Galois Finite Fields and the Advanced Encryption Standard (AES)
71 points by ankitg12 3 days ago | 9 comments
prideout 7 minutes ago
Is this post popular because of a recent mathematical discovery in this problem space?
replymmozeiko 47 minutes ago
> Inverses exist for each operation, which makes decryption possible.
replyIt is worth mentioning that in modern AES usage this feature does not really matter, because block cipher is used in CTR mode. In CTR block cipher is used to create long keystream bytes (pretty much one-time-pad key). This keystream gets xor'ed with plaintext to get ciphertext. Then getting from ciphertext back to plaintext is exactly the same xor operation with same keystream, no inverse AES operations needed.
ceroxylon 12 minutes ago
>Weirdly, members of a field of size pn are equivalent to polynomials of degree up to n
replyWouldn't it be equivalent to polynomials of degree less than n (up to n−1)?
dboreham 4 hours ago
I don't know that "Weirdly" is right for the polynomial equivalence -- isn't that what Galois was trying to do in the first place (prove things about polynomials such as the impossibility of analytic solutions to higher order polynomials, and the impossibility of constructing a heptagon)?
replytomsmeding 2 hours ago
I read the "weirdly" as "unexpectedly", i.e. "you might not see this coming, but it's still true (and there's deep theory behind it)".
replybananaflag 3 hours ago
A bit, yeah. I deleted a comment where I said it's not quite the same but it kinda is, and figuring it out is very rewarding.
reply
Back in the day it was tractable to build AES lookup tables for computing MixColumns and SubBytes similar to the site's multiplication table. Then djb found a remote timing attack, so processors started implementing hardware AES instructions.