test: known-answer coverage for HD derivation and the vault (closes #159)

wallet.js and vault.js — the two modules that hold user funds — had no
derivation or encryption tests. Add them, pinned to published vectors
rather than to whatever the implementation returns today.

wallet.js: hdWalletFromMnemonic, hdWalletFromXprv, deriveAddressFromXpub
and getSignerForAddress are pinned to the standard development recovery
phrase's first three accounts at m/44'/60'/0'/0/n and to the BIP-39
all-zero-entropy phrase's first address; addressFromPrivateKey is pinned
to the published key/address pairs, so the HD path and the bare-key path
must meet at the same address from two directions. isValidMnemonic and
isValidXprv cover bad checksum, wrong word count, wrong key type and
empty/garbage input. The absolute-vs-relative path asymmetry between
hdWalletFromMnemonic and hdWalletFromXprv is proven harmless: for the
same master key both reach the same xpub and the same addresses.

vault.js: round trip (including non-ASCII and an empty password), wrong
password rejected as a rejected promise with no partial plaintext,
tampered ciphertext / auth tag / nonce / salt rejected, truncated and
spliced blobs rejected, missing fields rejected, fresh salt and nonce per
encryption, the documented { salt, nonce, ciphertext } shape, and no
trace of the plaintext or password anywhere in the serialized blob. The
Argon2id cost parameters are pinned three ways — the INTERACTIVE
constants still mean 2 passes over 64 MiB, a key independently derived at
that cost opens the vault, and both encrypt and decrypt are observed
calling crypto_pwhash with those constants — because the KDF cost is the
vault's only defence against offline attack on a stolen blob and nothing
else in the suite would notice it being lowered. The tamper cases share
one encrypted fixture to stay inside script/test's 30-second budget.

One test is skipped: isValidXprv accepts an extended private key with a
one-character typo, because ethers skips base58 checksum verification for
the usual 82-byte payload. That defect is tracked separately and is not
fixed here; the skipped test asserts the correct behaviour and cites the
issue.

README.md

@@ -1173,8 +1173,8 @@ Currently supported:
 
 ### Testing
 
-- [ ] Tests for mnemonic generation and address derivation
-- [ ] Tests for xpub derivation and child address generation
+- [x] Tests for mnemonic generation and address derivation
+- [x] Tests for xpub derivation and child address generation
 - [ ] Test on Firefox (Manifest V2)
 
 ### Scam List

TODO.md

@@ -44,6 +44,9 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: Known-answer test coverage for the crypto core — BIP-39/BIP-32
+  derivation in `wallet.js` and the Argon2id vault in `vault.js`
+  ([#159](https://git.eeqj.de/sneak/AutistMask/issues/159)).
 - 2026-08-11: Three `README.md` claims corrected against the code — blocklist
   attribution, token-display rule, navigation model
   ([#213](https://git.eeqj.de/sneak/AutistMask/issues/213)).

tests/vault.test.js

@@ -0,0 +1,346 @@
+// Tests for src/shared/vault.js: the Argon2id + XSalsa20-Poly1305 encryption
+// that protects recovery phrases and private keys at rest.
+//
+// The properties that matter here are the ones whose failure is silent. A
+// vault that decrypts under the wrong password, that hands back plaintext from
+// a ciphertext an attacker edited, that reuses a nonce, or that leaves the
+// recovery phrase readable somewhere in the stored blob all look exactly like
+// a working vault from the UI. So each test below asserts a negative: the
+// thing that must not happen.
+//
+// Cost: every encrypt and decrypt runs one Argon2id pwhash at the production
+// interactive parameters, which the module hardcodes. The parameters are not
+// weakened or overridden anywhere in this file — they are pinned by the "key
+// derivation cost" tests, since they are the vault's only defence against an
+// offline attack on a stolen blob. The suite is kept inside script/test's
+// 30-second budget by sharing one encrypted fixture across the tamper cases
+// instead of re-encrypting per test.
+
+const sodium = require("libsodium-wrappers-sumo");
+const {
+    encryptWithPassword,
+    decryptWithPassword,
+} = require("../src/shared/vault");
+
+// A publicly known development phrase. Never fund it.
+const SECRET = "test test test test test test test test test test test junk";
+const PASSWORD = "correct horse battery staple";
+const WRONG_PASSWORD = "correct horse battery stapl";
+
+const SALT_BYTES = 16;
+const NONCE_BYTES = 24;
+const POLY1305_TAG_BYTES = 16;
+
+const BASE64 = /^[A-Za-z0-9+/_-]+={0,2}$/;
+
+function b64decode(s) {
+    return sodium.from_base64(s);
+}
+
+// A shallow copy with one field replaced, so the shared fixture is never
+// mutated by a tamper test.
+function withField(blob, field, value) {
+    return { ...blob, [field]: value };
+}
+
+// Flip the low bit of one byte of a base64-encoded field.
+function flipByte(b64, index) {
+    const bytes = b64decode(b64);
+    bytes[index] ^= 0x01;
+    return sodium.to_base64(bytes);
+}
+
+let vault;
+
+beforeAll(async () => {
+    await sodium.ready;
+    vault = await encryptWithPassword(SECRET, PASSWORD);
+});
+
+describe("stored blob shape", () => {
+    test("is exactly the documented { salt, nonce, ciphertext }", () => {
+        expect(Object.keys(vault).sort()).toEqual([
+            "ciphertext",
+            "nonce",
+            "salt",
+        ]);
+    });
+
+    test("every field is a base64 string", () => {
+        for (const field of ["salt", "nonce", "ciphertext"]) {
+            expect(typeof vault[field]).toBe("string");
+            expect(vault[field]).toMatch(BASE64);
+        }
+    });
+
+    test("salt and nonce are full length", () => {
+        expect(b64decode(vault.salt)).toHaveLength(SALT_BYTES);
+        expect(b64decode(vault.nonce)).toHaveLength(NONCE_BYTES);
+    });
+
+    test("ciphertext carries a Poly1305 authentication tag", () => {
+        expect(b64decode(vault.ciphertext)).toHaveLength(
+            SECRET.length + POLY1305_TAG_BYTES,
+        );
+    });
+
+    test("the blob survives JSON storage unchanged", async () => {
+        const stored = JSON.parse(JSON.stringify(vault));
+
+        await expect(decryptWithPassword(stored, PASSWORD)).resolves.toBe(
+            SECRET,
+        );
+    });
+});
+
+describe("no plaintext leakage", () => {
+    test("the secret does not appear in the serialized vault", () => {
+        const serialized = JSON.stringify(vault);
+
+        expect(serialized).not.toContain(SECRET);
+        for (const word of new Set(SECRET.split(" "))) {
+            expect(serialized).not.toContain(word);
+        }
+    });
+
+    test("the ciphertext bytes do not contain the secret bytes", () => {
+        const bytes = Buffer.from(b64decode(vault.ciphertext));
+
+        expect(bytes.includes(Buffer.from(SECRET, "utf8"))).toBe(false);
+        // Not even the first word, which would betray an unencrypted prefix.
+        expect(bytes.includes(Buffer.from("test test", "utf8"))).toBe(false);
+    });
+
+    test("the password does not appear in the serialized vault", () => {
