tls13: Add definition of mbedtls_ssl_{write,read}_early_data

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

docs/architecture/tls13-support.md

@@ -478,3 +478,175 @@ outbound message on server side as well.
 
 * state change: the state change is done in the main state handler to ease
 the navigation of the state machine transitions.
+
+
+Writing and reading early or 0-RTT data
+---------------------------------------
+
+An application function to write and send a buffer of data to a server through
+TLS may plausibly look like:
+
+```
+int write_data( mbedtls_ssl_context *ssl,
+                const unsigned char *data_to_write,
+                size_t data_to_write_len,
+                size_t *data_written )
+{
+    *data_written = 0;
+
+    while( *data_written < data_to_write_len )
+    {
+        ret = mbedtls_ssl_write( ssl, data_to_write + *data_written,
+                                 data_to_write_len - *data_written );
+
+        if( ret < 0 &&
+            ret != MBEDTLS_ERR_SSL_WANT_READ &&
+            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
+        {
+            return( ret );
+        }
+
+        *data_written += ret;
+    }
+
+    return( 0 );
+}
+```
+where ssl is the SSL context to use, data_to_write the address of the data
+buffer and data_to_write_len the number of data bytes. The handshake may
+not be completed, not even started for the SSL context ssl when the function is
+called and in that case the mbedtls_ssl_write() API takes care transparently of
+completing the handshake before to write and send data to the server. The
+mbedtls_ssl_write() may not been able to write and send all data in one go thus
+the need for a loop calling it as long as there are still data to write and
+send.
+
+An application function to write and send early data and only early data,
+data sent during the first flight of client messages while the handshake is in
+its initial phase, would look completely similar but the call to
+mbedtls_ssl_write_early_data() instead of mbedtls_ssl_write().
+```
+int write_early_data( mbedtls_ssl_context *ssl,
+                      const unsigned char *data_to_write,
+                      size_t data_to_write_len,
+                      size_t *data_written )
+{
+    *data_written = 0;
+
+    while( *data_written < data_to_write_len )
+    {
+        ret = mbedtls_ssl_write_early_data( ssl, data_to_write + *data_written,
+                                            data_to_write_len - *data_written );
+
+        if( ret < 0 &&
+            ret != MBEDTLS_ERR_SSL_WANT_READ &&
+            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
+        {
+            return( ret );
+        }
+
+        *data_written += ret;
+    }
+
+    return( 0 );
+}
+```
+Note that compared to write_data(), write_early_data() can also return
+MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA and that should be handled
+specifically by the user of write_early_data(). A fresh SSL context (typically
+just after a call to mbedtls_ssl_setup() or mbedtls_ssl_session_reset()) would
+be expected when calling `write_early_data`.
+
+All together, code to write and send a buffer of data as long as possible as
+early data and then as standard post-handshake application data could
+plausibly look like:
+
+```
+ret = write_early_data( ssl, data_to_write, data_to_write_len,
+                        &early_data_written );
+if( ret < 0 &&
+    ret != MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA )
+{
+    goto error;
+}
+
+ret = write_data( ssl, data_to_write + early_data_written,
+                  data_to_write_len - early_data_written, &data_written );
+if( ret < 0 )
+    goto error;
+
+data_written += early_data_written;
+```
+
+Finally, taking into account that the server may reject early data, application
+code to write and send a buffer of data could plausibly look like:
+```
+ret = write_early_data( ssl, data_to_write, data_to_write_len,
+                        &early_data_written );
+if( ret < 0 &&
+    ret != MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA )
+{
+    goto error;
+}
+
+/*
+ * Make sure the handshake is completed as it is a requisite to
+ * mbedtls_ssl_get_early_data_status().
+ */
+while( !mbedtls_ssl_is_handshake_over( ssl ) )
+{
+    ret = mbedtls_ssl_handshake( ssl );
+    if( ret < 0 &&
+        ret != MBEDTLS_ERR_SSL_WANT_READ &&
+        ret != MBEDTLS_ERR_SSL_WANT_WRITE )
+    {
+        goto error;
+    }
+}
+
+ret = mbedtls_ssl_get_early_data_status( ssl );
+if( ret < 0 )
+    goto error;
+
+if( ret == MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED )
+   early_data_written = 0;
+
+ret = write_data( ssl, data_to_write + early_data_written,
+                  data_to_write_len - early_data_written, &data_written );
+if( ret < 0 )
+    goto error;
+
+data_written += early_data_written;
+```
+
+Basically, the same holds for reading early data on the server side without the
+complication of possible rejection. An application function to read early data
+into a given buffer could plausibly look like:
+```
+int read_early_data( mbedtls_ssl_context *ssl,
+                     unsigned char *buffer,
+                     size_t buffer_size,
+                     size_t *data_len )
+{
+    *data_len = 0;
+
+    while( *data_len < buffer_size )
+    {
+        ret = mbedtls_ssl_read_early_data( ssl, buffer + *data_len,
+                                           buffer_size - *data_len );
+
+        if( ret < 0 &&
+            ret != MBEDTLS_ERR_SSL_WANT_READ &&
+            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
+        {
+            return( ret );
+        }
+
+        *data_len += ret;
+    }
+
+    return( 0 );
+}
+```
+with again calls to read_early_data() expected to be done with a fresh SSL
+context.

