rc6: RC6 Implementation

[ashWhiteHat]

Part of #1

Hi there.
I implemented rc6.

Implementation and documentation are aligned with rc5 implementation.

I referred following document for key schedule, encryption and decryption algorithms.
https://www.grc.com/r&d/rc6.pdf

I checked whether it works by following test vectors.
https://datatracker.ietf.org/doc/html/draft-krovetz-rc6-rc5-vectors-00#section-3

I would appreciate it if you could confirm.
Thank you!

[NamedNeon]

+1

RustCrypto apparently already owns the crate "rc6" and just isn't doing anything with it, so not merging this seems somewhat silly.

[tarcieri]

The reason this hasn't been merged yet is because it hasn't yet been code reviewed, which is especially important for cryptographic code.

Please give us time to review it.

If you'd like to be helpful, it would be good to get some feedback on if it worked in a particular application you're interested in.

[NamedNeon]

The reason this hasn't been merged yet is because it hasn't yet been code reviewed, which is especially important for cryptographic code.

Please give us time to review it.

Oh, yeah, 100%. I apologize if I came off as impatient. My intent was to express interest in the feature, not to rush you guys.

If you'd like to be helpful, it would be good to get some feedback on if it worked in a particular application you're interested in.

I am planning on using it in an application soon, hence my original comment. I'll report back with my results.

[NamedNeon]

The RC6 standard doesn't mandate any key size, round count, or word length, but this implementation does. RustCrypto's RC5 implementation has a similar problem which I've opened an issue for (#381). RC5 and RC6 are very similar so most of what I wrote there should be applicable here.