Verify Email/Phone Number

Authentication API that can be called from your mobile apps. This API utilizes an in-app webview with cookies sharing to allow a single-sign-on for bypassing email and phone number verification.
Concepts: Learn about how Sign in with Email/Phone Number works.


Verifying email and phone number in your mobile app using our Authentication API consists of the following steps:
  1. 1.
    Open a WebView within your app with shared cookies
  2. 2.
    Direct users to Cotter's Auth page
  3. 3.
    Redirect back to your app with an authorization code
  4. 4.
    Call Cotter API with the authorization code
  5. 5.
    Get back the user's email or phone number, and whether or not it's verified
Here's an example on opening the in-app Browser from iOS and Android

What you're building

Authentication API for Android and iOS

Authorization Flow

For mobile apps, we're going to use the OAuth 2.0 Authorization Code Flow with Proof Key for Code Exchange (PKCE). This flow is recommended for Mobile Apps because:
  • Mobile apps can't securely store the Secret Key. This is because decompiling the App will reveal the Secret Key, and there's only one secret key so it'll be the same for all users.
  • Sending tokens to Custom URL schemes (ex. YourApp://) will potentially expose the tokens to malicious apps.


  1. 1.
    Create a code_verifier and a code_challenge
  2. 2.
    Request Authorization from Cotter: Redirect user to Cotter to verify their email/phone and receive an authorization_code back to your app.
  3. 3.
    Request Tokens and Identity: Send your authorization_code and code_verifier to Cotter server and get back a token and the user's email or phone number.
  4. 4.
    Include Token to your server: The token contains the user's verified email/phone number and a signature. Include this to your signup/login request to your backend

Step 1: Create a Code Verifier

A code_verifier is a cryptographically-random key that will be sent to Cotter along with the authorization_code on Step 3. Read more about what are code challenge and verifier.
Swift 3
function dec2hex(dec) {
return ('0' + dec.toString(16)).substr(-2)
function generateRandomString() {
var array = new Uint32Array(56/2);
return Array.from(array, dec2hex).join('');
var verifier = generateRandomString();
import os
import base64
verifier_bytes = os.urandom(32)
code_verifier = base64.urlsafe_b64encode(verifier_bytes).rstrip(b'=')
// import android.util.Base64;
SecureRandom sr = new SecureRandom();
byte[] code = new byte[32];
String verifier = Base64.encodeToString(code, Base64.URL_SAFE | Base64.NO_WRAP | Base64.NO_PADDING);
var buffer = [UInt8](repeating: 0, count: 32)
_ = SecRandomCopyBytes(kSecRandomDefault, buffer.count, &buffer)
let verifier = Data(bytes: buffer).base64EncodedString()
.replacingOccurrences(of: "+", with: "-")
.replacingOccurrences(of: "/", with: "\_")
.replacingOccurrences(of: "=", with: "")
.trimmingCharacters(in: .whitespaces)
NSMutableData *data = [NSMutableData dataWithLength:32];
int result __attribute__((unused)) = SecRandomCopyBytes(kSecRandomDefault, 32, data.mutableBytes);
NSString *verifier = [[[[data base64EncodedStringWithOptions:0]
stringByReplacingOccurrencesOfString:@"+" withString:@"-"]
stringByReplacingOccurrencesOfString:@"/" withString:@"_"]
stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"="]];

Step 1-b: Create a Code Challenge from Code Verifier

A code_challenge is the hashed version of your code_verifier. We will send this hash on step 2 when you're requesting an authentication from Cotter.
Swift 3
function sha256(plain) { // returns promise ArrayBuffer
const encoder = new TextEncoder();
const data = encoder.encode(plain);
return window.crypto.subtle.digest('SHA-256', data);
function base64urlencode(a) {
var str = "";
var bytes = new Uint8Array(a);
var len = bytes.byteLength;
for (var i = 0; i < len; i++) {
str += String.fromCharCode(bytes[i]);
return btoa(str)
.replace(/\+/g, "-")
.replace(/\//g, "_")
.replace(/=+$/, "");
async function challenge_from_verifier(v) {
hashed = await sha256(v);
base64encoded = base64urlencode(hashed);
return base64encoded;
var challenge = await challenge_from_verifier(verifier);
import hashlib
import base64
challenge_bytes = hashlib.sha256(code_verifier).digest()
code_challenge = base64.urlsafe_b64encode(challenge_bytes).rstrip(b'=')
// import android.util.Base64;
byte[] codeVerifierBytes = codeVerifier.getBytes("US-ASCII");
MessageDigest md = MessageDigest.getInstance("SHA-256");
byte[] codeChallengeBytes = md.digest();
String codeChallenge = Base64.encodeToString(codeChallengeBytes, Base64.URL_SAFE | Base64.NO_WRAP | Base64.NO_PADDING);
// Dependency: Apple Common Crypto library
guard let data = .utf8) else { return nil }
var buffer = [UInt8](repeating: 0, count: Int(CC_SHA256_DIGEST_LENGTH))
data.withUnsafeBytes {
_ = CC_SHA256($0, CC_LONG(data.count), &buffer)
let hash = Data(bytes: buffer)
let challenge = hash.base64EncodedString()
.replacingOccurrences(of: "+", with: "-")
.replacingOccurrences(of: "/", with: "\_")
.replacingOccurrences(of: "=", with: "")
.trimmingCharacters(in: .whitespaces)
// Dependency: Apple Common Crypto library
u_int8_t buffer[CC_SHA256_DIGEST_LENGTH * sizeof(u_int8_t)];
memset(buffer, 0x0, CC_SHA256_DIGEST_LENGTH);
NSData *data = [verifier dataUsingEncoding:NSUTF8StringEncoding];
CC_SHA256([data bytes], (CC_LONG)[data length], buffer);
NSData *hash = [NSData dataWithBytes:buffer length:CC_SHA256_DIGEST_LENGTH];
NSString *challenge = [[[[hash base64EncodedStringWithOptions:0]
stringByReplacingOccurrencesOfString:@"+" withString:@"-"]
stringByReplacingOccurrencesOfString:@"/" withString:@"_"]
stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"="]];
The code_challenge is sent first so that later in step 3, Cotter's server can verify that hash(code_verifier) is the same as code_challenge and that you are indeed made the original request.

