Unformatted text preview: hash functions don’t have a key). Alice can immediately examine the signed timestamped hash she receives in step (4), so she will immediately catch any transmission errors. The only problem remaining is that Alice and Trent can still collude to produce any timestamp they want. Linking Protocol
One way to solve this problem is to link Alice’s timestamp with timestamps previously generated by Trent. These timestamps will most probably be generated for people other than Alice. Since the order that Trent receives the different timestamp requests can’t be known in advance, Alice’s timestamp must have occurred after the previous one. And since the request that came after is linked with Alice’s timestamp, then hers must have occurred before. This sandwiches Alice’s request in time. If A is Alice’s name, the hash value that Alice wants timestamped is Hn, and the previous time stamp is Tn - 1, then the protocol is: (1) Alice sends Trent Hn and A. (2) Trent sends back to Alice: Tn = SK(n,A,Hn,tn,In - 1,Hn - 1,Tn - 1,Ln) where Ln consists of the following hashed linking information: Ln = H(In - 1,Hn - 1,Tn - 1,Ln - 1) SK indicates that the message is signed with Trent’s private key. Alice’s name identifies her as the originator of the request. The parameter n indicates the sequence of the request: This is the nth timestamp Trent has issued. The parameter tn is the time. The additional information is the identification, original hash, time, and hashed timestamp of the previous document Trent stamped. (3) After Trent stamps the next document, he sends Alice the identification of the originator of that document: In + 1. If someone challenges Alice’s timestamp, she just contacts the originators of the previous and following documents: In - 1 and In + 1. If their documents are called into question, they can get in touch with In - 2 and In + 2, and so on. Every person can show that their document was timestamped after the one that came before and before the one that came after. Previous Table of Contents Next Products | Contac...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips