Add support for twofish cipher

[tomaka]

Adds support for the twofish cipher mode.

There are random data corruption in polkadot right now, and that might be a problem in the aes library.
Let's add support for twofish in secio, and later, once it's deployed, we can try using it in our local node to see if the data corruption goes away.

[tomaka]

This is an addition to the specifications of secio, as we add support for an algorithm that Go and JS don't support, with the name TwofishCTR.

I don't know if I should inform someone of that.
cc @mgoelzer

The reason why I went for twofish and not blowfish (which Go and JS support) is because I couldn't find any CBC library in Rust.

[ghost]

/cc @Stebalien

[Stebalien]

This shouldn't break anything so I don't really have any objections. However, I don't think there are really any plans to add twofish support to the other implementations.

Are you just using this for testing?

[tomaka]

This shouldn't break anything so I don't really have any objections.

Sure, but maybe we should document somewhere what the possible algorithms are, instead of having people figure out by reading the source code of the various implementations.

Are you just using this for testing?

There's probably a bug in the AES crypto library we're using, and support for Blowfish would be too long to implement.

[burdges]

I suppose this does not matter given your reasons, but if it's only twofish in CTR mode being used then that's just a stream cipher, so maybe ChaCha20 works too.

[tomaka]

(merging this because the Polkadot network is on fire, but we can keep discussing)

ChaCha20 works too.

I went with the algorithm that was the most straight-forward to add support for 😅

[Stebalien]

Yes, we should totally document this.

[ghost]

Yes, we should totally document this.

Unless we create an issue where docs people will see it, this will likely never be discovered by them. I made one at libp2p/docs#9. AFAIK, libp2p currently has no docs repo or docs backlog (waffle or zen). So we need to fix that.