The Publication Code provides the mathematical basis for independent evidence.

Each GuardTime Keyless Signature is cryptographically related to each of the newspaper publications that have appeared after its publication. GuardTime publishes the Publication Code once a month in newspapers distributed worldwide. A new Publication Code for the GuardTime Calendar is generated by GuardTime at the 15th day of every month at 00:00:00 UTC time. The code is printed in newspapers within the next 5 days depending on the publishers’ schedules.

The Publication Code that is published in newspapers contains a hash value as well as a date and time record. The hash value is an aggregate hash summary of all the hash values of the GuardTime Calendar beginning with the creation of the GuardTime Calendar through to the published date and time.

Verification using the electronic Calendar is typically sufficient for regular use. Publication in widely witnessed media is an additional mechanism which provides truly independent verification; the integrity of the media acts as a trust anchor for the GuardTime Keyless Signature service.

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