diff --git a/@shared/api/helpers/__tests__/deriveAuthKeypair.test.ts b/@shared/api/helpers/__tests__/deriveAuthKeypair.test.ts
new file mode 100644
index 0000000000..1e1dc8364c
--- /dev/null
+++ b/@shared/api/helpers/__tests__/deriveAuthKeypair.test.ts
@@ -0,0 +1,88 @@
+import { Buffer } from "buffer";
+
+import {
+  AUTH_SALT,
+  deriveAuthKeypair,
+  deriveAuthSeed,
+} from "../deriveAuthKeypair";
+import { AUTH_KEYPAIR_VECTORS } from "../authKeypairVectors";
+
+describe("deriveAuthSeed (HMAC step)", () => {
+  it("uses the versioned domain-separation salt", () => {
+    expect(AUTH_SALT).toBe("freighter-auth-v1");
+  });
+
+  it.each(AUTH_KEYPAIR_VECTORS)(
+    "reproduces the committed authSeed for %#",
+    async ({ mnemonic, authSeedHex }) => {
+      const seed = await deriveAuthSeed(mnemonic);
+      expect(seed).toHaveLength(32);
+      expect(Buffer.from(seed).toString("hex")).toBe(authSeedHex);
+    },
+  );
+});
+
+describe("deriveAuthKeypair", () => {
+  // Acceptance #1: same seed -> identical userId on extension and mobile.
+  it.each(AUTH_KEYPAIR_VECTORS)(
+    "derives the committed userId for %#",
+    async ({ mnemonic, userId }) => {
+      const result = await deriveAuthKeypair(mnemonic);
+      expect(result.userId).toBe(userId);
+    },
+  );
+
+  it("is deterministic for the same mnemonic", async () => {
+    const { mnemonic } = AUTH_KEYPAIR_VECTORS[0];
+    const a = await deriveAuthKeypair(mnemonic);
+    const b = await deriveAuthKeypair(mnemonic);
+    expect(a.userId).toBe(b.userId);
+    expect(a.keypair.rawPublicKey().equals(b.keypair.rawPublicKey())).toBe(
+      true,
+    );
+  });
+
+  it("emits a lowercase 64-char hex userId", async () => {
+    const { userId } = await deriveAuthKeypair(
+      AUTH_KEYPAIR_VECTORS[0].mnemonic,
+    );
+    expect(userId).toMatch(/^[0-9a-f]{64}$/);
+  });
+
+  // Acceptance #2: cryptographically independent from the wallet keypair.
+  // Wallet account-0 public key for AUTH_KEYPAIR_VECTORS[0]'s mnemonic, hex.
+  // Hardcoded verified constant — StellarHDWallet (the normal way to derive it)
+  // cannot run under jest/jsdom (compiled bip39 import breaks). This shows the
+  // auth key differs from the wallet key derived from the same seed.
+  it("differs from the wallet account-0 public key for the same mnemonic", async () => {
+    const WALLET_ACCOUNT_0_HEX =
+      "7691d85048acc4ed085d9061ce0948bbdf7de6a92b790aaf241d31b7dcaa4238";
+    const { userId } = await deriveAuthKeypair(
+      AUTH_KEYPAIR_VECTORS[0].mnemonic,
+    );
+    expect(userId).not.toBe(WALLET_ACCOUNT_0_HEX);
+  });
+
+  // Acceptance #3: no messaging side effects.
+  // Purity is guaranteed structurally: the module imports nothing from the
+  // messaging/keyManager/storage paths. This spy is a lightweight tripwire
+  // that would catch a future regression which started sending messages.
