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Public key or asymmetric cryptography is a form of encryption using separate keys for encryption and decryption, neither of which can be derived from the other. The public key can be used to encrypt messages which only the holder of the corresponding private key can decrypt. Alternatively, the holder of a private key can "digitally sign" a message by encrypting a digest of the message with his private key. By successfully decrypting the digest with the previously published public key, a recipient can prove the authorship of the message.