+        expect(JSON.stringify(vault)).not.toContain(PASSWORD);
+    });
+});
+
+describe("round trip", () => {
+    test("decrypts back to the original secret", async () => {
+        await expect(decryptWithPassword(vault, PASSWORD)).resolves.toBe(
+            SECRET,
+        );
+    });
+
+    test("survives a non-ASCII plaintext byte for byte", async () => {
+        const unicode = "recovery phrase é中文\u{1f600}";
+
+        const blob = await encryptWithPassword(unicode, PASSWORD);
+
+        await expect(decryptWithPassword(blob, PASSWORD)).resolves.toBe(
+            unicode,
+        );
+    });
+
+    test("an empty password still round-trips and is not a bypass", async () => {
+        const blob = await encryptWithPassword(SECRET, "");
+
+        await expect(decryptWithPassword(blob, "")).resolves.toBe(SECRET);
+        // An empty password must not act as a skeleton key on other vaults,
+        // nor may a real password open an empty-password vault.
+        await expect(decryptWithPassword(vault, "")).rejects.toThrow();
+        await expect(decryptWithPassword(blob, PASSWORD)).rejects.toThrow();
+    });
+});
+
+describe("fresh salt and nonce", () => {
+    test("two encryptions of the same plaintext differ in all three fields", async () => {
+        const second = await encryptWithPassword(SECRET, PASSWORD);
+
+        expect(second.salt).not.toBe(vault.salt);
+        expect(second.nonce).not.toBe(vault.nonce);
+        expect(second.ciphertext).not.toBe(vault.ciphertext);
+        await expect(decryptWithPassword(second, PASSWORD)).resolves.toBe(
+            SECRET,
+        );
+    });
+});
+
+describe("key derivation cost", () => {
+    // Argon2id's opslimit and memlimit are the whole of the vault's resistance
+    // to an offline attack on a stolen blob, and lowering them breaks nothing
+    // any other test here can see — the suite merely runs faster. So pin them
+    // directly, both to libsodium's INTERACTIVE constants and to the absolute
+    // values those constants must keep meaning.
+    const INTERACTIVE_OPSLIMIT = 2;
+    const INTERACTIVE_MEMLIMIT = 64 * 1024 * 1024;
+
+    test("the interactive constants still mean 2 passes over 64 MiB", () => {
+        expect(sodium.crypto_pwhash_OPSLIMIT_INTERACTIVE).toBe(
+            INTERACTIVE_OPSLIMIT,
+        );
+        expect(sodium.crypto_pwhash_MEMLIMIT_INTERACTIVE).toBe(
+            INTERACTIVE_MEMLIMIT,
+        );
+        // The floor these must never quietly be swapped for: _MIN is one pass
+        // over 8 KiB, an 8192x reduction in memory cost.
+        expect(sodium.crypto_pwhash_OPSLIMIT_MIN).toBeLessThan(
+            INTERACTIVE_OPSLIMIT,
+        );
+        expect(sodium.crypto_pwhash_MEMLIMIT_MIN).toBeLessThan(
+            INTERACTIVE_MEMLIMIT,
+        );
+    });
+
+    test("a key derived at the interactive parameters opens the vault", () => {
+        // Independent of any spy, and of the module's own code path: derive
+        // the key here from the vault's published salt at the interactive cost
+        // and open its ciphertext directly. A vault whose key came from any
+        // other opslimit, memlimit or Argon2id variant yields a different key
+        // and cannot be opened this way.
+        const key = sodium.crypto_pwhash(
+            sodium.crypto_secretbox_KEYBYTES,
+            PASSWORD,
+            b64decode(vault.salt),
+            INTERACTIVE_OPSLIMIT,
+            INTERACTIVE_MEMLIMIT,
+            sodium.crypto_pwhash_ALG_ARGON2ID13,
+        );
+        const opened = sodium.crypto_secretbox_open_easy(
+            b64decode(vault.ciphertext),
+            b64decode(vault.nonce),
+            key,
+        );
+
+        expect(sodium.to_string(opened)).toBe(SECRET);
+    });
+
+    test.each([
+        [
+            "encrypt",
+            async () => {
+                await encryptWithPassword(SECRET, PASSWORD);
+            },
+        ],
+        [
+            "decrypt",
+            async () => {
+                await decryptWithPassword(vault, PASSWORD);
+            },
+        ],
+    ])("%s derives exactly one key at the interactive cost", async (_, run) => {