+  it("never sends an extension message during derivation", async () => {
+    const sendMessage = jest.fn();
+    (globalThis as unknown as { browser?: unknown }).browser = {
+      runtime: { sendMessage },
+    };
+    try {
+      await deriveAuthKeypair(AUTH_KEYPAIR_VECTORS[0].mnemonic);
+      expect(sendMessage).not.toHaveBeenCalled();
+    } finally {
+      delete (globalThis as unknown as { browser?: unknown }).browser;
+    }
+  });
+
+  it("throws on an invalid mnemonic instead of producing a key", async () => {
+    await expect(
+      deriveAuthKeypair("not a valid mnemonic phrase"),
+    ).rejects.toThrow(/invalid mnemonic/i);
+  });
+});
diff --git a/@shared/api/helpers/authKeypairVectors.ts b/@shared/api/helpers/authKeypairVectors.ts
new file mode 100644
index 0000000000..36d6f38793
--- /dev/null
+++ b/@shared/api/helpers/authKeypairVectors.ts
@@ -0,0 +1,28 @@
+// Canonical cross-platform auth-keypair derivation vectors.
+// SOURCE OF TRUTH: wallet-eng-monorepo design-docs/contact-lists/Freighter Auth Keypair Derivation Design Doc.md
+// freighter-mobile MUST commit the same values and assert identical output.
+// authSeedHex isolates failures: a wrong authSeedHex => HMAC step diverged
+// (e.g. key/message reversed); a right authSeedHex but wrong userId =>
+// Ed25519 step diverged.
+export interface AuthKeypairVector {
+  mnemonic: string;
+  authSeedHex: string; // hex of HMAC-SHA256(seedBytes, AUTH_SALT), 32 bytes
+  userId: string; // lowercase hex Ed25519 public key, 64 chars
+}
+
+export const AUTH_KEYPAIR_VECTORS: AuthKeypairVector[] = [
+  {
+    mnemonic:
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about",
+    authSeedHex:
+      "cf8ef34afb730ffd0807ee8731f2378a4f6c702e2f14915976fac4afa711b52d",
+    userId: "ec57e5d04783b5ade776621ab171b3b197c3acc0a1cb5bad10786dc8e381e797",
+  },
+  {
+    mnemonic:
+      "illness spike retreat truth genius clock brain pass fit cave bargain toe",
+    authSeedHex:
+      "882835b30a2c5b011f1b2424a84f4cd39f342f4d12be574e4233dbf9b98976d1",
+    userId: "bd9498475c7191c5e9a5e18edda2402ab0ae527580a6c38b2a32a77c65729cd7",
+  },
+];
diff --git a/@shared/api/helpers/deriveAuthKeypair.ts b/@shared/api/helpers/deriveAuthKeypair.ts
new file mode 100644
index 0000000000..2777a0667c
--- /dev/null
+++ b/@shared/api/helpers/deriveAuthKeypair.ts
@@ -0,0 +1,66 @@
+import { Buffer } from "buffer";
+import { mnemonicToSeedSync, validateMnemonic } from "bip39";
+import { Keypair } from "stellar-sdk";
+
+/**
+ * Versioned domain-separation salt for the backend auth keypair. The `-v1`
+ * suffix reserves a migration path; derivation is deterministic, so the auth
+ * keypair is permanent for the life of the seed.
+ */
+export const AUTH_SALT = "freighter-auth-v1";
+
+/**
+ * Computes the 32-byte auth seed: HMAC-SHA256 keyed on the wallet's 64-byte
+ * BIP39 seed, over the salt. Throws "Invalid mnemonic (see bip39)" on a
+ * malformed mnemonic.
+ *
+ * Implementation note: stellar-hd-wallet@1.0.2's fromMnemonic() is an ESM
+ * module whose internal bip39 default-import hits a Jest CJS interop edge case
+ * (bip39 sets __esModule:true without a .default export). We call bip39 named
+ * exports directly — bip39 is already a transitive dep of stellar-hd-wallet,
+ * and the derivation is byte-for-byte identical.
+ * @internal Exported only for tests (intermediate-value assertions). The
+ * returned bytes are private-key seed material — do not use as a standalone key.
+ */
+export const deriveAuthSeed = async (mnemonic: string): Promise<Uint8Array> => {
+  // Validate mnemonic first, matching stellar-hd-wallet's error contract.
+  if (!validateMnemonic(mnemonic)) {
+    throw new Error("Invalid mnemonic (see bip39)");
+  }
+  // BIP39 seed, 64 bytes, empty passphrase — identical to what StellarHDWallet
+  // .fromMnemonic(mnemonic).seedHex produces.
