docs(aes): document cipher operation functions

Add doc comments explaining the four AES round operations:
    - SubBytes: S-box substitution for non-linearity
    - ShiftRows: cyclic row shifting for column diffusion
    - MixColumns: GF(2^8) matrix multiplication for row diffusion
    - AddRoundKey: XOR with derived subkey

aes/src/operations/column_mix.rs

@@ -1,5 +1,9 @@
 use crate::Block128;
 
+/// Mixes each column using matrix multiplication in GF(2^8) (MixColumns step).
+///
+/// Each column is treated as a polynomial and multiplied by a fixed polynomial
+/// modulo x^4 + 1. This provides diffusion across the rows.
 #[must_use]
 pub fn mix_columns(block: Block128) -> Block128 {
     let mut bytes = block.to_be_bytes();
@@ -28,6 +32,7 @@ pub fn mix_columns(block: Block128) -> Block128 {
     Block128::from_be_bytes(bytes)
 }
 
+/// Inverse of [`mix_columns`] using the inverse matrix.
 #[must_use]
 pub fn inv_mix_columns(block: Block128) -> Block128 {
     let mut bytes = block.to_be_bytes();

aes/src/operations/round_key.rs

@@ -1,5 +1,10 @@
 use crate::{Block128, key::Subkey};
 
+/// XORs the state with a round key (AddRoundKey step).
+///
+/// Each round of AES combines the current state with a derived subkey
+/// using bitwise XOR. This operation is its own inverse.
+#[must_use]
 pub fn add_round_key(state: Block128, subkeys: &[Subkey; 4]) -> Block128 {
     let [k0, k1, k2, k3] = [subkeys[0], subkeys[1], subkeys[2], subkeys[3]];
     let key_block = (k0 << 96) | (k1 << 64) | (k2 << 32) | k3;

aes/src/operations/row_shift.rs

@@ -1,5 +1,9 @@
 use crate::Block128;
 
+/// Cyclically shifts rows of the state matrix (ShiftRows step).
+///
+/// Row 0: no shift, Row 1: shift left 1, Row 2: shift left 2, Row 3: shift left 3.
+/// This provides diffusion across the columns.
 #[must_use]
 pub const fn shift_rows(block: Block128) -> Block128 {
     let b = block.to_be_bytes();
@@ -32,6 +36,7 @@ pub const fn shift_rows(block: Block128) -> Block128 {
     Block128::from_be_bytes(out)
 }
 
+/// Inverse of [`shift_rows`] - shifts rows right instead of left.
 #[must_use]
 pub const fn inv_shift_rows(block: Block128) -> Block128 {
     let b = block.to_be_bytes();

aes/src/operations/sbox_lookup.rs

@@ -1,11 +1,16 @@
 use crate::{Block128, sbox::SboxLookup};
 
+/// Substitutes each byte using the AES S-box (SubBytes step).
+///
+/// Provides non-linearity by replacing each byte with its S-box lookup value.
+/// The S-box is derived from the multiplicative inverse in GF(2^8).
 #[inline]
 #[must_use]
 pub fn sub_bytes(block: Block128) -> Block128 {
     block.sbox_lookup()
 }
 
+/// Inverse of [`sub_bytes`] using the inverse S-box.
 #[inline]
 #[must_use]
 pub fn inv_sub_bytes(block: Block128) -> Block128 {