+        const spy = jest.spyOn(sodium, "crypto_pwhash");
+        try {
+            await run();
+
+            expect(spy).toHaveBeenCalledTimes(1);
+            const [keyBytes, , salt, opslimit, memlimit, alg] =
+                spy.mock.calls[0];
+            expect(keyBytes).toBe(sodium.crypto_secretbox_KEYBYTES);
+            expect(salt).toHaveLength(SALT_BYTES);
+            expect(opslimit).toBe(sodium.crypto_pwhash_OPSLIMIT_INTERACTIVE);
+            expect(memlimit).toBe(sodium.crypto_pwhash_MEMLIMIT_INTERACTIVE);
+            expect(alg).toBe(sodium.crypto_pwhash_ALG_ARGON2ID13);
+        } finally {
+            spy.mockRestore();
+        }
+    });
+});
+
+describe("wrong password", () => {
+    test("is rejected, and rejects cleanly", async () => {
+        // rejects.toThrow asserts a rejected promise, not a synchronous throw
+        // and not an unhandled rejection: the caller can catch this.
+        await expect(
+            decryptWithPassword(vault, WRONG_PASSWORD),
+        ).rejects.toThrow();
+    });
+
+    test("returns no plaintext, not even partially", async () => {
+        const result = await decryptWithPassword(vault, WRONG_PASSWORD).catch(
+            (err) => err,
+        );
+
+        expect(result).toBeInstanceOf(Error);
+        expect(String(result)).not.toContain("test");
+    });
+
+    test("the empty password is rejected on a password-protected vault", async () => {
+        await expect(decryptWithPassword(vault, "")).rejects.toThrow();
+    });
+});
+
+describe("tampering", () => {
+    test("a flipped ciphertext bit is rejected by the auth tag", async () => {
+        const tampered = withField(
+            vault,
+            "ciphertext",
+            flipByte(vault.ciphertext, 0),
+        );
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a flipped bit in the authentication tag itself is rejected", async () => {
+        const tagStart = b64decode(vault.ciphertext).length - 1;
+        const tampered = withField(
+            vault,
+            "ciphertext",
+            flipByte(vault.ciphertext, tagStart),
+        );
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a flipped nonce bit is rejected", async () => {
+        const tampered = withField(vault, "nonce", flipByte(vault.nonce, 0));
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a flipped salt bit is rejected", async () => {
+        const tampered = withField(vault, "salt", flipByte(vault.salt, 0));
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a truncated ciphertext is rejected", async () => {
+        const bytes = b64decode(vault.ciphertext);
+        const tampered = withField(
+            vault,
+            "ciphertext",
+            sodium.to_base64(bytes.slice(0, bytes.length - 4)),
+        );
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a ciphertext shorter than the auth tag is rejected", async () => {
+        const tampered = withField(
+            vault,
+            "ciphertext",
+            sodium.to_base64(b64decode(vault.ciphertext).slice(0, 4)),
+        );
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a truncated nonce is rejected", async () => {
+        const tampered = withField(
+            vault,
+            "nonce",
+            sodium.to_base64(b64decode(vault.nonce).slice(0, NONCE_BYTES - 1)),
+        );
+
+        await expect(decryptWithPassword(tampered, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a ciphertext from another vault is rejected", async () => {
+        const other = await encryptWithPassword("a different secret", PASSWORD);
+        const spliced = withField(vault, "ciphertext", other.ciphertext);
+
+        await expect(decryptWithPassword(spliced, PASSWORD)).rejects.toThrow();
+    });
+
+    test("a missing field is rejected rather than decrypted", async () => {
+        for (const field of ["salt", "nonce", "ciphertext"]) {
+            const broken = { ...vault };
+            delete broken[field];
+
+            await expect(
+                decryptWithPassword(broken, PASSWORD),
+            ).rejects.toThrow();
+        }
+    });
+});

