iclouddrive: replace plaintext signin with SRP authentication

Apple has deprecated the legacy /appleauth/auth/signin endpoint and
now blocks it, causing "Invalid Session Token" errors for all users
when their trust token expires. The browser login flow now requires
SRP (Secure Remote Password), a cryptographic handshake that never
transmits the password.

Replace Session.SignIn() with a multi-step SRP-6a flow:
1. authStart - initialize session at /authorize/signin
2. authFederate - submit account name to /federate
3. authSRPInit - exchange client public value for salt/B at /signin/init
4. authSRPComplete - send M1/M2 proofs to /signin/complete

The SRP implementation uses the RFC 5054 2048-bit group with SHA-256
and Apple's NoUserNameInX variant. Password derivation supports both
s2k and s2k_fo protocols via SHA-256 + PBKDF2.

The 2FA and trust token flow is unchanged. Auth headers for all
idmsa.apple.com requests now include X-Apple-Auth-Attributes,
X-Apple-Frame-Id, and use Origin/Referer of https://idmsa.apple.com.

Fixes #8587

backend/iclouddrive/api/client.go

@@ -118,12 +118,19 @@ func (c *Client) Authenticate(ctx context.Context) error {
 
 	fs.Debugf("icloud", "Authenticating as %s\n", c.appleID)
 	err := c.Session.SignIn(ctx, c.appleID, c.password)
+	if err != nil {
+		return err
+	}
 
-	if err == nil {
+	// If 2FA is required, don't try AuthWithToken yet — the caller
-		err = c.Session.AuthWithToken(ctx)
+	// must complete 2FA first, then call AuthWithToken.
-		if err == nil && c.sessionSaveCallback != nil {
+	if c.Session.Requires2FA() {
-			c.sessionSaveCallback(c.Session)
+		return nil
-		}
+	}
+
+	err = c.Session.AuthWithToken(ctx)
+	if err == nil && c.sessionSaveCallback != nil {
+		c.sessionSaveCallback(c.Session)
 	}
 	return err
 }

backend/iclouddrive/api/session.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"context"
+	"encoding/base64"
 	"fmt"
 	"maps"
 	"net/http"
@@ -9,11 +10,15 @@ import (
 	"slices"
 	"strings"
 
+	"github.com/google/uuid"
 	"github.com/oracle/oci-go-sdk/v65/common"
+	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/fshttp"
 	"github.com/rclone/rclone/lib/rest"
 )
 
+const iCloudUserAgent = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.3.1 Safari/605.1.15"
+
 // Session represents an iCloud session
 type Session struct {
 	SessionToken   string         `json:"session_token"`
@@ -22,10 +27,22 @@ type Session struct {
 	AccountCountry string         `json:"account_country"`
 	TrustToken     string         `json:"trust_token"`
 	ClientID       string         `json:"client_id"`
+	AuthAttributes string         `json:"auth_attributes"`
+	FrameID        string         `json:"frame_id"`
 	Cookies        []*http.Cookie `json:"cookies"`
 	AccountInfo    AccountInfo    `json:"account_info"`
 
-	srv *rest.Client `json:"-"`
+	srv      *rest.Client `json:"-"`
+	needs2FA bool         `json:"-"` // set when SRP signin returns 409
+}
+
+// srpInitResponse is the server response from /auth/signin/init
+type srpInitResponse struct {
+	Iteration int    `json:"iteration"`
+	Salt      string `json:"salt"`
+	Protocol  string `json:"protocol"`
+	B         string `json:"b"`
+	C         string `json:"c"`
 }
 
 // String returns the session as a string
@@ -57,24 +74,217 @@ func (s *Session) Request(ctx context.Context, opts rest.Opts, request any, resp
 	if val := resp.Header.Get("scnt"); val != "" {
 		s.Scnt = val
 	}
+	if val := resp.Header.Get("X-Apple-Auth-Attributes"); val != "" {
+		s.AuthAttributes = val
+	}
 
 	return resp, nil
 }
 
 // Requires2FA returns true if the session requires 2FA
 func (s *Session) Requires2FA() bool {
-	return s.AccountInfo.DsInfo.HsaVersion == 2 && s.AccountInfo.HsaChallengeRequired
+	if s.needs2FA {
+		return true
+	}
+	return s.AccountInfo.DsInfo != nil && s.AccountInfo.DsInfo.HsaVersion == 2 && s.AccountInfo.HsaChallengeRequired
 }
 
