Key usage and extended key usage
Key usage extensions define the purpose of the public key
contained in a certificate. You can use them to restrict the public
key to as few or as many operations as needed. For example, if you
have a key used only for signing or verifying a signature, enable
the digital signature and/or non-repudiation extensions. Alternatively,
if a key is used only for key management, enable key encipherment.
Key usage
The following table describes the key
usage extensions available for certificates created using the CA process.
Note: The digital signature and data encipherment key usage
extensions are enabled by default for all Internet certificates.
Key usage extension
|
Description
|
|---|---|
Digital signature
|
Use when the public key is used with a digital
signature mechanism to support security services other than non-repudiation,
certificate signing, or CRL signing. A digital signature is often
used for entity authentication and data origin authentication with
integrity.
|
Non-repudiation
|
Use when the public key is used to verify digital
signatures used to provide a non-repudiation service. Non-repudiation
protects against the signing entity falsely denying some action (excluding
certificate or CRL signing).
|
Key encipherment
|
Use when a certificate will be used with a
protocol that encrypts keys. An example is S/MIME enveloping, where
a fast (symmetric) key is encrypted with the public key from the certificate.
SSL protocol also performs key encipherment.
|
Data encipherment
|
Use when the public key is used for encrypting
user data, other than cryptographic keys.
|
Key agreement
|
Use when the sender and receiver of the public
key need to derive the key without using encryption. This key can
then can be used to encrypt messages between the sender and receiver.
Key agreement is typically used with Diffie-Hellman ciphers.
|
Certificate signing
|
Use when the subject public key is used to
verify a signature on certificates. This extension can be used only
in CA certificates.
|
CRL signing
|
Use when the subject public key is to verify
a signature on revocation information, such as a CRL.
|
Encipher only
|
Use only when key agreement is also enabled.
This enables the public key to be used only for enciphering data while
performing key agreement.
|
Decipher only
|
Use only when key agreement is also enabled.
This enables the public key to be used only for deciphering data while
performing key agreement.
|
Extended key usage
Extended key usage further refines key usage
extensions. An extended key is either critical or non-critical. If
the extension is critical, the certificate must be used only
for the indicated purpose or purposes. If the certificate is used
for another purpose, it is in violation of the CA's policy.
If
the extension is non-critical, it indicates the intended purpose or
purposes of the key and may be used in finding the correct key/certificate
of an entity that has multiple keys/certificates. The extension is
then only an informational field and does not imply that the CA restricts
use of the key to the purpose indicated. Nevertheless, applications
that use certificates may require that a particular purpose be indicated
in order for the certificate to be acceptable.
If a certificate
contains both a critical key usage field and a critical extended key
usage field, both fields must be processed independently, and
the certificate be used only for a purpose consistent with both fields.
If there is no purpose consistent with both fields, the certificate
must not be used for any purpose.
Extended key
|
Enable for these key usage extensions
|
|---|---|
TLS Web server authentication
|
Digital signature, key encipherment or key
agreement
|
TLS Web client authentication
|
Digital signature and/or key agreement
|
Sign (downloadable) executable code
|
Digital signature
|
Email protection
|
Digital signature, non-repudiation, and/or
key encipherment or key agreement
|
IPSEC End System (host or router)
|
Digital signature and/or key encipherment or
key agreement
|
IPSEC Tunnel
|
Digital signature and/or key encipherment or
key agreement
|
IPSEC User
|
Digital signature and/or key encipherment or
key agreement
|
Timestamping
|
Digital signature, non-repudiation.
|
