EncipherOnly

EncipherOnly is a boolean flag defined as part of the KeyUsage extension in X.509 certificates. It is one of several usage bits that specify what a certificate’s private key may be used for within the public key infrastructure.

Meaning and usage: The encipherOnly bit is meaningful only when the KeyUsage extension also has the keyAgreement bit set. When encipherOnly is true, it indicates that, in the context of a key agreement protocol, the key may be used to encipher data. The complementary decipherOnly bit indicates that the key may be used to decipher data in a key agreement. If neither encipherOnly nor decipherOnly is set, the certificate’s key usage for key agreement is not restricted to encipherment or decipherment by that bit alone, and other usage bits govern its behavior. In practice, the encipherOnly bit is infrequently used and many certificates omit it entirely.

Relation to other bits: The encipherOnly bit works in conjunction with the keyAgreement bit. Together they describe the directionality of data flow in key exchange operations. The KeyUsage extension also includes other bits such as digitalSignature, keyEncipherment, and dataEncipherment, which define additional concrete capabilities of the key for signature, encryption, or key transport purposes.

Implementation and standards: EncipherOnly is defined in the X.509 PKI framework and is described in standards such as RFC 5280. It is encoded as part of the KeyUsage extension’s bit string and is a boolean flag that certificate authorities may set to constrain how the associated private key can be used in key exchange scenarios.

Practical notes: In modern PKI deployments, the encipherOnly bit is rarely relied upon, and many certificates do not include it. Its presence mainly affects compatibility with systems that implement strict key agreement usage policies.