diff --git a/README.md b/README.md
index 86f61d3..3f4fdbf 100644
--- a/README.md
+++ b/README.md
@@ -606,16 +606,16 @@ Phase 2: crypto primitives
 
 Phase 3: SRP + auth
 
-- [ ] SRP-6a client using `secure-remote-password` with the same group as the
-      server
-- [ ] `beginLogin(email, password)` returning a `LoginChallenge`
-- [ ] `requestEmailOTP` and `submitEmailOTP` for accounts without SRP
-- [ ] `submitTOTP(sessionID, code)`
+- [x] SRP-6a client using `fast-srp-hap` with the 4096-bit group (matching the
+      upstream Ente web client)
+- [x] `beginLogin(api, email, password)` returning a `LoginChallenge`
+- [x] `requestEmailOTP` and `submitEmailOTP` for accounts without SRP
+- [x] `submitTOTP(api, sessionID, code)`
 - [x] `unwrapAuth(response, password)` returning master key, secret key, public
       key, and decrypted token (URL-safe-no-padding base64)
 - [x] `src/auth/types.ts` with `KeyAttributes`, `SRPAttributes`,
       `AuthorizationResponse`, and `LoginChallenge`
-- [ ] Tests against recorded HTTP fixtures
+- [x] Tests with mock SRP server performing real 4096-bit math end-to-end
 
 Phase 4: HTTP client + endpoints
 
diff --git a/package.json b/package.json
index 5ae29a9..21e4328 100644
--- a/package.json
+++ b/package.json
@@ -38,6 +38,7 @@
         "vitest": "2.1.9"
     },
     "dependencies": {
+        "fast-srp-hap": "2.0.4",
         "libsodium-wrappers-sumo": "0.8.4"
     }
 }
diff --git a/src/auth/login.ts b/src/auth/login.ts
new file mode 100644
index 0000000..5a53f42
--- /dev/null
+++ b/src/auth/login.ts
@@ -0,0 +1,130 @@
+import { SRP, SrpClient } from "fast-srp-hap";
+import type { ApiClient } from "../api/client.js";
+import { deriveKEK, deriveLoginSubkey, fromBase64 } from "../crypto/index.js";
+import type {
+    AuthorizationResponse,
+    LoginChallenge,
+    SRPAttributes,
+} from "./types.js";
+
+// Ente uses the 4096-bit SRP group. Both client and server must agree.
+const SRP_PARAMS = SRP.params["4096"];
+
+// Wrap fast-srp-hap's callback-based genKey into a Promise.
+const genKey = (): Promise<Buffer> => SRP.genKey();
+
+// Create an SRP client from the derived login subkey and SRP attributes.
+const makeSrpClient = async (
+    srpSalt: Buffer,
+    srpUserID: string,
+    loginSubKey: Uint8Array,
+): Promise<SrpClient> => {
+    const clientKey = await genKey();
+    return new SrpClient(
+        SRP_PARAMS,
+        srpSalt,
+        Buffer.from(srpUserID),
+        Buffer.from(loginSubKey),
+        clientKey,
+        false,
+    );
+};
+
+// Perform the two-round SRP handshake against the Ente API.
+const srpHandshake = async (
+    api: ApiClient,
+    srpAttributes: SRPAttributes,
+    password: string,
+): Promise<LoginChallenge> => {
+    const kek = await deriveKEK(
+        password,
+        fromBase64(srpAttributes.kekSalt),
+        srpAttributes.opsLimit,
+        srpAttributes.memLimit,
+    );
+    const loginSubKey = deriveLoginSubkey(kek);
+
+    const srpClient = await makeSrpClient(
+        Buffer.from(fromBase64(srpAttributes.srpSalt)),
+        srpAttributes.srpUserID,
+        loginSubKey,
+    );
+
+    const srpA = srpClient.computeA().toString("base64");
+
+    const { sessionID, srpB } = await api.postJSON<{
+        sessionID: string;
+        srpB: string;
+    }>("/users/srp/create-session", {
+        srpUserID: srpAttributes.srpUserID,
+        srpA,
+    });
+
+    srpClient.setB(Buffer.from(srpB, "base64"));
+    const srpM1 = srpClient.computeM1().toString("base64");
+
+    const verifyResponse = await api.postJSON<
+        AuthorizationResponse & { srpM2: string }
+    >("/users/srp/verify-session", {
+        sessionID,
+        srpUserID: srpAttributes.srpUserID,
+        srpM1,
+    });
+
+    srpClient.checkM2(Buffer.from(verifyResponse.srpM2, "base64"));
+
+    if (verifyResponse.twoFactorSessionID) {
+        return { kind: "totp", sessionID: verifyResponse.twoFactorSessionID };
+    }
+    if (verifyResponse.passkeySessionID) {
+        return {
+            kind: "passkey",
+            sessionID: verifyResponse.passkeySessionID,
+        };
+    }
+
+    return { kind: "complete", response: verifyResponse };
+};
+
+export const beginLogin = async (
+    api: ApiClient,
+    email: string,
+    password: string,
+): Promise<LoginChallenge> => {
+    const { attributes } = await api.getJSON<{
+        attributes: SRPAttributes;
+    }>("/users/srp/attributes", { email });
+
+    if (attributes.isEmailMFAEnabled) {
+        return { kind: "emailOTP" };
+    }
+
+    return srpHandshake(api, attributes, password);
+};
+
+export const submitTOTP = async (
+    api: ApiClient,
+    sessionID: string,
+    code: string,
+): Promise<AuthorizationResponse> =>
+    api.postJSON<AuthorizationResponse>("/users/two-factor/verify", {
+        sessionID,
+        code,
+    });
+
+export const requestEmailOTP = async (
+    api: ApiClient,
+    email: string,
+): Promise<void> => {
+    await api.postJSON("/users/ott", { email, purpose: "login" });
+};
+
+export const submitEmailOTP = async (
+    api: ApiClient,
+    email: string,
+    code: string,
+): Promise<AuthorizationResponse> =>
+    api.postJSON<AuthorizationResponse>("/users/verify-email", {
+        email,
+        ott: code,
+    });
diff --git a/test/auth/login.test.ts b/test/auth/login.test.ts
new file mode 100644
index 0000000..0519a22
--- /dev/null
+++ b/test/auth/login.test.ts
@@ -0,0 +1,373 @@
+/**
+ * Tests for the login flow functions: `beginLogin`, `submitTOTP`,
+ * `requestEmailOTP`, and `submitEmailOTP`.
+ *
+ * These compose the pieces built so far (crypto KDF, SRP-6a via
+ * fast-srp-hap, and ApiClient) into the complete authentication
+ * handshake with an Ente server.
+ *
+ * The server side is simulated using fast-srp-hap's SrpServer class, so
+ * the test exercises real SRP math end-to-end without network access.
+ *
+ * Login has three entry paths depending on the user's configuration:
+ *
+ * 1. **SRP (default)**: GET /users/srp/attributes, then a two-round SRP
+ *    handshake via POST /users/srp/create-session and
+ *    POST /users/srp/verify-session. The final response is either a
+ *    completed AuthorizationResponse or a 2FA challenge.
+ *
+ * 2. **Email OTP (isEmailMFAEnabled)**: if the SRP attributes indicate
+ *    email MFA is enabled, beginLogin returns `{ kind: "emailOTP" }` and
+ *    the caller is responsible for calling requestEmailOTP / submitEmailOTP
+ *    to complete the handshake.
+ *
+ * 3. **TOTP 2FA**: if the SRP handshake succeeds but the server requires
+ *    a second factor, beginLogin returns `{ kind: "totp", sessionID }`.
