@@ -27,12 +27,14 @@
 [features]
 default = ["blake3/default", "std", "winter_crypto/default", "winter_math/default", "winter_utils/default"]
 std = ["blake3/std", "winter_crypto/std", "winter_math/std", "winter_utils/std"]
+serde = ["winter_math/serde", "dep:serde", "serde/alloc"]
 
 [dependencies]
 blake3 = { version = "1.4", default-features = false }
 winter_crypto = { version = "0.6", package = "winter-crypto", default-features = false }
 winter_math = { version = "0.6", package = "winter-math", default-features = false }
 winter_utils = { version = "0.6", package = "winter-utils", default-features = false }
+serde = { version = "1.0", features = [ "derive" ], optional = true, default-features = false }
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }

@@ -1,5 +1,8 @@
 use super::{Digest, ElementHasher, Felt, FieldElement, Hasher, StarkField};
-use crate::utils::{ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable};
+use crate::utils::{
+    bytes_to_hex_string, hex_to_bytes, string::String, ByteReader, ByteWriter, Deserializable,
+    DeserializationError, HexParseError, Serializable,
+};
 use core::{
     mem::{size_of, transmute, transmute_copy},
     ops::Deref,
@@ -24,6 +27,8 @@ ///
 /// Note: `N` can't be greater than `32` because [`Digest::as_bytes`] currently supports only 32
 /// bytes.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(into = "String", try_from = "&str"))]
 pub struct Blake3Digest<const N: usize>([u8; N]);
 
 impl<const N: usize> Default for Blake3Digest<N> {
@@ -49,6 +54,20 @@
 impl<const N: usize> From<[u8; N]> for Blake3Digest<N> {
     fn from(value: [u8; N]) -> Self {
         Self(value)
+    }
+}
+
+impl<const N: usize> From<Blake3Digest<N>> for String {
+    fn from(value: Blake3Digest<N>) -> Self {
+        bytes_to_hex_string(value.as_bytes())
+    }
+}
+
+impl<const N: usize> TryFrom<&str> for Blake3Digest<N> {
+    type Error = HexParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        hex_to_bytes(value).map(|v| v.into())
     }
 }
 

@@ -1,13 +1,19 @@
 use super::{Digest, Felt, StarkField, DIGEST_SIZE, ZERO};
 use crate::utils::{
-    string::String, ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable,
+    bytes_to_hex_string, hex_to_bytes, string::String, ByteReader, ByteWriter, Deserializable,
+    DeserializationError, HexParseError, Serializable,
 };
 use core::{cmp::Ordering, fmt::Display, ops::Deref};
 
+/// The number of bytes needed to encoded a digest
+pub const DIGEST_BYTES: usize = 32;
+
 // DIGEST TRAIT IMPLEMENTATIONS
 // ================================================================================================
 
 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(into = "String", try_from = "&str"))]
 pub struct RpoDigest([Felt; DIGEST_SIZE]);
 
 impl RpoDigest {
@@ -19,7 +25,7 @@ pub fn as_elements(&self) -> &[Felt] {
         self.as_ref()
     }
 
-    pub fn as_bytes(&self) -> [u8; 32] {
+    pub fn as_bytes(&self) -> [u8; DIGEST_BYTES] {
         <Self as Digest>::as_bytes(self)
     }
 
@@ -32,8 +38,8 @@ }
 }
 
 impl Digest for RpoDigest {
-    fn as_bytes(&self) -> [u8; 32] {
-        let mut result = [0; 32];
+    fn as_bytes(&self) -> [u8; DIGEST_BYTES] {
+        let mut result = [0; DIGEST_BYTES];
 
         result[..8].copy_from_slice(&self.0[0].as_int().to_le_bytes());
         result[8..16].copy_from_slice(&self.0[1].as_int().to_le_bytes());
@@ -107,18 +113,73 @@ ]
     }
 }
 
-impl From<&RpoDigest> for [u8; 32] {
+impl From<&RpoDigest> for [u8; DIGEST_BYTES] {
     fn from(value: &RpoDigest) -> Self {
         value.as_bytes()
     }
 }
 
-impl From<RpoDigest> for [u8; 32] {
+impl From<RpoDigest> for [u8; DIGEST_BYTES] {
     fn from(value: RpoDigest) -> Self {
         value.as_bytes()
     }
 }
 
+impl From<RpoDigest> for String {
+    fn from(value: RpoDigest) -> Self {
+        bytes_to_hex_string(value.as_bytes())
+    }
+}
+
+impl From<&RpoDigest> for String {
+    fn from(value: &RpoDigest) -> Self {
+        (*value).into()
+    }
+}
+
+impl TryFrom<[u8; DIGEST_BYTES]> for RpoDigest {
+    type Error = HexParseError;
+
+    fn try_from(value: [u8; DIGEST_BYTES]) -> Result<Self, Self::Error> {
+        // Note: the input length is known, the conversion from slice to array must succeed so the
+        // `unwrap`s below are safe
+        let a = u64::from_le_bytes(value[0..8].try_into().unwrap());
+        let b = u64::from_le_bytes(value[8..16].try_into().unwrap());
+        let c = u64::from_le_bytes(value[16..24].try_into().unwrap());
+        let d = u64::from_le_bytes(value[24..32].try_into().unwrap());
+
+        if [a, b, c, d].iter().any(|v| *v >= Felt::MODULUS) {
+            return Err(HexParseError::OutOfRange);
+        }
+
+        Ok(RpoDigest([Felt::new(a), Felt::new(b), Felt::new(c), Felt::new(d)]))
+    }
+}
+
+impl TryFrom<&str> for RpoDigest {
+    type Error = HexParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        hex_to_bytes(value).and_then(|v| v.try_into())
+    }
+}
+
+impl TryFrom<String> for RpoDigest {
+    type Error = HexParseError;
+
+    fn try_from(value: String) -> Result<Self, Self::Error> {
+        value.as_str().try_into()
+    }
+}
+
+impl TryFrom<&String> for RpoDigest {
+    type Error = HexParseError;
+
+    fn try_from(value: &String) -> Result<Self, Self::Error> {
+        value.as_str().try_into()
+    }
+}
+
 impl Deref for RpoDigest {
     type Target = [Felt; DIGEST_SIZE];
 
@@ -158,9 +219,8 @@ }
 
 impl Display for RpoDigest {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
-        for byte in self.as_bytes() {
-            write!(f, "{byte:02x}")?;
-        }
+        let encoded: String = self.into();
+        write!(f, "{}", encoded)?;
         Ok(())
     }
 }
@@ -170,8 +230,7 @@ // ================================================================================================
 
 #[cfg(test)]
 mod tests {
-
-    use super::{Deserializable, Felt, RpoDigest, Serializable};
+    use super::{Deserializable, Felt, RpoDigest, Serializable, DIGEST_BYTES};
     use crate::utils::SliceReader;
     use rand_utils::rand_value;
 
@@ -186,11 +245,27 @@ let d1 = RpoDigest([e1, e2, e3, e4]);
 
         let mut bytes = vec![];
         d1.write_into(&mut bytes);
-        assert_eq!(32, bytes.len());
+        assert_eq!(DIGEST_BYTES, bytes.len());
 
         let mut reader = SliceReader::new(&bytes);
         let d2 = RpoDigest::read_from(&mut reader).unwrap();
 
         assert_eq!(d1, d2);
+    }
+
+    #[cfg(feature = "std")]
+    #[test]
+    fn digest_encoding() {
+        let digest = RpoDigest([
+            Felt::new(rand_value()),
+            Felt::new(rand_value()),
+            Felt::new(rand_value()),
+            Felt::new(rand_value()),
+        ]);
+
+        let string: String = digest.into();
+        let round_trip: RpoDigest = string.try_into().expect("decoding failed");
+
+        assert_eq!(digest, round_trip);
     }
 }

@@ -13,6 +13,7 @@ /// [MerkleStoreDelta] stores a vector of ([RpoDigest], [MerkleTreeDelta]) tuples where the
 /// [RpoDigest] represents the root of the Merkle tree and [MerkleTreeDelta] represents the
 /// differences between the initial and final Merkle tree states.
 #[derive(Default, Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerkleStoreDelta(pub Vec<(RpoDigest, MerkleTreeDelta)>);
 
