feat: add generic Digest256 and Digest512 structs

CHANGELOG.md

@@ -27,6 +27,7 @@
 - [BREAKING] Changed sponge state layout from `[CAPACITY, RATE1, RATE0]` (BE) to `[RATE0, RATE1, CAPACITY]` (LE) ([#755](https://github.com/0xMiden/crypto/pull/755)).
 - [BREAKING] Added length-prefixing to Serializable/Deserializable impls for collections, fuzz deserialization for panics ([#757](https://github.com/0xMiden/crypto/pull/757)).
 - Added `SmtLeaf::try_from_elements()` ([#773](https://github.com/0xMiden/crypto/pull/773)).
+- Added const-generic `Digest<N>` struct for binary hash functions with `Digest256` and `Digest512` type aliases ([#777](https://github.com/0xMiden/crypto/pull/777)).
 - Copied `WordWrapper` macro from `miden-base` to `miden-crypto-derive`.
 
 # 0.20.1 (2025-12-29)

miden-crypto/src/hash/blake/mod.rs

@@ -1,14 +1,10 @@
 use alloc::string::String;
-use core::{
-    mem::size_of,
-    ops::Deref,
-    slice::{self, from_raw_parts},
-};
+use core::{mem::size_of, ops::Deref, slice};
 
 use p3_field::{BasedVectorSpace, PrimeField64};
 use p3_goldilocks::Goldilocks as Felt;
 
-use super::HasherExt;
+use super::{HasherExt, digest::prepare_merge};
 use crate::utils::{
     ByteReader, ByteWriter, Deserializable, DeserializationError, HexParseError, Serializable,
     bytes_to_hex_string, hex_to_bytes,
@@ -275,18 +271,3 @@ fn expand_bytes<const M: usize, const N: usize>(bytes: &[u8; M]) -> [u8; N] {
     expanded[..M].copy_from_slice(bytes);
     expanded
 }
-
-// Cast the slice into contiguous bytes.
-fn prepare_merge<const N: usize, D>(args: &[D; N]) -> &[u8]
-where
-    D: Deref<Target = [u8]>,
-{
-    // compile-time assertion
-    assert!(N > 0, "N shouldn't represent an empty slice!");
-    let values = args.as_ptr() as *const u8;
-    let len = size_of::<D>() * N;
-    // safety: the values are tested to be contiguous
-    let bytes = unsafe { from_raw_parts(values, len) };
-    debug_assert_eq!(args[0].deref(), &bytes[..len / N]);
-    bytes
-}