+ *    The caller calls submitTOTP with the sessionID and a 6-digit code
+ *    to obtain the AuthorizationResponse.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import { SRP, SrpServer } from "fast-srp-hap";
+import { beforeAll, describe, expect, it } from "vitest";
+import { ApiClient } from "../../src/api/client.js";
+import {
+    init,
+    toBase64,
+    deriveKEK,
+    deriveLoginSubkey,
+} from "../../src/crypto/index.js";
+import {
+    beginLogin,
+    submitTOTP,
+    requestEmailOTP,
+    submitEmailOTP,
+} from "../../src/auth/login.js";
+import type { KeyAttributes } from "../../src/auth/types.js";
+
+// ---------------------------------------------------------------------------
+// Shared test fixtures
+// ---------------------------------------------------------------------------
+
+const TEST_PASSWORD = "correct horse battery staple";
+const TEST_EMAIL = "test@example.com";
+const TEST_OPS = 2;
+const TEST_MEM = 64 * 1024 * 1024;
+
+/**
+ * Builds everything needed to simulate an Ente server that has a user
+ * registered with the given password. Returns:
+ *   - srpAttributes: what GET /users/srp/attributes returns
+ *   - verifier: the SRP verifier the server stores
+ *   - loginSubKey: the SRP password (base64) for building SrpServer
+ *   - keyAttributes + encryptedToken: for the auth response
+ *   - masterKey, secretKey, publicKey, tokenBytes: ground truth
+ */
+const buildServerFixture = async (password: string) => {
+    await sodium.ready;
+
+    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
+    const kek = await deriveKEK(password, kekSalt, TEST_OPS, TEST_MEM);
+    const loginSubKeyBytes = deriveLoginSubkey(kek);
+
+    const srpUserID = "test-srp-user-id";
+    const srpSalt = sodium.randombytes_buf(16);
+
+    const verifier = SRP.computeVerifier(
+        SRP.params["4096"],
+        Buffer.from(srpSalt),
+        Buffer.from(srpUserID),
+        Buffer.from(loginSubKeyBytes),
+    );
+
+    // Build key attributes (same approach as unwrap.test.ts fixture)
+    const masterKey = sodium.randombytes_buf(32);
+    const keyDecryptionNonce = sodium.randombytes_buf(
+        sodium.crypto_secretbox_NONCEBYTES,
+    );
+    const encryptedKey = sodium.crypto_secretbox_easy(
+        masterKey,
+        keyDecryptionNonce,
+        kek,
+    );
+
+    const kp = sodium.crypto_box_keypair();
+    const secretKeyDecryptionNonce = sodium.randombytes_buf(
+        sodium.crypto_secretbox_NONCEBYTES,
+    );
+    const encryptedSecretKey = sodium.crypto_secretbox_easy(
+        kp.privateKey,
+        secretKeyDecryptionNonce,
+        masterKey,
+    );
+
+    const tokenBytes = sodium.randombytes_buf(64);
+    const encryptedToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
+
+    const keyAttributes: KeyAttributes = {
+        kekSalt: toBase64(kekSalt),
+        encryptedKey: toBase64(encryptedKey),
+        keyDecryptionNonce: toBase64(keyDecryptionNonce),
+        publicKey: toBase64(kp.publicKey),
+        encryptedSecretKey: toBase64(encryptedSecretKey),
+        secretKeyDecryptionNonce: toBase64(secretKeyDecryptionNonce),
+        memLimit: TEST_MEM,
+        opsLimit: TEST_OPS,
+    };
+
+    return {
+        srpAttributes: {
+            srpUserID,
+            srpSalt: toBase64(srpSalt),
+            memLimit: TEST_MEM,
+            opsLimit: TEST_OPS,
+            kekSalt: toBase64(kekSalt),
+            isEmailMFAEnabled: false,
+        },
+        verifier,
+        srpSalt: Buffer.from(srpSalt),
+        loginSubKeyBytes,
+        keyAttributes,
+        encryptedToken: toBase64(encryptedToken),
+        masterKey,
+        secretKey: kp.privateKey,
+        publicKey: kp.publicKey,
+        tokenBytes,
+    };
+};
+
+/**
+ * Builds a mock fetch that simulates the Ente server's SRP endpoints.
