A pure-Haskell SSH server library

#6CBC ciphers disabled in OpenSSH 7.6

In OpenSSH 7.6 CBC ciphers are now disabled by default. This requires users to manually whitelist aes256-cbc,aes192-cbc,aes128-cbc in ~/.ssh/config if they want to connect to a server using this package. To fix that, CTR or XTS ciphers should be implemented.