private key
1. In a public-key cryptosystem, that key of a user’s key pair which is known only to that user. [After X9.42]
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. [After X9.42]
3. Encryption methodology in which the encryptor and decryptor use the same key, which must be kept secret. [INFOSEC-99]
4. A key used in an asymmetric algorithm. Possession of this key is restricted, usually to only one entity [10181-1].
5. The key, used in an asymmetric algorithm, that is known to only one entity[10181-2].