Gourt Home::Gourt :: Timestamp
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- For the ICMP message, see ICMP Timestamp.
Timestamp can refer to a time code or to a digitally signed timestamp whose signer vouches for the existence of the signed document or content at the time given as part of the digital signature.
Timestamps are very useful for logging events.
Examples 2005-10-30 10:45 Sat Jul 23 02:16:57 2005
Many sources also use the term timestamp to refer specifically to Unix time, the number of seconds since 00:00:00 UTC on January 1, 1970.
Trusted Timestamping
A trusted timestamp is a timestamp issued by a trusted third party (TTP) acting as a timestamping authority (TSA). It is used to prove the existence of certain data before a certain point (e.g. contracts, research data, medical records,...) without the possibility that the owner can backdate the timestamps.
Creating a timestamp
The technique is based on digital signatures and hash functions. First a hash is calculated from the data. A hash is a sort of digital fingerprint of the original data: a string of bits that is different for each set of data. If the original data is changed then this will result in a completely different hash. This hash is sent to the TSA. The TSA concatenates a timestamp to the hash and calculates the hash of this concatenation. This hash is in turn digitally signed with the private key of the TSA. This signed hash + the timestamp is send back to the requester of the timestamp who stores these with the original data (see diagram).Since the original data can not be calculated from the hash (because the hash function is a one way function), the TSA never gets to see the original data, which allows the use of this method for confidential data.
Checking the timestamp
It can now no longer be repudiated that the requester of the timestamp was in possession of the original data at the time given by the timestamp. To prove this (see diagram) the hash of the original data is calculated, the timestamp given by the TSA is appended to it and the hash of the result of this concatenation is calculated, call this hash A.Then the digital signature of the TSA is to be checked by decrypting the signed hash given by the TSA with the public key of the TSA. This results in the decrypted hash, let us call this hash B. If hash A equals hash B then the timestamp is unaltered and was issued by the TSA. If not, then either the timestamp was altered or the timestamp was not issued by the TSA.
External links
"Timestamp"