tests/wallet.test.js

@@ -1,4 +1,6 @@
-// Tests for the DEBUG build flag as it gates mnemonic generation.
+// Tests for src/shared/wallet.js: the DEBUG build flag as it gates mnemonic
+// generation (first two describes), and HD key derivation against published
+// known-answer vectors (rest of the file).
 //
 // The modules read the __BUILD_DEBUG__ global that esbuild replaces at bundle
 // time. Under jest the global is absent, which is exactly the release-build
@@ -92,3 +94,317 @@ describe("generateMnemonic in a debug build", () => {
         );
     });
 });
+
+// ---------------------------------------------------------------------------
+// Key derivation.
+//
+// Every address below is a published constant, not something this codebase
+// produced. Asserting against what the implementation happens to return today
+// would pass just as happily with the wrong coin type, the wrong path depth or
+// a non-empty seed passphrase, all of which silently send funds to addresses
+// no other wallet can recover.
+//
+// Vector sources:
+//
+//   VECTOR_PHRASE / VECTOR_ADDRESSES / VECTOR_PRIVATE_KEYS — the standard
+//   development recovery phrase and the first three accounts it yields at
+//   m/44'/60'/0'/0/n with an empty seed passphrase, as published in the
+//   Hardhat and Ganache documentation. Publicly known; never fund it.
+//
+//   ZERO_ENTROPY_PHRASE / ZERO_ENTROPY_ADDRESS — the BIP-39 all-zero-entropy
+//   phrase (Trezor's official BIP-39 vector set, first entry) and its
+//   m/44'/60'/0'/0/0 Ethereum address with an empty seed passphrase. A second,
+//   independently published phrase so the pin is not one vector deep.
+//
+//   BIP32_VECTOR_1_XPRV — the master key of BIP-32 test vector 1
+//   (seed 000102030405060708090a0b0c0d0e0f).
+//
+// The two Hardhat facts cross-check each other: VECTOR_PRIVATE_KEYS[n] is the
+// published key for VECTOR_ADDRESSES[n], so addressFromPrivateKey and the HD
+// path must meet at the same address from two different directions.
+
+const { HDNodeWallet, Mnemonic, verifyMessage } = require("ethers");
+const wallet = require("../src/shared/wallet");
+const { BIP44_ETH_PATH } = require("../src/shared/constants");
+
+const VECTOR_PHRASE =
+    "test test test test test test test test test test test junk";
+
+const VECTOR_ADDRESSES = [
+    "0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266",
+    "0x70997970C51812dc3A010C7d01b50e0d17dc79C8",
+    "0x3C44CdDdB6a900fa2b585dd299e03d12FA4293BC",
+];
+
+const VECTOR_PRIVATE_KEYS = [
+    "0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80",
+    "0x59c6995e998f97a5a0044966f0945389dc9e86dae88c7a8412f4603b6b78690d",
+    "0x5de4111afa1a4b94908f83103eb1f1706367c2e68ca870fc3fb9a804cdab365a",
+];
+
+const ZERO_ENTROPY_PHRASE =
+    "abandon abandon abandon abandon abandon abandon " +
+    "abandon abandon abandon abandon abandon about";
+const ZERO_ENTROPY_ADDRESS = "0x9858EfFD232B4033E47d90003D41EC34EcaEda94";
+
+const BIP32_VECTOR_1_XPRV =
+    "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqji" +
+    "ChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi";
+
+// The master (depth-0) extended private key for a phrase, which is what the
+// import-an-xprv flow is handed. Built with ethers rather than with the module
+// under test, so hdWalletFromXprv is not being checked against itself.
+function masterXprv(phrase, passphrase = "") {
+    return HDNodeWallet.fromSeed(
+        Mnemonic.fromPhrase(phrase, passphrase).computeSeed(),
+    ).extendedKey;
+}
+
+describe("hdWalletFromMnemonic", () => {
+    test("first address matches the published vector for m/44'/60'/0'/0/0", () => {
+        expect(wallet.hdWalletFromMnemonic(VECTOR_PHRASE).firstAddress).toBe(
+            VECTOR_ADDRESSES[0],
+        );
+    });
+
+    test("second published phrase derives its published address", () => {
+        expect(
+            wallet.hdWalletFromMnemonic(ZERO_ENTROPY_PHRASE).firstAddress,
+        ).toBe(ZERO_ENTROPY_ADDRESS);
+    });
+
+    test("returns the account-level xpub, which is watch-only", () => {
+        const { xpub } = wallet.hdWalletFromMnemonic(VECTOR_PHRASE);
+
+        expect(xpub.startsWith("xpub")).toBe(true);
+        // A neutered ethers node exposes no private key at all, so accept
+        // either absent or null rather than pinning which.
+        expect(