-// SignIn signs in the session
+// SignIn performs SRP-based authentication against Apple's idmsa endpoint.
 func (s *Session) SignIn(ctx context.Context, appleID, password string) error {
+	// Step 1: Initialize the auth session
+	if err := s.authStart(ctx); err != nil {
+		return fmt.Errorf("authStart: %w", err)
+	}
+
+	// Step 2: Federate (submit account name)
+	if err := s.authFederate(ctx, appleID); err != nil {
+		return fmt.Errorf("authFederate: %w", err)
+	}
+
+	// Step 3: SRP init — send client public value A, get salt + B
+	client := newSRPClient()
+	aBase64 := base64.StdEncoding.EncodeToString(client.getABytes())
+
+	initResp, err := s.authSRPInit(ctx, aBase64, appleID)
+	if err != nil {
+		return fmt.Errorf("authSRPInit: %w", err)
+	}
+
+	// Decode server values
+	serverB, err := base64.StdEncoding.DecodeString(initResp.B)
+	if err != nil {
+		return fmt.Errorf("decode B: %w", err)
+	}
+	salt, err := base64.StdEncoding.DecodeString(initResp.Salt)
+	if err != nil {
+		return fmt.Errorf("decode salt: %w", err)
+	}
+
+	// Step 4: Derive password key and process the SRP challenge
+	derivedKey, err := derivePassword(password, salt, initResp.Iteration, initResp.Protocol)
+	if err != nil {
+		return fmt.Errorf("derivePassword: %w", err)
+	}
+	client.processChallenge([]byte(appleID), derivedKey, salt, serverB)
+
+	// Step 5: Complete — send M1, M2 proofs
+	m1Base64 := base64.StdEncoding.EncodeToString(client.M1)
+	m2Base64 := base64.StdEncoding.EncodeToString(client.M2)
+
+	if err := s.authSRPComplete(ctx, appleID, m1Base64, m2Base64, initResp.C); err != nil {
+		return fmt.Errorf("authSRPComplete: %w", err)
+	}
+
+	return nil
+}
+
+// authStart initializes the SRP auth session by hitting the authorize/signin endpoint.
+func (s *Session) authStart(ctx context.Context) error {
+	if s.FrameID == "" {
+		s.FrameID = strings.ToLower(uuid.New().String())
+	}
+	frameTag := "auth-" + s.FrameID
+
+	params := url.Values{}
+	params.Set("frame_id", frameTag)
+	params.Set("language", "en_US")
+	params.Set("skVersion", "7")
+	params.Set("iframeId", frameTag)
+	params.Set("client_id", s.ClientID)
+	params.Set("redirect_uri", "https://www.icloud.com")
+	params.Set("response_type", "code")
+	params.Set("response_mode", "web_message")
+	params.Set("state", frameTag)
+	params.Set("authVersion", "latest")
+
+	opts := rest.Opts{
+		Method:     "GET",
+		Path:       "/authorize/signin",
+		Parameters: params,
+		ExtraHeaders: map[string]string{
+			"Accept":     "*/*",
+			"User-Agent": iCloudUserAgent,
+		},
+		RootURL:    authEndpoint,
+		NoResponse: true,
+	}
+
+	resp, err := s.srv.Call(ctx, &opts)
+	if err != nil {
+		return err
+	}
+	_ = resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("authStart: unexpected status %s", resp.Status)
+	}
+
+	if val := resp.Header.Get("X-Apple-Auth-Attributes"); val != "" {
+		s.AuthAttributes = val
+	}
+	if val := resp.Header.Get("scnt"); val != "" {
+		s.Scnt = val
+	}
+	if val := resp.Header.Get("X-Apple-ID-Session-Id"); val != "" {
+		s.SessionID = val
+	}
+
+	return nil
+}
+
+// authFederate submits the account name to Apple's federate endpoint.
+func (s *Session) authFederate(ctx context.Context, accountName string) error {
+	values := map[string]any{
+		"accountName": accountName,
+		"rememberMe":  true,
+	}
+	body, err := IntoReader(values)
+	if err != nil {
+		return err
+	}
+
+	opts := rest.Opts{
+		Method:       "POST",
+		Path:         "/federate",
+		Parameters:   url.Values{"isRememberMeEnabled": {"true"}},
+		ExtraHeaders: s.getSRPAuthHeaders(),
+		RootURL:      authEndpoint,
+		Body:         body,
+		NoResponse:   true,
+	}
+
+	resp, err := s.srv.Call(ctx, &opts)
+	if err != nil {
+		return err
+	}
+	_ = resp.Body.Close()
+
+	if val := resp.Header.Get("X-Apple-Auth-Attributes"); val != "" {
+		s.AuthAttributes = val
+	}
+	if val := resp.Header.Get("scnt"); val != "" {
+		s.Scnt = val
+	}
+	if val := resp.Header.Get("X-Apple-ID-Session-Id"); val != "" {
+		s.SessionID = val
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("authFederate: unexpected status %s", resp.Status)
+	}
+	return nil
+}
+
+// authSRPInit sends the client's public value A to the server and retrieves
+// the salt, server public value B, iteration count, protocol, and challenge.
+func (s *Session) authSRPInit(ctx context.Context, aBase64, accountName string) (*srpInitResponse, error) {
+	values := map[string]any{
+		"a":           aBase64,
+		"accountName": accountName,
+		"protocols":   []string{"s2k", "s2k_fo"},
+	}
+	body, err := IntoReader(values)
+	if err != nil {
+		return nil, err
+	}
+
+	opts := rest.Opts{
+		Method:       "POST",
+		Path:         "/signin/init",
+		ExtraHeaders: s.getSRPAuthHeaders(),
+		RootURL:      authEndpoint,
+		Body:         body,
+	}
+
+	var initResp srpInitResponse
+	resp, err := s.srv.CallJSON(ctx, &opts, nil, &initResp)
+	if err != nil {
+		return nil, err
+	}
+
+	if val := resp.Header.Get("scnt"); val != "" {
+		s.Scnt = val
+	}
+	if val := resp.Header.Get("X-Apple-ID-Session-Id"); val != "" {
+		s.SessionID = val
+	}
+
+	return &initResp, nil
+}
+
+// authSRPComplete sends the SRP proofs M1 and M2 to complete authentication.
+// Returns nil on success (200 or 409/2FA needed).
+func (s *Session) authSRPComplete(ctx context.Context, accountName, m1Base64, m2Base64, c string) error {
 	trustTokens := []string{}
 	if s.TrustToken != "" {
 		trustTokens = []string{s.TrustToken}
 	}
+
 	values := map[string]any{
-		"accountName": appleID,
+		"accountName": accountName,
-		"password":    password,
+		"m1":          m1Base64,
+		"m2":          m2Base64,
+		"c":           c,
 		"rememberMe":  true,
 		"trustTokens": trustTokens,
 	}
@@ -82,21 +292,93 @@ func (s *Session) SignIn(ctx context.Context, appleID, password string) error {
 	if err != nil {
 		return err
 	}
+
 	opts := rest.Opts{
 		Method:       "POST",
-		Path:         "/signin",
+		Path:         "/signin/complete",
-		Parameters:   url.Values{},
+		Parameters:   url.Values{"isRememberMeEnabled": {"true"}},
-		ExtraHeaders: s.GetAuthHeaders(map[string]string{}),
+		ExtraHeaders: s.getSRPAuthHeaders(),
 		RootURL:      authEndpoint,
-		IgnoreStatus: true, // need to handle 409 for hsa2
+		IgnoreStatus: true,
 		NoResponse:   true,
 		Body:         body,
 	}
-	opts.Parameters.Set("isRememberMeEnabled", "true")
-	_, err = s.Request(ctx, opts, nil, nil)
 
-	return err
+	resp, err := s.srv.Call(ctx, &opts)
+	if err != nil {
+		return err
+	}
+	_ = resp.Body.Close()
 
+	// Extract updated headers
+	if val := resp.Header.Get("X-Apple-Auth-Attributes"); val != "" {
+		s.AuthAttributes = val
+	}
+	if val := resp.Header.Get("X-Apple-Session-Token"); val != "" {
+		s.SessionToken = val
+	}
+	if val := resp.Header.Get("scnt"); val != "" {
+		s.Scnt = val
+	}
+	if val := resp.Header.Get("X-Apple-ID-Session-Id"); val != "" {
+		s.SessionID = val
+	}
+	if val := resp.Header.Get("X-Apple-ID-Account-Country"); val != "" {
+		s.AccountCountry = val
+	}
+	if val := resp.Header.Get("X-Apple-TwoSV-Trust-Token"); val != "" {
+		s.TrustToken = val
+	}
+
+	switch resp.StatusCode {
+	case http.StatusOK:
+		fs.Debugf("icloud", "SRP sign in successful")
+		return nil
+	case http.StatusConflict:
+		// 409 = 2FA required, this is expected
+		fs.Debugf("icloud", "SRP sign in requires 2FA")
+		s.needs2FA = true
+		return nil
+	case http.StatusForbidden:
+		return fmt.Errorf("sign in failed: incorrect username or password")
+	default:
+		return fmt.Errorf("sign in failed: %s", resp.Status)
+	}
+}
+
+// getSRPAuthHeaders returns headers needed for SRP auth requests to idmsa.apple.com.
+func (s *Session) getSRPAuthHeaders() map[string]string {
+	frameTag := "auth-" + s.FrameID
+	headers := map[string]string{
+		"Accept":                           "application/json",
+		"Content-Type":                     "application/json",
+		"User-Agent":                       iCloudUserAgent,
+		"Origin":                           "https://idmsa.apple.com",
+		"Referer":                          "https://idmsa.apple.com/",
+		"X-Apple-Widget-Key":               s.ClientID,
+		"X-Apple-OAuth-Client-Id":          s.ClientID,
+		"X-Apple-OAuth-Client-Type":        "firstPartyAuth",
+		"X-Apple-OAuth-Redirect-URI":       "https://www.icloud.com",
+		"X-Apple-OAuth-Require-Grant-Code": "true",
+		"X-Apple-OAuth-Response-Mode":      "web_message",
+		"X-Apple-OAuth-Response-Type":      "code",
+		"X-Apple-OAuth-State":              frameTag,
+		"X-Apple-Frame-Id":                 frameTag,
+		"X-Requested-With":                 "XMLHttpRequest",
+		"X-Apple-Mandate-Security-Upgrade": "0",
+		"X-Apple-I-Require-UE":             "true",
+		"X-Apple-I-FD-Client-Info":         `{"U":"` + iCloudUserAgent + `","L":"en-US","Z":"GMT-05:00","V":"1.1","F":""}`,
+	}
+	if s.AuthAttributes != "" {
+		headers["X-Apple-Auth-Attributes"] = s.AuthAttributes
+	}
+	if s.Scnt != "" {
+		headers["scnt"] = s.Scnt
+	}
+	if s.SessionID != "" {
+		headers["X-Apple-ID-Session-Id"] = s.SessionID
+	}
+	return headers
 }
 
 // AuthWithToken authenticates the session
@@ -135,14 +417,10 @@ func (s *Session) Validate2FACode(ctx context.Context, code string) error {
 		return err
 	}
 
-	headers := s.GetAuthHeaders(map[string]string{})
-	headers["scnt"] = s.Scnt
-	headers["X-Apple-ID-Session-Id"] = s.SessionID
-
 	opts := rest.Opts{
 		Method:       "POST",
 		Path:         "/verify/trusteddevice/securitycode",
-		ExtraHeaders: headers,
+		ExtraHeaders: s.GetAuthHeaders(map[string]string{}),
 		RootURL:      authEndpoint,
 		Body:         body,
 		NoResponse:   true,
@@ -162,14 +440,10 @@ func (s *Session) Validate2FACode(ctx context.Context, code string) error {
 
