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ITIS 6200/8200

ITIS 6200/8200. time-stamping services. Difficult to verify the creation date and accurate contents of a digital file Required properties of time-stamping Data, instead of the storage medium, should be stamped Minor change in source file leads to major changes in stamp

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ITIS 6200/8200

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  1. ITIS 6200/8200

  2. time-stamping services • Difficult to verify the creation date and accurate contents of a digital file • Required properties of time-stamping • Data, instead of the storage medium, should be stamped • Minor change in source file leads to major changes in stamp • Low probability of conflict

  3. Time-stamping using TTP • Alice sends a file to T • T keeps the file, date, and time in the record • When it is needed, T can provide the evidence • Problems: • Need a TTP • What about data error during transmission? • Need huge storage space • Why should Alice tell the secret to T? • Collusion between Alice and T

  4. Time-stamping using TTP • Using hash result and digital signature we can fix most of the problems • Alice asks T to sign the hash result of the file • T sends the digital signature back to Alice, T does not need to record the file • Alice verifies the signature and make sure that no error happens during transmission

  5. How to defend against collusion • Linking protocol: every signature is restricted by the previous one and the following one • T will sign: • Alice will also receive the owner of the next hash result In+1 • Q: Why do we need the Ln entry? Will I, H and t be enough?

  6. How does the linking protocol mitigate the collusion between T and Alice • The timestamp is restricted by the previous one and the next one • T cannot predict the order of the requesters • Possible way to compromise this method: • T generates fake event sequences and leaves some gaps for future use • Counteraction: linking a longer sequence

  7. Removing TTP: using Distributed hash table • Alice uses Hn as seed to generate a group of node ID using a pseudo random number generator • Alice sends out Hn to these nodes • These nodes will sign with their digital signatures and send it back • Alice can use these signatures to prove the timestamp

  8. Why it is difficult for Alice to collude with all these nodes? • The node IDs are generated through a pseudo random number generator based on Hn, Alice cannot predict those IDs • Similar ideas have been used in P2P systems and location-based routing for wireless networks

  9. The generation of a hash tree • Need to timestamp a large number of files • Using the hash values to construct a tree • Publish only the root of the tree • Provide corresponding entries in the tree to the end users so that they can verify

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