feat: bytes to fields and back (#8590)

Adds functionality to convert byte array to field array in Noir and back.

noir-projects/aztec-nr/aztec/src/utils/bytes.nr

@@ -0,0 +1,212 @@
+use std::static_assert;
+
+// Converts the input bytes into an array of fields. A Field is ~254 bits meaning that each field can store 31 bytes.
+// This implies that M = ceil(N / 31).
+//
+// Each 31 byte chunk is converted into a Field as if the chunk was the Field's big endian representation. If the last chunk
+// is less than 31 bytes long, then only the relevant bytes are conisdered.
+// For example, [1, 10, 3] is encoded as [1 * 256^2 + 10 * 256 + 3]
+pub fn bytes_to_fields<let N: u32, let M: u32>(input: [u8; N]) -> [Field; M] {
+    static_assert(N <= 31 * M, "Bytes do not fit into fields");
+    let mut dst = [0; M];
+
+    for dst_index in 0..M {
+        let mut field_value = 0;
+
+        for i in 0..31 {
+            let byte_index = dst_index * 31 + i;
+            if byte_index < N {
+                // Shift the existing value left by 8 bits and add the new byte
+                field_value = field_value * 256 + input[byte_index] as Field;
+            }
+        }
+
+        dst[dst_index] = field_value;
+    }
+
+    dst
+}
+
+// Converts an input array of fields into bytes. Each field of input has to contain only 31 bytes.
+// TODO(#8618): Optimize for public use.
+pub fn fields_to_bytes<let N: u32, let M: u32>(input: [Field; M]) -> [u8; N] {
+    let mut dst = [0; N];
+
+    for src_index in 0..M {
+        let field = input[src_index];
+
+        // We expect that the field contains at most 31 bytes of information.
+        field.assert_max_bit_size::<248>();
+
+        // Now we can safely convert the field to 31 bytes.
+        let src: [u8; 31] = field.to_be_bytes();
+
+        // Since some of the bytes might not be occupied (if the source value requiring less than 31 bytes),
+        // we have to compute the start index from which to copy.
+        let remaining_bytes = N - src_index * 31;
+        let src_start_index = if remaining_bytes < 31 {
+            // If the remaining bytes are less than 31, we only copy the remaining bytes
+            31 - remaining_bytes
+        } else {
+            0
+        };
+
+        // Note: I tried combining this check with `assert_max_bit_size` above but `assert_max_bit_size` expects
+        // the argument to be a constant. Using comptime block to derive the number of bits also does not work
+        // because comptime is evaluated before generics.
+        for i in 0..src_start_index {
+            assert(src[i] == 0, "Field does not fit into remaining bytes");
+        }
+
+        for i in 0..31 {
+            let byte_index = src_index * 31 + i;
+            if byte_index < N {
+                dst[byte_index] = src[src_start_index + i];
+            }
+        }
+    }
+
+    dst
+}
+
+mod test {
+    use crate::utils::bytes::{bytes_to_fields, fields_to_bytes};
+
+    #[test]
+    fn test_bytes_to_1_field() {
+        let input = [
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
+        ];
+        let output = bytes_to_fields::<31, 1>(input);
+
+        assert_eq(output[0], 0x0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f);
+    }
+
+    #[test]
+    fn test_1_field_to_bytes() {
+        let input = [0x0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f];
+        let output = fields_to_bytes::<31, 1>(input);
+
+        assert_eq(
+            output, [
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
+        ]
+        );
+    }
+
+    #[test]
+    fn test_3_small_fields_to_bytes() {
+        let input = [1, 2, 3];
+        let output = fields_to_bytes::<93, 3>(input);
+
+        // Each field should occupy 31 bytes with the non-zero value being placed in the last one.
+        assert_eq(
+            output, [
+            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3
+        ]
+        );
+    }
+
+    #[test]
+    fn test_3_small_fields_to_less_bytes() {
+        let input = [1, 2, 3];
+        let output = fields_to_bytes::<63, 3>(input);
+
+        // First 2 fields should occupy 31 bytes with the non-zero value being placed in the last one while the last
+        // field should occupy 1 byte. There is not information destruction here because the last field fits into
+        // 1 byte.
+        assert_eq(
+            output, [
+            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 3
+        ]
+        );
+    }
+
+    #[test]
+    fn test_bytes_to_2_fields() {
+        let input = [
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59
+        ];
+        let output = bytes_to_fields::<59, 2>(input);
+
+        assert_eq(output[0], 0x0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f);
+        assert_eq(output[1], 0x202122232425262728292a2b2c2d2e2f303132333435363738393a3b);
+    }
+
+    #[test]
+    fn test_2_fields_to_bytes() {
+        let input = [
+            0x0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f, 0x202122232425262728292a2b2c2d2e2f303132333435363738393a3b
+        ];
+        let output = fields_to_bytes::<62, 2>(input);
+
+        assert_eq(
+            output, [
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 0, 0, 0, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59
+        ]
+        );
+    }
+
+    #[test]
+    fn test_large_random_input_to_fields_and_back(input: [u8; 128]) {
+        let output = bytes_to_fields::<128, 5>(input);
+        let input_back = fields_to_bytes::<128, 5>(output);
+
+        assert_eq(input, input_back);
+    }
+
+    // I need to get an array of random values lower than 2^248 on input and since there is no u248 type and modulo
+    // operation is not supported on a Field (to do field % 2^248), I will take multiple smaller values and combine
+    // them to get a value lower than 2^248.
+    #[test]
+    fn test_large_random_input_to_bytes_and_back(
+        input1: [u64; 5],
+        input2: [u64; 5],
+        input3: [u64; 5],
+        input4: [u32; 5],
+        input5: [u16; 5],
+        input6: [u8; 5]
+    ) {
+        let mut input = [0; 5];
+        for i in 0..5 {
+            input[i] = (input1[i] as Field * 2.pow_32(184)) + (input2[i] as Field * 2.pow_32(120)) + (input3[i] as Field * 2.pow_32(56)) + (input4[i] as Field * 2.pow_32(24)) + (input5[i] as Field * 2.pow_32(8)) + input6[i] as Field;
+        }
+
+        let output = fields_to_bytes::<155, 5>(input);
+        let input_back = bytes_to_fields::<155, 5>(output);
+
+        assert_eq(input, input_back);
+    }
+
+    #[test(should_fail_with = "Argument is false")]
+    fn test_too_few_destination_fields() {
+        // This should fail because we need 2 fields to store 32 bytes but we only provide 1.
+        let input = [0 as u8; 32];
+        let _ignored_result = bytes_to_fields::<32, 1>(input);
+    }
+
+    #[test(should_fail_with = "Field does not fit into remaining bytes")]
+    fn test_too_few_destination_bytes() {
+        // We should get an error here because first field gets converted to 31 bytes and the second field needs
+        // at least 2 bytes but we provide it with 1.
+        let input = [1, 256];
+        let _ignored_result = fields_to_bytes::<32, 2>(input);
+    }
+
+    #[test(should_fail_with = "call to assert_max_bit_size")]
+    fn test_fields_to_bytes_value_too_large() {
+        let input = [2.pow_32(248)];
+        let _ignored_result = fields_to_bytes::<31, 1>(input);
+    }
+
+    #[test]
+    fn test_fields_to_bytes_max_value() {
+        let input = [2.pow_32(248) - 1];
+        let result = fields_to_bytes::<31, 1>(input);
+
+        // We check that all the bytes were set to max value (255)
+        for i in 0..31 {
+            assert_eq(result[i], 255);
+        }
+    }
+}

noir-projects/aztec-nr/aztec/src/utils/mod.nr

@@ -1,7 +1,9 @@
+mod bytes;
 mod collapse_array;
 mod comparison;
 mod point;
 mod test;
 mod to_bytes;
 
 pub use crate::utils::collapse_array::collapse_array;
+pub use crate::utils::bytes::{bytes_to_fields, fields_to_bytes};