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SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cbhandle client certificate callback function

#include <openssl/ssl.h>

SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey));

(*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey);

(*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);

() sets the client_cert_cb() callback that is called when a client certificate is requested by a server and no certificate was yet set for the SSL object.

When client_cert_cb is NULL, no callback function is used.

() returns a pointer to the currently set callback function.

() is the application-defined callback. If it wants to set a certificate, a certificate/private key combination must be set using the x509 and pkey arguments and 1 must be returned. The certificate will be installed into ssl. If no certificate should be set, 0 has to be returned and no certificate will be sent. A negative return value will suspend the handshake and the handshake function will return immediately. SSL_get_error(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate that the handshake was suspended. The next call to the handshake function will again lead to the call of client_cert_cb(). It is the job of the client_cert_cb() to store information about the state of the last call, if required to continue.

During a handshake (or renegotiation) a server may request a certificate from the client. A client certificate must only be sent when the server did send the request.

When a certificate has been set using the SSL_CTX_use_certificate(3) family of functions, it will be sent to the server. The TLS standard requires that only a certificate is sent if it matches the list of acceptable CAs sent by the server. This constraint is violated by the default behavior of the OpenSSL library. Using the callback function it is possible to implement a proper selection routine or to allow a user interaction to choose the certificate to be sent.

If a callback function is defined and no certificate was yet defined for the SSL object, the callback function will be called. If the callback function returns a certificate, the OpenSSL library will try to load the private key and certificate data into the SSL object using the () and () functions. Thus it will permanently install the certificate and key for this SSL object. It will not be reset by calling SSL_clear(3). If the callback returns no certificate, the OpenSSL library will not send a certificate.

ssl(3), SSL_clear(3), SSL_CTX_add_extra_chain_cert(3), SSL_CTX_use_certificate(3), SSL_free(3), SSL_get_client_CA_list(3)

SSL_CTX_set_client_cert_cb() and SSL_CTX_get_client_cert_cb() first appeared in SSLeay 0.6.6 and have been available since OpenBSD 2.4.

The client_cert_cb() cannot return a complete certificate chain; it can only return one client certificate. If the chain only has a length of 2, the root CA certificate may be omitted according to the TLS standard and thus a standard conforming answer can be sent to the server. For a longer chain, the client must send the complete chain (with the option to leave out the root CA certificate). This can be accomplished only by either adding the intermediate CA certificates into the trusted certificate store for the SSL_CTX object (resulting in having to add CA certificates that otherwise maybe would not be trusted), or by adding the chain certificates using the SSL_CTX_add_extra_chain_cert(3) function, which is only available for the SSL_CTX object as a whole and that therefore probably can only apply for one client certificate, making the concept of the callback function (to allow the choice from several certificates) questionable.

Once the SSL object has been used in conjunction with the callback function, the certificate will be set for the SSL object and will not be cleared even when SSL_clear(3) is called. It is therefore to destroy the SSL object using SSL_free(3) and create a new one to return to the previous state.

March 27, 2018 OpenBSD-current