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A cryptosystem that combines a widely available public key and a private key controlled only by its holder. A message encrypted by the public key can only be decrypted by the private key that corresponds to that public key. For example, when someone wants to send a message to someone else, on the sender's side, the public key of the receiver's side is used to encrypt the message; on the receiver's side, the message is decrypted by using the private key of the receiver.
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. Some examples of public key cryptosystems are HIME(R) developed by Hitachi, RSA cryptosystem and Elliptic Curve Cryptosystem.