1. In a public-key cryptosystem, that key of a user's key pair which is known only to that user. 2. A cryptographic key created and kept private by a party. Note: A private key may be used for the following: to compute the corresponding public key; to make a digital signature which may be verified by the corresponding public key; to decrypt the message encrypted by corresponding public key; or, together with other information, to compute a piece of common shared secret information. 3. Encryption methodology in which the encryptor and decryptor use the same key, which must be kept secret. 4. A key used in an asymmetric algorithm. Possession of this key is restricted, usually to only one entity. 5. The key, used in an asymmetric algorithm, that is known to only one entity.