The keyUsage as delineated in RFC 5280 specifies the the purpose of the key (public key) contained in the certificate.
- “keyEncipherment” implies that the public key is used to encrypt private or secret keys.
- “digitalSignature” implies that the public key can be used to validate the digital signatures.
- “keyAgreement” implies that the public key is used for key agreement as in the DH case. The key agreement algorithm could be ECDH (Elliptic Curve DH) where the public key of the end-entity certificate is a ECDH public key. The certificate could be signed by any normal CA – for example with it’s ECDSA or RSA private keys. So in the case of a ECC certificate or any certificate containing an ECC public key, one would find the same ECC public key being utilized for key agreement as in the ECDH (not ECDHE) case. Note that ECDHE does not require this keyUsage bit to be set.
For the other bits in the keyUsage extension, please see the RFC.