 // TrustSession trusts the session
 func (s *Session) TrustSession(ctx context.Context) error {
-	headers := s.GetAuthHeaders(map[string]string{})
-	headers["scnt"] = s.Scnt
-	headers["X-Apple-ID-Session-Id"] = s.SessionID
-
 	opts := rest.Opts{
 		Method:        "GET",
 		Path:          "/2sv/trust",
-		ExtraHeaders:  headers,
+		ExtraHeaders:  s.GetAuthHeaders(map[string]string{}),
 		RootURL:       authEndpoint,
 		NoResponse:    true,
 		ContentLength: common.Int64(0),
@@ -201,25 +475,9 @@ func (s *Session) ValidateSession(ctx context.Context) error {
 }
 
 // GetAuthHeaders returns the authentication headers for the session.
-//
+// Used for 2FA validation and trust requests to idmsa.apple.com.
-// It takes an `overwrite` map[string]string parameter which allows
-// overwriting the default headers. It returns a map[string]string.
 func (s *Session) GetAuthHeaders(overwrite map[string]string) map[string]string {
-	headers := map[string]string{
+	headers := s.getSRPAuthHeaders()
-		"Accept":                           "application/json",
-		"Content-Type":                     "application/json",
-		"X-Apple-OAuth-Client-Id":          s.ClientID,
-		"X-Apple-OAuth-Client-Type":        "firstPartyAuth",
-		"X-Apple-OAuth-Redirect-URI":       "https://www.icloud.com",
-		"X-Apple-OAuth-Require-Grant-Code": "true",
-		"X-Apple-OAuth-Response-Mode":      "web_message",
-		"X-Apple-OAuth-Response-Type":      "code",
-		"X-Apple-OAuth-State":              s.ClientID,
-		"X-Apple-Widget-Key":               s.ClientID,
-		"Origin":                           homeEndpoint,
-		"Referer":                          fmt.Sprintf("%s/", homeEndpoint),
-		"User-Agent":                       "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:103.0) Gecko/20100101 Firefox/103.0",
-	}
 	maps.Copy(headers, overwrite)
 	return headers
 }
@@ -248,7 +506,7 @@ func GetCommonHeaders(overwrite map[string]string) map[string]string {
 		"Content-Type": "application/json",
 		"Origin":       baseEndpoint,
 		"Referer":      fmt.Sprintf("%s/", baseEndpoint),
-		"User-Agent":   "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:103.0) Gecko/20100101 Firefox/103.0",
+		"User-Agent":   iCloudUserAgent,
 	}
 	maps.Copy(headers, overwrite)
 	return headers
@@ -281,9 +539,16 @@ func GetCookiesForDomain(url *url.URL, cookies []*http.Cookie) ([]*http.Cookie,
 
 // NewSession creates a new Session instance with default values.
 func NewSession() *Session {
-	session := &Session{}
+	session := &Session{
-	session.srv = rest.NewClient(fshttp.NewClient(context.Background())).SetRoot(baseEndpoint)
+		FrameID: strings.ToLower(uuid.New().String()),
-	//session.ClientID = "auth-" + uuid.New().String()
+	}
+	httpClient := fshttp.NewClient(context.Background())
+	if tr, ok := httpClient.Transport.(*fshttp.Transport); ok {
+		tr.SetRequestFilter(func(req *http.Request) {
+			req.Header.Set("User-Agent", iCloudUserAgent)
+		})
+	}
+	session.srv = rest.NewClient(httpClient).SetRoot(baseEndpoint)
 	return session
 }
 

