The timestamp is the signer's assertion that a piece of electronic data existed at or before a particular time.
One application of the protocol is to show that a digital signature was issued before a point in time, for example before the corresponding certificate was revoked.
In the protocol a Time Stamp Authority (TSA) is a trusted third party that can provide a timestamp to be associated with a hashed version of some data.
This is sent to the TSA and the response contains a Time Stamp Token (TST) which itself includes the hash of the data, a unique serial number, a timestamp and a digital signature.
The protocol can operate over a number of different transports, including email, TCP sockets or HTTP.