Unformatted text preview: timestamp a document. (1) Alice transmits a copy of the document to Trent. (2) Trent records the date and time he received the document and retains a copy of the document for safekeeping. Now, if anyone calls into question Alice’s claim of when the document was created, she just has to call up Trent. He will produce his copy of the document and verify that he received the document on the date and time stamped. This protocol works, but has some obvious problems. First, there is no privacy. Alice has to give a copy of the document to Trent. Anyone listening in on the communications channel could read it. She could encrypt it, but still the document has to sit in Trent’s database. Who knows how secure that database is? Second, the database itself would have to be huge. And the bandwidth requirements to send large documents to Trent would be unwieldy. The third problem has to do with the potential errors. An error in transmission, or an electromagnetic bomb detonating somewhere in Trent’s central computers, could completely invalidate Alice’s claim of a timestamp. And fourth, there might not be someone as honest as Trent to run the timestamping service. Maybe Alice is using Bob’s Timestamp and Taco Stand. There is nothing to stop Alice and Bob from colluding and timestamping a document with any time that they want. Improved Arbitrated Solution
One-way hash functions and digital signatures can clear up most of these problems easily: (1) Alice produces a one-way hash of the document. (2) Alice transmits the hash to Trent. (3) Trent appends the date and time he received the hash onto the hash and then digitally signs the result. (4) Trent sends the signed hash with timestamp back to Alice. This solves every problem but the last. Alice no longer has to worry about revealing the contents of her document; the hash is sufficient. Trent no longer has to store copies of the document (or even of the hash), so the massive storage requirements and security problems are solved (remember, one-way...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips