fix: sign the ERC-20 amount the send screen displayed (closes #305)

The wallet's own Send screen renders the amount, the balance and the symbol
from the block explorer's cached decimals, but confirmTx encoded the transfer
from decimals() read off the contract at signing time and nothing compared the
two. A token whose on-chain scale disagrees with the cached one -- an
upgradeable or proxy token, a caller-dependent one, a stale or wrong explorer
entry, a compromised Blockscout -- therefore signed an amount that was never
displayed, off by a power of ten for every decimal place of disagreement. The
reproduction on the issue approves 0.25 and signs 250,000,000,000.

The scale is now carried forward on the pending transaction, taken from the
same tokenBalances entry the screen's own numbers come from, and the contract's
decimals() is read at signing time only to be compared with it. A disagreement
is a refusal that names both numbers, never a preference for either: both
candidate transfers move an amount nobody approved. New
src/shared/transferAmount.js holds that check, as the confirmTx counterpart to
approvalVerify.js, and takes the same stance on an absent or unusable value --
a quantity that cannot be compared with what was displayed has not been
checked. The gas estimate encodes from the same carried value and no longer
reads decimals() at all, so the estimate is for the transfer that would be
signed.

Nothing in the e2e suite had ever clicked #btn-confirm-send, so the popup's own
Send -> ConfirmTx -> Sign & Send -> WaitTx path had no coverage at all, which is
how this shipped. It is now driven end to end to a broadcast, with the
transfer() amount hand-decoded out of the raw signed bytes and asserted against
the amount read off the confirmation screen, plus a case where the fixture's
decimals() starts answering 18 after the screen was built and nothing reaches
eth_sendRawTransaction. The fixture gains that override and a receipt, so the
wait screen resolves to the success view instead of polling for the rest of the
run.

README.md

@@ -992,6 +992,13 @@ on ConfirmTx, DeleteWallet, ApproveTx and ApproveSign.
 - **Transitions**:
     - "Sign & Send" (correct password) → broadcast tx → **WaitTx**
     - "Sign & Send" (correct password) → broadcast fails → **ErrorTx**
+    - "Sign & Send" on an ERC-20 whose contract answers `decimals()` with a
+      different number than the amount above was displayed with → nothing is
+      signed → **ErrorTx** naming both numbers. The transfer is encoded from the
+      decimals the screen rendered, carried forward on the pending transaction;
+      the contract's own answer is read at signing time only to be compared with
+      it, and a disagreement is a refusal rather than a preference for either
+      value (`src/shared/transferAmount.js`)
     - "Sign & Send" (wrong password) → "Wrong password." on the password error
       line, no screen change
     - "Back" → **Send**

TODO.md

@@ -44,6 +44,27 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-20: The wallet's own ERC-20 send signs the amount it displayed
+  ([#305](https://git.eeqj.de/sneak/AutistMask/issues/305)). The confirmation
+  screen renders from the block explorer's cached decimals; the transfer was
+  encoded from `decimals()` read off the contract at signing time, and nothing
+  compared the two, so a token whose on-chain scale disagreed — an upgradeable
+  or proxy token, a stale explorer entry, a compromised Blockscout — signed an
+  amount that was never on screen, off by a power of ten per decimal place of
+  disagreement. The scale is now carried forward on the pending transaction from
+  the same balance entry the screen's amount, balance and symbol come from, and
+  the contract's answer is read at signing time only to be compared with it: a
+  disagreement is a refusal naming both numbers, never a preference for either
+  (`src/shared/transferAmount.js`, the `confirmTx` counterpart to
+  `approvalVerify.js`). The gas estimate encodes from the same carried value and
+  no longer reads `decimals()` at all. Nothing in the e2e suite had ever clicked
+  `#btn-confirm-send`, which is how this shipped: the popup's own Send →
+  ConfirmTx → Sign & Send → WaitTx path now runs end to end to a broadcast, with
+  the `transfer()` amount decoded out of the raw signed bytes and asserted
+  against what the screen displayed, and a companion case where the contract
+  starts answering a different scale after the screen was built and nothing
+  reaches the RPC. Reverting only the signing-side comparison turns that second
+  case red and leaves the other 53 green.
 - 2026-08-17: The Settings screen is driven in a browser, and every element id
   the popup looks up is checked statically. Nothing exercised Settings in the
   e2e suite, and jest runs with no DOM, so the densest run of `$("...")` lookups

