Unformatted text preview: as the identification, I, of another person. She sends Hn to each of these people. (3) Each of these people attaches the date and time to the hash, signs the result, and sends it back to Alice. (4) Alice collects and stores all the signatures as the timestamp. The cryptographically secure pseudo-random-number generator in step (1) prevents Alice from deliberately choosing corrupt Is as verifiers. Even if she makes trivial changes in her document in an attempt to construct a set of corrupt Is, her chances of getting away with this are negligible. The hash function randomizes the Is; Alice cannot force them. This protocol works because the only way for Alice to fake a timestamp would be to convince all of the k people to cooperate. Since she chose them at random in step (1), the odds against this are very high. The more corrupt society is, the higher a number k should be. Additionally, there should be some mechanism for dealing with people who can’t promptly return the timestamp. Some subset of k is all that would be required for a valid timestamp. The details depend on the implementation. Further Work
Further improvements to timestamping protocols are presented in . The authors use binary trees to increase the number of timestamps that depend on a given timestamp, reducing even further the possibility that someone could create a chain of fictitious timestamps. They also recommend publishing a hash of the day’s timestamps in a public place, such as a newspaper. This serves a function similar to sending the hash to random people in the distributed protocol. In fact, a timestamp has appeared in every Sunday’s New York Times since 1992. These timestamping protocols are patented [684, 685, 686]. A Bellcore spin-off company called Surety Technologies owns the patents and markets a Digital Notary System to support these protocols. In their first version, clients send “certify” requests to a central coordinating server. Following Merkle’s technique of using hash functions to build trees , the server builds a tree...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips