fix: eth_sign's warning and the README say what it signs (closes #409)

eth_sign is signed as an EIP-191 personal message over its data, which is
what the Ethereum JSON-RPC specification defines it to return. Its warning
on the signature prompt said the site was asking to sign a raw hash that
could drain funds, and the README said the same; neither was true. The
warning now says the site used the older eth_sign method and that the data
will be signed as a personal message, and the README's SignApproval section
says so too. A test pins that eth_sign is read address first and data
second, and that the page receives the personal message signature over the
data.

Model: opus-5-5

README.md

@@ -2124,7 +2124,12 @@ view would leave a wallet one click from deletion.
       blocklist)
     - Site origin (bold, scheme and port included) + "wants you to sign a
       message"
-    - Danger warning box (shown for `eth_sign`, which signs a raw hash)
+    - Warning box, shown for `eth_sign`: "This site used the older eth_sign
+      method. The data will be signed as a personal message." An `eth_sign`
+      request names the address first and the data second. Its data is signed
+      exactly as a `personal_sign` message is, as an EIP-191 personal message,
+      which is what the Ethereum JSON-RPC specification defines `eth_sign` to
+      return.
     - Type: "Personal message" or "Typed data (EIP-712)"
     - From: color dot + full address + etherscan link
     - Message: for `personal_sign` and `eth_sign`, the text the message's bytes

src/background/index.js

@@ -984,9 +984,8 @@ async function handleRpc(method, params, origin) {
 
         if (method === "eth_sign") {
             signParams.dangerWarning =
-                "\u26a0\ufe0f DANGER: This site is requesting to sign a raw hash. " +
-                "This can be used to sign transactions that drain your funds. " +
-                "Only proceed if you fully understand what you are signing.";
+                "This site used the older eth_sign method. " +
+                "The data will be signed as a personal message.";
         }
 
         if (findPendingApproval(origin, "sign")) {

src/popup/views/approval.js

@@ -721,7 +721,7 @@ function showSignApproval(details) {
             "This message is plain text, not hex, so it cannot be signed.";
     }
 
-    // Display danger warning for eth_sign (raw hash signing)
+    // The warning the background attaches to an eth_sign request.
     const warningEl = $("approve-sign-danger-warning");
     if (warningEl) {
         if (sp.dangerWarning) {

tests/backgroundApproval.test.js

@@ -1551,6 +1551,53 @@ describe("what the approval is verified against", () => {
     });
 });
 
+// eth_sign names the address first and the data second, the reverse of
+// personal_sign, and is answered as the JSON-RPC specification defines it:
+// with an EIP-191 personal message signature over the data, the same
+// signature personal_sign returns
+// (https://git.eeqj.de/sneak/AutistMask/issues/409).
+describe("eth_sign", () => {
+    test("is read address first, and returns a personal message signature over the data", async () => {
+        const bg = loadBackground();
+        const page = bg.send(
+            {
+                type: "AUTISTMASK_RPC",
+                method: "eth_sign",
+                params: [signer.address, MESSAGE],
+            },
+            { origin: ORIGIN },
+        );
+        await settle();
+        const id = approvalIdIn(bg.created.map((c) => c.url));
+
+        const details = bg.send(
+            { type: "AUTISTMASK_GET_APPROVAL", id },
+            { url: bg.fromPopup.url },
+        );
+        const { signParams } = details.sendResponse.mock.calls[0][0];
+        expect(signParams).toMatchObject({
+            method: "eth_sign",
+            message: MESSAGE,
+            from: signer.address,
+        });
+        expect(signParams.dangerWarning).toBe(
+            "This site used the older eth_sign method. " +
+                "The data will be signed as a personal message.",
+        );
+
+        const signature = await signer.signMessage(
+            Buffer.from(MESSAGE.slice(2), "hex"),
+        );
+        bg.send(
+            { type: "AUTISTMASK_SIGN_RESPONSE", id, approved: true, signature },
+            { url: bg.fromPopup.url },
+        );
+        await settle();
+
+        expect(page.sendResponse).toHaveBeenCalledWith({ result: signature });
+    });
+});
+
 // The interlock must not cost the retry the approval exists to allow.
 describe("the interlock releases a failed attempt", () => {
     test("a retryable failure before the broadcast leaves the approval usable", async () => {