Red Hat CERTIFICATE SYSTEM 7.3 - ADMINISTRATION, Administration Manual

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Appendix A. Certificate and CRL Extensions
468
A.3.8.3. Discussion
The Key Usage extension defines the purpose of the key contained in the certificate. The Key Usage,
Extended Key Usage, and Basic Constraints extensions act together to specify the purposes for which
a certificate can be used.
If this extension is included at all, set the bits as follows:
•
digitalSignature
(
0
) for SSL client certificates, S/MIME signing certificates, and object-signing
certificates.
•
nonRepudiation
(
1
) for some S/MIME signing certificates and object-signing certificates.
WARNING
Use of this bit is controversial. Carefully consider the legal consequences of its use
before setting it for any certificate.
•
keyEncipherment
(
2
) for SSL server certificates and S/MIME encryption certificates.
•
dataEncipherment
(
3
) when the subject's public key is used to encrypt user data instead of key
material.
•
keyAgreement
(
4
) when the subject's public key is used for key agreement.
•
keyCertSign
(
5
) for all CA signing certificates.
•
cRLSign
(
6
) for CA signing certificates that are used to sign CRLs.
•
encipherOnly
(
7
) if the public key is used only for enciphering data. If this bit is set,
keyAgreement
should also be set.
•
decipherOnly
(
8
) if the public key is used only for deciphering data. If this bit is set,
keyAgreement
should also be set.
Table A.3, “Certificate Uses and Corresponding Key Usage Bits”
summarizes the guidelines for typical
certificate uses.
Purpose of Certificate Required Key Usage Bit
CA Signing •
keyCertSign
•
cRLSign
SSL Client
digitalSignature
SSL Server
keyEncipherment
S/MIME Signing
digitalSignature
S/MIME Encryption
keyEncipherment
Certificate Signing
keyCertSign
Table A.3. Certificate Uses and Corresponding Key Usage Bits
If the
keyUsage
extension is present and marked critical, then it is used to enforce the usage of the
certificate and key. The extension is used to limit the usage of a key; if the extension is not present or
not critical, all types of usage are allowed.