Cryptosystem modules have been really used in various systems, all of which would not be constructed
in such a way that a used cryptosystem module would be changed to another one.
However, if a cryptosystem module is attacked, then it must be actually replaced to another one.
Furthermore, it is important to renew cryptosystem modules, maintaining the entire system security.
Therefore, it is necessary to construct a new scheme in such a way that a cryptosystem module is renewed
easily without reducing security.
In this study, we investigate a renewal of public key cryptosystem modules in the open network architecture.
First, we discuss a serious attack in renewing a cryptosystem module, then we propose a new protocol system
structure that are strong against the attack and enable to replace a cryptosystem module flexibly.
Our scheme assumes that each entity renews cryptosystem modules individually.
In this case, it is important to keep synchronization of cryptosystem modules used among entities.
Therefore we investigates an algorithm to synchronize them among entities.
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