feat: add LedgerStarknetApp type for Ledger specific operations (#621)

Author: xJonathanLEI

starknet-signers/src/ledger.rs

@@ -22,12 +22,19 @@ const EIP_2645_PATH_LENGTH: usize = 6;
 const PUBLIC_KEY_SIZE: usize = 65;
 const SIGNATURE_SIZE: usize = 65;
 
+/// Ledger app wrapper that implements the [`Signer`] trait.
 #[derive(Debug)]
 pub struct LedgerSigner {
-    transport: Ledger,
+    app: LedgerStarknetApp,
     derivation_path: DerivationPath,
 }
 
+/// A handle for communicating with the Ledger Starknet app.
+#[derive(Debug)]
+pub struct LedgerStarknetApp {
+    transport: Ledger,
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum LedgerError {
     #[error("derivation path is empty, not prefixed with m/2645', or is not 6-level long")]
@@ -77,16 +84,14 @@ impl LedgerSigner {
     ///
     /// Currently, the Ledger app only enforces the length and the first level of the path.
     pub async fn new(derivation_path: DerivationPath) -> Result<Self, LedgerError> {
-        let transport = Ledger::init().await?;
-
         if !matches!(derivation_path.iter().next(), Some(&EIP_2645_PURPOSE))
             || derivation_path.len() != EIP_2645_PATH_LENGTH
         {
             return Err(LedgerError::InvalidDerivationPath);
         }
 
         Ok(Self {
-            transport,
+            app: LedgerStarknetApp::new().await?,
             derivation_path,
         })
     }
@@ -98,12 +103,57 @@ impl Signer for LedgerSigner {
     type SignError = LedgerError;
 
     async fn get_public_key(&self) -> Result<VerifyingKey, Self::GetPublicKeyError> {
+        self.app
+            .get_public_key(self.derivation_path.clone(), false)
+            .await
+    }
+
+    async fn sign_hash(&self, hash: &Felt) -> Result<Signature, Self::SignError> {
+        self.app.sign_hash(self.derivation_path.clone(), hash).await
+    }
+
+    fn is_interactive(&self) -> bool {
+        true
+    }
+}
+
+impl LedgerStarknetApp {
+    /// Initializes the Starknet Ledger app. Attempts to find and connect to a Ledger device. The
+    /// device must be unlocked and have the Starknet app open.
+    pub async fn new() -> Result<Self, LedgerError> {
+        let transport = Ledger::init().await?;
+
+        Ok(Self { transport })
+    }
+
+    /// Gets a public key from the app for a particular derivation path, with optional on-device
+    /// confirmation for extra security.
+    ///
+    /// The derivation path _must_ follow EIP-2645, i.e. having `2645'` as its "purpose" level as
+    /// per BIP-44, as the Ledger app does not allow other paths to be used.
+    ///
+    /// The path _must_ also be 6-level in length. An example path for Starknet would be:
+    ///
+    /// `m/2645'/1195502025'/1470455285'/0'/0'/0`
+    ///
+    /// where:
+    ///
+    /// - `2645'` is the EIP-2645 prefix
+    /// - `1195502025'`, decimal for `0x4741e9c9`, is the 31 lowest bits for `sha256(starknet)`
+    /// - `1470455285'`, decimal for `0x57a55df5`, is the 31 lowest bits for `sha256(starkli)`
+    ///
+    /// Currently, the Ledger app only enforces the length and the first level of the path.
+    async fn get_public_key(
+        &self,
+        derivation_path: DerivationPath,
+        display: bool,
+    ) -> Result<VerifyingKey, LedgerError> {
         let response = self
             .transport
             .exchange(
                 &GetPubKeyCommand {
-                    display: false,
-                    path: self.derivation_path.clone(),
+                    display,
+                    path: derivation_path,
                 }
                 .into(),
             )
@@ -123,13 +173,34 @@ impl Signer for LedgerSigner {
         Ok(VerifyingKey::from_scalar(pubkey_x))
     }
 
-    async fn sign_hash(&self, hash: &Felt) -> Result<Signature, Self::SignError> {
+    /// Requests a signature for a **raw hash** with a certain derivation path. Currently the Ledger
+    /// app only supports blind signing raw hashes.
+    ///
+    /// The derivation path _must_ follow EIP-2645, i.e. having `2645'` as its "purpose" level as
+    /// per BIP-44, as the Ledger app does not allow other paths to be used.
+    ///
+    /// The path _must_ also be 6-level in length. An example path for Starknet would be:
+    ///
+    /// `m/2645'/1195502025'/1470455285'/0'/0'/0`
+    ///
+    /// where:
+    ///
+    /// - `2645'` is the EIP-2645 prefix
+    /// - `1195502025'`, decimal for `0x4741e9c9`, is the 31 lowest bits for `sha256(starknet)`
+    /// - `1470455285'`, decimal for `0x57a55df5`, is the 31 lowest bits for `sha256(starkli)`
+    ///
+    /// Currently, the Ledger app only enforces the length and the first level of the path.
+    async fn sign_hash(
+        &self,
+        derivation_path: DerivationPath,
+        hash: &Felt,
+    ) -> Result<Signature, LedgerError> {
         get_apdu_data(
             &self
                 .transport
                 .exchange(
                     &SignHashCommand1 {
-                        path: self.derivation_path.clone(),
+                        path: derivation_path,
                     }
                     .into(),
                 )
@@ -163,10 +234,6 @@ impl Signer for LedgerSigner {
 
         Ok(signature)
     }
-
-    fn is_interactive(&self) -> bool {
-        true
-    }
 }
 
 impl From<coins_ledger::LedgerError> for LedgerError {