Hash calendar

It can be thought of special kind of Merkle or hash tree, with the property that at any given moment, the tree contains a leaf node for each second since 1970‑01‑01 00:00:00 UTC.

By periodically publishing the root of the hash-tree is it possible to use a hash calendar as the basis of a hash-linking based digital timestamping scheme.

The hash calendar construct was invented by Estonian cryptographers Ahto Buldas and Mart Saarepera based on their research on the security properties of cryptographic hash functions and hash-linking based digital timestamping.

In the second phase, the multiple unconnected trees are turned into a single tree by merging the roots of the initial trees, but this time starting from the right and adding new parent nodes as needed (red nodes).

In order to ensure a high availability service it is possible to have multiple calendars in different physical locations all of which communicate with each other to ensure that each calendar contains identical hash values.