Ensure the size of our locking script never changes

swap/Cargo.toml

@@ -80,6 +80,7 @@ get-port = "3"
 hyper = "0.14"
 monero-harness = { path = "../monero-harness" }
 port_check = "0.1"
+proptest = "1"
 serde_cbor = "0.11"
 spectral = "0.6"
 tempfile = "3"

swap/src/bitcoin/lock.rs

@@ -7,11 +7,11 @@ use ::bitcoin::{OutPoint, TxIn, TxOut, Txid};
 use anyhow::{bail, Result};
 use bdk::database::BatchDatabase;
 use bitcoin::Script;
-use ecdsa_fun::fun::Point;
 use miniscript::{Descriptor, DescriptorTrait};
-use rand::thread_rng;
 use serde::{Deserialize, Serialize};
 
+const SCRIPT_SIZE: usize = 34;
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct TxLock {
     inner: PartiallySignedTransaction,
@@ -112,12 +112,7 @@ impl TxLock {
 
     /// Calculate the size of the script used by this transaction.
     pub fn script_size() -> usize {
-        build_shared_output_descriptor(
+        SCRIPT_SIZE
-            Point::random(&mut thread_rng()),
-            Point::random(&mut thread_rng()),
-        )
-        .script_pubkey()
-        .len()
     }
 
     pub fn script_pubkey(&self) -> Script {
@@ -245,6 +240,17 @@ mod tests {
         result.expect_err("PSBT to be invalid");
     }
 
+    proptest::proptest! {
+        #[test]
+        fn estimated_tx_lock_script_size_never_changes(a in crate::proptest::ecdsa_fun::point(), b in crate::proptest::ecdsa_fun::point()) {
+            proptest::prop_assume!(a != b);
+
+            let computed_size = build_shared_output_descriptor(a, b).script_pubkey().len();
+
+            assert_eq!(computed_size, SCRIPT_SIZE);
+        }
+    }
+
     /// Helper function that represents Bob's action of constructing the PSBT.
     ///
     /// Extracting this allows us to keep the tests concise.

swap/src/lib.rs

@@ -32,3 +32,6 @@ pub mod tor;
 pub mod tracing_ext;
 
 mod monero_ext;
+
+#[cfg(test)]
+mod proptest;

swap/src/proptest.rs

@@ -0,0 +1,17 @@
+use proptest::prelude::*;
+
+pub mod ecdsa_fun {
+    use super::*;
+    use ::ecdsa_fun::fun::marker::{Mark, NonZero, Normal};
+    use ::ecdsa_fun::fun::{Point, Scalar, G};
+
+    pub fn point() -> impl Strategy<Value = Point> {
+        scalar().prop_map(|mut scalar| Point::from_scalar_mul(&G, &mut scalar).mark::<Normal>())
+    }
+
+    pub fn scalar() -> impl Strategy<Value = Scalar> {
+        prop::array::uniform32(0..255u8).prop_filter_map("generated the 0 element", |bytes| {
+            Scalar::from_bytes_mod_order(bytes).mark::<NonZero>()
+        })
+    }
+}