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I have one question regarding the OCSP protocol to check if the certificate is revoked or not. The question is about checking whether the intermediate CA certificate immediately below the root CA is valid or not. I know that when we send an OCSP request to validate a certificate, the corresponding response must be signed with the private key of the issuer of the certificate we are checking. So in this case it must be the root CA's private key. Also, I know that the private key of the root CA (and the whole root CA) is offline, so it will not be signed with that key, but with the private key of the authority that the root CA has delegated to be the OCSP responder (the root CA will create a certificate for this delegated authority and will sign it). So my question is:

If the response is signed with the delegated authority's private key, how can the client verify that the response is valid or not when the client does not have the delegated authority's certificate (so it does not have its public key)? The client only has the certificate (and public key) of the root CA, so how will it get the certificate (and public key) of the delegated authority to confirm the response?

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So in this case it must be the root CA's private key

this statement is not correct. It must be the signing certificate which can be delegated or CA (not necessary root) certificate.

how can the client verify that the response is valid or not when the client does not have the delegated authority's certificate

First of all: at OCSP request time, client knows issuing CA certificate because it is part of the original certificate's chain.

Second, at a minimum, OCSP server includes signing certificate into OCSP response. Either, OCSP signing certificate must match original issuer, or original issuer must be the issuer of OCSP signing certificate. This information is already available at OCSP request construction time.

Third, OCSP responder may include additional CA certificates to help clients to build and validate the OCSP signing certificate's chain.

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  • First of all, thanks for your reply. So, the signature of the OCSP response must come from either a CA that is the issuer of the certificate being verified or a delegated authority that has a certificate signed by the issuer of the certificate that is being verified. In this case, since it is a certificate just below the root, the OCSP response can be signed either by the root certificate or by a delegated authorized authority (that authorized authority will have a certificate signed by the root certificate). Am I right? The signature can't come from the another inter. CA right below root CA?
    – dassd
    Feb 6, 2023 at 9:39
  • Additionally, when the signer is a delegated authority, his certificate will be returned with the OCSP response in order to have a public key to check the integrity of the response?
    – dassd
    Feb 6, 2023 at 9:39
  • since it is a certificate just below the root -- it really depends on PKI hierarchy. It can be several levels below root.
    – Crypt32
    Feb 6, 2023 at 13:57

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