backend/iclouddrive/api/srp.go

@@ -0,0 +1,229 @@
+package api
+
+import (
+	"crypto/pbkdf2"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"hash"
+	"math/big"
+)
+
+// RFC 5054 2048-bit group parameters for SRP-6a (g=2, SHA-256).
+var (
+	srpN = mustParseBigHex(
+		"AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050" +
+			"A37329CBB4A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50" +
+			"E8083969EDB767B0CF6095179A163AB3661A05FBD5FAAAE82918A9962F0B93B8" +
+			"55F97993EC975EEAA80D740ADBF4FF747359D041D5C33EA71D281E446B14773B" +
+			"CA97B43A23FB801676BD207A436C6481F1D2B9078717461A5B9D32E688F87748" +
+			"544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB37861602790" +
+			"04E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8" +
+			"E9DBFBB694B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F" +
+			"9E4AFF73")
+	srpG         = big.NewInt(2)
+	srpNLenBytes = 2048 / 8
+	srpHashFunc  = sha256.New
+)
+
+func mustParseBigHex(s string) *big.Int {
+	b, ok := new(big.Int).SetString(s, 16)
+	if !ok {
+		panic("srp: bad hex constant for N")
+	}
+	return b
+}
+
+// srpClient implements the client side of SRP-6a for Apple's iCloud auth.
+type srpClient struct {
+	a  *big.Int // client secret
+	A  *big.Int // client public value
+	k  *big.Int // multiplier
+	M1 []byte
+	M2 []byte
+	K  []byte
+}
+
+// newSRPClient generates a random 32-byte secret and computes the public value A.
+func newSRPClient() *srpClient {
+	secret := make([]byte, 32)
+	if _, err := rand.Read(secret); err != nil {
+		panic(fmt.Sprintf("srp: rand.Read failed: %v", err))
+	}
+
+	a := new(big.Int).SetBytes(secret)
+	A := new(big.Int).Exp(srpG, a, srpN)
+	k := getMultiplier()
+
+	return &srpClient{
+		a: a,
+		A: A,
+		k: k,
+	}
+}
+
+// getABytes returns the padded public value A.
+func (c *srpClient) getABytes() []byte {
+	return padToN(c.A)
+}
+
+// processChallenge computes the session key and proof values from the server challenge.
+// username is the Apple ID, derivedKey is the output of derivePassword, salt and B come
+// from the server's init response (raw bytes, already base64-decoded).
+func (c *srpClient) processChallenge(username, derivedKey, salt, serverB []byte) {
+	B := new(big.Int).SetBytes(serverB)
+
+	// Validate B
+	if B.Cmp(big.NewInt(0)) <= 0 || B.Cmp(srpN) >= 0 {
+		panic("srp: invalid server-supplied B, must be 1..N-1")
+	}
+
+	x := calculateX(salt, derivedKey)
+	u := calculateU(c.A, B)
+	S := calculateS(c.k, x, c.a, B, u)
+	c.K = calculateK(S)
+
+	aBytes := padToN(c.A)
+	bBytes := padToN(B)
+	c.M1 = calculateM1(username, salt, aBytes, bBytes, c.K)
+	c.M2 = calculateM2(aBytes, c.M1, c.K)
+}
+
+// derivePassword performs Apple's password key derivation.
+// For "s2k": PBKDF2(SHA256(password), salt, iterations, 32, SHA256)
+// For "s2k_fo": PBKDF2(hex(SHA256(password)), salt, iterations, 32, SHA256)
+func derivePassword(password string, salt []byte, iterations int, protocol string) ([]byte, error) {
+	passHash := sha256.Sum256([]byte(password))
+
+	var passInput string
+	switch protocol {
+	case "s2k_fo":
+		passInput = hex.EncodeToString(passHash[:])
+	default: // "s2k"
+		passInput = string(passHash[:])
+	}
+	return pbkdf2.Key(sha256.New, passInput, salt, iterations, 32)
+}
+
+// padToN pads a big.Int to the SRP group size (256 bytes for 2048-bit).
+func padToN(n *big.Int) []byte {
+	b := n.Bytes()
+	if len(b) >= srpNLenBytes {
+		return b
+	}
+	padded := make([]byte, srpNLenBytes)
+	copy(padded[srpNLenBytes-len(b):], b)
+	return padded
+}
+
+func srpHash(data []byte) []byte {
+	h := srpHashFunc()
+	h.Write(data)
+	return h.Sum(nil)
+}
+
+func hashToInt(h hash.Hash) *big.Int {
+	return new(big.Int).SetBytes(h.Sum(nil))
+}
+
+// getMultiplier computes k = H(N | pad(g))
+func getMultiplier() *big.Int {
+	h := srpHashFunc()
+	nBytes := srpN.Bytes()
+	gBytes := srpG.Bytes()
+	// pad g to same length as N
+	for len(gBytes) < len(nBytes) {
+		gBytes = append([]byte{0}, gBytes...)
+	}
+	h.Write(nBytes)
+	h.Write(gBytes)
+	return hashToInt(h)
+}
+
+// calculateX computes x = H(salt | H(":" | password))
+// Apple's variant: NoUserNameInX — username is omitted from the inner hash.
+// The "password" here is actually the derived key from derivePassword.
+func calculateX(salt, derivedKey []byte) *big.Int {
+	h := srpHashFunc()
+	// Apple omits username (NoUserNameInX), only writes ":" + derivedKey
+	h.Write([]byte(":"))
+	h.Write(derivedKey)
+	digest := h.Sum(nil)
+
+	h2 := srpHashFunc()
+	h2.Write(salt)
+	h2.Write(digest)
+	return new(big.Int).SetBytes(h2.Sum(nil))
+}
+
+// calculateU computes u = H(pad(A) | pad(B))
+func calculateU(A, B *big.Int) *big.Int {
+	h := srpHashFunc()
+	h.Write(padToN(A))
+	h.Write(padToN(B))
+	return hashToInt(h)
+}
+
+// calculateS computes the client-side shared secret:
+// S = (B - k * g^x) ^ (a + u*x) mod N
+func calculateS(k, x, a, B, u *big.Int) []byte {
+	// g^x mod N
+	gx := new(big.Int).Exp(srpG, x, srpN)
+
+	// k * g^x
+	kgx := new(big.Int).Mul(k, gx)
+
+	// B - k*g^x
+	diff := new(big.Int).Sub(B, kgx)
+
+	// u*x
+	ux := new(big.Int).Mul(u, x)
+
+	// a + u*x
+	exp := new(big.Int).Add(a, ux)
+
+	// (B - k*g^x) ^ (a + u*x) mod N
+	S := new(big.Int).Exp(diff, exp, srpN)
+
+	// Ensure positive
+	S.Mod(S, srpN)
+
+	return padToN(S)
+}
+
+// calculateK computes K = H(S)
+func calculateK(S []byte) []byte {
+	return srpHash(S)
+}
+
+// calculateM1 computes the client proof (Apple/iCloud variant):
+// M1 = H(H(g) XOR H(N) | H(username) | salt | A | B | K)
+func calculateM1(username, salt, A, B, K []byte) []byte {
+	hg := srpHash(padToN(srpG))
+	hn := srpHash(srpN.Bytes())
+	hi := srpHash(username)
+
+	hxor := make([]byte, len(hg))
+	for i := range hg {
+		hxor[i] = hg[i] ^ hn[i]
+	}
+
+	h := srpHashFunc()
+	h.Write(hxor)
+	h.Write(hi)
+	h.Write(salt)
+	h.Write(A)
+	h.Write(B)
+	h.Write(K)
+	return h.Sum(nil)
+}
+
+// calculateM2 computes the server proof: M2 = H(A | M1 | K)
+func calculateM2(A, M1, K []byte) []byte {
+	h := srpHashFunc()
+	h.Write(A)
+	h.Write(M1)
+	h.Write(K)
+	return h.Sum(nil)
+}