miden-crypto/src/hash/digest.rs

@@ -0,0 +1,243 @@
+//! Generic digest types for binary hash functions.
+//!
+//! This module provides a reusable const-generic digest struct for hash functions with
+//! fixed-size outputs. The default size is 32 bytes (256 bits), suitable for SHA-256,
+//! Blake3-256, etc. For 64-byte outputs (e.g., SHA-512), use `Digest<64>`.
+
+use alloc::string::String;
+use core::{mem::size_of, ops::Deref, slice};
+
+use crate::utils::{
+    ByteReader, ByteWriter, Deserializable, DeserializationError, HexParseError, Serializable,
+    bytes_to_hex_string, hex_to_bytes,
+};
+
+// CONSTANTS
+// ================================================================================================
+
+/// Size of a 256-bit digest in bytes.
+pub const DIGEST256_BYTES: usize = 32;
+
+/// Size of a 512-bit digest in bytes.
+pub const DIGEST512_BYTES: usize = 64;
+
+// TYPE ALIASES
+// ================================================================================================
+
+/// A 256-bit (32-byte) digest. Type alias for `Digest<32>`.
+pub type Digest256 = Digest<DIGEST256_BYTES>;
+
+/// A 512-bit (64-byte) digest. Type alias for `Digest<64>`.
+pub type Digest512 = Digest<DIGEST512_BYTES>;
+
+// DIGEST
+// ================================================================================================
+
+/// A fixed-size digest for binary hash functions.
+///
+/// This struct provides a generic, reusable digest type for hash functions that produce
+/// fixed-size outputs. The const parameter `N` specifies the digest size in bytes,
+/// defaulting to 32 bytes (256 bits).
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(into = "String", try_from = "&str"))]
+#[repr(transparent)]
+pub struct Digest<const N: usize = DIGEST256_BYTES>([u8; N]);
+
+impl<const N: usize> Digest<N> {
+    /// Creates a new digest from the given bytes.
+    #[inline]
+    pub const fn new(bytes: [u8; N]) -> Self {
+        Self(bytes)
+    }
+
+    /// Returns the digest as a byte array reference.
+    #[inline]
+    pub fn as_bytes(&self) -> &[u8; N] {
+        &self.0
+    }
+
+    /// Converts a slice of digests into a contiguous byte slice.
+    pub fn digests_as_bytes(digests: &[Digest<N>]) -> &[u8] {
+        let p = digests.as_ptr();
+        let len = digests.len() * N;
+        // SAFETY: Digest<N> is repr(transparent) over [u8; N], so this is safe
+        unsafe { slice::from_raw_parts(p as *const u8, len) }
+    }
+}
+
+impl<const N: usize> Default for Digest<N> {
+    fn default() -> Self {
+        Self([0; N])
+    }
+}
+
+impl<const N: usize> Deref for Digest<N> {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<const N: usize> From<Digest<N>> for [u8; N] {
+    fn from(value: Digest<N>) -> Self {
+        value.0
+    }
+}
+
+impl<const N: usize> From<[u8; N]> for Digest<N> {
+    fn from(value: [u8; N]) -> Self {
+        Self(value)
+    }
+}
+
+impl<const N: usize> From<Digest<N>> for String {
+    fn from(value: Digest<N>) -> Self {
+        bytes_to_hex_string(value.0)
+    }
+}
+
+impl<const N: usize> TryFrom<&str> for Digest<N> {
+    type Error = HexParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        hex_to_bytes(value).map(Self)
+    }
+}
+
+impl<const N: usize> Serializable for Digest<N> {
+    fn write_into<W: ByteWriter>(&self, target: &mut W) {
+        target.write_bytes(&self.0);
+    }
+}
+
+impl<const N: usize> Deserializable for Digest<N> {
+    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
+        source.read_array().map(Self)
+    }
+}
+
+// HELPER FUNCTIONS
+// ================================================================================================
+
+/// Cast the slice into contiguous bytes.
+///
+/// This function is used by hash implementations to efficiently convert an array of digests
+/// into a byte slice for hashing in merge operations.
+pub fn prepare_merge<const N: usize, D>(args: &[D; N]) -> &[u8]
+where
+    D: Deref<Target = [u8]>,
+{
+    // compile-time assertion
+    assert!(N > 0, "N shouldn't represent an empty slice!");
+    let values = args.as_ptr() as *const u8;
+    let len = size_of::<D>() * N;
+    // safety: the values are tested to be contiguous
+    let bytes = unsafe { slice::from_raw_parts(values, len) };
+    debug_assert_eq!(args[0].deref(), &bytes[..len / N]);
+    bytes
+}
+
+// TESTS
+// ================================================================================================
+
+#[cfg(test)]
+mod tests {
+    use core::mem::{align_of, size_of};
+
+    use super::*;
+
+    #[test]
+    fn test_memory_layout_assumptions() {
+        // Verify that Digest<N> has the same size and alignment as [u8; N].
+        // The unsafe code in digests_as_bytes and prepare_merge relies on this.
+        assert_eq!(size_of::<Digest<32>>(), size_of::<[u8; 32]>());
+        assert_eq!(align_of::<Digest<32>>(), align_of::<[u8; 32]>());
+
+        assert_eq!(size_of::<Digest<64>>(), size_of::<[u8; 64]>());
+        assert_eq!(align_of::<Digest<64>>(), align_of::<[u8; 64]>());
+
+        // Verify type aliases as well
+        assert_eq!(size_of::<Digest256>(), 32);
+        assert_eq!(size_of::<Digest512>(), 64);
+    }
+
+    #[test]
+    fn test_digest_default_32() {
+        let digest = Digest::<32>::default();
+        assert_eq!(digest.as_bytes(), &[0u8; 32]);
+    }
+
+    #[test]
+    fn test_digest_default_64() {
+        let digest = Digest::<64>::default();
+        assert_eq!(digest.as_bytes(), &[0u8; 64]);
+    }
+
+    #[test]
+    fn test_digest256_alias() {
+        let digest = Digest256::default();
+        assert_eq!(digest.as_bytes(), &[0u8; 32]);
+    }
+
+    #[test]
+    fn test_digest512_alias() {
+        let digest = Digest512::default();
+        assert_eq!(digest.as_bytes(), &[0u8; 64]);
+    }
+
+    #[test]
+    fn test_digest_from_bytes_32() {
+        let bytes = [1u8; 32];
+        let digest = Digest::<32>::from(bytes);
+        assert_eq!(digest.as_bytes(), &bytes);
+    }
+
+    #[test]
+    fn test_digest_from_bytes_64() {
+        let bytes = [1u8; 64];
+        let digest = Digest::<64>::from(bytes);
+        assert_eq!(digest.as_bytes(), &bytes);
+    }
+
+    #[test]
+    fn test_digest_hex_roundtrip_32() {
+        let bytes = [0xab; 32];
+        let digest = Digest::<32>::from(bytes);
+        let hex: String = digest.into();
+        let recovered = Digest::<32>::try_from(hex.as_str()).unwrap();
+        assert_eq!(recovered.as_bytes(), &bytes);
+    }
+
+    #[test]
+    fn test_digest_hex_roundtrip_64() {
+        let bytes = [0xcd; 64];
+        let digest = Digest::<64>::from(bytes);
+        let hex: String = digest.into();
+        let recovered = Digest::<64>::try_from(hex.as_str()).unwrap();
+        assert_eq!(recovered.as_bytes(), &bytes);
+    }
+
+    #[test]
+    fn test_digest_digests_as_bytes_32() {
+        let d1 = Digest::<32>::from([1u8; 32]);
+        let d2 = Digest::<32>::from([2u8; 32]);
+        let digests = [d1, d2];
+        let bytes = Digest::<32>::digests_as_bytes(&digests);
+        assert_eq!(bytes.len(), 64);
+        assert_eq!(&bytes[0..32], &[1u8; 32]);
+        assert_eq!(&bytes[32..64], &[2u8; 32]);
+    }
+
+    #[test]
+    fn test_digest_digests_as_bytes_64() {
+        let d1 = Digest::<64>::from([1u8; 64]);
+        let d2 = Digest::<64>::from([2u8; 64]);
+        let digests = [d1, d2];
+        let bytes = Digest::<64>::digests_as_bytes(&digests);
+        assert_eq!(bytes.len(), 128);
+        assert_eq!(&bytes[0..64], &[1u8; 64]);
+        assert_eq!(&bytes[64..128], &[2u8; 64]);
+    }
+}