# Key pair usage

A key pair is bound to a usage profile by its public key certificate.

* Public keys are seldom multi-purpose.

Typical usages.

* **Authentication** / key distribution.
  * Digital signature, Key encipherment, Data encipherment, Key agreement.
* **Document signing.**
  * Digital signature, Non-repudiation.
* **Certificate issuing.**
  * Certificate signing, CRL signing.

Public key certificates have an extension for this.

* Key usage (critical).
