Add pyca/cryptography backend

[tomato42]

https://github.com/pyca/cryptography provides a lot of algorithms we use. Implement backends to those algorithms:

• [ ] RC4
• [ ] 3DES-CBC
• [ ] AES-CBC
• [ ] AES-GCM
• [ ] AES-CCM
• [ ] ChaCha20-Poly1305
• [ ] RSA
• [ ] DH (? – using gmpy2 should give faster code than pyca/cryptography but side-channel vulnerable)
• [ ] ECDHE (see also #367)
• [ ] ECDSA (see also #367)
• [ ] EdDSA (?) (likely depends on EdDSA being implemented first, see #231)

in general, it would be nice to first implement #309, so that we can see if and when the backends are used (and if use of them is not counter-productive – while using code that is side-channel secure but is, say, 2 times slower than the alternative side-channel insecure code is most-likely ok, using secure code that is 10 times slower, is not a good trade

checking if it works with version of pyca/cryptography present in CentOS/EPEL would also be nice in travis

[HMaker]

Is this being worked on? Really all cryptographic tasks should be delegated to other libraries since this project core is the TLS protocol.

[tomato42]

it's not, and while the core of the project is TLS, the second goal is portability, which we don't get by depending unconditionally on other libraries

[HMaker]

TLSLite has defined interfaces for all these algorithms? What is the integration complexity?

[tomato42]

yes, it has interfaces for most of them

What is the integration complexity?

Not very high, here's an example of using M2Crypto for aes-cbc: https://github.com/tlsfuzzer/tlslite-ng/blob/master/tlslite/utils/openssl_aes.py