feat: Merkle tree list hashing

.vscode/settings.json

@@ -1,5 +1,6 @@
 {
     "cSpell.words": [
+        "hasher",
         "Merkle",
         "Precertificate"
     ],

src/merkle/hash.rs

@@ -1 +1,37 @@
+use sha2::{Digest, Sha256};
 
+use super::types::MerkleTreeLeaf;
+
+pub enum MerkleTreeNode<'a> {
+	Leaf(MerkleTreeLeaf<'a>),
+	Intermediate(String)
+}
+
+/// Hashes a merkle leaf list. This takes in a slice of u8 slices, each
+/// representing the binary form of a merkle tree leaf. This implementation is
+/// not optimized for extra-long lists as its implementation is recursive,
+/// however, the current-largest CT log as of 11/5/24 would theoretically only
+/// go 32 levels deep.
+pub fn hash_merkle_leaf_list(leaves: &[&[u8]]) -> [u8; 32] {
+	let mut hasher = Sha256::default();
+
+	// The hash of an empty list is the empty hash, no branch necessary
+	if leaves.len() == 1 {
+		// The hash of one element is 0x00 + the hash of its data
+		hasher.update([0x00u8]);
+		hasher.update(leaves[0]);
+	} else if leaves.len() > 1 {
+		// For n > 1, let k be the largest power of two smaller than n (i.e., k < n <=
+		// 2k)
+		// MTH(D[n]) = SHA-256(0x01 || MTH(D[0:k]) || MTH(D[k:n]))
+
+		// This can be calculated by taking log_2(n), flooring it, then taking 2
+		// to the power of it again
+		let k = 2usize.pow(leaves.len().ilog2());
+		hasher.update([0x01u8]);
+		hasher.update(hash_merkle_leaf_list(&leaves[0..k]));
+		hasher.update(hash_merkle_leaf_list(&leaves[k..leaves.len()]));
+	}
+
+	return hasher.finalize().into();
+}