Add server certificate selection callback

https://github.com/ARMmbed/mbedtls/issues/5430

Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>

ChangeLog.d/mbedtls_ssl_cert_cb.txt

@@ -0,0 +1,3 @@
+Features
+   * Add server certificate selection callback near end of Client Hello.
+     Register callback with mbedtls_ssl_conf_cert_cb().

include/mbedtls/ssl.h

@@ -1475,6 +1475,10 @@ struct mbedtls_ssl_config
      * access it afterwards.
      */
     mbedtls_ssl_user_data_t MBEDTLS_PRIVATE(user_data);
+
+#if defined(MBEDTLS_SSL_SRV_C)
+    int (*MBEDTLS_PRIVATE(f_cert_cb))(mbedtls_ssl_context *); /*!< certificate selection callback */
+#endif /* MBEDTLS_SSL_SRV_C */
 };
 
 struct mbedtls_ssl_context
@@ -2220,6 +2224,28 @@ void mbedtls_ssl_set_timer_cb( mbedtls_ssl_context *ssl,
                                mbedtls_ssl_set_timer_t *f_set_timer,
                                mbedtls_ssl_get_timer_t *f_get_timer );
 
+#if defined(MBEDTLS_SSL_SRV_C)
+/**
+ * \brief           Set the certificate selection callback (server-side only).
+ *
+ *                  If set, the callback is always called for each handshake,
+ *                  after `ClientHello` processing has finished.
+ *
+ *                  The callback has the following parameters:
+ *                  - \c mbedtls_ssl_context*: The SSL context to which
+ *                                             the operation applies.
+ *                  The return value of the callback is 0 if successful,
+ *                  or a specific MBEDTLS_ERR_XXX code, which will cause
+ *                  the handshake to be aborted.
+ *
+ * \param conf      The SSL configuration to register the callback with.
+ * \param f_cert_cb The callback for selecting server certificate after
+ *                  `ClientHello` processing has finished.
+ */
+void mbedtls_ssl_conf_cert_cb( mbedtls_ssl_config *conf,
+                               int (*f_cert_cb)(mbedtls_ssl_context *) );
+#endif /* MBEDTLS_SSL_SRV_C */
+
 /**
  * \brief           Callback type: generate and write session ticket
  *

library/ssl_srv.c

@@ -1870,10 +1870,20 @@ read_record_header:
         return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
     }
 
+    /*
+     * Server certification selection (after processing TLS extensions)
+     */
+    if( ssl->conf->f_cert_cb && ( ret = ssl->conf->f_cert_cb( ssl ) ) != 0 )
+    {
+        MBEDTLS_SSL_DEBUG_RET( 1, "f_cert_cb", ret );
+        return( ret );
+    }
+
     /*
      * Search for a matching ciphersuite
      * (At the end because we need information from the EC-based extensions
-     * and certificate from the SNI callback triggered by the SNI extension.)
+     * and certificate from the SNI callback triggered by the SNI extension
+     * or certificate from server certificate selection callback.)
      */
     got_common_suite = 0;
     ciphersuites = ssl->conf->ciphersuite_list;

library/ssl_tls.c

@@ -1232,6 +1232,14 @@ void mbedtls_ssl_set_timer_cb( mbedtls_ssl_context *ssl,
     mbedtls_ssl_set_timer( ssl, 0 );
 }
 
+#if defined(MBEDTLS_SSL_SRV_C)
+void mbedtls_ssl_conf_cert_cb( mbedtls_ssl_config *conf,
+                               int (*f_cert_cb)(mbedtls_ssl_context *) )
+{
+    conf->f_cert_cb = f_cert_cb;
+}
+#endif /* MBEDTLS_SSL_SRV_C */
+
 #if defined(MBEDTLS_SSL_SRV_C)
 void mbedtls_ssl_conf_session_cache( mbedtls_ssl_config *conf,
                                      void *p_cache,