implement inclusion proof verif

crates/simplicityhl-core/src/assets/test-tx-incl-block.hex

@@ -0,0 +1 @@
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crates/simplicityhl-core/src/lib.rs

@@ -8,6 +8,7 @@ mod error;
 mod fee_rate_fetcher;
 mod runner;
 mod scripts;
+mod tx_inclusion;
 
 #[cfg(feature = "encoding")]
 pub mod encoding {
@@ -72,6 +73,7 @@ pub use error::EncodingError;
 
 pub use runner::*;
 pub use scripts::*;
+pub use tx_inclusion::*;
 
 pub use fee_rate_fetcher::*;
 

crates/simplicityhl-core/src/tx_inclusion.rs

@@ -0,0 +1,180 @@
+//! Transaction inclusion verification using Merkle proofs for Liquid/Elements blocks.
+//!
+//! This module provides SPV (Simplified Payment Verification) functionality to prove
+//! a transaction exists in a block without downloading all transactions.
+
+use simplicityhl::elements::hashes::{Hash, HashEngine};
+use simplicityhl::elements::{Block, TxMerkleNode, Txid};
+
+/// Merkle proof: (`transaction_index`, `sibling_hashes`)
+pub type MerkleProof = (usize, Vec<TxMerkleNode>);
+
+/// Constructs a Merkle inclusion proof (Merkle branch) for a transaction TXID
+/// in a block, using Bitcoin consensus Merkle tree construction rules
+/// (pairwise double-SHA256 hashing with odd-hash duplication).
+///
+/// Liquid inherits the same Merkle tree semantics via the Elements codebase:
+/// <https://developer.bitcoin.org/reference/block_chain.html>
+///
+/// Returns `None` if the transaction is not present in the block.
+#[must_use]
+pub fn merkle_branch(tx: &Txid, block: &Block) -> Option<MerkleProof> {
+    if block.txdata.is_empty() {
+        return None;
+    }
+
+    let tx_index = block.txdata.iter().position(|t| &t.txid() == tx)?;
+
+    Some((tx_index, build_merkle_branch(tx_index, block)))
+}
+
+/// Verifies a Merkle inclusion proof (Merkle branch) for a transaction TXID
+/// against the given Merkle root using Bitcoin consensus Merkle tree rules
+/// (pairwise double-SHA256 hashing with left/right ordering).
+///
+/// Liquid inherits the same Merkle tree semantics via the Elements codebase:
+/// <https://developer.bitcoin.org/reference/block_chain.html>
+///
+/// Returns `true` if the proof commits the transaction to the given root.
+#[must_use]
+pub fn verify_tx(tx: &Txid, root: &TxMerkleNode, proof: &MerkleProof) -> bool {
+    root.eq(&compute_merkle_root_from_branch(tx, proof.0, &proof.1))
+}
+
+fn build_merkle_branch(tx_index: usize, block: &Block) -> Vec<TxMerkleNode> {
+    if block.txdata.is_empty() || block.txdata.len() == 1 {
+        return vec![];
+    }
+
+    let mut branch = vec![];
+    let mut layer = block
+        .txdata
+        .iter()
+        .map(|tx| TxMerkleNode::from_raw_hash(*tx.txid().as_raw_hash()))
+        .collect::<Vec<_>>();
+    let mut index = tx_index;
+
+    // Bottom-up traversal: pair nodes, hash parents, collect siblings along path to root
+    while layer.len() > 1 {
+        let mut next_layer = vec![];
+
+        for i in (0..layer.len()).step_by(2) {
+            let left = layer[i];
+            let right = if i + 1 < layer.len() {
+                layer[i + 1]
+            } else {
+                layer[i]
+            };
+
+            let mut eng = TxMerkleNode::engine();
+            eng.input(left.as_raw_hash().as_byte_array());
+            eng.input(right.as_raw_hash().as_byte_array());
+
+            next_layer.push(TxMerkleNode::from_engine(eng));
+
+            if index / 2 == i / 2 {
+                let sibling = if index.is_multiple_of(2) { right } else { left };
+                branch.push(sibling);
+            }
+        }
+
+        index /= 2;
+        layer = next_layer;
+    }
+
+    branch
+}
+
+fn compute_merkle_root_from_branch(
+    tx: &Txid,
+    tx_index: usize,
+    branch: &[TxMerkleNode],
+) -> TxMerkleNode {
+    let mut res = TxMerkleNode::from_raw_hash(*tx.as_raw_hash());
+    let mut pos = tx_index;
+
+    for leaf in branch {
+        let mut eng = TxMerkleNode::engine();
+
+        res = if pos & 1 == 0 {
+            eng.input(res.as_raw_hash().as_byte_array());
+            eng.input(leaf.as_raw_hash().as_byte_array());
+            TxMerkleNode::from_engine(eng)
+        } else {
+            eng.input(leaf.as_raw_hash().as_byte_array());
+            eng.input(res.as_raw_hash().as_byte_array());
+            TxMerkleNode::from_engine(eng)
+        };
+
+        pos >>= 1;
+    }
+
+    res
+}
+
+#[cfg(test)]
+mod test {
+
+    use super::*;
+
+    /// Taken from rust-elements
+    /// <https://github.com/ElementsProject/rust-elements/blob/master/src/internal_macros.rs>
+    macro_rules! hex_deserialize(
+        ($e:expr) => ({
+            use simplicityhl::elements::encode::deserialize;
+
+            fn hex_char(c: char) -> u8 {
+                match c {
+                    '0' => 0,
+                    '1' => 1,
+                    '2' => 2,
+                    '3' => 3,
+                    '4' => 4,
+                    '5' => 5,
+                    '6' => 6,
+                    '7' => 7,
+                    '8' => 8,
+                    '9' => 9,
+                    'a' | 'A' => 10,
+                    'b' | 'B' => 11,
+                    'c' | 'C' => 12,
+                    'd' | 'D' => 13,
+                    'e' | 'E' => 14,
+                    'f' | 'F' => 15,
+                    x => panic!("Invalid character {} in hex string", x),
+                }
+            }
+
+            let mut ret = Vec::with_capacity($e.len() / 2);
+            let mut byte = 0;
+            for (ch, store) in $e.chars().zip([false, true].iter().cycle()) {
+                byte = (byte << 4) + hex_char(ch);
+                if *store {
+                    ret.push(byte);
+                    byte = 0;
+                }
+            }
+            deserialize(&ret).expect("deserialize object")
+        });
+    );
+
+    // Unfortunately, `hex_deserialize` macro aforehead returns error trying deserialize
+    // blocks from elements-cli regtest, so this block, taken from `elements::Block::block`, is
+    // the only test case I have found so far.
+    const BLOCK_STR: &str = include_str!("./assets/test-tx-incl-block.hex");
+
+    #[test]
+    fn test_merkle_branch_construction() {
+        let block: Block = hex_deserialize!(BLOCK_STR);
+
+        assert_eq!(block.txdata.len(), 3);
+
+        let tx = block.txdata[1].txid();
+        let proof = merkle_branch(&tx, &block).expect("Failed to find tx in block");
+
+        assert!(
+            verify_tx(&tx, &block.header.merkle_root, &proof),
+            "Invalid merkle proof"
+        );
+    }
+}