Abstract: At the time when generating a secret key of each entity by using each divided identification information (ID division vector) obtained by dividing identification information (ID vector) of each entity into a plurality of blocks and by using a secret symmetric matrix of each key generating agency (center), a part of the components of the symmetric matrix is extracted in accordance with each divided identification information (ID division vector) so that the extracted components are synthesized with a random number particular to each entity so as to generate a secret key of each entity.

Abstract: In an ID-NIKS using an electronic mail address as the identification information (ID information) of each entity, when generating a common key at each entity, if the electronic mail address of the communicating party does not contain a domain name, the common key is generated after adding the same domain name as the domain name in the electronic mail address of the entity to the electronic mail address of the communicating party.

Abstract: When generating a common key for use in an encryption process of encrypting a plaintext into a ciphertext and a decryption process of decrypting the ciphertext into the plaintext, components which are contained in the secret keys of one entity and correspond to other entity as a communicating party are extracted and composition of all the extracted components is performed while shifting the components to generate a common key. Thus, the common key consisting of a larger number of bits than the number of bits in each of the extracted components is generated. A common key of any size is generated by adjusting the amount of shift.

Abstract: A secret key of an entity is generated at each key generating agency (center) by using divided identification information obtained by dividing identification information of the entity into a plurality of blocks and a hash value which consists of a plurality of bits and is set for each key generating agency (center). A plurality of bits of any order are selected for each key generating agency (center) from an original hash-value sequence consisting of a predetermined sequence of a plurality of bits so as to set the hash value for each key generating agency (center). When a new key generating agency (center) is added to a plurality of existing key generating agencies (centers), a hash value is set for the new key generating agency (center) without changing the hash values of the existing key generating agencies (centers).