Merge #575: descriptor: make it possible to reuse xpubs at different …

…derivation paths

28049d4 descriptor: make it possible to use the same xpub at different paths (Antoine Poinsot)

Pull request description:

  This is unnecessary to bump a hardened step to get another xpub for the same participant:
  - It's more hurdle when sharing it
  - It's more hurdle when verifying it on the device's screen
  - It's for the same person within the same script anyways

  Fix this by allowing to use the same xpub multiple times in a descriptor as long as the derivation path is different. Then the GUI will be able to use a new unhardened derivation step instead of bumping the BIP48 "accounts". A unit test is introduced that showcases this.

  See also #542.

ACKs for top commit:
  edouardparis:
    ACK 28049d4

Tree-SHA512: e864a8193998667d36b34d96e6e32bfff1c507b3c31faa324b7f264604c41b5d2d4aefeec3b71a1d3edf5b0242966861887baf97d7a410e43e7449f22c3453f4

src/descriptors/analysis.rs

@@ -67,7 +67,7 @@ fn is_single_key_or_multisig(policy: &SemanticPolicy<descriptor::DescriptorPubli
 }
 
 struct DescKeyChecker {
-    keys_set: HashSet<bip32::ExtendedPubKey>,
+    keys_set: HashSet<(bip32::ExtendedPubKey, descriptor::DerivPaths)>,
 }
 
 impl DescKeyChecker {
@@ -85,11 +85,12 @@ impl DescKeyChecker {
     ///  - Have an xpub that is not a duplicate.
     pub fn check(&mut self, key: &descriptor::DescriptorPublicKey) -> Result<(), LianaPolicyError> {
         if let descriptor::DescriptorPublicKey::MultiXPub(ref xpub) = *key {
+            let key_identifier = (xpub.xkey, xpub.derivation_paths.clone());
             // First make sure it's not a duplicate and record seeing it.
-            if self.keys_set.contains(&xpub.xkey) {
+            if self.keys_set.contains(&key_identifier) {
                 return Err(LianaPolicyError::DuplicateKey(key.clone().into()));
             }
-            self.keys_set.insert(xpub.xkey);
+            self.keys_set.insert(key_identifier);
             // Then perform the contextless checks.
             let der_paths = xpub.derivation_paths.paths();
             let first_der_path = der_paths.get(0).expect("Cannot be empty");

src/descriptors/mod.rs

@@ -437,30 +437,33 @@ mod tests {
         .unwrap();
         assert_eq!(LianaDescriptor::new(policy).to_string(), "wsh(or_d(pk([abcdef01]xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*),and_v(v:pkh([abcdef01]xpub688Hn4wScQAAiYJLPg9yH27hUpfZAUnmJejRQBCiwfP5PEDzjWMNW1wChcninxr5gyavFqbbDjdV1aK5USJz8NDVjUy7FRQaaqqXHh5SbXe/<0;1>/*),older(52560))))#g7vk9r5l");
 
-        // A decaying multisig after 6 months. Note we can't duplicate the keys, so different ones
-        // are used. In practice they would both be controlled by the same entity.
+        // A 3-of-3 multisig decaying into a 2-of-3 multisig after 6 months. Trying to mimic a
+        // real situation, we use keys from 3 different origins (in practice, 3 different devices
+        // held by 3 different persons).
+        // Since there is 2 keys per origin, we increase a unhardened derivation step we put before
+        // the change/receive derivation step.
         let primary_keys = PathInfo::Multi(
             3,
             vec![
-                descriptor::DescriptorPublicKey::from_str("[abcdef01]xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*").unwrap(),
-                descriptor::DescriptorPublicKey::from_str("[aabb0011/10/4893]xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<0;1>/*").unwrap(),
-                descriptor::DescriptorPublicKey::from_str("[abcdef01]xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<0;1>/*").unwrap()
+                descriptor::DescriptorPublicKey::from_str("[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/0/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*").unwrap()
             ]
         );
         let recovery_keys = PathInfo::Multi(
             2,
             vec![
-                descriptor::DescriptorPublicKey::from_str("[abcdef01]xpub69cP4Y7S9TWcbSNxmk6CEDBsoaqr3ZEdjHuZcHxEFFKGh569RsJNr2V27XGhsbH9FXgWUEmKXRN7c5wQfq2VPjt31xP9VsYnVUyU8HcVevm/<0;1>/*").unwrap(),
-                descriptor::DescriptorPublicKey::from_str("[abcdef01]xpub6AA2N8RALRYgLD6jT1iXYCEDkndTeZndMtWPbtNX6sY5dPiLtf2T88ahdxrGXMUPoNadgR86sFhBXWQVgifPzDYbY9ZtwK4gqzx4y5Da1DW/<0;1>/*").unwrap(),
-                descriptor::DescriptorPublicKey::from_str("[aabb0011/10/4893]xpub6AyxexvxizZJffF153evmfqHcE9MV88fCNCAtP3jQjXJHwrAKri71Tq9jWUkPxj9pja4u6AkCPHY7atgxzSEa2HtDwJfrRWKK4fsfQg4o77/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*").unwrap(),
             ],
         );
         let policy = LianaPolicy::new(
             primary_keys,
             [(26352, recovery_keys)].iter().cloned().collect(),
         )
         .unwrap();
-        assert_eq!(LianaDescriptor::new(policy).to_string(), "wsh(or_d(multi(3,[abcdef01]xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*,[aabb0011/10/4893]xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<0;1>/*,[abcdef01]xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<0;1>/*),and_v(v:thresh(2,pkh([abcdef01]xpub69cP4Y7S9TWcbSNxmk6CEDBsoaqr3ZEdjHuZcHxEFFKGh569RsJNr2V27XGhsbH9FXgWUEmKXRN7c5wQfq2VPjt31xP9VsYnVUyU8HcVevm/<0;1>/*),a:pkh([abcdef01]xpub6AA2N8RALRYgLD6jT1iXYCEDkndTeZndMtWPbtNX6sY5dPiLtf2T88ahdxrGXMUPoNadgR86sFhBXWQVgifPzDYbY9ZtwK4gqzx4y5Da1DW/<0;1>/*),a:pkh([aabb0011/10/4893]xpub6AyxexvxizZJffF153evmfqHcE9MV88fCNCAtP3jQjXJHwrAKri71Tq9jWUkPxj9pja4u6AkCPHY7atgxzSEa2HtDwJfrRWKK4fsfQg4o77/<0;1>/*)),older(26352))))#hmsqemgr");
+        assert_eq!(LianaDescriptor::new(policy).to_string(), "wsh(or_d(multi(3,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/0/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/0/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/0/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/1/<0;1>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/1/<0;1>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/1/<0;1>/*)),older(26352))))#prj7nktq");
 
         // We prevent footguns with timelocks by requiring a u16. Note how the following wouldn't
         // compile: