Cryptography and SSL/TLS Toolkit



SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure


 #include <openssl/ssl.h>

 void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx,
                                       int (*callback)(X509_STORE_CTX *, void *),
                                       void *arg);


SSL_CTX_set_cert_verify_callback() sets the verification callback function for ctx. SSL objects that are created from ctx inherit the setting valid at the time when SSL_new(3) is called.


Whenever a certificate is verified during a SSL/TLS handshake, a verification function is called. If the application does not explicitly specify a verification callback function, the built-in verification function is used. If a verification callback callback is specified via SSL_CTX_set_cert_verify_callback(), the supplied callback function is called instead. By setting callback to NULL, the default behaviour is restored.

When the verification must be performed, callback will be called with the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg). The argument arg is specified by the application when setting callback.

callback should return 1 to indicate verification success and 0 to indicate verification failure. If SSL_VERIFY_PEER is set and callback returns 0, the handshake will fail.

In client mode callback may also return -1, typically on failure verifying the server certificate. This makes the handshake suspend and return control to the calling application with SSL_ERROR_WANT_RETRY_VERIFY. The app can for instance fetch further certificates or cert status information needed for the verification. Calling SSL_connect(3) again resumes the connection attempt by retrying the server certificate verification step. This process may even be repeated if need be.

As the verification procedure may allow the connection to continue in the case of failure (by always returning 1) the verification result must be set in any case using the error member of x509_store_ctx so that the calling application will be informed about the detailed result of the verification procedure!

Within x509_store_ctx, callback has access to the verify_callback function set using SSL_CTX_set_verify(3).


SSL_CTX_set_cert_verify_callback() does not return a value.


Do not mix the verification callback described in this function with the verify_callback function called during the verification process. The latter is set using the SSL_CTX_set_verify(3) family of functions.

Providing a complete verification procedure including certificate purpose settings etc is a complex task. The built-in procedure is quite powerful and in most cases it should be sufficient to modify its behaviour using the verify_callback function.


SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.


ssl(7), SSL_CTX_set_verify(3), SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3)

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