Extended Key Usage (EKU) Extension Explained

The Extended Key Usage (EKU) extension (OID 2.5.29.37) refines the permitted uses of a certificate's public key at the application layer. While Key Usage deals with cryptographic operations, EKU specifies the application context — the protocols and purposes for which the certificate is authorized.

Common EKU Values

• TLS Web Server Authentication (OID 1.3.6.1.5.5.7.3.1): The certificate can authenticate a TLS server. Required on all public HTTPS certificates.
• TLS Web Client Authentication (OID 1.3.6.1.5.5.7.3.2): The certificate can authenticate a TLS client. Used in mutual TLS (mTLS).
• Code Signing (OID 1.3.6.1.5.5.7.3.3): For authenticating signed executables and scripts.
• Email Protection (OID 1.3.6.1.5.5.7.3.4): Used in S/MIME email signing and encryption.
• Time Stamping (OID 1.3.6.1.5.5.7.3.8): For trusted time stamp tokens (RFC 3161).
• OCSP Signing (OID 1.3.6.1.5.5.7.3.9): For signing OCSP responses.

What Happens Without the Right EKU

If a certificate is used for a purpose not listed in its EKU, the TLS handshake will fail. A certificate with only clientAuth cannot be used as a server certificate. This is enforced by browsers and the CA/Browser Forum's Baseline Requirements.

Reading EKU in the Decoder

The EKU extension appears in the Extensions section of the decoder output. A typical HTTPS certificate will show something like: TLS Web Server Authentication, TLS Web Client Authentication. The inclusion of both server and client authentication is common — it allows the same certificate to be used for mutual TLS if needed.