Checking your code challenge and verifier

To check if your code_challenge and code_verifier are correctly generated and formatted, try comparing it with codes generated here

Step 2: Request Authorization from Cotter

Open Cotter's Auth URL from a WebView from your app.
Query Parameter
Your app's URL scheme where Cotter Auth will redirect back your users to your app
Example: or
The code_challenge you created in Step 1
A random string that you generate from your application before opening to Cotter's Auth (ex. abcXYZ456). This is not the same as your code_verifier. You need to check if the state included by Cotter in the redirect_url is the same as the initial state that you set to make sure the request is for you. Learn more about state.
Make sure the scheme of your redirect_url (the front part before ://) doesn't have an underscore or other special characters. To test it out, enter your redirect_url here:
Here's an example on opening the in-app Browser from iOS and Android
// Using ASWebAuthenticationSession
guard let authURL = URL(string: "<api_key_id>&redirect_url=yourapp://&type=PHONE&code_challenge=<code_challenge>&state=<state>") else { return }
let scheme = "yourapp://"
self.authSession = ASWebAuthenticationSession(url: authURL, callbackURLScheme: scheme)
{ callbackURL, error in
// Handle the callback.
if #available(iOS 13.0, *) {
self.authSession?.presentationContextProvider = self
} else {
// Fallback on earlier versions
// Dependencies: Trusted Web Activity
//build.gradle (Module:app)
android {
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
dependencies {
implementation ''
public class MainActivity extends AppCompatActivity {
static Uri LAUNCH_URI = Uri.parse("<api_key_id>&redirect_url=yourapp://&type=PHONE&code_challenge=<code_challenge>&state=<state>");
public void login(View view) {
new TwaLauncher(this).launch(LAUNCH_URI);
  1. 1.
    Open the user's browser with the URL above, also display it so the user can click it if opening the browser doesn't work.
  2. 2.
    Listen to the localhost port that you specified in the redirect URL
  3. 3.
    When the browser redirect back to you with the response below, handle the request and use the code, state, and challenge_id to continue to Step 3.


After the user's email or phone is verified, Cotter will redirect back to your app using redirect_url that you specified in step 2.
You should check that the state is the same as the initial state you passed in to the URL here.

Check out how to Handle the URL Scheme.

Step 3: Request Tokens and Identity

In this step, you'll use your code_verifier , authorization_code and the challenge_id to request tokens and the user's email or phone number from Cotter's server.
Your authorization_token is valid for 5 minutes, and can only be used once.
curl -XPOST \
-H 'Content-type: application/json' \
-H 'API_KEY_ID: <api_key_id>' \
-d '{
"code_verifier": "<code_verifier>",
"authorization_code": "<authorization_code>",
"challenge_id": <challenge_id>,
"redirect_url": "<redirect_url>"
}' ''
Request Tokens

Step 4: Include the Token to your Server

Now that the email or phone number is verified, you can continue your Sign Up or Login process by submitting the email or phone number to your server, either now or after the user enters more information.
You should include this oauth_tokens into your call to your backend for Login or Registration. Your backend should then verify that the access token is valid.

Validating Cotter's Access Token

Check out how to verify the OAuth Tokens from Cotter here:

🎉 You're done!

Securing your Project

Since you'll be using your API Key from a front-end website or mobile app, your API_KEY_ID is exposed to anyone inspecting your code. Here are some ways to prevent abuse:

What are State and Code Challenge?


Your app generates state=XYZ in the beginning of the auth flow. You should expect that Cotter's response on Step 2 when Cotter redirect back to your redirect_url, the state is the same (state == XYZ). This makes sure that the redirect was in response to your initial authentication request.

code_challenge / verifier:

This is needed for installed apps / SPA because they cannot store the Api Secret Key securely, so the code_challenge and code_verifier is for Cotter to make sure that the original App that requested authentication on Step 2 is the same as the one that asked for access token on Step 3.