src/popup/views/confirmTx.js

@@ -25,6 +25,10 @@ const {
     getFullWarnings,
 } = require("../../shared/addressWarnings");
 const { ERC20_ABI, isBurnAddress } = require("../../shared/constants");
+const {
+    displayedDecimals,
+    transferAmountUnits,
+} = require("../../shared/transferAmount");
 const {
     CODES,
     FEE_PENDING,
@@ -302,8 +306,17 @@ async function estimateGas(txInfo) {
             });
         } else {
             const contract = new Contract(txInfo.token, ERC20_ABI, provider);
-            const decimals = await contract.decimals();
-            const amount = parseUnits(txInfo.amount, decimals);
+            // The scale the screen is rendering with, not the contract's own
+            // answer: the estimate has to be for the transfer that would be
+            // signed, and that one is encoded from what was displayed. See
+            // transferAmount.js. A pending transaction that carries no usable
+            // scale throws here, which reports the fee as unknown and leaves
+            // Send blocked — an amount that cannot be checked against the
+            // screen is never estimated for, let alone sent.
+            const amount = parseUnits(
+                txInfo.amount,
+                displayedDecimals(txInfo.tokenDecimals),
+            );
             gasLimit = await contract.transfer.estimateGas(txInfo.to, amount, {
                 from: txInfo.from,
             });
@@ -445,8 +458,16 @@ function init(_ctx) {
                     ERC20_ABI,
                     connectedSigner,
                 );
-                const decimals = await contract.decimals();
-                const amount = parseUnits(pendingTx.amount, decimals);
+                // The contract's decimals() is read to be COMPARED with the
+                // scale the screen rendered this amount at, not to encode with:
+                // encoding from it signs whatever the contract answers now,
+                // which is not what the user read. A disagreement throws and is
+                // reported on the error screen. See transferAmount.js.
+                const amount = transferAmountUnits(
+                    pendingTx.amount,
+                    pendingTx.tokenDecimals,
+                    await contract.decimals(),
+                );
                 tx = await contract.transfer(pendingTx.to, amount);
             }
 

src/popup/views/send.js

@@ -220,6 +220,11 @@ function init(_ctx) {
 
         let tokenSymbol = null;
         let tokenBalance = null;
+        // The scale the amount and the balance below are rendered at, carried
+        // forward so the transfer is encoded with the number the user read
+        // rather than with whatever the contract answers at signing time. See
+        // src/shared/transferAmount.js.
+        let tokenDecimals = null;
         if (token !== "ETH") {
             const tb = (addr.tokenBalances || []).find(
                 (t) => t.address.toLowerCase() === token.toLowerCase(),
@@ -230,6 +235,7 @@ function init(_ctx) {
                 state.trackedTokens,
             );
             tokenBalance = tb ? tb.balance || "0" : "0";
+            tokenDecimals = tb ? tb.decimals : null;
         }
 
         ctx.showConfirmTx({
@@ -241,6 +247,7 @@ function init(_ctx) {
             balance: addr.balance,
             tokenSymbol: tokenSymbol,
             tokenBalance: tokenBalance,
+            tokenDecimals: tokenDecimals,
         });
     });
 

