fix lamport merkle depth footgun

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.2.2] - 2026-09-01
+
+### Fixed
+
+- Merkle-tree Lamport keys generated with `Builder::new_from_random_bytes` now stamp the mandatory `depth` attribute (value 1). Previously such keys were generated with a depth-1 tree but carried no `depth` attribute, so every merkle operation on them (`to_public_key`, `sign_advance`, `merkle_state_view`) failed with `AttributesError::MissingThreshold`. Use `new_from_random_bytes_with_depth` for depths 2 or 3.
+
 ## [1.2.0] - 2026-09-01
 
 ### Added
@@ -222,6 +228,9 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Initial published release on crates.io as `multi-key`.
 
 [1.1.1]: https://github.com/cryptidtech/multi-key/compare/v1.1.0...v1.1.1
+[1.2.2]: https://github.com/cryptidtech/multi-key/compare/v1.2.1...v1.2.2
+[1.2.1]: https://github.com/cryptidtech/multi-key/compare/v1.2.0...v1.2.1
+[1.2.0]: https://github.com/cryptidtech/multi-key/compare/v1.1.1...v1.2.0
 [1.1.0]: https://github.com/cryptidtech/multi-key/compare/v1.0.9...v1.1.0
 [1.0.9]: https://github.com/cryptidtech/multi-key/compare/v1.0.8...v1.0.9
 [1.0.8]: https://github.com/cryptidtech/multi-key/compare/v1.0.7...v1.0.8

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "multi-key"
-version = "1.2.1"
+version = "1.2.2"
 edition = "2024"
 rust-version = "1.96"
 authors = ["Dave Grantham <dwg@linuxprogrammer.org>"]

src/mk.rs

@@ -2207,7 +2207,7 @@ impl Builder {
             | Codec::LamportMerkleBlake3256Priv
             | Codec::LamportMerkleShake128Priv
             | Codec::LamportMerkleShake256Priv => {
-                lamport_merkle::generate_private_key(codec)?.to_vec()
+                lamport_merkle::generate_private_key_with_depth(codec, 1)?.to_vec()
             }
             #[cfg(feature = "xmss")]
             Codec::XmssSha210256Priv | Codec::XmssSha216256Priv | Codec::XmssSha220256Priv => {
@@ -2217,13 +2217,39 @@ impl Builder {
         };
         let mut attributes = Attributes::new();
         attributes.insert(AttrId::KeyData, key_bytes.into());
+        // Merkle-tree Lamport keys embed a mandatory depth byte; a depth-1
+        // (capacity 2) tree is the default here, mirroring
+        // `new_from_random_bytes_with_depth`.
+        #[cfg(feature = "lamport")]
+        if Self::lamport_merkle_codec(codec) {
+            attributes.insert(AttrId::Depth, Zeroizing::new(vec![1]));
+        }
         Ok(Builder {
             codec,
             attributes: Some(attributes),
             ..Default::default()
         })
     }
 
+    /// True when `codec` is a merkle-tree Lamport private key codec.
+    #[cfg(feature = "lamport")]
+    fn lamport_merkle_codec(codec: Codec) -> bool {
+        matches!(
+            codec,
+            Codec::LamportMerkleSha3256Priv
+                | Codec::LamportMerkleSha3384Priv
+                | Codec::LamportMerkleSha3512Priv
+                | Codec::LamportMerkleSha2256Priv
+                | Codec::LamportMerkleSha2384Priv
+                | Codec::LamportMerkleSha2512Priv
+                | Codec::LamportMerkleBlake2B512Priv
+                | Codec::LamportMerkleBlake2S256Priv
+                | Codec::LamportMerkleBlake3256Priv
+                | Codec::LamportMerkleShake128Priv
+                | Codec::LamportMerkleShake256Priv
+        )
+    }
+
     /// new builder for a merkle-tree Lamport key of the given depth (1..=3)
     ///
     /// The depth is validated against the scheme's limit and stamped as the

src/views/lamport_merkle.rs

@@ -989,6 +989,10 @@ fn signing_key_material_size_for(codec: Codec) -> Result<usize, Error> {
     Ok(2 * bits * digest)
 }
 
+/// Generate a depth-1 merkle-Lamport private key (unused since 1.2.2: the
+/// plain `Builder::new_from_random_bytes` path stamps depth inline; kept for
+/// symmetry with the other view helpers).
+#[allow(dead_code)]
 pub(crate) fn generate_private_key(codec: Codec) -> Result<Zeroizing<Vec<u8>>, Error> {
     generate_private_key_with_depth(codec, 1)
 }
@@ -1101,6 +1105,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_merkle_plain_ctor_stamps_depth() {
+        // The plain constructor generates a depth-1 tree and must stamp the
+        // mandatory depth attribute (fixed in 1.2.2).
+        let sk =
+            Builder::new_from_random_bytes(Codec::LamportMerkleBlake3256Priv, &mut rand::rng())
+                .unwrap()
+                .try_build()
+                .unwrap();
+        assert_eq!(sk.attributes.get(&AttrId::Depth).map(|b| b[0]), Some(1));
+
+        // the key must be fully usable: public derivation and signing
+        let pk = sk.conv_view().unwrap().to_public_key().unwrap();
+        let msg = b"plain ctor roundtrip";
+        let (ms, advanced) = sk
+            .sign_view()
+            .unwrap()
+            .sign_advance(msg, false, None)
+            .unwrap();
+        pk.verify_view().unwrap().verify(&ms, Some(msg)).unwrap();
+        let av = advanced.merkle_state_view().unwrap();
+        assert_eq!(av.next_index().unwrap(), 1);
+        assert_eq!(av.remaining_signatures().unwrap(), 1);
+    }
+
     #[test]
     fn test_merkle_depth3_sha3_256() {
         let sk = build_priv(Codec::LamportMerkleSha3256Priv, 3);