Restructure outgoing CliKeyExch: Add frame for new structure

This commit adds declarations and dummy implementations for
the restructured outgoing client key exchange handling that
will replace the previous ssl_write_client_key_exchange().

The entry point for the CliKeyExchange handling that is called
from the handshake state machine is

   `ssl_process_client_key_exchange()`,

splitting the processing into the following steps:

- Preparation
  Compute the keying material to be sent.
  * For (EC)DH: Pick parameters and compute PMS.
  * For ECJPAKE: Run round 2
  * For RSA: Encrypt PMS
- Writing: Prepare the writing of a new messae.
- Postprocessing: Update handstate state machine.

The subsequent commits will scatter the code from the previous
monolithic function ssl_write_client_key_exchange() among those
dedicated functions, commenting out each part of
ssl_write_client_key_exchange() that has already been dealt with.
This gradual progression is meant to ease reviewing. Once all
code has been moved and all changes explained,
ssl_write_client_key_exchange() will be removed.

include/mbedtls/ssl_internal.h

@@ -1806,4 +1806,6 @@ MBEDTLS_ALWAYS_INLINE static inline void mbedtls_ssl_pend_fatal_alert(
     ssl->pending_fatal_alert_msg = message;
 }
 
+#define SSL_PROC_CHK(f) do { if( ( ret = f ) < 0 ) goto cleanup; } while( 0 )
+
 #endif /* ssl_internal.h */

library/ssl_cli.c

@@ -3187,6 +3187,97 @@ static int ssl_parse_server_hello_done( mbedtls_ssl_context *ssl )
     return( 0 );
 }
 
+/*
+ *
+ * STATE HANDLING: Client Key Exchange
+ *
+ */
+
+/*
+ * Overview
+ */
+
+/* Main entry point; orchestrates the other functions */
+static int ssl_process_client_key_exchange( mbedtls_ssl_context *ssl );
+
+/* Preparation
+ * - For ECDH: Generate client params and derive premater secret
+ * - For RSA-suites: Encrypt PMS
+ * - For ECJPAKE: Do Round 2
+ */
+static int ssl_client_key_exchange_prepare( mbedtls_ssl_context *ssl );
+static int ssl_client_key_exchange_write( mbedtls_ssl_context *ssl,
+                                          unsigned char *buf,
+                                          size_t buflen,
+                                          size_t *olen );
+static int ssl_client_key_exchange_postprocess( mbedtls_ssl_context *ssl );
+
+/*
+ * Implementation
+ */
+
+static int ssl_process_client_key_exchange( mbedtls_ssl_context *ssl )
+{
+    int ret = 0;
+    MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> process client key exchange" ) );
+
+    SSL_PROC_CHK( ssl_client_key_exchange_prepare( ssl ) );
+
+     /* Prepare CertificateVerify message in output buffer. */
+    SSL_PROC_CHK( ssl_client_key_exchange_write( ssl, ssl->out_msg,
+                                                 MBEDTLS_SSL_MAX_CONTENT_LEN,
+                                                 &ssl->out_msglen ) );
+
+    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
+    ssl->out_msg[0]  = MBEDTLS_SSL_HS_CLIENT_KEY_EXCHANGE;
+
+    /* Update state */
+    SSL_PROC_CHK( ssl_client_key_exchange_postprocess( ssl ) );
+
+    /* Dispatch message */
+
+    SSL_PROC_CHK( mbedtls_ssl_write_handshake_msg( ssl ) );
+
+    /* NOTE: For the new messaging layer, the postprocessing step
+     *       might come after the dispatching step if the latter
+     *       doesn't send the message immediately.
+     *       At the moment, we must do the postprocessing
+     *       prior to the dispatching because if the latter
+     *       returns WANT_WRITE, we want the handshake state
+     *       to be updated in order to not enter
+     *       this function again on retry. */
+
+cleanup:
+
+    MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= process client key exchange" ) );
+    return( ret );
+}
+
+static int ssl_client_key_exchange_prepare( mbedtls_ssl_context *ssl )
+{
+    /* TBD */
+}
+
+static int ssl_client_key_exchange_write( mbedtls_ssl_context *ssl,
+                                          unsigned char *buf,
+                                          size_t buflen,
+                                          size_t *olen )
+{
+    /* TBD */
+}
+
+static int ssl_client_key_exchange_postprocess( mbedtls_ssl_context *ssl )
+{
+    /* TBD */
+}
+
+/* OLD CODE
+ *
+ * Temporarily included to gradually move it to the correct
+ * place in the restructured code.
+ *
+ */
+
 static int ssl_write_client_key_exchange( mbedtls_ssl_context *ssl )
 {
     int ret;
@@ -3892,7 +3983,7 @@ int mbedtls_ssl_handshake_client_step( mbedtls_ssl_context *ssl )
            break;
 
        case MBEDTLS_SSL_CLIENT_KEY_EXCHANGE:
-           ret = ssl_write_client_key_exchange( ssl );
+           ret = ssl_process_client_key_exchange( ssl );
            break;
 
        case MBEDTLS_SSL_CERTIFICATE_VERIFY: