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.
When a peer certificate has been received during a SSL/TLS handshake, a verification function is called regardless of the verification mode. 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 with the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg). The argument arg is specified by the application when setting callback. By setting callback to NULL, the default behaviour is restored.
callback should return 1 to indicate verification success and 0 to indicate verification failure. In server mode, a return value of 0 leads to handshake failure. In client mode, the behaviour is as follows. All values, including 0, are ignored if the verification mode is SSL_VERIFY_NONE. Otherwise, when the return value is less than or equal to 0, the handshake will fail.
In client mode callback may also call the SSL_set_retry_verify(3) function on the SSL object set in the x509_store_ctx ex data (see SSL_get_ex_data_X509_STORE_CTX_idx(3)) and return 1. This would be typically done in case the certificate verification was not yet able to succeed. 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.
In any case a viable verification result value must be reflected in the error member of x509_store_ctx, which can be done using X509_STORE_CTX_set_error(3). This is particularly important in case the callback allows the connection to continue (by returning 1). Note that the verification status in the store context is a possibly durable indication of the chain's validity! This gets recorded in the SSL session (and thus also in session tickets) and the validity of the originally presented chain is then visible on resumption, even though no chain is presented int that case. Moreover, the calling application will be informed about the detailed result of the verification procedure and may elect to base further decisions on it.
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), X509_STORE_CTX_set_error(3), SSL_get_verify_result(3), SSL_set_retry_verify(3), SSL_CTX_load_verify_locations(3)
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