KeyUsages

KeyUsages (often referred to as the KeyUsage extension in X.509 certificates) defines the purposes for which the public key contained in a certificate may be used. It is typically encoded as a bit string where each bit corresponds to a specific allowed operation. The extension helps constrain a certificate holder’s cryptographic actions to align with policy and risk requirements.

The standard bits include digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment, keyAgreement, keyCertSign, and cRLSign. Some implementations also use encipherOnly and decipherOnly, which are meaningful only if keyAgreement is set. When a certificate is presented, software checks these bits to determine whether a requested operation (for example, signing data or encrypting a session key) is permitted by the certificate’s key usage.

KeyUsages are commonly marked as a critical extension. If the extension is marked critical and an application cannot interpret its value, validation fails. If it is not marked critical, applications may still proceed by applying their own default handling. In practice, the set of key usages is chosen to reflect intended roles, such as server authentication, client authentication, code signing, or email protection, often in combination with the ExtendedKeyUsage extension for more precise purposes.

Relation to other PKI constraints: KeyUsages limit how the key can be used, while ExtendedKeyUsage provides more specific, higher-level purposes. Together they help enforce security policies and reduce the risk if a certificate’s private key is compromised.

See also: X.509, ExtendedKeyUsage, PKI, TLS certificates.