Key Usage Extensions in X.509

Key Usage and Extended Key Usage

X.509 certificates contain two complementary extensions that restrict how the certificate's public key can be used: Key Usage and Extended Key Usage (EKU). These extensions prevent a single certificate from being misused for unintended purposes.

Key Usage Extension

The Key Usage extension is a bit field with up to 9 possible values:

X509v3 Key Usage: critical
    Digital Signature, Key Encipherment

Bit	Purpose
digitalSignature	Verify digital signatures (TLS handshake, authentication)
nonRepudiation	Signature that the signer cannot later deny (rare)
keyEncipherment	Encrypt symmetric keys (RSA key exchange in TLS)
dataEncipherment	Directly encrypt data (uncommon)
keyAgreement	Key agreement protocols (DH, ECDH)
keyCertSign	Sign other certificates (CA certificates only)
cRLSign	Sign Certificate Revocation Lists (CA certificates only)
encipherOnly	Used with keyAgreement (encrypt only)
decipherOnly	Used with keyAgreement (decrypt only)

Extended Key Usage (EKU)

EKU provides finer-grained purpose restrictions using OIDs (Object Identifiers):

X509v3 Extended Key Usage:
    TLS Web Server Authentication, TLS Web Client Authentication

OID	Name	Purpose
1.3.6.1.5.5.7.3.1	serverAuth	TLS server certificate
1.3.6.1.5.5.7.3.2	clientAuth	TLS client certificate (mTLS)
1.3.6.1.5.5.7.3.3	codeSigning	Digitally sign executables
1.3.6.1.5.5.7.3.4	emailProtection	S/MIME email signing/encryption
1.3.6.1.5.5.7.3.8	timeStamping	Trusted timestamping
1.3.6.1.5.5.7.3.9	OCSPSigning	Sign OCSP responses

Critical vs Non-Critical

When Key Usage is marked critical, the certificate MUST NOT be used for any purpose not listed. Clients that encounter a critical extension they do not understand must reject the certificate. Most CA-issued certificates mark Key Usage as critical.

Typical Key Usage for Different Certificate Types

TLS Server Certificate:

• Key Usage: Digital Signature, Key Encipherment
• EKU: TLS Web Server Authentication

CA Certificate:

• Key Usage: Certificate Sign, CRL Sign
• EKU: usually absent (CAs are not restricted by EKU)

Code Signing Certificate:

• Key Usage: Digital Signature
• EKU: Code Signing

Why Key Usage Matters

Without proper Key Usage restrictions, a TLS server certificate could theoretically be used to sign other certificates, creating a security vulnerability. The Key Usage and EKU extensions implement the principle of least privilege at the certificate level.