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Certificate extensions

Extensions provide additional context for a certificate.

Some of the more common extension types include the following:

Subject key identifier

Holds a unique identifier for the certificate, which is generally derived from the certificate’s public key.

Authority key identifier

Holds the subject key identifier for the issuer certificate. This extension type helps to identify the issuer certificate, especially when presented with an incomplete certificate chain.

Subject alternative name

Holds a list of ways that clients are expected to reference a server when establishing a connection to it.

Clients must take this information into account when deciding whether to trust a server’s certificate.

The most common types of values include DNS names, IP addresses, and URIs. DNS names must be fully qualified, but can optionally use an asterisk in the leftmost component to match any single name in that component. For example, *.example.com could match www.example.com or ldap.example.com, but would not match ldap.east.example.com or example.com.
Key usage

Provides information about the manner in which the certificate is expected to be used. The following key usages are allowed:

digitalSignature

Indicates that the certificate can be used for digitally signing data, excluding certificates and certificate revocation lists (CRL).

nonRepudiation

Indicates that the certificate can be used to prevent denying the authenticity of a message. nonRepudiation is also known as contentCommitment.

keyEncipherment

Indicates that the certificate can be used to protect encryption keys, such as symmetric keys that are derived during TLS key agreement.

dataEncipherment

Indicates that the certificate can be used for encrypting data directly.

keyAgreement

Indicates that the certificate’s public key can be used for key agreement, such as deriving the symmetric key that protects TLS communication.

keyCertSign

Indicates that the certificate can act as a certification authority and be used for signing other certificates.

cRLSign

Indicates that the certificate can be used to sign CRLs.

encipherOnly

When used in conjunction with keyEncipherment, indicates that the public key can be used only for encrypting data during key agreement.

decipherOnly

When used in conjunction with keyEncipherment, indicates that the public key can be used only for decrypting data during key agreement.

Extended key usage

Acts as an alternative to the key usage extension and provides additional high-level functionality. The following extended key usages are allowed:

serverAuth

Indicates that the server can present the certificate to the client during TLS negotiation.

clientAuth

Indicates that the client can present the certificate to the server during TLS negotiation.

codeSigning

Indicates that the certificate can be used to sign source and compiled code.

emailProtection

Indicates that the certificate can be used to sign or encrypt email messages.

timeStamping

Indicates that the certificate can be used to assert the time that an event occurred.

ocspSigning

Indicates that the certificate can be used to sign an online certificate status protocol (OCSP) response.

Basic constraints

Indicates whether the certificate can act as a certification authority and, if so, the maximum number of intermediate certificates that can follow it in a certificate chain.