src/shared/transferAmount.js

@@ -0,0 +1,116 @@
+// The base-unit amount an ERC-20 transfer from the wallet's own Send screen is
+// encoded with.
+//
+// A token amount is a decimal string plus a scale, and the two come from
+// different places. The confirmation screen renders the amount, the balance and
+// the symbol from the block explorer's cached metadata (see
+// fetchTokenBalances() in balances.js); the transfer used to be encoded from
+// decimals() read off the contract at signing time, and nothing compared the
+// two. A token whose on-chain scale differs from the cached one — an
+// upgradeable or proxy token, a caller-dependent one, a stale or wrong explorer
+// entry — therefore signed an amount that was never displayed, off by a power
+// of ten for every decimal place of disagreement.
+//
+// So the scale used to encode is the scale the screen rendered with, carried
+// forward on the pending transaction, and the contract's own answer is read
+// only to be compared with it. A disagreement is a refusal, never a preference
+// for either number: the wallet cannot tell which of the two the user meant,
+// and both candidate transfers move an amount nobody approved.
+//
+// This is the confirmTx counterpart to approvalVerify.js, which does the same
+// job for the dApp approval path, and it takes the same stance: a quantity that
+// cannot be compared with what was displayed has not been checked, so an absent
+// or unusable value is refused rather than filled in.
+//
+// Every message here is shown to the user on the transaction error screen, so
+// each is a full sentence and names the numbers it is refusing over.
+
+const { parseUnits } = require("ethers");
+
+// Solidity's decimals() returns a uint8, so anything outside that range is not
+// an answer this wallet can use.
+const MAX_DECIMALS = 255;
+
+const UNKNOWN_DISPLAYED_DECIMALS_MESSAGE =
+    "The transfer was not sent, because the number of decimal places this" +
+    " amount was shown with is unknown, so the amount that would be signed" +
+    " cannot be shown to be the amount that was displayed.";
+
+const UNREADABLE_CONTRACT_DECIMALS_MESSAGE =
+    "The transfer was not sent, because the token contract did not report a" +
+    " usable number of decimal places, so the amount that would be signed" +
+    " cannot be checked against the amount that was displayed.";
+
+function mismatchMessage(displayed, onChain) {
+    return (
+        "The transfer was not sent. The token contract reports " +
+        onChain +
+        " decimal places, but the amount was displayed using " +
+        displayed +
+        ", so signing it would move a different amount than the one shown." +
+        " Reopen the wallet to reload this token's details and try again."
+    );
+}
+
+// A decimals value from either source as a number, or null if it is not one.
+// decimals() comes back from ethers as a bigint and the explorer's copy arrives
+// as a string, so both of those are accepted alongside a plain number; anything
+// fractional, negative, out of uint8 range, or of any other type at all is not.
+//
+// The types are enumerated rather than coerced because Number() is far too
+// willing: Number([]) is 0 and Number(true) is 1, so a coercing check would
+// admit an empty array as a scale of zero and encode a whole-token transfer
+// against it.
+function toDecimals(value) {
+    let n;
+    if (typeof value === "number") {
+        n = value;
+    } else if (typeof value === "bigint") {
+        if (value < 0n || value > BigInt(MAX_DECIMALS)) return null;
+        n = Number(value);
+    } else if (typeof value === "string") {
+        if (!/^[0-9]+$/.test(value)) return null;
+        n = Number(value);
+    } else {
+        return null;
+    }
+    if (!Number.isInteger(n) || n < 0 || n > MAX_DECIMALS) return null;
+    return n;
+}
+
+// The decimals the confirmation screen rendered an amount with, as a number.
+// Throws when the pending transaction does not carry a usable one — which is
+// also what keeps the gas estimate from quietly estimating a different transfer
+// than the one that would be signed.
+function displayedDecimals(value) {
+    const displayed = toDecimals(value);
+    if (displayed === null) {
+        throw new Error(UNKNOWN_DISPLAYED_DECIMALS_MESSAGE);
+    }
+    return displayed;
+}
+
+// The transfer amount in the token's base units, or a throw. `amount` is the
+// decimal string the user typed and the screen displayed, `displayed` is the
+// scale it was displayed at, and `onChain` is what the contract's decimals()
+// answered at signing time. The two scales must agree.
+function transferAmountUnits(amount, displayed, onChain) {
+    const shown = displayedDecimals(displayed);
+    const reported = toDecimals(onChain);
+    if (reported === null) {
+        throw new Error(UNREADABLE_CONTRACT_DECIMALS_MESSAGE);
+    }
+    if (reported !== shown) {
+        throw new Error(mismatchMessage(shown, reported));
+    }
+    return parseUnits(String(amount), shown);
+}
+
+module.exports = {
+    displayedDecimals,
+    transferAmountUnits,
+    mismatchMessage,
+    MAX_DECIMALS,
+    UNKNOWN_DISPLAYED_DECIMALS_MESSAGE,
+    UNREADABLE_CONTRACT_DECIMALS_MESSAGE,
+};