 // MERKLE TREE DELTA
@@ -26,6 +27,7 @@ /// - cleared_slots: indexes of slots where values were set to [ZERO; 4].
 /// - updated_slots: index-value pairs of slots where values were set to non [ZERO; 4] values.
 #[cfg(not(test))]
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerkleTreeDelta {
     depth: u8,
     cleared_slots: Vec<u64>,
@@ -107,6 +109,7 @@ // INTERNALS
 // --------------------------------------------------------------------------------------------
 #[cfg(test)]
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerkleTreeDelta {
     pub depth: u8,
     pub cleared_slots: Vec<u64>,

@@ -21,6 +21,7 @@ ///
 /// The root is represented by the pair $(0, 0)$, its left child is $(1, 0)$ and its right child
 /// $(1, 1)$.
 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct NodeIndex {
     depth: u8,
     value: u64,

@@ -8,6 +8,7 @@ // ================================================================================================
 
 /// A fully-balanced binary Merkle tree (i.e., a tree where the number of leaves is a power of two).
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerkleTree {
     nodes: Vec<RpoDigest>,
 }

@@ -4,6 +4,7 @@ Felt, MmrProof, Rpo256, Word,
 };
 
 #[derive(Debug, Clone, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MmrPeaks {
     /// The number of leaves is used to differentiate accumulators that have the same number of
     /// peaks. This happens because the number of peaks goes up-and-down as the structure is used

@@ -29,6 +29,7 @@ ///
 /// Since this is a full representation of the MMR, elements are never removed and the MMR will
 /// grow roughly `O(2n)` in number of leaf elements.
 #[derive(Debug, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Mmr {
     /// Refer to the `forest` method documentation for details of the semantics of this value.
     pub(super) forest: usize,

@@ -3,6 +3,7 @@ use super::super::MerklePath;
 use super::full::{high_bitmask, leaf_to_corresponding_tree};
 
 #[derive(Debug, Clone, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MmrProof {
     /// The state of the MMR when the MmrProof was created.
     pub forest: usize,

@@ -2,6 +2,7 @@ use crate::hash::rpo::RpoDigest;
 
 /// Representation of a node with two children used for iterating over containers.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct InnerNodeInfo {
     pub value: RpoDigest,
     pub left: RpoDigest,

@@ -28,6 +28,7 @@ /// Tree allows to create Merkle Tree by providing Merkle paths of different lengths.
 ///
 /// The root of the tree is recomputed on each new leaf update.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct PartialMerkleTree {
     max_depth: u8,
     nodes: BTreeMap<NodeIndex, RpoDigest>,

@@ -6,6 +6,7 @@ // ================================================================================================
 
 /// A merkle path container, composed of a sequence of nodes of a Merkle tree.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerklePath {
     nodes: Vec<RpoDigest>,
 }

@@ -13,6 +13,7 @@ /// A sparse Merkle tree with 64-bit keys and 4-element leaf values, without compaction.
 ///
 /// The root of the tree is recomputed on each new leaf update.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct SimpleSmt {
     depth: u8,
     root: RpoDigest,
@@ -265,6 +266,7 @@ // BRANCH NODE
 // ================================================================================================
 
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 struct BranchNode {
     left: RpoDigest,
     right: RpoDigest,

@@ -19,6 +19,7 @@ /// A [MerkleStore] with recording capabilities which uses [RecordingMap] as the backing storage.
 pub type RecordingMerkleStore = MerkleStore<RecordingMap<RpoDigest, StoreNode>>;
 
 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct StoreNode {
     left: RpoDigest,
     right: RpoDigest,
@@ -87,6 +88,7 @@ /// // only 10 new entries, corresponding to the 10 unique internal nodes of these trees.
 /// assert_eq!(store.num_internal_nodes() - 255, 10);
 /// ```
 #[derive(Debug, Clone, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct MerkleStore<T: KvMap<RpoDigest, StoreNode> = BTreeMap<RpoDigest, StoreNode>> {
     nodes: T,
 }

