A periodical multi-secret threshold cryptosystem enables a sender to encrypt a mes-
sage by using a sequence of keys shared by n parties and updated periodically with a cyclic period,
so that any subset of t + 1 parties can decrypt the message only in the periodical time-frames,
while no subset of t corrupted parties can control the system (in particular, none can learn the
decryption key). This scheme can be applied to a timed-release cryptosystem whose release time
is determined by the number of share update phases. The system is implemented by sharing a
pseudo-random sequence generator function. It realizes n > 3t robustness, and therefore is
secure against an adversary who can corrupt at most one third of the parties.
By: Masayuki Numao
Published in: RT0305 in 2002
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