tests/e2e/network.js

@@ -221,11 +221,6 @@ const RPC_RESULTS = {
     eth_estimateGas: hex(GAS_LIMIT),
     eth_getTransactionCount: "0x0",
     eth_maxPriorityFeePerGas: hex(PRIORITY_FEE_WEI),
-    // "not mined yet", which is what a node answers for a transaction it has
-    // only just accepted. The wait screen the dApp transaction approval hands
-    // off to polls this every 10 seconds; leaving it unstubbed would report
-    // the poll as escaping traffic the moment a test outlived one tick.
-    eth_getTransactionReceipt: null,
 };
 
 // The "latest" block, which ethers' getFeeData() reads baseFeePerGas from
@@ -253,17 +248,22 @@ function latestBlock() {
 const SELECTOR_DECIMALS = "0x313ce567";
 
 // Every eth_call still answers with a zero word except decimals() on the
-// stub token. ethers reads that before it can encode an ERC-20 transfer,
-// and a zero there makes parseUnits() reject any fractional amount — so the
-// ERC-20 confirmation path would fail its gas estimate for a reason that
-// has nothing to do with what is being tested.
-function ethCallResult(req) {
+// stub token, which the wallet reads back at signing time to compare with
+// the scale the confirmation screen rendered (issue #305).
+//
+// opts.tokenDecimalsOverride is the lying contract: set it and decimals()
+// answers something other than the value this same fixture reports through
+// Blockscout, which is exactly the disagreement the wallet must refuse to
+// sign over. It is read at request time, so a test flips it on the options
+// object the route was registered with — after the confirmation screen has
+// been built — without re-registering anything.
+function ethCallResult(req, opts) {
     const call = Array.isArray(req.params) ? req.params[0] : null;
     if (!call || typeof call !== "object") return ZERO_WORD;
     const data = String(call.data || call.input || "").toLowerCase();
     const to = String(call.to || "").toLowerCase();
     if (data.startsWith(SELECTOR_DECIMALS) && to === STUB_TOKEN.address) {
-        return word(STUB_TOKEN.decimals);
+        return word(opts.tokenDecimalsOverride || STUB_TOKEN.decimals);
     }
     return ZERO_WORD;
 }
@@ -346,6 +346,38 @@ function transactionDetails(hash) {
     };
 }
 
