A form of encryption that utilises a unique pair of keys, one (the "public key") being openly known, and the other (the "private key",) being known only to the recipient of an encrypted message. Note 1: At the recipient's discretion, the public key is made available to those who may have occasion to send an encrypted message to that recipient. The sender uses the recipient's public key to encrypt a message. The encrypted message, which cannot be decrypted by means of the public key, is then delivered by conventional means to the recipient, who uses the matching private key to decrypt the message. Note 2: Public-key encryption can also be used to add a digital signature to publicly posted electronic messages. The poster of an electronic message feeds the text of the message into the encryption program, along with the poster's private key. A unique block of text (the digital signature) is generated and attached to the end of the message. Any other reader of the message can use the poster's public key to analyse the message text and the signature block. The encryption software will indicate whether or not the message text matches the digital signature. Note 3: Users of public-key encryption systems may register their public keys in several public databases.