backend/iclouddrive/api/srp_test.go

@@ -0,0 +1,379 @@
+package api
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPadToN(t *testing.T) {
+	t.Run("pads small value to 256 bytes", func(t *testing.T) {
+		result := padToN(big.NewInt(1))
+		assert.Equal(t, srpNLenBytes, len(result))
+		assert.Equal(t, byte(0), result[0])
+		assert.Equal(t, byte(1), result[srpNLenBytes-1])
+	})
+
+	t.Run("pads zero to 256 bytes", func(t *testing.T) {
+		result := padToN(big.NewInt(0))
+		assert.Equal(t, srpNLenBytes, len(result))
+		for _, b := range result {
+			assert.Equal(t, byte(0), b)
+		}
+	})
+
+	t.Run("preserves value that fills N", func(t *testing.T) {
+		// Use N itself — already 256 bytes
+		result := padToN(srpN)
+		assert.Equal(t, srpNLenBytes, len(result))
+		assert.Equal(t, srpN.Bytes(), result)
+	})
+
+	t.Run("round-trips through big.Int", func(t *testing.T) {
+		original := big.NewInt(123456789)
+		padded := padToN(original)
+		recovered := new(big.Int).SetBytes(padded)
+		assert.Equal(t, 0, original.Cmp(recovered))
+	})
+}
+
+func TestGetMultiplier(t *testing.T) {
+	k := getMultiplier()
+
+	// k must be non-zero and less than N
+	assert.True(t, k.Sign() > 0, "multiplier must be positive")
+	assert.True(t, k.Cmp(srpN) < 0, "multiplier must be less than N")
+
+	// k is deterministic
+	k2 := getMultiplier()
+	assert.Equal(t, 0, k.Cmp(k2), "multiplier must be deterministic")
+
+	// k = H(N | pad(g)) — verify by manual computation
+	h := srpHashFunc()
+	nBytes := srpN.Bytes()
+	gBytes := srpG.Bytes()
+	for len(gBytes) < len(nBytes) {
+		gBytes = append([]byte{0}, gBytes...)
+	}
+	h.Write(nBytes)
+	h.Write(gBytes)
+	expected := new(big.Int).SetBytes(h.Sum(nil))
+	assert.Equal(t, 0, expected.Cmp(k), "multiplier must equal H(N|pad(g))")
+}
+
+func TestDerivePassword(t *testing.T) {
+	salt := []byte("test_salt_value!")
+	password := "test_password"
+
+	t.Run("s2k produces 32-byte key", func(t *testing.T) {
+		key, err := derivePassword(password, salt, 1000, "s2k")
+		require.NoError(t, err)
+		assert.Equal(t, 32, len(key))
+	})
+
+	t.Run("s2k_fo produces 32-byte key", func(t *testing.T) {
+		key, err := derivePassword(password, salt, 1000, "s2k_fo")
+		require.NoError(t, err)
+		assert.Equal(t, 32, len(key))
+	})
+
+	t.Run("s2k and s2k_fo produce different keys", func(t *testing.T) {
+		key1, err := derivePassword(password, salt, 1000, "s2k")
+		require.NoError(t, err)
+		key2, err := derivePassword(password, salt, 1000, "s2k_fo")
+		require.NoError(t, err)
+		assert.NotEqual(t, key1, key2, "s2k and s2k_fo must produce different keys")
+	})
+
+	t.Run("deterministic output", func(t *testing.T) {
+		key1, err := derivePassword(password, salt, 1000, "s2k")
+		require.NoError(t, err)
+		key2, err := derivePassword(password, salt, 1000, "s2k")
+		require.NoError(t, err)
+		assert.Equal(t, key1, key2)
+	})
+
+	t.Run("different passwords produce different keys", func(t *testing.T) {
+		key1, err := derivePassword("password1", salt, 1000, "s2k")
+		require.NoError(t, err)
+		key2, err := derivePassword("password2", salt, 1000, "s2k")
+		require.NoError(t, err)
+		assert.NotEqual(t, key1, key2)
+	})
+
+	t.Run("s2k uses raw SHA256 bytes", func(t *testing.T) {
+		// Verify that s2k uses SHA256(password) as the PBKDF2 input.
+		// Two passwords with the same SHA256 hash would produce the same key,
+		// while two passwords with different SHA256 hashes must produce different keys.
+		key1, err := derivePassword("hello", salt, 10, "s2k")
+		require.NoError(t, err)
+		key2, err := derivePassword("world", salt, 10, "s2k")
+		require.NoError(t, err)
+		assert.NotEqual(t, key1, key2)
+	})
+
+	t.Run("s2k_fo uses hex-encoded SHA256", func(t *testing.T) {
+		// For s2k_fo, the PBKDF2 input is hex(SHA256(password)), which is
+		// a 64-character ASCII string, not raw 32-byte hash.
+		key, err := derivePassword("hello", salt, 10, "s2k_fo")
+		require.NoError(t, err)
+		assert.Equal(t, 32, len(key))
+	})
+}
+
+func TestCalculateX(t *testing.T) {
+	salt := []byte("some_salt")
+	derivedKey := []byte("some_derived_key_value_32_bytes!")
+
+	x := calculateX(salt, derivedKey)
+
+	// x must be positive and less than the hash output space
+	assert.True(t, x.Sign() > 0, "x must be positive")
+
+	// x is deterministic
+	x2 := calculateX(salt, derivedKey)
+	assert.Equal(t, 0, x.Cmp(x2), "x must be deterministic")
+
+	// Verify manually: x = H(salt | H(":" | derivedKey))
+	h1 := srpHashFunc()
+	h1.Write([]byte(":"))
+	h1.Write(derivedKey)
+	inner := h1.Sum(nil)
+
+	h2 := srpHashFunc()
+	h2.Write(salt)
+	h2.Write(inner)
+	expected := new(big.Int).SetBytes(h2.Sum(nil))
+	assert.Equal(t, 0, expected.Cmp(x), "x must equal H(salt | H(\":\" | derivedKey))")
+}
+
+func TestCalculateU(t *testing.T) {
+	A := big.NewInt(12345)
+	B := big.NewInt(67890)
+
+	u := calculateU(A, B)
+	assert.True(t, u.Sign() > 0, "u must be positive")
+
+	// u is deterministic
+	u2 := calculateU(A, B)
+	assert.Equal(t, 0, u.Cmp(u2))
+
+	// u changes when inputs change
+	u3 := calculateU(A, big.NewInt(99999))
+	assert.NotEqual(t, 0, u.Cmp(u3), "u must change when B changes")
+
+	// Verify: u = H(pad(A) | pad(B))
+	h := srpHashFunc()
+	h.Write(padToN(A))
+	h.Write(padToN(B))
+	expected := new(big.Int).SetBytes(h.Sum(nil))
+	assert.Equal(t, 0, expected.Cmp(u))
+}
+
+func TestCalculateK(t *testing.T) {
+	S := []byte("some_shared_secret_value_32bytes")
+
+	K := calculateK(S)
+
