
signing the time stamp receipt with its private signature key.