include/mbedtls/ssl.h

@@ -96,15 +96,16 @@
 /* Error space gap */
 /** Processing of the Certificate handshake message failed. */
 #define MBEDTLS_ERR_SSL_BAD_CERTIFICATE                   -0x7A00
+/* Error space gap */
 /**
  * Received NewSessionTicket Post Handshake Message.
  * This error code is experimental and may be changed or removed without notice.
  */
 #define MBEDTLS_ERR_SSL_RECEIVED_NEW_SESSION_TICKET       -0x7B00
-/* Error space gap */
+/** Not possible to read early data */
-/* Error space gap */
+#define MBEDTLS_ERR_SSL_CANNOT_READ_EARLY_DATA            -0x7B80
-/* Error space gap */
+/** Not possible to write early data */
-/* Error space gap */
+#define MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA           -0x7C00
 /* Error space gap */
 /* Error space gap */
 /* Error space gap */
@@ -806,14 +807,6 @@ typedef struct mbedtls_ssl_key_cert mbedtls_ssl_key_cert;
 typedef struct mbedtls_ssl_flight_item mbedtls_ssl_flight_item;
 #endif
 
-#if defined(MBEDTLS_SSL_EARLY_DATA) && defined(MBEDTLS_SSL_CLI_C)
-#define MBEDTLS_SSL_EARLY_DATA_STATUS_UNKNOWN           0
-#define MBEDTLS_SSL_EARLY_DATA_STATUS_NOT_SENT          1
-#define MBEDTLS_SSL_EARLY_DATA_STATUS_INDICATION_SENT   2
-#define MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED          3
-#define MBEDTLS_SSL_EARLY_DATA_STATUS_ACCEPTED          4
-#endif
-
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3) && defined(MBEDTLS_SSL_SESSION_TICKETS)
 typedef uint8_t mbedtls_ssl_tls13_ticket_flags;
 
@@ -4897,6 +4890,151 @@ int mbedtls_ssl_send_alert_message( mbedtls_ssl_context *ssl,
  */
 int mbedtls_ssl_close_notify( mbedtls_ssl_context *ssl );
 
