Add AES-GCM(includes GHASH)

[mrdaybird]

closes #140 (Bounty: AES-GCM)

Worklist:

• Implement GHASH
  • Use Polynomial lib for polynomial multiplication.
• Implement GCM
• Add doc + comments for GCM+GHASH
• Add tests to CTR (using NIST test vectors)
• Add to README
• (optional) GHASH : faster polynomial multiplication

Notes:

1. Made some changes to already existing CTR(ctr.rs) implementation, which were required for GCM. The GCM spec requires a nonce of 96-bit and counter size of 32-bit (=> total of 128-bit), but the previous CTR implementation used a 50-50 split of block size for nonce and counter.
2. Removed an "assert" in AES implementation which failed when encrypting a string of zeros.

[mrdaybird]

@0xJepsen @brunny-eth I think this PR is ready for review!

There are a couple of more things, like updating the CTR section in README and adding a faster polynomial hash algorithm, but that can be done in another PR, this PR is already too big.

It was a fun week learning Rust and playing with bits! 😄

[0xJepsen]

I wonder if there is a better way to store this

[mrdaybird]

I was looking into it for a good hour, though I couldn't find anything better. :(
Maybe I should add a test to sanity check the coefficients.

[0xJepsen]

If you can push up a change so that the lint passes I am happy to merge this! Like you mentioned i think a little bit of love on the readme would be good. I spent some time drawing some diagrams in ascii for GCTR here, as well as for GHASH here if you like you can grab them and put them where you think they fit if they are helpful. Also adding a link to the root readme and linking to the one you wrote would be a nice touch!

Thanks for everything! After some final touches on docs we @brunny-eth can work with you on getting you the bounty!

[brunny-eth]

hey @mrdaybird -- great work here, we're really happy that you chose to contribute to our project!

we're going to be awarding you with a $750 bounty for the work you've done here. Please share your preferred contact info (e-mail, telegram, etc.) here so I can reach out and coordinate payment

thanks again for your contributions! Hope you decide to stick around 😃

[mrdaybird]

hey @mrdaybird -- great work here, we're really happy that you chose to contribute to our project! we're going to be awarding you with a $750 bounty for the work you've done here

@brunny-eth woah! awesome!

you can email me at [email protected]

Hope you decide to stick around 😃

Definitely! this is an amazing initiative!
I learned a lot from this project, and I am sure that I will be using this to learn about cryptography in the future.

EDIT: I have pinged you on telegram, just in case!