Understanding Online Certificate Status Protocol

OCSP is used to check the revocation status of X509 certificates. OCSP provides revocation status on certificates in real time and is useful in time-sensitive situations such as bank transactions and stock trades.

The revocation status of a certificate is checked by sending a request to an OCSP server that resides outside of an SRX Series device. Based on the response from the server, the VPN connection is allowed or denied. OCSP responses are not cached on SRX Series devices.

The OCSP server can be the certificate authority (CA) that issues a certificate or a designated authorized responder. The location of the OCSP server can be configured manually or extracted from the certificate that is being verified. Requests are sent first to OCSP server locations that are manually configured in CA profiles with the ocsp url statement at the [edit security pki ca-profile profile-name revocation-check] hierarchy level; up to two locations can be configured for each CA profile. If the first configured OCSP server is not reachable, the request is sent to the second OCSP server. If the second OCSP server is not reachable, the request is then sent to the location in the certificate's AuthorityInfoAccess extension field. The use-ocsp option must also be configured, as certificate revocation list (CRL) is the default checking method.

SRX Series devices accept only signed OCSP responses from the CA or authorized responder. The response received is validated using trusted certificates. The response is validated as follows:

The CA certificate enrolled for the configured CA profile is used to validate the response.
The OCSP response might contain a certificate to validate the OCSP response. The received certificate must be signed by a CA certificate enrolled in the SRX Series device. After the received certificate is validated by the CA certificate, it is used to validate the OCSP response.

The response from the OCSP server can be signed by different CAs. The following scenarios are supported:

The CA server that issues the end entity certificate for a device also signs the OCSP revocation status response. The SRX Series device verifies the OCSP response signature using the CA certificate enrolled in the SRX Series device. After the OCSP response is validated, the certificate revocation status is checked.
An authorized responder signs the OCSP revocation status response. The certificate for the authorized responder and the end entity certificate being verified must be issued by the same CA. The authorized responder is first verified using the CA certificate enrolled in the SRX Series device. The OCSP response is validated using the responder’s CA certificate. The SRX Series device then uses the OCSP response to check the revocation status of the end entity certificate.
There are different CA signers for the end entity certificate being verified and the OCSP response. The OCSP response is signed by a CA in the certificate chain for the end entity certificate being verified. (All peers participating in an IKE negotiation need to have at least one common trusted CA in their respective certificate chains.) The OCSP responder’s CA is verified using a CA in the certificate chain. After validating the responder CA certificate, the OCSP response is validated using the responder’s CA certificate.

To prevent replay attacks, a nonce payload can be sent in an OCSP request. Nonce payloads are sent by default unless it is explicitly disabled. If enabled, the SRX Series device expects the OCSP response to contain a nonce payload, otherwise the revocation check fails. If OCSP responders are not capable of responding with a nonce payload, then the nonce payload must be disabled on the SRX Series device.

Understanding Online Certificate Status Protocol

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