fix: explain a stored non-master xprv wallet instead of throwing at signing time (closes #234)

src/shared/wallet.js

@@ -120,9 +120,24 @@ function getSignerForAddress(walletData, addrIndex, decryptedSecret) {
         return node.deriveChild(addrIndex);
     }
     if (walletData.type === "xprv") {
-        const node =
-            masterXprvOrThrow(decryptedSecret).derivePath(BIP44_ETH_PATH);
-        return node.deriveChild(addrIndex);
+        // Checked here rather than through masterXprvOrThrow so the message
+        // fits the situation: nobody is importing anything at signing time,
+        // and this wallet is already in storage. src/shared/walletDefects.js
+        // catches it at list-render time; this is the backstop behind that.
+        const node = parseExtendedKey(decryptedSecret);
+        if (!node || !node.privateKey) {
+            throw new Error(
+                "This wallet's stored key is not a valid extended private " +
+                    "key, so it cannot sign.",
+            );
+        }
+        if (node.depth !== MASTER_DEPTH) {
+            throw new Error(
+                "This wallet was imported from an extended private key that " +
+                    "is not a master key, so it cannot sign.",
+            );
+        }
+        return node.derivePath(BIP44_ETH_PATH).deriveChild(addrIndex);
     }
     return new Wallet(decryptedSecret);
 }
@@ -142,6 +157,7 @@ function walletHasRecoveryPhrase(walletData) {
 
 module.exports = {
     generateMnemonic,
+    parseExtendedKey,
     deriveAddressFromXpub,
     hdWalletFromMnemonic,
     hdWalletFromXprv,

src/shared/walletDefects.js

@@ -0,0 +1,86 @@
+// Wallets already in stored state whose key cannot be used, and the copy that
+// explains them.
+//
+// Refusing a non-master extended private key at import time does nothing for a
+// wallet imported before that refusal existed. Such a wallet is detected here,
+// at wallet-list render time, so the user meets the explanation on the list
+// screen rather than an exception on the send screen. Nothing here modifies or
+// removes a wallet: the record is the user's data.
+
+const { parseExtendedKey } = require("./wallet");
+
+const NON_MASTER_XPRV = "non-master-xprv";
+
+// An "xprv" wallet stores the neutered BIP-44 Ethereum node, four levels below
+// the key that was imported: the current import path derives the absolute
+// m/44'/60'/0'/0 from a depth-0 key, and the pre-#210 path derived the same
+// four levels as a relative path beneath whatever depth it was given. A master
+// import therefore stores a depth-4 xpub and a depth-d import stores depth
+// d + 4, which makes the stored xpub an exact read on the imported key's
+// depth — and it is readable without the password, unlike the key itself.
+const BIP44_ETH_XPUB_DEPTH = 4;
+
+const DEFECTS = {
+    [NON_MASTER_XPRV]: {
+        id: NON_MASTER_XPRV,
+        heading: "This wallet's addresses were derived incorrectly.",
+        paragraphs: [
+            "This wallet was imported from an extended private key that is " +
+                "not a master key. An earlier version applied the Ethereum " +
+                "derivation path beneath that key instead of from a master " +
+                "key, so the addresses listed here are not the ones that key " +
+                "produces under the standard path.",
+            "Signing and sending are disabled for this wallet. The addresses " +
+                "do descend from the extended private key you imported, so " +
+                "anything they hold is still reachable by software that " +
+                "repeats the same non-standard derivation. Check them in a " +
+                "block explorer before deciding what to do.",
+            "To see the addresses this key produces under the standard path, " +
+                "import the master extended private key, or the recovery " +
+                "phrase it came from, as a new wallet. Nothing here has been " +
+                "changed or removed, and this wallet stays until you delete " +
+                "it yourself.",
+        ],
+        // One sentence for the places that have room for one: the flash on a
+        // blocked Send, the inline error on the approval screens.
+        shortMessage:
+            "This wallet cannot sign, because it was imported from an " +
+            "extended private key that is not a master key. The wallet list " +
+            "explains what happened.",
+    },
+};
+
+// The defect record for a wallet, or null if there is nothing wrong with it
+// that this module can see. Read-only.
+//
+// A wallet whose xpub will not parse gets null rather than a defect: there is
+// no basis in that case to tell the user their key was not a master key, and a
+// wrong explanation is worse than none.
+function walletDefect(walletData) {
+    if (!walletData || walletData.type !== "xprv") return null;
+    const node = parseExtendedKey(walletData.xpub);
+    if (!node) return null;
+    if (node.depth === BIP44_ETH_XPUB_DEPTH) return null;
+    return DEFECTS[NON_MASTER_XPRV];
+}
+
+// The notice block for the wallet list, or "" for a wallet with no defect.
+// The copy is fixed text from this module, so it needs no escaping.
+function walletDefectHtml(walletData) {
+    const defect = walletDefect(walletData);
+    if (!defect) return "";
+    let html =
+        '<div class="border border-red-500 border-dashed p-2 my-1 text-xs text-red-500">';
+    html += `<div class="font-bold mb-1">${defect.heading}</div>`;
+    for (const p of defect.paragraphs) {
+        html += `<p class="mb-1">${p}</p>`;
+    }
+    html += "</div>";
+    return html;
+}
+
+module.exports = {
+    NON_MASTER_XPRV,
+    walletDefect,
+    walletDefectHtml,
+};