+// The receipt for a transaction this run broadcast.
+//
+// eth_getTransactionReceipt otherwise answers null — "not mined yet", which is
+// what a node says about a transaction it has only just accepted, and what the
+// wait screen has to keep polling through. opts.seedReceipt confirms it
+// instead, which is how a test that drives the popup's own send to a broadcast
+// gets off the wait screen: the wait resolves to the success view, which has a
+// Done button, rather than polling for a receipt for the rest of the suite.
+//
+// Every field ethers' receipt formatter requires is present. A receipt it
+// cannot parse throws inside the poll, which the wallet reports through
+// log.errorf — i.e. console.error — and the harness fails the run on, so a
+// half-populated fixture here would surface as an unrelated-looking failure.
+function transactionReceipt(hash) {
+    return {
+        transactionHash: hash,
+        transactionIndex: "0x0",
+        blockHash: "0x" + "33".repeat(32),
+        blockNumber: hex(STUB_BLOCK_NUMBER),
+        from: STUB_COUNTERPARTY,
+        to: STUB_TOKEN.address,
+        cumulativeGasUsed: hex(GAS_LIMIT),
+        gasUsed: hex(GAS_LIMIT),
+        effectiveGasPrice: hex(GAS_PRICE_WEI),
+        contractAddress: null,
+        logs: [],
+        logsBloom: "0x" + "00".repeat(256),
+        status: "0x1",
+        type: "0x2",
+    };
+}
+
 function jsonResponse(route, body) {
     return route.fulfill({
         status: 200,
@@ -408,7 +440,13 @@ function rpcReply(req, opts, report) {
         });
     }
     if (req.method === "eth_call") {
-        return Object.assign(envelope, { result: ethCallResult(req) });
+        return Object.assign(envelope, { result: ethCallResult(req, opts) });
+    }
+    if (req.method === "eth_getTransactionReceipt") {
+        const hash = Array.isArray(req.params) ? req.params[0] : null;
+        return Object.assign(envelope, {
+            result: opts.seedReceipt && hash ? transactionReceipt(hash) : null,
+        });
     }
     if (req.method === "eth_getBlockByNumber") {
         return Object.assign(envelope, { result: latestBlock() });
@@ -555,6 +593,11 @@ function traceEnabled(raw) {
  *   eth_estimateGas until this is cleared again.
  * @param {string[]} [opts.broadcastTransactions]  every raw signed
  *   transaction handed to eth_sendRawTransaction, appended in order.
+ * @param {string} [opts.tokenDecimalsOverride]  what decimals() answers for
+ *   the stub token, in place of the value Blockscout reports for it. This is
+ *   the token that lies about its scale; read at request time.
+ * @param {boolean} [opts.seedReceipt]  answer eth_getTransactionReceipt with a
+ *   confirmed receipt instead of null, so a wait screen resolves.
  * @returns {Promise<{waitForServiceWorkerTraffic: (ms: number) =>
  *   Promise<string|null>}>}
  */

tests/e2e/run.js

@@ -12,10 +12,12 @@
 const {
     Transaction,
     formatEther,
+    formatUnits,
     getAddress,
     getBytes,
     hexlify,
     parseEther,
+    parseUnits,
     toQuantity,
     toUtf8Bytes,
     verifyMessage,
@@ -1964,6 +1966,208 @@ test("ConfirmTx reports a failed ERC-20 estimate as unknown, not as a fee proble
     );
 });
 