+            HDNodeWallet.fromExtendedKey(xpub).privateKey ?? null,
+        ).toBeNull();
+        expect(wallet.isValidXprv(xpub)).toBe(false);
+    });
+
+    test("the account path is the documented BIP-44 Ethereum path", () => {
+        expect(BIP44_ETH_PATH).toBe("m/44'/60'/0'/0");
+    });
+
+    test("rejects an invalid recovery phrase rather than deriving from it", () => {
+        expect(() => wallet.hdWalletFromMnemonic("not a phrase")).toThrow();
+    });
+});
+
+describe("deriveAddressFromXpub", () => {
+    const { xpub } = wallet.hdWalletFromMnemonic(VECTOR_PHRASE);
+
+    test.each([0, 1, 2])(
+        "child %i matches the published vector address",
+        (index) => {
+            expect(wallet.deriveAddressFromXpub(xpub, index)).toBe(
+                VECTOR_ADDRESSES[index],
+            );
+        },
+    );
+
+    test("agrees with hdWalletFromMnemonic at index 0", () => {
+        expect(wallet.deriveAddressFromXpub(xpub, 0)).toBe(
+            wallet.hdWalletFromMnemonic(VECTOR_PHRASE).firstAddress,
+        );
+    });
+
+    test("rejects garbage instead of returning an address", () => {
+        expect(() =>
+            wallet.deriveAddressFromXpub("xpub-nonsense", 0),
+        ).toThrow();
+    });
+});
+
+describe("hdWalletFromMnemonic seed passphrase handling", () => {
+    // The vectors above are only reproducible with an empty BIP-39 seed
+    // passphrase. This pins that the empty string reaching
+    // HDNodeWallet.fromPhrase is load-bearing: with any passphrase applied the
+    // published address is unreachable, and a wallet derived that way could
+    // not be restored anywhere else from the phrase alone.
+    test("a non-empty seed passphrase would yield a different address", () => {
+        const withPassphrase = HDNodeWallet.fromPhrase(
+            VECTOR_PHRASE,
+            "TREZOR",
+            BIP44_ETH_PATH,
+        ).deriveChild(0).address;
+
+        expect(withPassphrase).not.toBe(VECTOR_ADDRESSES[0]);
+    });
+});
+
+describe("hdWalletFromXprv", () => {
+    // hdWalletFromMnemonic derives the absolute path "m/44'/60'/0'/0" while
+    // hdWalletFromXprv derives the relative path "44'/60'/0'/0". For a
+    // depth-0 master key the two are the same derivation; these tests pin that
+    // equivalence to a published address rather than assuming it.
+    test("master xprv for the vector phrase yields the vector address", () => {
+        expect(
+            wallet.hdWalletFromXprv(masterXprv(VECTOR_PHRASE)).firstAddress,
+        ).toBe(VECTOR_ADDRESSES[0]);
+    });
+
+    test("agrees with hdWalletFromMnemonic on xpub and address", () => {
+        const fromPhrase = wallet.hdWalletFromMnemonic(VECTOR_PHRASE);
+        const fromXprv = wallet.hdWalletFromXprv(masterXprv(VECTOR_PHRASE));
+
+        expect(fromXprv).toEqual(fromPhrase);
+    });
+
+    test("derived xpub generates the same child addresses", () => {
+        const { xpub } = wallet.hdWalletFromXprv(masterXprv(VECTOR_PHRASE));
+
+        expect(
+            [0, 1, 2].map((i) => wallet.deriveAddressFromXpub(xpub, i)),
+        ).toEqual(VECTOR_ADDRESSES);
+    });
+
+    test("accepts the BIP-32 test vector 1 master key", () => {
+        const { xpub, firstAddress } =
+            wallet.hdWalletFromXprv(BIP32_VECTOR_1_XPRV);
+
+        expect(xpub.startsWith("xpub")).toBe(true);
+        expect(firstAddress).toMatch(/^0x[0-9a-fA-F]{40}$/);
+    });
+
+    test("rejects a watch-only xpub", () => {
+        const { xpub } = wallet.hdWalletFromMnemonic(VECTOR_PHRASE);
+
+        expect(() => wallet.hdWalletFromXprv(xpub)).toThrow();
+    });
+
+    test("rejects garbage", () => {
+        expect(() => wallet.hdWalletFromXprv("nonsense")).toThrow();
+    });
+});
+
+describe("isValidXprv", () => {
+    test.each([
+        ["BIP-32 test vector 1 master key", BIP32_VECTOR_1_XPRV, true],
+        ["the empty string", "", false],
+        ["garbage", "not-a-key", false],
+        ["a bare private key", VECTOR_PRIVATE_KEYS[0], false],
+        ["a truncated xprv", BIP32_VECTOR_1_XPRV.slice(0, -6), false],
+        ["an xprv with an extra character", BIP32_VECTOR_1_XPRV + "a", false],
+    ])("%s -> %s", (_name, key, expected) => {
+        expect(wallet.isValidXprv(key)).toBe(expected);
+    });
+
+    test("a watch-only xpub is not an xprv", () => {
