None

  A mathematical key (kept secret by the holder) used to create digital signatures and, depending upon the algorithm, to decrypt messages or files encrypted (for confidentiality) with the corresponding public key.
Source(s):
CNSSI 4009-2015 CNSSI 1300

  A cryptographic key, used with a public key cryptographic algorithm, that is uniquely associated with an entity and is not made public.
Source(s):
NIST SP 800-12 Rev. 1 under Private Key FIPS 140-2

  A cryptographic key used with an asymmetric-key (public-key) cryptographic algorithm that is not made public and is uniquely associated with an entity that is authorized to use it. In an asymmetric-key cryptosystem, the private key is associated with a public key. Depending on the algorithm that employs the private key, it may be used to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified using the corresponding public key, 3. Decrypt data that was encrypted using the corresponding public key, or 4. Compute a key derivation key, which may then be used as an input to a key derivation process.
Source(s):
NIST SP 800-133 under Private key [Superseded]

  A cryptographic key used by a public-key (asymmetric) cryptographic algorithm that is uniquely associated with an entity and is not made public.
Source(s):
NIST SP 800-152 under Private key

  (1) The key of a signature key pair used to create a digital signature. (2) The key of an encryption key pair that is used to decrypt confidential information. In both cases, this key must be kept secret.
Source(s):
NIST SP 800-32 under Private Key

  A cryptographic key that is kept secret and is used with a public-key cryptographic algorithm. A private key is associated with a public key.
Source(s):
NIST SP 800-56B Rev. 2 under Private key
NIST SP 800-56B Rev. 1 under Private key [Superseded]

  The secret part of an asymmetric key pair that is used to digitally sign or decrypt data.
Source(s):
NIST SP 1800-17b under Private Key
NIST SP 1800-17c under Private Key
NIST SP 800-63-3 under Private Key
NIST SP 800-63-2 under Private Key [Superseded]

  A cryptographic key used with an asymmetric-key (public-key) cryptographic algorithm that is not made public and is uniquely associated with an entity that is authorized to use it. In an asymmetric-key cryptosystem, the private key is associated with a public key. Depending on the algorithm that employs the private key, it may be used to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified using the corresponding public key, 3. Decrypt data that was encrypted using the corresponding public key, or 4. Compute a key derivation key, which may then be used as an input to a key derivation process.
Source(s):
NIST SP 800-133 Rev.1 under Private key [Superseded]

  The secret part of an asymmetric key pair that is typically used to digitally sign or decrypt data.
Source(s):
NIST SP 1800-12b

  A cryptographic key that is used with an asymmetric (public key) cryptographic algorithm. For digital signatures, the private key is uniquely associated with the owner and is not made public. The private key is used to compute a digital signature that may be verified by the corresponding public key.
Source(s):
NIST SP 800-89 under Private key

  A cryptographic key used with a public-key cryptographic algorithm that is uniquely associated with an entity and is not made public. The private key has a corresponding public key. Depending on the algorithm, the private key may be used to: 1. Compute the corresponding public key, 2. Compute a digital signautre taht may be verified by the corresponding public key, 3. Decrypt keys that were encrypted by the corresponding public key, or 4. Compute a shared secret that during a key agreement transaction.
Source(s):
NIST SP 800-57 Part 2 Rev.1 under Private key

  A cryptographic key, used with a public key cryptographic algorithm that is uniquely associated with an entity and is not made public. In an asymmetric (public) cryptosystem, the private key is associated with a public key. The private key is known only by the owner of the key pair and is used to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified by the corresponding public key, 3. Decrypt data that was encrypted by the corresponding public key, or 4. Compute a piece of common shared data, together with other information.
Source(s):
NIST SP 800-57 Part 2 under Private key [Superseded]

  A cryptographic key, used with a public-key cryptographic algorithm that is uniquely associated with an entity and is not made public. In an asymmetric (public) cryptosystem, the private key has a corresponding public key. Depending on the algorithm, the private key may be used, for example, to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified by the corresponding public key, 3. Decrypt keys that were encrypted by the corresponding public key, or 4. Compute a shared secret during a key-agreement transaction.
Source(s):
NIST SP 800-57 Part 1 Rev. 4 under Private key [Superseded]

  A cryptographic key used with a public key cryptographic algorithm that is uniquely associated with an entity and is not made public. In an asymmetric (public) key cryptosystem, the private key is associated with a public key. Depending on the algorithm, the private key may be used to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified by the corresponding public key, 3. Decrypt data that was encrypted by the corresponding public key, or 4. Compute a shared secret during a key-agreement process.
Source(s):
NIST SP 800-175B Rev. 1 under Private key

  A cryptographic key used with a public-key cryptographic algorithm that is uniquely associated with an entity and is not made public. In an asymmetric-key (public-key) cryptosystem, the private key has a corresponding public key. Depending on the algorithm, the private key may be used, for example, to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified by the corresponding public key, 3. Decrypt keys that were encrypted by the corresponding public key, or 4. Compute a shared secret during a key-agreement transaction.
Source(s):
NIST SP 800-57 Part 1 Rev. 5 under Private key

  A cryptographic key, used with a public-key cryptographic algorithm, which is uniquely associated with an entity and is not made public. In an asymmetric (public) cryptosystem, the private key is associated with a public key. Depending on the algorithm, the private key may be used, for example, to: 1. Compute the corresponding public key, 2. Compute a digital signature that may be verified by the corresponding public key, 3. Decrypt keys that were encrypted by the corresponding public key, or 4. Compute a shared secret during a key-agreement transaction.
Source(s):
NIST SP 800-57 Part 1 Rev. 3 under Private key [Superseded]