+// ------------------------- the popup's own send, end to end (#305)
+//
+// Everything above this point stops at the confirmation screen. Nothing in
+// the suite had ever clicked #btn-confirm-send, so the wallet's own Send ->
+// ConfirmTx -> Sign & Send -> WaitTx path had no coverage at all, and issue
+// #305 shipped through the gap: the screen was rendered from the explorer's
+// decimals while the transfer was encoded from decimals() read off the
+// contract at signing time, with nothing comparing the two.
+//
+// These two tests drive that path to a broadcast and read the amount out of
+// the bytes the node was handed. The first asserts those bytes against what
+// the screen displayed; the second makes the contract answer a different
+// scale after the screen was built, and requires that nothing is broadcast.
+
+// keccak("transfer(address,uint256)")[0:4].
+const SELECTOR_TRANSFER = "0xa9059cbb";
+
+// What decimals() starts answering once the confirmation screen has been
+// built. The explorer reports 6 for the same token, so a wallet that encodes
+// from the contract signs 10^12 times the amount it displayed.
+const LYING_DECIMALS = "18";
+
+const TOKEN_DECIMALS = Number(STUB_TOKEN.decimals);
+
+// The transfer() call inside a raw signed transaction, hand-decoded.
+//
+// Deliberately not run through an ethers Interface built from the
+// extension's own ABI: what is under assertion is the bytes that reached the
+// node, and the fewer assumptions the wallet and the assertion share, the
+// less room there is for both to be wrong in the same direction.
+function decodeTransfer(rawSignedTx) {
+    const signed = Transaction.from(rawSignedTx);
+    const data = signed.data.toLowerCase();
+    assert(
+        data.startsWith(SELECTOR_TRANSFER) && data.length === 10 + 128,
+        "the broadcast transaction is not an ERC-20 transfer() call: " + data,
+    );
+    return {
+        signed,
+        recipient: getAddress("0x" + data.slice(34, 74)),
+        rawAmount: BigInt("0x" + data.slice(74)),
+    };
+}
+
+// The amount the confirmation screen is showing, verbatim.
+async function shownAmount(page) {
+    return (await page.locator("#confirm-amount").innerText()).trim();
+}
+
+async function fillPasswordAndSend(page) {
+    await page.fill("#confirm-tx-password", PASSWORD);
+    await page.click("#btn-confirm-send");
+}
+
+async function goToTokenConfirm(env) {
+    await goToConfirm(env.page, {
+        token: STUB_TOKEN.address,
+        balance: TOKEN_BALANCE_TEXT + " " + STUB_TOKEN.symbol,
+        amount: TOKEN_AMOUNT,
+    });
+    await waitForEstimate(env.page);
+    const shown = await shownAmount(env.page);
+    assert(
+        shown === TOKEN_AMOUNT + " " + STUB_TOKEN.symbol,
+        "the confirmation screen is not showing the amount that was entered: " +
+            JSON.stringify(shown),
+    );
+    return shown;
+}
+
+test("the popup's own ERC-20 send broadcasts the amount it displayed (#305)", async (env) => {
+    // The previous test left the ETH balance at the fee-only fixture, which
+    // blocks sending outright; this one has to be able to press Send.
+    env.routeOpts.ethBalanceWei = toHexWei(FUNDED_ETH_WEI);
+    await settleOnMain(env, { ethWei: FUNDED_ETH_WEI, expectToken: true });
+    const shown = await goToTokenConfirm(env);
+
+    const before = env.routeOpts.broadcastTransactions.length;
+    // Confirm the transaction once it is broadcast, so the wait screen
+    // resolves to the success view instead of polling for the rest of the run.
+    env.routeOpts.seedReceipt = true;
+    await fillPasswordAndSend(env.page);
+    await visible(env.page, "#view-wait-tx", 60000);
+
+    const broadcast = env.routeOpts.broadcastTransactions;
+    assert(
+        broadcast.length === before + 1,
+        "expected exactly one raw transaction to reach the RPC, got " +
+            (broadcast.length - before),
+    );
+    const { signed, recipient, rawAmount } = decodeTransfer(
+        broadcast[broadcast.length - 1],
+    );
+
+    // The measurement, printed on every run: the amount the user read, and
+    // what the signed bytes mean at each of the two candidate scales. Under
+    // the defect these three lines disagree.
+    console.log(
+        "# erc-20 send artifact: displayed=" +
+            JSON.stringify(shown) +
+            " rawAmount=" +
+            rawAmount +
+            " asIf" +
+            TOKEN_DECIMALS +
+            "Decimals=" +
