records, ...) without the possibility that the owner can backdate the timestamps. Multiple TSAs can be used to increase reliability and reduce vulnerability. The newer ANSI ASC X9.95 Standard for trusted timestamps augments the RFC 3161 standard with data-level security requirements to ensure data integrity against a reliable time source that is provable to any third party. This standard has been applied to authenticating digitally signed data for regulatory compliance, financial transactions, and legal evidence. 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 practically impossible to duplicate with any other 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 sent back to the requester of the timestamp who stores these with the original data (see diagram). Since the original data cannot 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 Anyone trusting the timestamper can then verify that the document was not created after the date that the timestamper vouches. It can also 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 needs to be validated. This is done by decrypting the digital signature using public key of TSA, producing hash B. Hash A is then compared with hash B inside the signed TSA message to confirm they are equal, proving that the timestamp and message is unaltered and was issued by the TSA. If not, then either the timestamp was altered or the timestamp was not issued by the