descriptors: allow the multipath step to be different than '<0;1>'

Ledger and wallet policies disallow having more than 2 depth after the
placeholder, therefore we can't do `@1/0/<0;1>/*`, `@1/1/<0;1>/*`, ..
Instead we have to do `@1/<0;1>/*`, `@1/<2;3>/*`, ..

Why not? Salvatore also says the cost of deriving another depth is
non-trivial on a signing device.

Don't pick a fight with Salvatore, instead just let the GUI (or whatever
creates the desc) use different multipath steps for keys derived from
the same xpubs.

src/descriptors/analysis.rs

@@ -80,9 +80,9 @@ impl DescKeyChecker {
     /// We require the descriptor key to:
     ///  - Be deriveable (to contain a wildcard)
     ///  - Be multipath (to contain a step in the derivation path with multiple indexes)
-    ///  - The multipath step to only contain two indexes, 0 and 1.
+    ///  - The multipath step to only contain two indexes. These can be any indexes, which is
+    ///     useful for deriving multiple keys from the same xpub.
     ///  - Be 'signable' by an external signer (to contain an origin)
-    ///  - 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());
@@ -94,8 +94,6 @@ impl DescKeyChecker {
             // Then perform the contextless checks.
             let der_paths = xpub.derivation_paths.paths();
             let first_der_path = der_paths.get(0).expect("Cannot be empty");
-            // Rust-miniscript enforces BIP389 which states that all paths must have the same len.
-            let len = first_der_path.len();
             // Technically the xpub could be for the master xpub and not have an origin. But it's
             // unlikely (and easily fixable) while users shooting themselves in the foot by
             // forgetting to provide the origin is so likely that it's worth ruling out xpubs
@@ -104,8 +102,6 @@ impl DescKeyChecker {
             let valid = xpub.origin.is_some()
                 && xpub.wildcard == descriptor::Wildcard::Unhardened
                 && der_paths.len() == 2
-                && der_paths[0][len - 1] == 0.into()
-                && der_paths[1][len - 1] == 1.into()
                 && first_der_path.into_iter().all(|step| step.is_normal());
             if valid {
                 return Ok(());

src/descriptors/mod.rs

@@ -465,6 +465,33 @@ mod tests {
         .unwrap();
         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");
 
+        // The same pseudo-realistic situation as above, but instead of using another derivation
+        // depth to derive from the same xpub, we reuse the multipath step.
+        // Note: this is required by Ledger and the wallet policies BIP
+        // (https://github.com/bitcoin/bips/pull/1389)
+        let primary_keys = PathInfo::Multi(
+            3,
+            vec![
+                descriptor::DescriptorPublicKey::from_str("[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<0;1>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<0;1>/*").unwrap()
+            ]
+        );
+        let recovery_keys = PathInfo::Multi(
+            2,
+            vec![
+                descriptor::DescriptorPublicKey::from_str("[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<2;3>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<2;3>/*").unwrap(),
+                descriptor::DescriptorPublicKey::from_str("[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<2;3>/*").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,[aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<0;1>/*,[aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<0;1>/*,[aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<0;1>/*),and_v(v:thresh(2,pkh([aabb0011/48'/0'/0'/2']xpub6Eze7yAT3Y1wGrnzedCNVYDXUqa9NmHVWck5emBaTbXtURbe1NWZbK9bsz1TiVE7Cz341PMTfYgFw1KdLWdzcM1UMFTcdQfCYhhXZ2HJvTW/<2;3>/*),a:pkh([aabb0012/48'/0'/0'/2']xpub6Bw79HbNSeS2xXw1sngPE3ehnk1U3iSPCgLYzC9LpN8m9nDuaKLZvkg8QXxL5pDmEmQtYscmUD8B9MkAAZbh6vxPzNXMaLfGQ9Sb3z85qhR/<2;3>/*),a:pkh([aabb0013/48'/0'/0'/2']xpub67zuTXF9Ln4731avKTBSawoVVNRuMfmRvkL7kLUaLBRqma9ZqdHBJg9qx8cPUm3oNQMiXT4TmGovXNoQPuwg17RFcVJ8YrnbcooN7pxVJqC/<2;3>/*)),older(26352))))#d2h994td");
+
         // We prevent footguns with timelocks by requiring a u16. Note how the following wouldn't
         // compile:
         //LianaPolicy::new(owner_key.clone(), heir_key.clone(), 0x00_01_0f_00).unwrap_err();