Add SSL configuration API for trusted CA callbacks

include/mbedtls/ssl.h

@@ -2071,6 +2071,63 @@ void mbedtls_ssl_conf_ca_chain( mbedtls_ssl_config *conf,
                                mbedtls_x509_crt *ca_chain,
                                mbedtls_x509_crl *ca_crl );
 
+#if defined(MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK)
+/**
+ * \brief          Set the trusted certificate callback.
+ *
+ *                 This API allows to register the set of trusted certificates
+ *                 through a callback, instead of a linked list as configured
+ *                 by mbedtls_ssl_conf_ca_chain().
+ *
+ *                 This is useful for example in contexts where a large number
+ *                 of CAs are used, and the inefficiency of maintaining them
+ *                 in a linked list cannot be tolerated. It is also useful when
+ *                 the set of trusted CAs needs to be modified frequently.
+ *
+ *                 See the documentation of `mbedtls_x509_crt_ca_cb_t` for
+ *                 more information.
+ *
+ * \param conf     The SSL configuration to register the callback with.
+ * \param f_ca_cb  The trusted certificate callback to use when verifying
+ *                 certificate chains.
+ * \param p_ca_cb  The context to be passed to \p f_ca_cb (for example,
+ *                 a reference to a trusted CA database).
+ *
+ * \note           This API is incompatible with mbedtls_ssl_conf_ca_chain():
+ *                 Any call to this function overwrites the values set through
+ *                 earlier calls to mbedtls_ssl_conf_ca_chain() or
+ *                 mbedtls_ssl_conf_ca_cb().
+ *
+ * \note           This API is incompatible with CA indication in
+ *                 CertificateRequest messages: A server-side SSL context which
+ *                 is bound to an SSL configuration that uses a CA callback
+ *                 configured via mbedtls_ssl_conf_ca_cb(), and which requires
+ *                 client authentication, will send an empty CA list in the
+ *                 corresponding CertificateRequest message.
+ *
+ * \note           This API is incompatible with mbedtls_ssl_set_hs_ca_chain():
+ *                 If an SSL context is bound to an SSL configuration which uses
+ *                 CA callbacks configured via mbedtls_ssl_conf_ca_cb(), then
+ *                 calls to mbedtls_ssl_set_hs_ca_chain() have no effect.
+ *
+ * \note           The use of this API disables the use of restartable ECC
+ *                 during X.509 CRT signature verification (but doesn't affect
+ *                 other uses).
+ *
+ * \warning        This API is incompatible with the use of CRLs. Any call to
+ *                 mbedtls_ssl_conf_ca_cb() unsets CRLs configured through
+ *                 earlier calls to mbedtls_ssl_conf_ca_chain().
+ *
+ * \warning        In multi-threaded environments, the callback \p f_ca_cb
+ *                 must be thread-safe, and it is the user's responsibility
+ *                 to guaranteee this (for example through a mutex
+ *                 contained in the callback context pointed to by \p p_ca_cb).
+ */
+void mbedtls_ssl_conf_ca_cb( mbedtls_ssl_config *conf,
+                             mbedtls_x509_crt_ca_cb_t f_ca_cb,
+                             void *p_ca_cb );
+#endif /* MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK */
+
 /**
  * \brief          Set own certificate chain and private key
  *

library/ssl_srv.c

@@ -2818,6 +2818,11 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
 
     if( ssl->conf->cert_req_ca_list ==  MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED )
     {
+        /* NOTE: If trusted certificates are provisioned
+         *       via a CA callback (configured through
+         *       `mbedtls_ssl_conf_ca_cb()`, then the
+         *       CertificateRequest is currently left empty. */
+
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
         if( ssl->handshake->sni_ca_chain != NULL )
             crt = ssl->handshake->sni_ca_chain;