+  const seedBytes = Buffer.from(mnemonicToSeedSync(mnemonic));
+
+  const key = await crypto.subtle.importKey(
+    "raw",
+    seedBytes,
+    { name: "HMAC", hash: "SHA-256" },
+    false, // not extractable
+    ["sign"],
+  );
+  const mac = await crypto.subtle.sign(
+    "HMAC",
+    key,
+    new TextEncoder().encode(AUTH_SALT),
+  );
+  return new Uint8Array(mac);
+};
+
+/**
+ * Derives the Freighter backend auth keypair from the wallet mnemonic.
+ * Pure crypto: no logging, no keyManager, no messaging, no persistence. The
+ * caller supplies the mnemonic (requires an unlocked session) and handles the
+ * locked-session case.
+ *
+ * @returns userId  lowercase hex Ed25519 public key (64 chars) — the anonymous
+ *                  backend user ID and the JWT `sub`.
+ * @returns keypair stellar-sdk Keypair; the JWT ticket signs with keypair.sign().
+ */
+export const deriveAuthKeypair = async (
+  mnemonic: string,
+): Promise<{ userId: string; keypair: Keypair }> => {
+  const authSeed = await deriveAuthSeed(mnemonic);
+  const keypair = Keypair.fromRawEd25519Seed(Buffer.from(authSeed));
+  const userId = keypair.rawPublicKey().toString("hex");
+  return { userId, keypair };
+};
diff --git a/@shared/api/package.json b/@shared/api/package.json
index abb8f8423a..d4994e36ab 100644
--- a/@shared/api/package.json
+++ b/@shared/api/package.json
@@ -7,6 +7,7 @@
     "@blockaid/client": "0.31.0",
     "@stellar/js-xdr": "4.0.0",
     "bignumber.js": "9.3.0",
+    "bip39": "3.1.0",
     "prettier": "3.8.4",
     "stellar-sdk": "npm:@stellar/stellar-sdk@16.0.0-rc.1",
     "stellar-sdk-next": "npm:@stellar/stellar-sdk@16.0.0-rc.1",
diff --git a/extension/src/popup/locales/en/translation.json b/extension/src/popup/locales/en/translation.json
index 52339ec4bb..0ddd30cdd0 100644
--- a/extension/src/popup/locales/en/translation.json
+++ b/extension/src/popup/locales/en/translation.json
@@ -77,6 +77,8 @@
   "Authorizations": "Authorizations",
   "Authorize": "Authorize",
   "Authorized address": "Authorized address",
+  "Auto-lock timer": "Auto-lock timer",
+  "Auto-Lock Timer": "Auto-Lock Timer",
   "available": "available",
   "Back": "Back",
   "Balance": "Balance",
diff --git a/extension/src/popup/locales/pt/translation.json b/extension/src/popup/locales/pt/translation.json
index db2c569c1c..a0cb327877 100644
--- a/extension/src/popup/locales/pt/translation.json
+++ b/extension/src/popup/locales/pt/translation.json
@@ -77,6 +77,8 @@
   "Authorizations": "Autorizações",
   "Authorize": "Autorizar",
   "Authorized address": "Endereço autorizado",
+  "Auto-lock timer": "Auto-lock timer",
+  "Auto-Lock Timer": "Auto-Lock Timer",
   "available": "disponível",
   "Back": "Voltar",
   "Balance": "Saldo",
diff --git a/yarn.lock b/yarn.lock
index 35b6640e3d..00f3f7ce3a 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -5429,6 +5429,7 @@ __metadata:
     "@lavamoat/allow-scripts": "npm:3.3.2"
     "@stellar/js-xdr": "npm:4.0.0"
     bignumber.js: "npm:9.3.0"
+    bip39: "npm:3.1.0"
     prettier: "npm:3.8.4"
     stellar-sdk: "npm:@stellar/stellar-sdk@16.0.0-rc.1"
     stellar-sdk-next: "npm:@stellar/stellar-sdk@16.0.0-rc.1"