Abstract out ComputeTapbranchHash

Author: roconnor-blockstream

src/script/interpreter.cpp

@@ -1830,6 +1830,17 @@ uint256 ComputeTapleafHash(uint8_t leaf_version, Span<const unsigned char> scrip
     return (HashWriter{HASHER_TAPLEAF} << leaf_version << CompactSizeWriter(script.size()) << script).GetSHA256();
 }
 
+uint256 ComputeTapbranchHash(Span<const unsigned char> a, Span<const unsigned char> b)
+{
+    HashWriter ss_branch{HASHER_TAPBRANCH};
+    if (std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end())) {
+        ss_branch << a << b;
+    } else {
+        ss_branch << b << a;
+    }
+    return ss_branch.GetSHA256();
+}
+
 uint256 ComputeTaprootMerkleRoot(Span<const unsigned char> control, const uint256& tapleaf_hash)
 {
     assert(control.size() >= TAPROOT_CONTROL_BASE_SIZE);
@@ -1839,14 +1850,8 @@ uint256 ComputeTaprootMerkleRoot(Span<const unsigned char> control, const uint25
     const int path_len = (control.size() - TAPROOT_CONTROL_BASE_SIZE) / TAPROOT_CONTROL_NODE_SIZE;
     uint256 k = tapleaf_hash;
     for (int i = 0; i < path_len; ++i) {
-        HashWriter ss_branch{HASHER_TAPBRANCH};
         Span node{Span{control}.subspan(TAPROOT_CONTROL_BASE_SIZE + TAPROOT_CONTROL_NODE_SIZE * i, TAPROOT_CONTROL_NODE_SIZE)};
-        if (std::lexicographical_compare(k.begin(), k.end(), node.begin(), node.end())) {
-            ss_branch << k << node;
-        } else {
-            ss_branch << node << k;
-        }
-        k = ss_branch.GetSHA256();
+        k = ComputeTapbranchHash(k, node);
     }
     return k;
 }

src/script/interpreter.h

@@ -334,6 +334,9 @@ class DeferringSignatureChecker : public BaseSignatureChecker
 
 /** Compute the BIP341 tapleaf hash from leaf version & script. */
 uint256 ComputeTapleafHash(uint8_t leaf_version, Span<const unsigned char> script);
+/** Compute the BIP341 tapbranch hash from two branches.
+  * Spans must be 32 bytes each. */
+uint256 ComputeTapbranchHash(Span<const unsigned char> a, Span<const unsigned char> b);
 /** Compute the BIP341 taproot script tree Merkle root from control block and leaf hash.
  *  Requires control block to have valid length (33 + k*32, with k in {0,1,..,128}). */
 uint256 ComputeTaprootMerkleRoot(Span<const unsigned char> control, const uint256& tapleaf_hash);

src/script/standard.cpp

@@ -370,12 +370,7 @@ bool IsValidDestination(const CTxDestination& dest) {
         leaf.merkle_branch.push_back(a.hash);
         ret.leaves.emplace_back(std::move(leaf));
     }
-    /* Lexicographically sort a and b's hash, and compute parent hash. */
-    if (a.hash < b.hash) {
-        ret.hash = (HashWriter{HASHER_TAPBRANCH} << a.hash << b.hash).GetSHA256();
-    } else {
-        ret.hash = (HashWriter{HASHER_TAPBRANCH} << b.hash << a.hash).GetSHA256();
-    }
+    ret.hash = ComputeTapbranchHash(a.hash, b.hash);
     return ret;
 }
 
@@ -607,7 +602,7 @@ std::optional<std::vector<std::tuple<int, std::vector<unsigned char>, int>>> Inf
             node.done = true;
             stack.pop_back();
         } else if (node.sub[0]->done && !node.sub[1]->done && !node.sub[1]->explored && !node.sub[1]->hash.IsNull() &&
-                   (HashWriter{HASHER_TAPBRANCH} << node.sub[1]->hash << node.sub[1]->hash).GetSHA256() == node.hash) {
+                   ComputeTapbranchHash(node.sub[1]->hash, node.sub[1]->hash) == node.hash) {
             // Whenever there are nodes with two identical subtrees under it, we run into a problem:
             // the control blocks for the leaves underneath those will be identical as well, and thus
             // they will all be matched to the same path in the tree. The result is that at the location

src/test/script_tests.cpp

@@ -1817,7 +1817,14 @@ BOOST_AUTO_TEST_CASE(bip341_keypath_test_vectors)
         }
 
     }
+}
 
+BOOST_AUTO_TEST_CASE(compute_tapbranch)
+{
+    uint256 hash1 = uint256S("8ad69ec7cf41c2a4001fd1f738bf1e505ce2277acdcaa63fe4765192497f47a7");
+    uint256 hash2 = uint256S("f224a923cd0021ab202ab139cc56802ddb92dcfc172b9212261a539df79a112a");
+    uint256 result = uint256S("a64c5b7b943315f9b805d7a7296bedfcfd08919270a1f7a1466e98f8693d8cd9");
+    BOOST_CHECK_EQUAL(ComputeTapbranchHash(hash1, hash2), result);
 }
 
 BOOST_AUTO_TEST_SUITE_END()