+            formatUnits(rawAmount, TOKEN_DECIMALS) +
+            " asIf" +
+            LYING_DECIMALS +
+            "Decimals=" +
+            formatUnits(rawAmount, Number(LYING_DECIMALS)),
+    );
+
+    assert(
+        getAddress(signed.to) === getAddress(STUB_TOKEN.address),
+        "the broadcast transaction does not call the token contract: " +
+            signed.to,
+    );
+    assert(
+        recipient === getAddress(STUB_COUNTERPARTY),
+        "the broadcast transfer goes to " + recipient,
+    );
+    // What the whole issue turns on: the signed amount, read back at the
+    // scale the SCREEN rendered with, is the number the screen rendered.
+    const wanted = parseUnits(shown.split(" ")[0], TOKEN_DECIMALS);
+    assert(
+        rawAmount === wanted,
+        "the broadcast transfer moves " +
+            rawAmount +
+            " base units, which is " +
+            formatUnits(rawAmount, TOKEN_DECIMALS) +
+            " " +
+            STUB_TOKEN.symbol +
+            " at the scale the confirmation screen displayed — but the screen" +
+            " displayed " +
+            JSON.stringify(shown) +
+            ", i.e. " +
+            wanted +
+            " base units (#305)",
+    );
+
+    const summary = (
+        await env.page.locator("#wait-tx-summary").innerText()
+    ).trim();
+    assert(
+        summary === shown,
+        "the wait screen summarises the send as " +
+            JSON.stringify(summary) +
+            ", not as the approved " +
+            JSON.stringify(shown),
+    );
+
+    await visible(env.page, "#view-success-tx", 60000);
+    await env.page.click("#btn-success-tx-done");
+    await visible(env.page, "#view-address");
+    env.routeOpts.seedReceipt = false;
+});
+
+test("a token that lies about decimals() at signing time broadcasts nothing (#305)", async (env) => {
+    const shown = await goToTokenConfirm(env);
+
+    // Only now, with the screen already built and its estimate already taken
+    // at the explorer's scale, does the contract start answering differently.
+    // This is the whole shape of the defect: a value read at signing time that
+    // nothing on screen was ever derived from.
+    env.routeOpts.tokenDecimalsOverride = LYING_DECIMALS;
+    const before = env.routeOpts.broadcastTransactions.length;
+    await fillPasswordAndSend(env.page);
+    await visible(env.page, "#view-error-tx", 60000);
+    env.routeOpts.tokenDecimalsOverride = null;
+
+    assert(
+        env.routeOpts.broadcastTransactions.length === before,
+        "a transfer encoded against a contract that contradicts the " +
+            "confirmation screen still reached the RPC (#305)",
+    );
+
+    const message = (
+        await env.page.locator("#error-tx-message").innerText()
+    ).trim();
+    console.log(
+        "# erc-20 decimals refusal: displayed=" +
+            JSON.stringify(shown) +
+            " contract=" +
+            LYING_DECIMALS +
+            " message=" +
+            JSON.stringify(message),
+    );
+    assert(
+        message.includes("reports " + LYING_DECIMALS + " decimal places") &&
+            message.includes("displayed using " + STUB_TOKEN.decimals),
+        "the refusal does not name both scales it is refusing over: " +
+            JSON.stringify(message),
+    );
+    assert(
+        /^[A-Z].*\.$/s.test(message),
+        "the refusal is not a full sentence: " + JSON.stringify(message),
+    );
+
+    await env.page.click("#btn-error-tx-done");
+    await visible(env.page, "#view-address");
+});
+
 // ------------------------------------------- dApp round trips (#183)
 //
 // The seam. Everything above drives the popup on its own; this section is
@@ -3094,6 +3298,13 @@ async function main() {
         ethBalanceWei: null,
         failGasEstimate: false,
         holdGasEstimate: false,
+        // What decimals() answers for the stub token, when it is to answer
+        // something other than the value the same fixture reports through
+        // Blockscout. The token that lies about its scale (#305).
+        tokenDecimalsOverride: null,
+        // Whether eth_getTransactionReceipt confirms a transaction rather than
+        // answering "not mined yet".
+        seedReceipt: false,
         // Every raw signed transaction handed to eth_sendRawTransaction, in
         // order. The dApp transaction round trip asserts against these bytes
         // rather than against anything the extension reported about them.