+        const { xpub } = wallet.hdWalletFromMnemonic(VECTOR_PHRASE);
+
+        expect(wallet.isValidXprv(xpub)).toBe(false);
+    });
+
+    // Skipped: this asserts the correct behaviour, which the code does not
+    // currently have. isValidXprv gates the paste-your-extended-private-key
+    // import in src/popup/views/addWallet.js:215, and it accepts a key with a
+    // one-character typo: ethers' HDNodeWallet.fromExtendedKey skips base58
+    // checksum verification whenever the decoded payload is the usual 82
+    // bytes, which is the whole point of that checksum. Measured on this
+    // vector: changing any one of the last 14 characters passes validation,
+    // and for 9 of those 14 positions the import silently yields a *different*
+    // wallet (e.g. 0x3F334f0a356d6B46B1d70B590E7437D77100d28D instead of
+    // 0x022b971dFF0C43305e691DEd7a14367AF19D6407) with no error shown.
+    // Tracked as https://git.eeqj.de/sneak/AutistMask/issues/210; out of scope
+    // here, which is tests only. Unskip when it is fixed.
+    test.skip("rejects an extended key with a one-character typo", () => {
+        const index = BIP32_VECTOR_1_XPRV.length - 8;
+        const typo =
+            BIP32_VECTOR_1_XPRV.slice(0, index) +
+            (BIP32_VECTOR_1_XPRV[index] === "a" ? "b" : "a") +
+            BIP32_VECTOR_1_XPRV.slice(index + 1);
+
+        expect(wallet.isValidXprv(typo)).toBe(false);
+    });
+});
+
+describe("isValidMnemonic", () => {
+    test.each([
+        ["the vector phrase", VECTOR_PHRASE, true],
+        ["the BIP-39 zero-entropy phrase", ZERO_ENTROPY_PHRASE, true],
+        [
+            "a 12-word phrase with a bad checksum",
+            "abandon abandon abandon abandon abandon abandon " +
+                "abandon abandon abandon abandon abandon abandon",
+            false,
+        ],
+        ["an 11-word phrase", "abandon ".repeat(10) + "about", false],
+        ["a word outside the wordlist", VECTOR_PHRASE + " zzzzzz", false],
+        ["the empty string", "", false],
+        ["garbage", "correct horse battery staple", false],
+    ])("%s -> %s", (_name, phrase, expected) => {
+        expect(wallet.isValidMnemonic(phrase)).toBe(expected);
+    });
+});
+
+describe("addressFromPrivateKey", () => {
+    test.each([0, 1, 2])(
+        "published key %i yields its published address",
+        (index) => {
+            expect(
+                wallet.addressFromPrivateKey(VECTOR_PRIVATE_KEYS[index]),
+            ).toBe(VECTOR_ADDRESSES[index]);
+        },
+    );
+
+    test("rejects a key of the wrong length", () => {
+        expect(() => wallet.addressFromPrivateKey("0xdeadbeef")).toThrow();
+    });
+
+    test("rejects the empty string", () => {
+        expect(() => wallet.addressFromPrivateKey("")).toThrow();
+    });
+});
+
+describe("getSignerForAddress", () => {
+    test.each([0, 1, 2])("hd wallet, address index %i", (index) => {
+        const signer = wallet.getSignerForAddress(
+            { type: "hd" },
+            index,
+            VECTOR_PHRASE,
+        );
+
+        expect(signer.address).toBe(VECTOR_ADDRESSES[index]);
+        expect(signer.privateKey).toBe(VECTOR_PRIVATE_KEYS[index]);
+    });
+
+    test.each([0, 1, 2])("xprv wallet, address index %i", (index) => {
+        const signer = wallet.getSignerForAddress(
+            { type: "xprv" },
+            index,
+            masterXprv(VECTOR_PHRASE),
+        );
+
+        expect(signer.address).toBe(VECTOR_ADDRESSES[index]);
+        expect(signer.privateKey).toBe(VECTOR_PRIVATE_KEYS[index]);
+    });
+
+    test("single private key ignores the address index", () => {
+        for (const index of [0, 1, 2]) {
+            const signer = wallet.getSignerForAddress(
+                { type: "privkey" },
+                index,
+                VECTOR_PRIVATE_KEYS[1],
+            );
+
+            expect(signer.address).toBe(VECTOR_ADDRESSES[1]);
+        }
+    });
+
+    test("the returned signer signs recoverably as the expected address", async () => {
+        const signer = wallet.getSignerForAddress(
+            { type: "hd" },
+            1,
+            VECTOR_PHRASE,
+        );
+        const message = "AutistMask derivation test";
+
+        const signature = await signer.signMessage(message);
+
+        expect(verifyMessage(message, signature)).toBe(VECTOR_ADDRESSES[1]);
+    });
+});