+	// K is a SHA-256 hash, so 32 bytes
+	assert.Equal(t, sha256.Size, len(K))
+
+	// K is deterministic
+	K2 := calculateK(S)
+	assert.Equal(t, K, K2)
+
+	// Verify: K = H(S)
+	expected := srpHash(S)
+	assert.Equal(t, expected, K)
+}
+
+func TestCalculateM1(t *testing.T) {
+	username := []byte("test@example.com")
+	salt := []byte("test_salt")
+	A := make([]byte, srpNLenBytes)
+	B := make([]byte, srpNLenBytes)
+	K := make([]byte, 32)
+
+	// Fill with non-zero values
+	A[srpNLenBytes-1] = 0x42
+	B[srpNLenBytes-1] = 0x43
+	K[0] = 0x44
+
+	m1 := calculateM1(username, salt, A, B, K)
+
+	// M1 is a SHA-256 hash
+	assert.Equal(t, sha256.Size, len(m1))
+
+	// M1 is deterministic
+	m1b := calculateM1(username, salt, A, B, K)
+	assert.Equal(t, m1, m1b)
+
+	// M1 changes when username changes
+	m1c := calculateM1([]byte("other@example.com"), salt, A, B, K)
+	assert.NotEqual(t, m1, m1c)
+
+	// Verify: M1 = H(H(g) XOR H(N) | H(username) | salt | A | B | K)
+	hg := srpHash(padToN(srpG))
+	hn := srpHash(srpN.Bytes())
+	hi := srpHash(username)
+	hxor := make([]byte, len(hg))
+	for i := range hg {
+		hxor[i] = hg[i] ^ hn[i]
+	}
+	h := srpHashFunc()
+	h.Write(hxor)
+	h.Write(hi)
+	h.Write(salt)
+	h.Write(A)
+	h.Write(B)
+	h.Write(K)
+	expected := h.Sum(nil)
+	assert.Equal(t, expected, m1)
+}
+
+func TestCalculateM2(t *testing.T) {
+	A := make([]byte, srpNLenBytes)
+	M1 := make([]byte, 32)
+	K := make([]byte, 32)
+
+	A[srpNLenBytes-1] = 0x01
+	M1[0] = 0x02
+	K[0] = 0x03
+
+	m2 := calculateM2(A, M1, K)
+	assert.Equal(t, sha256.Size, len(m2))
+
+	// Verify: M2 = H(A | M1 | K)
+	h := srpHashFunc()
+	h.Write(A)
+	h.Write(M1)
+	h.Write(K)
+	assert.Equal(t, h.Sum(nil), m2)
+}
+
+func TestNewSRPClient(t *testing.T) {
+	client := newSRPClient()
+
+	// A must be non-zero and in range [1, N-1]
+	assert.True(t, client.A.Sign() > 0, "A must be positive")
+	assert.True(t, client.A.Cmp(srpN) < 0, "A must be less than N")
+
+	// A = g^a mod N
+	expectedA := new(big.Int).Exp(srpG, client.a, srpN)
+	assert.Equal(t, 0, expectedA.Cmp(client.A), "A must equal g^a mod N")
+
+	// getABytes returns padded A
+	aBytes := client.getABytes()
+	assert.Equal(t, srpNLenBytes, len(aBytes))
+	recoveredA := new(big.Int).SetBytes(aBytes)
+	assert.Equal(t, 0, client.A.Cmp(recoveredA))
+
+	// Two clients should have different secrets (probabilistic, but 2^256 collision chance)
+	client2 := newSRPClient()
+	assert.NotEqual(t, 0, client.a.Cmp(client2.a), "two clients must have different secrets")
+}
+
+func TestProcessChallenge(t *testing.T) {
+	// Use a fixed secret for deterministic testing
+	fixedSecret := make([]byte, 32)
+	fixedSecret[31] = 0x42 // small non-zero value
+
+	a := new(big.Int).SetBytes(fixedSecret)
+	A := new(big.Int).Exp(srpG, a, srpN)
+	k := getMultiplier()
+
+	client := &srpClient{
+		a: a,
+		A: A,
+		k: k,
+	}
+
+	username := []byte("testuser@apple.com")
+	derivedKey := make([]byte, 32)
+	for i := range derivedKey {
+		derivedKey[i] = byte(i)
+	}
+	salt := []byte("fixed_salt_for_test!")
+
+	// Compute a valid B: B = (k*v + g^b) mod N where v = g^x mod N
+	x := calculateX(salt, derivedKey)
+	v := new(big.Int).Exp(srpG, x, srpN)
+
+	bSecret := make([]byte, 32)
+	bSecret[31] = 0x99
+	b := new(big.Int).SetBytes(bSecret)
+
+	// B = (k*v + g^b) mod N
+	kv := new(big.Int).Mul(k, v)
+	gb := new(big.Int).Exp(srpG, b, srpN)
+	B := new(big.Int).Add(kv, gb)
+	B.Mod(B, srpN)
+
+	serverB := padToN(B)
+
+	client.processChallenge(username, derivedKey, salt, serverB)
+
+	// M1, M2, K must be populated
+	assert.NotNil(t, client.M1)
+	assert.NotNil(t, client.M2)
+	assert.NotNil(t, client.K)
+	assert.Equal(t, sha256.Size, len(client.M1))
+	assert.Equal(t, sha256.Size, len(client.M2))
+	assert.Equal(t, sha256.Size, len(client.K))
+
+	// M1 and M2 must be different
+	assert.NotEqual(t, client.M1, client.M2)
+
+	// Results must be deterministic — run again with same inputs
+	client2 := &srpClient{
+		a: new(big.Int).Set(a),
+		A: new(big.Int).Set(A),
+		k: new(big.Int).Set(k),
+	}
+	client2.processChallenge(username, derivedKey, salt, serverB)
+
+	assert.Equal(t, client.M1, client2.M1, "M1 must be deterministic")
+	assert.Equal(t, client.M2, client2.M2, "M2 must be deterministic")
+	assert.Equal(t, client.K, client2.K, "K must be deterministic")
+
+	// Verify the server could also compute S and get the same K.
+	// Server computes: S_server = (A * v^u)^b mod N
+	u := calculateU(A, B)
+	vu := new(big.Int).Exp(v, u, srpN)
+	avu := new(big.Int).Mul(A, vu)
+	avu.Mod(avu, srpN)
+	sServer := new(big.Int).Exp(avu, b, srpN)
+	sServer.Mod(sServer, srpN)
+	kServer := calculateK(padToN(sServer))
+	assert.Equal(t, client.K, kServer, "client and server must derive the same session key")
+}
+
+func TestSRPGroupParameters(t *testing.T) {
+	// Verify the 2048-bit prime starts with known hex prefix
+	nHex := hex.EncodeToString(srpN.Bytes())
+	assert.True(t, len(nHex) > 0)
+	assert.Equal(t, "ac6bdb41", nHex[:8], "N must start with known prefix")
+
+	// g must be 2
+	assert.Equal(t, int64(2), srpG.Int64())
+
+	// N byte length must match expected
+	assert.Equal(t, 256, srpNLenBytes)
+	assert.Equal(t, 256, len(srpN.Bytes()))
+}
+
+func TestSRPHash(t *testing.T) {
+	data := []byte("hello world")
+	hash := srpHash(data)
+
+	// Must be SHA-256 output
+	assert.Equal(t, sha256.Size, len(hash))
+
+	// Must equal standard sha256
+	expected := sha256.Sum256(data)
+	assert.Equal(t, expected[:], hash)
+}

