chore: refactor keccak into subfunctions (#18)

Author: TomAFrench

src/keccak256.nr

@@ -4,6 +4,7 @@ mod benchmarks;
 
 use std::hash::keccakf1600;
 use std::runtime::is_unconstrained;
+use std::static_assert;
 
 global BLOCK_SIZE_IN_BYTES: u32 = 136; //(1600 - BITS * 2) / WORD_SIZE;
 global WORD_SIZE: u32 = 8; // Limbs are made up of u64s so 8 bytes each.
@@ -30,15 +31,26 @@ pub fn keccak256<let N: u32>(input: [u8; N], message_size: u32) -> [u8; 32] {
     }
 
     //1. format_input_lanes and apply padding
-    let max_blocks = (N + BLOCK_SIZE_IN_BYTES) / BLOCK_SIZE_IN_BYTES;
     let real_max_blocks = (message_size + BLOCK_SIZE_IN_BYTES) / BLOCK_SIZE_IN_BYTES;
 
     // Apply Keccak padding (0x01 after message, 0x80 at block end)
     apply_keccak_padding(&mut block_bytes, message_size, real_max_blocks);
 
+    let block_array = convert_to_u64_array(block_bytes);
+
+    let state = apply_keccak_permutations(block_array, real_max_blocks);
+
+    //3. sponge_squeeze
+    read_hash_from_state(state)
+}
+
+fn convert_to_u64_array<let N: u32>(input: [u8; N]) -> [u64; N / WORD_SIZE] {
+    static_assert(
+        N % WORD_SIZE == 0,
+        "Byte array is expected to cleanly divide into chunks",
+    );
     // populate a vector of 64-bit limbs from our byte array
-    let mut sliced_buffer =
-        [0; (((N / BLOCK_SIZE_IN_BYTES) + 1) * BLOCK_SIZE_IN_BYTES) / WORD_SIZE];
+    let mut sliced_buffer = [0; N / WORD_SIZE];
     for i in 0..sliced_buffer.len() {
         let limb_start = WORD_SIZE * i;
 
@@ -49,7 +61,7 @@ pub fn keccak256<let N: u32>(input: [u8; N], message_size: u32) -> [u8; 32] {
             WORD_SIZE,
             |i: u32| {
                 quote {
-                    sliced += v * (block_bytes[limb_start + $i] as Field);
+                    sliced += v * (input[limb_start + $i] as Field);
                     v *= 256;
                 }
             },
@@ -58,29 +70,36 @@ pub fn keccak256<let N: u32>(input: [u8; N], message_size: u32) -> [u8; 32] {
         sliced.assert_max_bit_size::<64>();
         sliced_buffer[i] = sliced as u64;
     }
+    sliced_buffer
+}
 
+fn apply_keccak_permutations<let N: u32>(
+    flattened_blocks_array: [u64; N],
+    num_blocks: u32,
+) -> [u64; NUM_KECCAK_LANES] {
     //2. sponge_absorb
     let mut state: [u64; NUM_KECCAK_LANES] = [0; NUM_KECCAK_LANES];
-    // `real_max_blocks` is guaranteed to at least be `1`
+    // `num_blocks` is guaranteed to at least be `1`
     // We peel out the first block as to avoid a conditional inside of the loop.
     // Otherwise, a dynamic predicate can cause a blowup in a constrained runtime.
     unroll_loop!(
         0u32,
         LIMBS_PER_BLOCK,
         |i: u32| {
             quote {
-            state[$i] = sliced_buffer[$i];
+            state[$i] = flattened_blocks_array[$i];
         }
         },
     );
     state = keccakf1600(state);
 
