Implement `aes128-ctr` cipher (and 192, 256)

[dswisher]

The openssh SFTP client by default does not include 3des-cbc in the key exchange. Implement aes128-ctr (and/or 192/256 bit lengths) so that the openssh SFTP can connect without having to explicitly override the allowed ciphers.

The .NET framework includes support for AES, but not the CTR cipher mode. We'll need to implement something. There are resources available:

• RFC 4344 - The Secure Shell (SSH) Transport Layer Encryption Modes
• RFC 4253, section 6.3 - original encryption RFC
• The .NET 3.1 AES class
• Can I use AES in CTR mode in .NET? - SO post
• The above SO post links to this gist which seems to have an implementation, but I'm not sure how to leverage it
• A duplicate SO post
• Wikipedia article on cipher modes
• The SSH.NET CtrCipherMode class, and AesCipher class (which does not use the framework AES implementation!)

It would be nice to write a unit test for this (and the other cipher/crypto bits).

[dswisher]

To write a unit test, it would be good to encrypt something with openssh and feed that into the unit test.

• How to use OpenSSL to encrypt/decrypt files?
• Blog post - Security basics with GPG, OpenSSH, OpenSSL and Keybase - explains some of the basics around encryption, keys, etc.

[dswisher]

FileZilla supports aes192-cbc, as does openssh (with the -oCiphers=aes192-cbc option).

Redo the way ciphers are handled in the code, to accept a CipherMode implementation. Implement cbc and get it working with 3des-cbc, then apply to aes192-cbc. Next, write a ctr mode and apply it to aes192-ctr.