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A cryptosystem that uses the same key for encrypting and decrypting data. Three main problems have been pointed out as regards symmetric key cryptosystems: (1) in the case of multiple persons, multiple keys are needed; (2) if key is stolen, security cannot be assured; and (3) handing over of keys must be strictly controlled.
The public key cryptosystem overcomes the issues related to secure key-sharing of symmetric key cryptosystems. However, in public key cryptosystem, since the encryption key differ from the corresponding decryption key, its encryption and decryption processes are complicated, and has less efficient compared with symmetric key cryptosystem. Therefore, it is common to use symmetric key cryptosystem to encrypt bulk data and to use public key cryptosystem to encrypt the encryption key which was used for bulk data encryption. Hitachi developed MUGI as symmetric key cryptosystem.