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A hash calendar is a data structure that is used to measure the passage of time by adding hash values to an append-only database with one hash value per elapsed second. 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. A hash tree with 8 leaf nodes and a hash calendar after 7 seconds. A hash calendar after 31 seconds consists of 5 disjoint hash trees. The leaves are numbered left to right starting from zero and new leaves are always added to the right. 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. == History == 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.〔(System and method for generating a digital certificate patent 8,312,528 )〕 Their design goal was to remove the need for a trusted third party i.e. that the time of the timestamp should be verifiable independently from the issuer of the timestamp.〔http://www.guardtime.com/resources/video-library/educational-series-on-hash-functions〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hash calendar」の詳細全文を読む スポンサード リンク
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