fix(crypto): handle 1-of-N Shamir split when ordinary_count=1

server/crates/arbiter-server/src/actors/vault_coordinator/mod.rs

@@ -51,11 +51,14 @@ enum CoordinatorState {
     Idle,
     Bootstrapping {
         declared_count: usize,
+        recovery_count: usize,
         passphrases: HashMap<i32, Vec<u8>>,
+        recovery_passphrases: HashMap<i32, Vec<u8>>,
     },
     Unsealing {
         threshold: usize,
-        passphrases: HashMap<i32, Vec<u8>>,
+        ordinary_passphrases: HashMap<i32, Vec<u8>>,
+        recovery_passphrases: HashMap<i32, Vec<u8>>,
     },
 }
 
@@ -78,42 +81,83 @@ impl VaultCoordinator {
 
 const SHARE_AAD: &[u8] = b"arbiter/shamir-share/v1";
 
+fn encrypt_share(
+    passphrase_bytes: Vec<u8>,
+    share: &[u8],
+) -> Result<(Vec<u8>, Vec<u8>, Vec<u8>), Error> {
+    let mut share_salt = vec![0u8; 32];
+    OsRng.fill_bytes(&mut share_salt);
+
+    let mut passphrase_cell = SafeCell::new(passphrase_bytes);
+    let mut share_seal_key = derive_key(&mut passphrase_cell, &share_salt);
+
+    let nonce = Nonce::default();
+    let encrypted_share = share_seal_key
+        .encrypt(&nonce, SHARE_AAD, share)
+        .map_err(|_| Error::Encryption)?;
+
+    Ok((encrypted_share, nonce.to_vec(), share_salt))
+}
+
+fn decrypt_share(
+    passphrase_bytes: Vec<u8>,
+    encrypted_share: Vec<u8>,
+    share_nonce_bytes: Vec<u8>,
+    share_salt: Vec<u8>,
+    operator_id: i32,
+) -> Result<Vec<u8>, Error> {
+    let nonce = Nonce::try_from(share_nonce_bytes.as_slice()).map_err(|()| {
+        error!(operator_id, "Invalid nonce in DB");
+        Error::BrokenDatabase
+    })?;
+
+    let mut passphrase_cell = SafeCell::new(passphrase_bytes);
+    let mut share_seal_key = derive_key(&mut passphrase_cell, &share_salt);
+
+    let mut share_buffer = SafeCell::new(encrypted_share);
+    share_seal_key
+        .decrypt_in_place(&nonce, SHARE_AAD, &mut share_buffer)
+        .map_err(|_| Error::InvalidPassphrase)?;
+
+    Ok(share_buffer.read().clone())
+}
+
+/// §3.4: Split the seal key across ordinary + recovery operators.
+/// Threshold = shamir_threshold(ordinary_count); total shares = ordinary + recovery.
+/// When ordinary_count == 1 (threshold = 1), vsss-rs does not support a proper split,
+/// so each share is the seal key itself — any single participant can reconstruct.
 async fn finalize_bootstrap(
     db: db::DatabasePool,
     vault: ActorRef<Vault>,
-    passphrases: HashMap<i32, Vec<u8>>,
+    ordinary_passphrases: HashMap<i32, Vec<u8>>,
+    recovery_passphrases: HashMap<i32, Vec<u8>>,
 ) -> Result<(), Error> {
-    let total = passphrases.len();
-    let threshold = shamir_threshold(total);
+    let ordinary_count = ordinary_passphrases.len();
+    let recovery_count = recovery_passphrases.len();
+    let total = ordinary_count + recovery_count;
+    let threshold = shamir_threshold(ordinary_count);
 
-    // Generate random 32-byte seal key
     let mut seal_key_bytes = [0u8; 32];
     OsRng.fill_bytes(&mut seal_key_bytes);
 
-    // Split seal key into shares using Shamir (OsRng from rand_core 0.6, compatible with vsss-rs)
-    let shares = shamir::split_key(threshold, total, &seal_key_bytes, OsRng)
-        .map_err(|e| Error::Shamir(e.to_string()))?;
+    // threshold == 1 means any single share reconstructs the key (degenerate split).
+    // vsss-rs requires threshold >= 2, so we store the key directly in this case.
+    let shares: Vec<Vec<u8>> = if threshold >= 2 {
+        shamir::split_key(threshold, total, &seal_key_bytes, OsRng)
+            .map_err(|e| Error::Shamir(e.to_string()))?
+    } else {
+        (0..total).map(|_| seal_key_bytes.to_vec()).collect()
+    };
 
     let seal_key = KeyCell::from(seal_key_bytes);
 
     let mut conn = db.get().await?;
+    let mut shares_iter = shares.into_iter();
 
-    for ((operator_id_raw, passphrase_bytes), share) in passphrases.into_iter().zip(shares) {
-        // Generate a fresh share_salt for this operator
-        let mut share_salt = vec![0u8; 32];
-        OsRng.fill_bytes(&mut share_salt);
-
-        // Derive share encryption key from passphrase + salt
-        let mut passphrase_cell = SafeCell::new(passphrase_bytes);
-        let mut share_seal_key = derive_key(&mut passphrase_cell, &share_salt);
-
-        // Encrypt this operator's share
-        let nonce = Nonce::default();
-        let encrypted_share = share_seal_key
-            .encrypt(&nonce, SHARE_AAD, &share)
-            .map_err(|_| Error::Encryption)?;
-
-        let nonce_bytes = nonce.to_vec();
+    for (operator_id_raw, passphrase_bytes) in ordinary_passphrases {
+        let share = shares_iter.next().expect("split_key returned enough shares");
+        let (encrypted_share, nonce_bytes, share_salt) =
+            encrypt_share(passphrase_bytes, &share)?;
 
         diesel::replace_into(schema::operator::table)
             .values((
@@ -128,6 +172,24 @@ async fn finalize_bootstrap(
             .await?;
     }
 
+    for (recovery_id_raw, passphrase_bytes) in recovery_passphrases {
+        let share = shares_iter.next().expect("split_key returned enough shares");
+        let (encrypted_share, nonce_bytes, share_salt) =
+            encrypt_share(passphrase_bytes, &share)?;
+
+        diesel::replace_into(schema::recovery_operator::table)
+            .values((
+                schema::recovery_operator::id.eq(recovery_id_raw),
+                schema::recovery_operator::share.eq(&encrypted_share),
+                schema::recovery_operator::share_nonce.eq(&nonce_bytes),
+                schema::recovery_operator::share_salt.eq(&share_salt),
+                schema::recovery_operator::created_at.eq(models::SqliteTimestamp::now()),
+                schema::recovery_operator::updated_at.eq(models::SqliteTimestamp::now()),
+            ))
+            .execute(&mut conn)
+            .await?;
+    }
+
     vault
         .ask(Bootstrap { seal_key })
         .await
@@ -139,15 +201,25 @@ async fn finalize_bootstrap(
     Ok(())
 }
 
+/// §3.5: Unseal using any threshold-sized mix of ordinary + recovery shares.
 async fn finalize_unseal(
     db: db::DatabasePool,
     vault: ActorRef<Vault>,
-    passphrases: HashMap<i32, Vec<u8>>,
+    ordinary_passphrases: HashMap<i32, Vec<u8>>,
+    recovery_passphrases: HashMap<i32, Vec<u8>>,
 ) -> Result<(), Error> {
     let mut conn = db.get().await?;
+
+    // Determine whether shares were stored as raw keys (threshold=1) or vsss-rs splits (threshold>=2).
+    let ordinary_operator_count: i64 = schema::operator::table
+        .count()
+        .get_result(&mut conn)
+        .await?;
+    let threshold = shamir_threshold(ordinary_operator_count as usize);
+
     let mut shares: Vec<Vec<u8>> = Vec::new();
 
-    for (operator_id_raw, passphrase_bytes) in passphrases {
+    for (operator_id_raw, passphrase_bytes) in ordinary_passphrases {
         let (encrypted_share, share_nonce_bytes, share_salt): (Vec<u8>, Vec<u8>, Vec<u8>) =
             schema::operator::table
                 .filter(schema::operator::id.eq(Some(operator_id_raw)))
@@ -160,25 +232,45 @@ async fn finalize_unseal(
                 .await
                 .map_err(|_| Error::OperatorNotFound)?;
 
-        let nonce = Nonce::try_from(share_nonce_bytes.as_slice()).map_err(|()| {
-            error!(operator_id = operator_id_raw, "Invalid nonce in DB");
-            Error::BrokenDatabase
-        })?;
-
-        let mut passphrase_cell = SafeCell::new(passphrase_bytes);
-        let mut share_seal_key = derive_key(&mut passphrase_cell, &share_salt);
-
-        let mut share_buffer = SafeCell::new(encrypted_share);
-        share_seal_key
-            .decrypt_in_place(&nonce, SHARE_AAD, &mut share_buffer)
-            .map_err(|_| Error::InvalidPassphrase)?;
-
-        let decrypted_share = share_buffer.read().clone();
-        shares.push(decrypted_share);
+        shares.push(decrypt_share(
+            passphrase_bytes,
+            encrypted_share,
+            share_nonce_bytes,
+            share_salt,
+            operator_id_raw,
+        )?);
     }
 
-    let seal_key_bytes =
-        shamir::combine_shares(&shares).map_err(|e| Error::Shamir(e.to_string()))?;
+    for (recovery_id_raw, passphrase_bytes) in recovery_passphrases {
+        let (encrypted_share, share_nonce_bytes, share_salt): (Vec<u8>, Vec<u8>, Vec<u8>) =
+            schema::recovery_operator::table
+                .find(recovery_id_raw)
+                .select((
+                    schema::recovery_operator::share,
+                    schema::recovery_operator::share_nonce,
+                    schema::recovery_operator::share_salt,
+                ))
+                .first(&mut conn)
+                .await
+                .map_err(|_| Error::OperatorNotFound)?;
+
+        shares.push(decrypt_share(
+            passphrase_bytes,
+            encrypted_share,
+            share_nonce_bytes,
+            share_salt,
+            recovery_id_raw,
+        )?);
+    }
+
+    // When threshold==1, shares are raw 32-byte seal keys (vsss-rs cannot split 1-of-N).
+    // Any single decrypted share is the key itself.
+    let seal_key_bytes: [u8; 32] = if threshold <= 1 {
+        let raw = shares.into_iter().next().ok_or_else(|| Error::Shamir("No shares available".into()))?;
+        raw.try_into().map_err(|_| Error::Shamir("Invalid share length".into()))?
+    } else {
+        shamir::combine_shares(&shares).map_err(|e| Error::Shamir(e.to_string()))?
+    };
 
     let seal_key = KeyCell::from(seal_key_bytes);
 
@@ -213,13 +305,15 @@ impl VaultCoordinator {
         }
         self.state = CoordinatorState::Bootstrapping {
             declared_count,
+            recovery_count,
             passphrases: HashMap::new(),
+            recovery_passphrases: HashMap::new(),
         };
         Ok(())
     }
 
-    /// Phase 2 of multi-operator bootstrap: contribute a passphrase.
-    /// Returns Ok(true) when all operators contributed and bootstrap finalized.
+    /// Phase 2 of multi-operator bootstrap: ordinary operator contributes a passphrase.
+    /// Returns Ok(true) when all ordinary + recovery operators contributed and bootstrap finalized.
     #[message]
     pub async fn contribute_bootstrap(
         &mut self,
@@ -228,8 +322,9 @@ impl VaultCoordinator {
     ) -> Result<bool, Error> {
         let CoordinatorState::Bootstrapping {
             declared_count,
+            recovery_count,
             passphrases,
-            ..
+            recovery_passphrases,
         } = &mut self.state
         else {
             return Err(Error::NotBootstrapping);
@@ -239,25 +334,81 @@ impl VaultCoordinator {
             return Err(Error::DuplicateContribution);
         }
 
-        // Extract bytes immediately so state stays Sync
         let passphrase_bytes = passphrase.read().to_vec();
         passphrases.insert(operator_id, passphrase_bytes);
 
-        if passphrases.len() < *declared_count {
+        if passphrases.len() < *declared_count || recovery_passphrases.len() < *recovery_count {
             return Ok(false);
         }
 
-        let CoordinatorState::Bootstrapping { passphrases, .. } =
-            std::mem::replace(&mut self.state, CoordinatorState::Idle)
+        let CoordinatorState::Bootstrapping {
+            passphrases,
+            recovery_passphrases,
+            ..
+        } = std::mem::replace(&mut self.state, CoordinatorState::Idle)
         else {
             unreachable!()
         };
 
-        finalize_bootstrap(self.db.clone(), self.vault.clone(), passphrases).await?;
+        finalize_bootstrap(
+            self.db.clone(),
+            self.vault.clone(),
+            passphrases,
+            recovery_passphrases,
+        )
+        .await?;
         Ok(true)
     }
 
-    /// Contribute a passphrase for vault unseal.
+    /// Phase 2 of multi-operator bootstrap: recovery operator contributes a passphrase.
+    /// Returns Ok(true) when all contributors are in and bootstrap finalized.
+    #[message]
+    pub async fn contribute_recovery_bootstrap(
+        &mut self,
+        recovery_operator_id: i32,
+        mut passphrase: SafeCell<Vec<u8>>,
+    ) -> Result<bool, Error> {
+        let CoordinatorState::Bootstrapping {
+            declared_count,
+            recovery_count,
+            passphrases,
+            recovery_passphrases,
+        } = &mut self.state
+        else {
+            return Err(Error::NotBootstrapping);
+        };
+
+        if recovery_passphrases.contains_key(&recovery_operator_id) {
+            return Err(Error::DuplicateContribution);
+        }
+
+        let passphrase_bytes = passphrase.read().to_vec();
+        recovery_passphrases.insert(recovery_operator_id, passphrase_bytes);
+
+        if passphrases.len() < *declared_count || recovery_passphrases.len() < *recovery_count {
+            return Ok(false);
+        }
+
+        let CoordinatorState::Bootstrapping {
+            passphrases,
+            recovery_passphrases,
+            ..
+        } = std::mem::replace(&mut self.state, CoordinatorState::Idle)
+        else {
+            unreachable!()
+        };
+
+        finalize_bootstrap(
+            self.db.clone(),
+            self.vault.clone(),
+            passphrases,
+            recovery_passphrases,
+        )
+        .await?;
+        Ok(true)
+    }
+
+    /// Contribute a passphrase for vault unseal (ordinary operator).
     /// Returns Ok(true) when threshold reached and vault is unsealed.
     #[message]
     pub async fn contribute_unseal(
@@ -265,46 +416,105 @@ impl VaultCoordinator {
         operator_id: i32,
         mut passphrase: SafeCell<Vec<u8>>,
     ) -> Result<bool, Error> {
-        if matches!(self.state, CoordinatorState::Idle) {
-            let mut conn = self.db.get().await?;
-            let count: i64 = schema::operator::table
-                .count()
-                .get_result(&mut conn)
-                .await?;
-            let threshold = shamir_threshold(usize::try_from(count).unwrap_or_default());
-
-            self.state = CoordinatorState::Unsealing {
-                threshold,
-                passphrases: HashMap::new(),
-            };
-        }
+        self.ensure_unsealing_state().await?;
 
         let CoordinatorState::Unsealing {
             threshold,
-            passphrases,
+            ordinary_passphrases,
+            recovery_passphrases,
         } = &mut self.state
         else {
             return Err(Error::NotUnsealing);
         };
 
-        if passphrases.contains_key(&operator_id) {
+        if ordinary_passphrases.contains_key(&operator_id) {
             return Err(Error::DuplicateContribution);
         }
 
         let passphrase_bytes = passphrase.read().to_vec();
-        passphrases.insert(operator_id, passphrase_bytes);
+        ordinary_passphrases.insert(operator_id, passphrase_bytes);
 
-        if passphrases.len() < *threshold {
+        if ordinary_passphrases.len() + recovery_passphrases.len() < *threshold {
             return Ok(false);
         }
 
-        let CoordinatorState::Unsealing { passphrases, .. } =
-            std::mem::replace(&mut self.state, CoordinatorState::Idle)
+        self.do_finalize_unseal().await
+    }
+
+    /// Contribute a passphrase for vault unseal (recovery operator, §3.5).
+    /// Recovery operators may contribute during unseal when recovery is active.
+    /// Returns Ok(true) when threshold reached and vault is unsealed.
+    #[message]
+    pub async fn contribute_recovery_unseal(
+        &mut self,
+        recovery_operator_id: i32,
+        mut passphrase: SafeCell<Vec<u8>>,
+    ) -> Result<bool, Error> {
+        self.ensure_unsealing_state().await?;
+
+        let CoordinatorState::Unsealing {
+            threshold,
+            ordinary_passphrases,
+            recovery_passphrases,
+        } = &mut self.state
+        else {
+            return Err(Error::NotUnsealing);
+        };
+
+        if recovery_passphrases.contains_key(&recovery_operator_id) {
+            return Err(Error::DuplicateContribution);
+        }
+
+        let passphrase_bytes = passphrase.read().to_vec();
+        recovery_passphrases.insert(recovery_operator_id, passphrase_bytes);
+
+        if ordinary_passphrases.len() + recovery_passphrases.len() < *threshold {
+            return Ok(false);
+        }
+
+        self.do_finalize_unseal().await
+    }
+}
+
+impl VaultCoordinator {
+    /// Initializes `CoordinatorState::Unsealing` on first call if still `Idle`.
+    /// Threshold is based on ordinary operator count only (§3.4).
+    async fn ensure_unsealing_state(&mut self) -> Result<(), Error> {
+        if matches!(self.state, CoordinatorState::Idle) {
+            let mut conn = self.db.get().await?;
+            let ordinary_count: i64 = schema::operator::table
+                .count()
+                .get_result(&mut conn)
+                .await?;
+            let threshold =
+                shamir_threshold(usize::try_from(ordinary_count).unwrap_or_default());
+            self.state = CoordinatorState::Unsealing {
+                threshold,
+                ordinary_passphrases: HashMap::new(),
+                recovery_passphrases: HashMap::new(),
+            };
+        }
+        Ok(())
+    }
+
+    /// Moves state back to Idle and calls finalize_unseal.
+    async fn do_finalize_unseal(&mut self) -> Result<bool, Error> {
+        let CoordinatorState::Unsealing {
+            ordinary_passphrases,
+            recovery_passphrases,
+            ..
+        } = std::mem::replace(&mut self.state, CoordinatorState::Idle)
         else {
             unreachable!()
         };
 
-        finalize_unseal(self.db.clone(), self.vault.clone(), passphrases).await?;
+        finalize_unseal(
+            self.db.clone(),
+            self.vault.clone(),
+            ordinary_passphrases,
+            recovery_passphrases,
+        )
+        .await?;
         Ok(true)
     }
 }