-    let state = if is_unconstrained() {
+    let max_blocks = N / LIMBS_PER_BLOCK;
+    if is_unconstrained() {
         // When in an unconstrained runtime we can take advantage of runtime loop bounds,
         // thus allowing us to simplify the loop body.
-        for i in 1..real_max_blocks {
+        for i in 1..num_blocks {
             for j in 0..LIMBS_PER_BLOCK {
-                state[j] = state[j] ^ sliced_buffer[i * LIMBS_PER_BLOCK + j];
+                state[j] = state[j] ^ flattened_blocks_array[i * LIMBS_PER_BLOCK + j];
             }
             state = keccakf1600(state);
         }
@@ -89,20 +108,42 @@ pub fn keccak256<let N: u32>(input: [u8; N], message_size: u32) -> [u8; 32] {
     } else {
         // We store the intermediate states in an array to avoid having a dynamic predicate
         // inside the loop, which can cause a blowup in a constrained runtime.
-        let mut intermediate_states = [state; (N + BLOCK_SIZE_IN_BYTES) / BLOCK_SIZE_IN_BYTES + 1];
+        let mut intermediate_states = [state; N / LIMBS_PER_BLOCK + 1];
         for i in 1..max_blocks {
             let mut previous_state = intermediate_states[i - 1];
             for j in 0..LIMBS_PER_BLOCK {
-                previous_state[j] = previous_state[j] ^ sliced_buffer[i * LIMBS_PER_BLOCK + j];
+                previous_state[j] =
+                    previous_state[j] ^ flattened_blocks_array[i * LIMBS_PER_BLOCK + j];
             }
             intermediate_states[i] = keccakf1600(previous_state);
         }
 
-        // We can then take the state as of `real_max_blocks`, ignoring later permutations.
-        intermediate_states[real_max_blocks - 1]
-    };
+        // We can then take the state as of `num_blocks`, ignoring later permutations.
+        intermediate_states[num_blocks - 1]
+    }
+}
 
-    //3. sponge_squeeze
+// Apply Keccak padding to the block_bytes array
+// Append 0x01 after message, then 0x80 at end of block
+// If both padding bytes collide at the same byte, combine them as 0x81
+#[inline_always]
+pub(crate) fn apply_keccak_padding<let BLOCK_BYTES: u32>(
+    block_bytes: &mut [u8; BLOCK_BYTES],
+    message_size: u32,
+    real_max_blocks: u32,
+) {
+    let real_blocks_bytes = real_max_blocks * BLOCK_SIZE_IN_BYTES;
+
+    if message_size == real_blocks_bytes - 1 {
+        // Combine both padding bits: 0x01 | 0x80 = 0x81
+        block_bytes[message_size] = 0x81;
+    } else {
+        block_bytes[message_size] = 0x01;
+        block_bytes[real_blocks_bytes - 1] = 0x80;
+    }
+}
+
+fn read_hash_from_state(state: [u64; NUM_KECCAK_LANES]) -> [u8; 32] {
     let mut result = [0; 32];
     unroll_loop!(
         0u32,
@@ -126,26 +167,6 @@ pub fn keccak256<let N: u32>(input: [u8; N], message_size: u32) -> [u8; 32] {
     result
 }
 
-// Apply Keccak padding to the block_bytes array
-// Append 0x01 after message, then 0x80 at end of block
-// If both padding bytes collide at the same byte, combine them as 0x81
-#[inline_always]
-pub(crate) fn apply_keccak_padding<let BLOCK_BYTES: u32>(
-    block_bytes: &mut [u8; BLOCK_BYTES],
-    message_size: u32,
-    real_max_blocks: u32,
-) {
-    let real_blocks_bytes = real_max_blocks * BLOCK_SIZE_IN_BYTES;
-
-    if message_size == real_blocks_bytes - 1 {
-        // Combine both padding bits: 0x01 | 0x80 = 0x81
-        block_bytes[message_size] = 0x81;
-    } else {
-        block_bytes[message_size] = 0x01;
-        block_bytes[real_blocks_bytes - 1] = 0x80;
-    }
-}
-
 comptime fn unroll_loop(start: u32, end: u32, body: fn(u32) -> Quoted) -> Quoted {
     let mut iterations: [Quoted] = &[];
     for i in start..end {