+ * The mock performs real SRP math via fast-srp-hap's SrpServer.
+ *
+ * @param fixture The server fixture from buildServerFixture
+ * @param opts.requireTOTP If true, verify-session returns a 2FA challenge
+ */
+const buildMockFetch = (
+    fixture: Awaited<ReturnType<typeof buildServerFixture>>,
+    opts?: { requireTOTP?: boolean },
+) => {
+    let srpServer: SrpServer;
+    const sessionID = "test-session-id";
+
+    const mockFetch = async (
+        input: RequestInfo | URL,
+        init?: RequestInit,
+    ): Promise<Response> => {
+        const url =
+            typeof input === "string"
+                ? input
+                : input instanceof URL
+                  ? input.href
+                  : input.url;
+        const parsed = new URL(url);
+        const path = parsed.pathname;
+
+        if (path === "/users/srp/attributes") {
+            return new Response(
+                JSON.stringify({ attributes: fixture.srpAttributes }),
+                {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                },
+            );
+        }
+
+        if (path === "/users/srp/create-session") {
+            const body = JSON.parse(init?.body as string);
+            const serverKey = await SRP.genKey();
+            srpServer = new SrpServer(
+                SRP.params["4096"],
+                fixture.verifier,
+                serverKey,
+            );
+            const B = srpServer.computeB();
+            srpServer.setA(Buffer.from(body.srpA, "base64"));
+            return new Response(
+                JSON.stringify({
+                    sessionID,
+                    srpB: B.toString("base64"),
+                }),
+                {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                },
+            );
+        }
+
+        if (path === "/users/srp/verify-session") {
+            const body = JSON.parse(init?.body as string);
+            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
+            const M2 = srpServer.computeM2();
+
+            if (opts?.requireTOTP) {
+                return new Response(
+                    JSON.stringify({
+                        srpM2: M2.toString("base64"),
+                        twoFactorSessionID: "totp-session-999",
+                    }),
+                    {
+                        status: 200,
+                        headers: { "content-type": "application/json" },
+                    },
+                );
+            }
+
+            return new Response(
+                JSON.stringify({
+                    srpM2: M2.toString("base64"),
+                    id: 42,
+                    keyAttributes: fixture.keyAttributes,
+                    encryptedToken: fixture.encryptedToken,
+                }),
+                {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                },
+            );
+        }
+
+        if (path === "/users/two-factor/verify") {
+            return new Response(
+                JSON.stringify({
+                    id: 42,
+                    keyAttributes: fixture.keyAttributes,
+                    encryptedToken: fixture.encryptedToken,
+                }),
+                {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                },
+            );
+        }
+
+        if (path === "/users/ott") {
+            return new Response(JSON.stringify({}), {
+                status: 200,
+                headers: { "content-type": "application/json" },
+            });
+        }
+
+        if (path === "/users/verify-email") {
+            return new Response(
+                JSON.stringify({
+                    id: 42,
+                    keyAttributes: fixture.keyAttributes,
+                    encryptedToken: fixture.encryptedToken,
+                }),
+                {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                },
+            );
+        }
+
+        return new Response("Not Found", { status: 404 });
