Cryptographic keys go through a life cycle. From the generation of keys, to their safe destruction, keys have to be managed according to the established policies and procedures.
Key management consists of four essential procedures concerning public and private keys. They are as follows:
- Secure generation of keys: Procedures to ensure that the private and public keys are generated in a secure manner.
- Secure storage of keys: Procedures to ensure that keys are stored securely.
- Secure distribution of keys: Procedures to ensure that keys are not lost or modified during distribution.
- Secure destruction of keys: Procedures to ensure that the keys are destroyed completely once the useful life of the key is over.
NIST Special Publication 800-57, titled Recommendation for Key Management - Part 1: General, specifies the following nineteen types of keys:
- Private signature keys: They are private keys of public key pairs and are used to generate digital signatures. They are used to provide authentication, integrity, and non-repudiation.
- Public signature verification key: This is the public key of the asymmetric (public) key pair. It is used to verify the digital signature.
- Symmetric authentication keys: They are used with symmetric key algorithms to provide the assurance of integrity and source of the messages.
- Private authentication key: This is the private key of the asymmetric (public) key pair. It is used to provide assurance of the integrity of information.
- Public authentication key: This is a public key of an asymmetric (public) pair that is used to determine the integrity of information; and to authenticate the identity of entities.
- Symmetric data encryption keys: They are used to apply confidentiality protection of information.
- Symmetric key wrapping keys: They are key encryption keys in the sense that they are used to encrypt other symmetric keys.
- Symmetric and asymmetric random number generation keys: They are used to generate random numbers.
- Symmetric master key: This is a master key that is used to derive other symmetric keys.
- Private key transport keys: They are the private keys of asymmetric (public) key pairs that are used to decrypt keys, which have been encrypted with the associated public key.
- Public key transport keys: They are the public keys of asymmetric (public) key pairs that are used to decrypt keys that have been encrypted with the associated public key.
- Symmetric agreement keys: They are used to establish keys such as key wrapping keys, data encryption keys, and so on using a symmetric key agreement algorithm.
- Private static key agreement keys: They are the private keys of asymmetric (public) key pairs that are used to establish keys such as key wrapping keys, data encryption keys, and so on.
- Public static key agreement key: They are public keys of asymmetric (public) key pairs that are used to establish keys such as key wrapping keys, data encryption keys, and so on.
- Private ephemeral key agreement keys: They are private keys of asymmetric (public) key pairs that are used only once to establish one or more keys such as key wrapping keys, data encryption keys, and so on.
- Public ephemeral key agreement key: They are the public keys of asymmetric (public) key pairs that are used in a single key establishment transaction to establish one or more keys.
- Symmetric authorization keys: These keys are used to provide privileges to an entity using the symmetric cryptographic method.
- Private authorization key: This is a private key of an asymmetric (public) key pair that is used to provide privileges to an entity.
- Public authorization key: It is the public key of an asymmetric (public) key pair that is used to verify privileges for an entity that knows the associated private authorization key.
The following are some of the best practices that are applicable to key management.
Key usage refers to using the key for a cryptographic process. This should be limited to using a single key for only one cryptographic process. This is to ensure that the security provided by the key is not weakened.
When a specific key is authorized for use by legitimate entities for a period of time or if the effect of a specific key for given system is only for a period of time, then the time span is known as a the crypto period. The purpose of defining a crypto period is to limit a successful cryptanalysis by a malicious entity.
The following assurance requirements are a part of the key management process:
- Integrity protection: This assures the source and format of the keying material by verification
- Domain parameter validity: This assures the parameters used by some public key algorithms during the generation of key pairs and digital signatures and during the generation of shared secrets that are subsequently used to derive keying material
- Public key validity: This assures that the public key is arithmetically mathematically correct
- Private key possession: This assures that the possession of a private key is obtained before using the public key
The cryptographic algorithm and the key-size selection are two important key management parameters that provide adequate protection to the system and data throughout their expected lifetime.
Cryptographic keys go through different states from their generation to destruction. These states are defined as key states. When a cryptographic key moves from one state to another, it is known as key transition.
NIST SP800-57 defines the following six key states:
- The pre-activation state: The key has been generated, but not yet authorized for use
- The active state: The key may be used to cryptographically protect information
- The deactivated state: The crypto period of the key is expired, but the key still needs to perform cryptographic operations
- The destroyed state: The key is destroyed here
- The compromised state: The key is released or determined by an unauthorized entity
- The destroyed compromised state: The key is destroyed after a compromise or the compromise is found after the key is destroyed
The key states or transitions can be grouped under four key management phases. They are as follows:
- The pre-operational phase: The keying material is not yet available for normal cryptographic operations
- The operational phase: The keying material is available for normal cryptographic operations and is in use
- The post-operational phase: The keying material is no longer in use, but access to the material is possible
- The destroyed phase: Keys are no longer available