Peer certificate verification in verify_callback

[Jason Schultz]

I have some questions about my application’s verify_callback() function and how I handle some of the OpenSSL errors.



For example, if my client application is presented a self-signed certificate in the handshake, verify_callback() is called with an error, for which X509_STORE_CTX_get_error() returns 18/X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT. In this case, my application searches its trusted store for this certificate, and if it finds a match, the error is cleared and the handshake is allow to proceed.



Other examples are cases where my client application is presented with a certificate chain. Let’s say the chain looks like root -> intermediate -> end-entity, but the server is configured to not send the root, so my client gets: intermediate -> end-entity in the handshake.



One case is where my client is presented these  certificates and has the end-entity certificate in its trusted store. In this case, the verify_callback() gets error 20/ X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY. For this error, my application will search its trusted store for the end-entity certificate, and when a match is found the error is cleared and the handshake is allowed to proceed.



A slightly different case is when the client has only the intermediate certificate in its trusted store, while the server presents the intermediate -> end-entity chain. In this case, verify_callback() is called with an error, and X509_STORE_CTX_get_error() returns 2/ X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT.



These last two cases seem very similar but get slightly different errors. Right now my application does not look for a match in the case of X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT. My plan is to add that error to the cases where the trusted store is searched for a match. Are there more subtle differences between these two errors that I’m missing? Or does my plan to have the application do the addition checking for this error make sense?

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