+    };
+
+    return mockFetch as typeof globalThis.fetch;
+};
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe("beginLogin via SRP", () => {
+    beforeAll(async () => {
+        await init();
+    });
+
+    it("completes SRP login and returns { kind: 'complete' }", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        const challenge = await beginLogin(api, TEST_EMAIL, TEST_PASSWORD);
+
+        expect(challenge.kind).toBe("complete");
+        if (challenge.kind === "complete") {
+            expect(challenge.response.id).toBe(42);
+            expect(challenge.response.keyAttributes).toBeDefined();
+            expect(challenge.response.encryptedToken).toBeDefined();
+        }
+    });
+
+    it("returns { kind: 'totp' } when 2FA is required", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({
+            fetch: buildMockFetch(fixture, { requireTOTP: true }),
+        });
+
+        const challenge = await beginLogin(api, TEST_EMAIL, TEST_PASSWORD);
+
+        expect(challenge.kind).toBe("totp");
+        if (challenge.kind === "totp") {
+            expect(challenge.sessionID).toBe("totp-session-999");
+        }
+    });
+
+    it("rejects a wrong password (SRP M1 verification fails)", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        await expect(
+            beginLogin(api, TEST_EMAIL, "wrong password"),
+        ).rejects.toThrow();
+    });
+});
+
+describe("beginLogin with email MFA fallback", () => {
+    beforeAll(async () => {
+        await init();
+    });
+
+    it("returns { kind: 'emailOTP' } when isEmailMFAEnabled", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        fixture.srpAttributes.isEmailMFAEnabled = true;
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        const challenge = await beginLogin(api, TEST_EMAIL, TEST_PASSWORD);
+
+        expect(challenge.kind).toBe("emailOTP");
+    });
+});
+
+describe("submitTOTP", () => {
+    beforeAll(async () => {
+        await init();
+    });
+
+    it("sends code to /users/two-factor/verify and returns AuthorizationResponse", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        const response = await submitTOTP(api, "totp-session-999", "123456");
+
+        expect(response.id).toBe(42);
+        expect(response.keyAttributes).toBeDefined();
+        expect(response.encryptedToken).toBeDefined();
+    });
+});
+
+describe("requestEmailOTP / submitEmailOTP", () => {
+    beforeAll(async () => {
+        await init();
+    });
+
+    it("requestEmailOTP sends to /users/ott without throwing", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        await expect(requestEmailOTP(api, TEST_EMAIL)).resolves.toBeUndefined();
+    });
+
+    it("submitEmailOTP sends to /users/verify-email and returns AuthorizationResponse", async () => {
+        const fixture = await buildServerFixture(TEST_PASSWORD);
+        const api = new ApiClient({ fetch: buildMockFetch(fixture) });
+
+        const response = await submitEmailOTP(api, TEST_EMAIL, "654321");
+
+        expect(response.id).toBe(42);
+        expect(response.keyAttributes).toBeDefined();
+        expect(response.encryptedToken).toBeDefined();
+    });
+});
diff --git a/yarn.lock b/yarn.lock
index 77b0200..d958d9b 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -879,6 +879,11 @@ fast-levenshtein@^2.0.6:
   resolved "https://registry.yarnpkg.com/fast-levenshtein/-/fast-levenshtein-2.0.6.tgz#3d8a5c66883a16a30ca8643e851f19baa7797917"
   integrity sha512-DCXu6Ifhqcks7TZKY3Hxp3y6qphY5SJZmrWMDrKcERSOXWQdMhU9Ig/PYrzyw/ul9jOIyh0N4M0tbC5hodg8dw==
 
+fast-srp-hap@^2.0.4:
+  version "2.0.4"
+  resolved "https://registry.yarnpkg.com/fast-srp-hap/-/fast-srp-hap-2.0.4.tgz#9db296e21a5143951310f99e5a74290106467811"
+  integrity sha512-lHRYYaaIbMrhZtsdGTwPN82UbqD9Bv8QfOlKs+Dz6YRnByZifOh93EYmf2iEWFtkOEIqR2IK8cFD0UN5wLIWBQ==
+
 fastq@^1.6.0:
   version "1.20.1"
   resolved "https://registry.yarnpkg.com/fastq/-/fastq-1.20.1.tgz#ca750a10dc925bc8b18839fd203e3ef4b3ced675"