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Write the keccak state in one literal #1000

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Write the keccak state in one literal #1000

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7b311e7
Write the keccak state in one literal
jotabulacios Sep 21, 2026
62c13a3
Make the literal's lane assumptions explicit
jotabulacios Sep 21, 2026
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61 changes: 45 additions & 16 deletions syscalls/src/keccak.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -174,27 +174,56 @@ pub fn keccak256(input: &[u8]) -> [u8; 32] {
/// Keccak-256 of exactly two concatenated 32-byte nodes (64 bytes) — the fixed
/// shape of every Merkle parent hash. 64 bytes fit the 136-byte rate in one
/// block, so this skips the incremental sponge entirely: load the eight data
/// lanes straight from `left`/`right`, XOR the `pad10*1` bits in place, run one
/// permutation, squeeze four lanes. Byte-identical to feeding `left` then
/// `right` through the streaming [`Keccak256`] and finalizing.
/// lanes straight from `left`/`right`, write the `pad10*1` bits into their
/// lanes, run one permutation, squeeze four lanes. Byte-identical to feeding
/// `left` then `right` through the streaming [`Keccak256`] and finalizing.
///
/// The nodes are only byte-aligned, so lanes are assembled with `from_le_bytes`
/// over owned arrays — never an aligned doubleword load, which the VM would trap
/// on at a misaligned address.
pub fn keccak256_pair(left: &[u8; 32], right: &[u8; 32]) -> [u8; 32] {
let mut state = [0u64; 25];
// Bytes 0..64 span rate lanes 0..8: lanes 0..4 from `left`, 4..8 from `right`.
for i in 0..4 {
let l: &[u8; 8] = left[i * 8..i * 8 + 8].try_into().unwrap();
state[i] = u64::from_le_bytes(*l);
let r: &[u8; 8] = right[i * 8..i * 8 + 8].try_into().unwrap();
state[4 + i] = u64::from_le_bytes(*r);
}
// pad10*1 for a 64-byte message at rate 136: delimiter at byte 64 (lane 8,
// low byte) and the final bit at the last rate byte (byte 135, lane 16 high
// byte). Both target lanes are still zero, so XOR == assignment.
state[8] ^= u64::from(DELIMITER);
state[RATE_LANES - 1] ^= FINAL_PAD_LANE_BIT;
let lane = |b: &[u8; 32], i: usize| -> u64 {
let w: &[u8; 8] = b[i * 8..i * 8 + 8].try_into().unwrap();
u64::from_le_bytes(*w)
};
// Lane indices the literal below hardcodes: the message fits one block, so
// pad10*1 puts the delimiter at byte 64 (lane 8, low byte) and the final
// bit at the last rate byte (byte 135, lane 16 high byte).
const MSG_BYTES: usize = 32 + 32;
const DELIMITER_LANE: usize = MSG_BYTES / 8;
const LAST_RATE_LANE: usize = RATE_LANES - 1;
const _: () = assert!(MSG_BYTES < RATE_BYTES && DELIMITER_LANE == 8 && LAST_RATE_LANE == 16);
// Every lane written once: zeroing all 25 and then overwriting 10 costs a
// memset per call, and this is the guest's hottest call. Bytes 0..64 span
// rate lanes 0..8 (0..4 from `left`, 4..8 from `right`). Both pad lanes
// start zero, so the XORs the sponge would do are plain assignments.
let mut state: [u64; 25] = [
lane(left, 0),
lane(left, 1),
lane(left, 2),
lane(left, 3),
lane(right, 0),
lane(right, 1),
lane(right, 2),
lane(right, 3),
u64::from(DELIMITER),
0,
0,
0,
0,
0,
0,
0,
FINAL_PAD_LANE_BIT,
0,
0,
0,
0,
0,
0,
0,
0,
];
keccak_permute(&mut state);

let mut out = [0u8; 32];
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