How to Implement OTP-Passwordless Login Using Either Email or Phone Number in Amazon Cognito?

I have successfully implemented an OTP-passwordless login flow following this tutorial with some adaptations.

When creating a new user in Amazon Cognito, I’m asked to choose between the user’s primary sign-in attribute:

I’ve also configured some POSTMAN scripts to run the flow successfully. Here’s the request to initiate the flow:

Request Details:

• Method: POST

• URL: https://cognito-idp.<region_id>.amazonaws.com/

• Headers:

  • Content-Type: application/x-amz-json-1.1
  • X-Amz-Target: AWSCognitoIdentityProviderService.InitiateAuth

• Body:

  {
    "AuthFlow": "CUSTOM_AUTH",
    "ClientId": <CLIENT-ID>,
    "AuthParameters": {
      "USERNAME": <email or phone>
    }
  }
  

Current Behavior: I can successfully complete the OTP login flow if:

• The username attribute is filled with the email, and the user was created with email as the primary sign-in attribute.
• The username attribute is filled with the phone number, and the user was created with the phone number as the primary sign-in attribute.

However, I cannot use both email and phone number interchangeably for a given user, as it depends on the primary sign-in attribute selected at the time of user creation.

Issue: I am trying to implement a flow where either the email or phone number can be used for a single user regardless of the primary sign-in attribute. Currently, this seems to be restricted by the sign-up configuration.

Details: In my implementation, the DefineAuthChallenge Lambda function receives an empty user and the userNotFound attribute set to true in the event request when trying to use the non-primary attribute.

Question: Is it possible to configure Amazon Cognito to allow either email or phone number for OTP-passwordless login for a single user? I couldn’t find any documentation on this specific use case. I found this question but I cannot get anything from this resolution.

Any guidance or references to relevant documentation would be greatly appreciated.

Thank you!

Firstly if this is a client side application that calls Cognito API directly, consider proxying via your own backend.

Now on the backend, you could use the username or email to lookup users using: ListUsers

Example request:

{
 "AttributesToGet": [ "email","phone_number" ],
 "Filter": "<<username|email>>",
 "Limit": number,
 "PaginationToken": "string",
 "UserPoolId": "string"
}

Expected Response:

{
"PaginationToken": "efgh5678EXAMPLE",
"Users": [
    {
        "Attributes": [
            {
                "Name": "sub",
                "Value": "eaad0219-2117-439f-8d46-4db20e59268f"
            },
            {
                "Name": "email",
                "Value": "testuser@example.com"
            }
        ],
        "Enabled": true,
        "UserCreateDate": 1682955829.578,
        "UserLastModifiedDate": 1689030181.63,
        "UserStatus": "CONFIRMED",
        "Username": "testuser"
    },
// TRUNCATED

Now simply use the Username from response to continue with the Authentication.