A digital time-stamping service (DTS) issues time-stamps which associate a date and time with a digital document in a cryptographically strong way. The digital time-stamp can be used at a later date to prove that an electronic document existed at the time stated on its time-stamp. For example, a physicist who has a brilliant idea can write about it with a word processes and have the document time-stamped. The time-stamp and document together can later prove that the scientist deserves the Nobel Prize, even through an arch rival may have been the first to publish.

Here’s one way such a system could work. Suppose Ramesh sings a document and wants it time-stamped. He computes a message digest of the document using a secure hash function and then sends the message digest (but not the document itself) to the DTS, which sends her in return a digital time-stamp consisting of the message digest, the date and time it was received at the DTS, and the signature of the DTS. Since the message digest does not reveal any information about the content of the document, the DTS cannot eavesdrop on the documents it time-stamp. Later, Ramesh can present the document and time-stamp together to prove when the document was write. A verifier computes the message digest of the document, makes sure it matches the digest n the time-stamp, and then verifies the signature of the DTS on the time-stamp.

The use of a DTS would appear to be extremely important, if not essential, for maintaining the validity of documents over many years. Suppose a landlord and tenant sign a twenty-year lease. The public keys used to sign the lease are set to expire after two years. Solutions such as recertifying the keys or resigning every two years with new keys require the cooperation of both parties several years after the original signing. If one party becomes dissatisfied with the lease, he or she may refuse to cooperate. The solution is to register the lease with the DTS at the time of the original signing; both parties would then receive a copy of the time-stamp, which can be used years later enforce the integrity of the original lease.

In the future, it is likely that a DTS will be used for everything from long-term corporate contracts to personal diaries and letters. Today, if an historian discovers some lost letters of Gandhi Ji, their authenticity is checked by physical means. But a similar find 100 years from now may consist of an author’s computer files; digital time-stamps may be the only way to authenticate the find.

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