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Note: This section is a copy/paste from RFC3280.

This extension MUST appear as a critical extension in all CA certificates that contain public keys used to validate digital signatures on certificates. This extension MAY appear as a critical or non-critical extension in CA certificates that contain public keys used exclusively for purposes other than validating digital signatures on certificates. Such CA certificates include ones that contain public keys used exclusively for validating digital signatures on CRLs and ones that contain key management public keys used with certificate enrollment protocols. This extension MAY appear as a critical or non-critical extension in end entity certificates.

The cA boolean indicates whether the certified public key belongs to a CA. If the cA boolean is not asserted, then the keyCertSign bit in the key usage extension MUST NOT be asserted.

The digitalSignature bit is asserted when the subject public key is used with a digital signature mechanism to support security services other than certificate signing (bit 5), or CRL signing (bit 6). Digital signature mechanisms are often used for entity authentication and data origin authentication with integrity.

The nonRepudiation bit is asserted when the subject public key is used to verify digital signatures used to provide a non- repudiation service which protects against the signing entity falsely denying some action, excluding certificate or CRL signing. In the case of later conflict, a reliable third party may determine the authenticity of the signed data.

Further distinctions between the digitalSignature and nonRepudiation bits may be provided in specific certificate policies.

The keyEncipherment bit is asserted when the subject public key is used for key transport. For example, when an RSA key is to be used for key management, then this bit is set.

The dataEncipherment bit is asserted when the subject public key is used for enciphering user data, other than cryptographic keys.

The keyAgreement bit is asserted when the subject public key is used for key agreement. For example, when a Diffie-Hellman key is to be used for key management, then this bit is set.

The keyCertSign bit is asserted when the subject public key is used for verifying a signature on public key certificates. If the keyCertSign bit is asserted, then the cA bit in the basic constraints extension (section 4.2.1.10) MUST also be asserted.

The cRLSign bit is asserted when the subject public key is used for verifying a signature on certificate revocation list (e.g., a CRL, delta CRL, or an ARL). This bit MUST be asserted in certificates that are used to verify signatures on CRLs.

The meaning of the encipherOnly bit is undefined in the absence of the keyAgreement bit. When the encipherOnly bit is asserted and the keyAgreement bit is also set, the subject public key may be used only for enciphering data while performing key agreement.

The meaning of the decipherOnly bit is undefined in the absence of the keyAgreement bit. When the decipherOnly bit is asserted and the keyAgreement bit is also set, the subject public key may be used only for deciphering data while performing key agreement.