docs/content/iclouddrive.md

@@ -12,13 +12,27 @@ The initial setup for an iCloud Drive backend involves getting a trust token/ses
 This can be done by simply using the regular iCloud password, and accepting the code
 prompt on another iCloud connected device.
 
-**IMPORTANT**: At the moment an app specific password won't be accepted. Only
+**IMPORTANT**: App-specific passwords are not accepted. Only use your
-use your regular password and 2FA.
+regular Apple ID password and 2FA.
 
 `rclone config` walks you through the token creation. The trust token is valid
 for 30 days. After which you will have to reauthenticate with `rclone reconnect`
 or `rclone config`.
 
+## Authentication
+
+rclone authenticates with Apple using [SRP (Secure Remote Password)](https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol),
+the same protocol used by the iCloud web interface. Your password is never
+sent to Apple's servers -- instead, a cryptographic proof is exchanged that
+verifies you know the password without revealing it.
+
+The authentication flow is:
+
+1. rclone initiates a session with Apple's identity service
+2. An SRP key exchange takes place (your password is used locally to derive a key)
+3. Apple sends a 2FA prompt to your trusted devices
+4. After you enter the 2FA code, rclone receives a trust token for future sessions
+
 Here is an example of how to make a remote called `iclouddrive`.  First run:
 
 ```console