+#if defined(MBEDTLS_SSL_EARLY_DATA)
+
+#if defined(MBEDTLS_SSL_SRV_C)
+/**
+ * \brief          Read at most 'len' application data bytes while performing
+ *                 the handshake (early data).
+ *
+ * \note           This function behaves mainly as mbedtls_ssl_read(). The
+ *                 specification of mbedtls_ssl_read() relevant to TLS 1.3
+ *                 (thus not the parts specific to (D)TLS 1.2) applies to this
+ *                 function and the present documentation is restricted to the
+ *                 differences with mbedtls_ssl_read().
+ *
+ * \param ssl      SSL context
+ * \param buf      buffer that will hold the data
+ * \param len      maximum number of bytes to read
+ *
+ * \return         One additional specific return value:
+ *                 #MBEDTLS_ERR_SSL_CANNOT_READ_EARLY_DATA.
+ *
+ *                 #MBEDTLS_ERR_SSL_CANNOT_READ_EARLY_DATA is returned when it
+ *                 is not possible to read early data for the SSL context
+ *                 \p ssl.
+ *
+ *                 It may have been possible and it is not possible
+ *                 anymore because the server received the End of Early Data
+ *                 message or the maximum number of allowed early data for the
+ *                 PSK in use has been reached.
+ *
+ *                 It may never have been possible and will never be possible
+ *                 for the SSL context \p ssl because the use of early data
+ *                 is disabled for that context or more generally the context
+ *                 is not suitably configured to enable early data or the
+ *                 client does not use early data or the first call to the
+ *                 function was done while the handshake was already too
+ *                 advanced to gather and accept early data.
+ *
+ *                 It is not possible to read early data for the SSL context
+ *                 \p ssl but this does not preclude for using it with
+ *                 mbedtls_ssl_write(), mbedtls_ssl_read() or
+ *                 mbedtls_ssl_handshake().
+ *
+ * \note           When a server wants to retrieve early data, it is expected
+ *                 that this function starts the handshake for the SSL context
+ *                 \p ssl. But this is not mandatory.
+ *
+ */
+int mbedtls_ssl_read_early_data( mbedtls_ssl_context *ssl,
+                                 unsigned char *buf, size_t len );
+#endif /* MBEDTLS_SSL_SRV_C */
+
+#if defined(MBEDTLS_SSL_CLI_C)
+/**
+ * \brief          Try to write exactly 'len' application data bytes while
+ *                 performing the handshake (early data).
+ *
+ * \note           This function behaves mainly as mbedtls_ssl_write(). The
+ *                 specification of mbedtls_ssl_write() relevant to TLS 1.3
+ *                 (thus not the parts specific to (D)TLS1.2) applies to this
+ *                 function and the present documentation is restricted to the
+ *                 differences with mbedtls_ssl_write().
+ *
+ * \param ssl      SSL context
+ * \param buf      buffer holding the data
+ * \param len      how many bytes must be written
+ *
+ * \return         One additional specific return value:
+ *                 #MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA.
+ *
+ *                 #MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA is returned when it
+ *                 is not possible to write early data for the SSL context
+ *                 \p ssl.
+ *
+ *                 It may have been possible and it is not possible
+ *                 anymore because the client received the server Finished
+ *                 message, the server rejected early data or the maximum
+ *                 number of allowed early data for the PSK in use has been
+ *                 reached.
+ *
+ *                 It may never have been possible and will never be possible
+ *                 for the SSL context \p ssl because the use of early data
+ *                 is disabled for that context or more generally the context
+ *                 is not suitably configured to enable early data or the first
+ *                 call to the function was done while the handshake was
+ *                 already completed.
+ *
+ *                 It is not possible to write early data for the SSL context
+ *                 \p ssl but this does not preclude for using it with
+ *                 mbedtls_ssl_write(), mbedtls_ssl_read() or
+ *                 mbedtls_ssl_handshake().
+ *
+ * \note           This function may write early data only if the SSL context
+ *                 has been configured for the handshake with a PSK for which
+ *                 early data is allowed.
+ *
+ * \note           To maximize the number of early data that can be written in
+ *                 the course of the handshake, it is expected that this
+ *                 function starts the handshake for the SSL context \p ssl.
+ *                 But this is not mandatory.
+ *
+ * \note           This function does not provide any information on whether
+ *                 the server has accepted or will accept early data or not.
+ *                 When it returns a positive value, it just means that it
+ *                 has written early data to the server. To know whether the
+ *                 server has accepted early data or not, you should call
+ *                 mbedtls_ssl_get_early_data_status() with the handshake
+ *                 completed.
+ */
+int mbedtls_ssl_write_early_data( mbedtls_ssl_context *ssl,
+                                  const unsigned char *buf, size_t len );
+
+#define MBEDTLS_SSL_EARLY_DATA_STATUS_NOT_SENT  0
+#define MBEDTLS_SSL_EARLY_DATA_STATUS_ACCEPTED  1
+#define MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED  2
+/**
+ * \brief Get the status of the negotiation of the use of early data.
+ *
+ * \param ssl      The SSL context to query
+ *
+ * \return         #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if this function is called
+ *                 from the server-side.
+ *
+ * \return         #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if this function is called
+ *                 prior to completion of the handshake.
+ *
+ * \return         #MBEDTLS_SSL_EARLY_DATA_STATUS_NOT_SENT if the client has
+ *                 not indicated the use of early data to the server.
+ *
+ * \return         #MBEDTLS_SSL_EARLY_DATA_STATUS_ACCEPTED if the client has
+ *                 indicated the use of early data and the server has accepted
+ *                 it.
+ *
+ * \return         #MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED if the client has
+ *                 indicated the use of early data but the server has rejected
+ *                 it. In this situation, the client may want to re-send the
+ *                 early data it may have tried to send by calling
+ *                 mbedtls_ssl_write_early_data() as ordinary post-handshake
+ *                 application data by calling mbedtls_ssl_write().
+ *
+ */
+int mbedtls_ssl_get_early_data_status( mbedtls_ssl_context *ssl );
+#endif /* MBEDTLS_SSL_CLI_C */
+
+#endif /* MBEDTLS_SSL_EARLY_DATA */
+
 /**
  * \brief          Free referenced items in an SSL context and clear memory
  *

library/ssl_tls13_client.c

@@ -1183,11 +1183,11 @@ int mbedtls_ssl_tls13_write_client_hello_exts( mbedtls_ssl_context *ssl,
             return( ret );
         p += ext_len;
 
-        /* Initializes the status to `indication sent`. It will be updated to
+        /* Initializes the status to `rejected`. It will be updated to
-         * `accepted` or `rejected` depending on whether the EncryptedExtension
+         * `accepted` if the EncryptedExtension message contain an early data
-         * message will contain an early data indication extension or not.
+         * indication extension.
          */
-        ssl->early_data_status = MBEDTLS_SSL_EARLY_DATA_STATUS_INDICATION_SENT;
+        ssl->early_data_status = MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED;
     }
     else
     {