@@ -43,6 +43,7 @@ /// - Internal nodes: hash(left_child, right_child).
 /// - Leaf node at depths 16, 32, or 64: hash(key, value, domain=depth).
 /// - Leaf node at depth 64: hash([key_0, value_0, ..., key_n, value_n], domain=64).
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct TieredSmt {
     root: RpoDigest,
     nodes: NodeStore,

@@ -24,6 +24,7 @@ /// The store contains information about all nodes as well as information about which of the nodes
 /// represent leaf nodes in a Tiered Sparse Merkle tree. In the current implementation, [BTreeSet]s
 /// are used to determine the position of the leaves in the tree.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct NodeStore {
     nodes: BTreeMap<NodeIndex, RpoDigest>,
     upper_leaves: BTreeSet<NodeIndex>,

@@ -26,6 +26,7 @@ ///
 /// The store supports lookup by the full key (i.e. [RpoDigest]) as well as by the 64-bit key
 /// prefix.
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct ValueStore {
     values: BTreeMap<u64, StoreEntry>,
 }
@@ -173,6 +174,7 @@ ///
 /// An entry can contain either a single key-value pair or a vector of key-value pairs sorted by
 /// key.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub enum StoreEntry {
     Single((RpoDigest, Word)),
     List(Vec<(RpoDigest, Word)>),

@@ -1,5 +1,5 @@
 use super::{utils::string::String, Word};
-use core::fmt::{self, Write};
+use core::fmt::{self, Display, Write};
 
 #[cfg(not(feature = "std"))]
 pub use alloc::{format, vec};
@@ -36,3 +36,78 @@ }
 
     Ok(s)
 }
+
+/// Renders an array of bytes as hex into a String.
+pub fn bytes_to_hex_string<const N: usize>(data: [u8; N]) -> String {
+    let mut s = String::with_capacity(N + 2);
+
+    s.push_str("0x");
+    for byte in data.iter() {
+        write!(s, "{byte:02x}").expect("formatting hex failed");
+    }
+
+    s
+}
+
+#[derive(Debug)]
+pub enum HexParseError {
+    InvalidLength { expected: usize, got: usize },
+    MissingPrefix,
+    InvalidChar,
+    OutOfRange,
+}
+
+impl Display for HexParseError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            HexParseError::InvalidLength { expected, got } => {
+                write!(f, "Hex encoded RpoDigest must have length 66, including the 0x prefix. expected {expected} got {got}")
+            }
+            HexParseError::MissingPrefix => {
+                write!(f, "Hex encoded RpoDigest must start with 0x prefix")
+            }
+            HexParseError::InvalidChar => {
+                write!(f, "Hex encoded RpoDigest must contain characters [a-zA-Z0-9]")
+            }
+            HexParseError::OutOfRange => {
+                write!(f, "Hex encoded values of an RpoDigest must be inside the field modulus")
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for HexParseError {}
+
+/// Parses a hex string into an array of bytes of known size.
+pub fn hex_to_bytes<const N: usize>(value: &str) -> Result<[u8; N], HexParseError> {
+    let expected: usize = (N * 2) + 2;
+    if value.len() != expected {
+        return Err(HexParseError::InvalidLength {
+            expected,
+            got: value.len(),
+        });
+    }
+
+    if !value.starts_with("0x") {
+        return Err(HexParseError::MissingPrefix);
+    }
+
+    let mut data = value.bytes().skip(2).map(|v| match v {
+        b'0'..=b'9' => Ok(v - b'0'),
+        b'a'..=b'f' => Ok(v - b'a' + 10),
+        b'A'..=b'F' => Ok(v - b'A' + 10),
+        _ => Err(HexParseError::InvalidChar),
+    });
+
+    let mut decoded = [0u8; N];
+    #[allow(clippy::needless_range_loop)]
+    for pos in 0..N {
+        // These `unwrap` calls are okay because the length was checked above
+        let high: u8 = data.next().unwrap()?;
+        let low: u8 = data.next().unwrap()?;
+        decoded[pos] = (high << 4) + low;
+    }
+
+    Ok(decoded)
+}