keyEncipherment

KeyEncipherment is a flag in the X.509 public key certificate KeyUsage extension defined by RFC 5280. It indicates that the subject public key is intended to be used to encipher (encrypt) private or secret keys, such as a symmetric session key, or to wrap keys for secure delivery. The keyEncipherment bit is commonly associated with RSA keys but is applicable to other algorithms capable of encrypting keys.

In practice, keyEncipherment matters in TLS and other public-key infrastructures. A server certificate with keyEncipherment enables certain key exchange methods, such as RSA key exchange, where the client uses the server’s public key to encrypt a pre-master secret that only the server can decrypt with its private key. In modern TLS configurations that employ ephemeral key exchanges (for example, ECDHE with RSA or ECDHE with ECDSA), the certificate is typically used to sign the key exchange parameters rather than to encipher data, so the digitalSignature bit becomes essential and keyEncipherment may be unused for the key.

KeyUsage is one of several bits in the extension, and it can be marked as critical. If a certificate is marked critical and a relying party cannot honor keyEncipherment, validation may fail. It is common for TLS server certificates to include both digitalSignature and keyEncipherment, though configurations vary by algorithm and policy.

See also RFC 5280, X.509, and TLS cipher suites.