Update README.mbedtls

Change-Id: Ia61c467d85d690752011bafcf112e39d5b252aa7
Signed-off-by: Max Fillinger <[email protected]>
Acked-by: Frank Lichtenheld <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg27295.html
Signed-off-by: Gert Doering <[email protected]>

README.mbedtls

@@ -1,13 +1,13 @@
-This version of OpenVPN has mbed TLS support. To enable follow the following
-instructions:
+This version of OpenVPN has mbed TLS support. To enable, follow the
+instructions below:
 
-To Build and Install,
+To build and install,
 
 	./configure --with-crypto-library=mbedtls
 	make
 	make install
 
-This version depends on mbed TLS 2.0 (and requires at least 2.0.0).
+This version requires mbed TLS version >= 2.0.0 or >= 3.2.1.
 
 *************************************************************************
 
@@ -16,17 +16,15 @@ Warning:
 As of mbed TLS 2.17, it can be licensed *only* under the Apache v2.0 license.
 That license is incompatible with OpenVPN's GPLv2.
 
-If you wish to distribute OpenVPN linked with mbed TLS, there are two options:
+We are currently in the process of resolving this problem, but for now, if you
+wish to distribute OpenVPN linked with mbed TLS, there are two options:
 
  * Ensure that your case falls under the system library exception in GPLv2, or
 
  * Use an earlier version of mbed TLS. Version 2.16.12 is the last release
    that may be licensed under GPLv2. Unfortunately, this version is
    unsupported and won't receive any more updates.
 
-If nothing changes about the license situation, mbed TLS support may be
-deprecated in a future release of OpenVPN.
-
 *************************************************************************
 
 Due to limitations in the mbed TLS library, the following features are missing
@@ -42,3 +40,22 @@ Plugin/Script features:
  * X.509 subject line has a different format than the OpenSSL subject line
  * X.509 certificate export does not work
  * X.509 certificate tracking
+
+*************************************************************************
+
+Mbed TLS 3 supports the TLS 1.3 protocol, but the implementation is not yet
+complete. Therefore, using TLS 1.3 in the mbed TLS build of OpenVPN is not yet
+supported.
+
+Nevertheless, here are some pointers to make it work with mbed TLS 3.5.0:
+
+ * The stock configuration of mbed TLS does not support TLS 1.3. To enable it,
+   uncomment `#define MBEDTLS_SSL_PROTO_TLS1_3` in your mbedtls_config.h before
+   compiling the library.
+ * An OpenVPN client with mbed TLS cannot connect to a server with OpenSSL
+   using TLS 1.3.
+ * An OpenVPN client with OpenSSL *can* connect to a server using mbed TLS with
+   TLS 1.3, but *only* if `#define MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE` has
+   been uncommented in mbedtls_config.h.
+
+Note that none of these limitations apply to TLS 1.2.