Porting to mbedTLS 2.16.5.

[AdrianMoranMontes]

Related with old #501 (can discard and apply this PR).

This is a porting of Paho C library to mbedTLS library, tested with last version (2.16.5). All CI tests pass in my Linux machine, except the two ones related to de HA (I see @icraggs is currently working on them).

I would be pleasure to help on developing this new SSL layer for Paho. I know this PR is a first approach, but it could be a good base to start with.

Please, let me know if this work is of interest for community in order to keep it updated.

Thank you.

[icraggs]

I think it could definitely be of interest. Do you use mbedTLS? Are there advantages over OpenSSL?

Does this PR enable mbedTLS to be used - so can we run the CI tests with these changes?

[jumoog]

OpenSSL license is a nightmare for commerical use. mbedTLS is dual-licensed with the Apache License version 2.0 and GPLv2.

[AdrianMoranMontes]

I know this version of Paho is not designed for embedded devices (we have https://github.com/eclipse/paho.mqtt.embedded-c for such purpose), but there are some cases in which the embedded device is not "so embedded" and it is better to use this "normal" Paho C library. However, OpenSSL is usually too big for such devices being mbedTLS much smaller than OpenSSL.

I think this porting will enable the use of Paho C is such scenarios.

Moreover, there are many developments that have mbedTLS in their framework and not OpenSSL (mbedTLS is much more portable than OpenSSL). Indeed, I performed this porting of Paho because of this need.

I would have liked to permorf this porting in a cleaner way (without so many "ifdef MBEDTLS"), but I didn't want to break current OpenSSL part. I think that the cleanest way would be to define a common SSL API (more or less what SSLSocket.h does) in a way that OpenSSL implementation could be performed in one .c file and mbedTLS in a separate .c file, letting CMake use one or another. However, I don't have enough knowledge about how Paho library internally works to do that by myself.

I have clear how mbedTLS handshake is performed, so if any of you wants to do this jobs, I would be pleasure of helping.

Regarding @jumoog question, you can download mbedTLS library (https://tls.mbed.org/download), compile it and then, compile Paho with my patch (replace PAHO_WITH_SSL=TRUE by PAHO_WITH_MBEDTLS=TRUE in cmake invokation). The next spets are the same. Indeed, I have been testing it with a copy of travis-build.sh script replacing that cmake defines. The tests run in the same way they run with OpenSSL. If you have issues to run them, let me know and I can explain in detail or even upload my modified script.

Thank you so much.

Adrian

Note: Provide MBEDTLS_ROOT_DIR in case mbedTLS is not located in common places, cmake needs to find it.

[icraggs]

Is this applicable to Linux only? Will the CI tests pass using mbedTLS?

I started merging to the latest develop branch and got as far as the static build building, but the CI tests not. The shared build failing at linking a sample. This is with mbedTLS enabled. On Ubuntu 18.04 with the Ubuntu mbedTLS package installed.

[AdrianMoranMontes]

Hi @icraggs, when I was working with this it worked both, in Linux/Windows and I would say that also with dynamic linkage (I am not 100% sure of this because the projects where I usually use Paho only support static linkage).

Now I am little bit busy, but I would like to help, so if you give me some steps to reproduce your errors I will give a try in the following days.

Thank you Ian.