To provide a method and a system for creating a mini time key from a time
key, a plurality of mini time keys are created within a unit time period.
First, a unit time decryption key is prepared immediately after the unit
time is created. Then, the last mini time key is created by applying a
one-way function to the unit time decryption key. A desired mini time key
is created by applying the one-way function to a mini time key following
the desired mini time key. In other words, the mini time keys are created
as a timed series arranged in a descending order beginning with the last
mini time key. In this manner, even when a specific mini time key is
externally leaked for a specific reason, a following mini time key in a
timed series can not be created by using this mini time key. In addition,
even when the mini time keys are sequentially published, the security of
the unit time decryption key is maintained.