tests/transferAmount.test.js

@@ -0,0 +1,128 @@
+// The scale an ERC-20 transfer from the wallet's own Send screen is encoded
+// with (issue #305). The screen renders from the block explorer's cached
+// decimals; the transfer used to be encoded from decimals() read off the
+// contract at signing time, with nothing comparing the two, so a token whose
+// on-chain scale differed signed an amount that was never displayed.
+
+const { parseUnits } = require("ethers");
+const {
+    displayedDecimals,
+    transferAmountUnits,
+    MAX_DECIMALS,
+    UNKNOWN_DISPLAYED_DECIMALS_MESSAGE,
+    UNREADABLE_CONTRACT_DECIMALS_MESSAGE,
+} = require("../src/shared/transferAmount");
+
+describe("displayedDecimals", () => {
+    test("accepts what the explorer and the contract each answer with", () => {
+        // A string is what fetchTokenBalances() parses out of Blockscout, a
+        // number is what it stores, and a bigint is what ethers hands back
+        // from a uint8 return.
+        expect(displayedDecimals("6")).toBe(6);
+        expect(displayedDecimals(6)).toBe(6);
+        expect(displayedDecimals(6n)).toBe(6);
+        expect(displayedDecimals(0)).toBe(0);
+        expect(displayedDecimals(MAX_DECIMALS)).toBe(MAX_DECIMALS);
+    });
+
+    test("refuses anything that is not a uint8", () => {
+        for (const bad of [
+            null,
+            undefined,
+            "",
+            "eighteen",
+            NaN,
+            6.5,
+            -1,
+            MAX_DECIMALS + 1,
+            true,
+            {},
+            [],
+        ]) {
+            expect(() => displayedDecimals(bad)).toThrow(
+                UNKNOWN_DISPLAYED_DECIMALS_MESSAGE,
+            );
+        }
+    });
+});
+
+describe("transferAmountUnits", () => {
+    test("encodes with the displayed scale when the contract agrees", () => {
+        expect(transferAmountUnits("0.25", 6, 6n)).toBe(parseUnits("0.25", 6));
+        expect(transferAmountUnits("0.25", "6", 6n)).toBe(
+            parseUnits("0.25", 6),
+        );
+        expect(transferAmountUnits("1.5", 18, 18n)).toBe(parseUnits("1.5", 18));
+    });
+
+    // The reproduction on the issue: 0.25 of a token displayed at 6 decimals,
+    // signed against a contract answering 18, moves 10^12 times the amount
+    // that was approved.
+    test("refuses the reproduction rather than signing either amount", () => {
+        expect(() => transferAmountUnits("0.25", 6, 18n)).toThrow(
+            /contract reports 18 decimal places, but the amount was displayed using 6/,
+        );
+    });
+
+    test("refuses a disagreement in the other direction too", () => {
+        expect(() => transferAmountUnits("0.25", 18, 6n)).toThrow(
+            /contract reports 6 decimal places, but the amount was displayed using 18/,
+        );
+    });
+
+    test("never returns the amount at either scale on a disagreement", () => {
+        // The point of the refusal: both candidate encodings exist, and the
+        // wallet must produce neither.
+        let thrown = null;
+        try {
+            transferAmountUnits("0.25", 6, 18n);
+        } catch (e) {
+            thrown = e;
+        }
+        expect(thrown).toBeInstanceOf(Error);
+        expect(thrown.message).toMatch(/was not sent/);
+    });
+
+    test("refuses when the screen's scale is unknown", () => {
+        expect(() => transferAmountUnits("0.25", null, 6n)).toThrow(
+            UNKNOWN_DISPLAYED_DECIMALS_MESSAGE,
+        );
+        expect(() => transferAmountUnits("0.25", undefined, 6n)).toThrow(
+            UNKNOWN_DISPLAYED_DECIMALS_MESSAGE,
+        );
+    });
+
+    test("refuses when the contract's answer is not a uint8", () => {
+        for (const bad of [null, undefined, "", "eighteen", 6.5, -1, 256]) {
+            expect(() => transferAmountUnits("0.25", 6, bad)).toThrow(
+                UNREADABLE_CONTRACT_DECIMALS_MESSAGE,
+            );
+        }
+    });
+
+    test("rejects an amount finer than the token's scale", () => {
+        // parseUnits' own refusal, reached only once the scales agree: a
+        // fractional base unit cannot be sent and must not be truncated.
+        expect(() => transferAmountUnits("0.0000001", 6, 6n)).toThrow();
+    });
+
+    test("every refusal is a full sentence", () => {
+        const messages = [];
+        for (const args of [
+            ["0.25", 6, 18n],
+            ["0.25", null, 6n],
+            ["0.25", 6, "eighteen"],
+        ]) {
+            try {
+                transferAmountUnits(...args);
+            } catch (e) {
+                messages.push(e.message);
+            }
+        }
+        expect(messages).toHaveLength(3);
+        for (const m of messages) {
+            expect(m).toMatch(/^[A-Z]/);
+            expect(m).toMatch(/\.$/);
+        }
+    });
+});