chore: Add hash_to_field Noir alternative (#3338)

noir_stdlib/src/field.nr

@@ -82,3 +82,20 @@ pub fn modulus_be_bytes() -> [u8] {}
 
 #[builtin(modulus_le_bytes)]
 pub fn modulus_le_bytes() -> [u8] {}
+
+// Convert a 32 byte array to a field element
+pub fn bytes32_to_field(bytes32 : [u8; 32]) -> Field {
+    // Convert it to a field element
+    let mut v = 1;
+    let mut high = 0 as Field;
+    let mut low = 0 as Field;
+
+    for i in 0..16 {
+        high = high + (bytes32[15 - i] as Field) * v;
+        low = low + (bytes32[16 + 15 - i] as Field) * v;
+        v = v * 256;
+    }
+
+    // Abuse that a % p + b % p = (a + b) % p and that low < p
+    low + high * v
+}

noir_stdlib/src/hash.nr

@@ -20,8 +20,19 @@ pub fn pedersen_hash<N>(input : [Field; N]) -> Field {
 #[foreign(pedersen_hash)]
 pub fn pedersen_hash_with_separator<N>(_input : [Field; N], _separator : u32) -> Field {}
 
-#[foreign(hash_to_field_128_security)]
-pub fn hash_to_field<N>(_input : [Field; N]) -> Field {}
+pub fn hash_to_field<N>(_input : [Field; N]) -> Field {
+    let mut inputs_as_bytes = [];
+
+    for i in 0..N {
+        let input_bytes = _input[i].to_le_bytes(32);
+        for i in 0..32 {
+            inputs_as_bytes = inputs_as_bytes.push_back(input_bytes[i]);
+        }
+    }
+
+    let hashed_input = blake2s(inputs_as_bytes);
+    crate::field::bytes32_to_field(hashed_input)
+}
 
 #[foreign(keccak256)]
 pub fn keccak256<N>(_input : [u8; N], _message_size: u32) -> [u8; 32] {}

tooling/nargo_cli/tests/execution_success/hash_to_field/src/main.nr

@@ -2,4 +2,4 @@ use dep::std;
 
 fn main(input : Field) -> pub Field {
     std::hash::hash_to_field([input])
-}
+}