Custom SMS Sender Lambda Trigger

The CustomSMSSender trigger is invoked from within your Lambda function code to enable a third-party provider to send SMS notifications to your users. Using this trigger involves five main steps:

Note

The CustomSMSSender trigger isn't available in the Amazon Cognito console.

• Create a Lambda function for the CustomSMSSender.

• Create an encryption key in AWS KMS.

• Grant Amazon Cognito service principal cognito-idp.amazonaws.com access to invoke the Lambda function.

• Edit the code in your Lambda function to include third-party providers.

• Update your user pool to add custom triggers.

Important

For additional security you must configure a symmetric customer master key in AWS KMS, when CustomEmailSender or CustomSMSSender is configured with your user pool. Amazon Cognito uses your configured KMS key to encrypt codes or temporary passwords. Amazon Cognito sends the base64 encoded ciphertext to your Lambda functions. For more information see, Symmetric customer master keys

Enable the CustomSMSSender Lambda trigger

You can enable the CustomSMSSender trigger using a Lambda function.

Step 1: Create a Lambda function

Create a Lambda function for the CustomSMSSender trigger. Amazon Cognito uses the AWS Encryption SDK to encrypt the secrets (temporary passwords or authorization codes).

Step 2: Create an encryption key in AWS KMS

Create an encryption key in AWS KMS. This key will be used to encrypt temporary passwords and authorization codes that Amazon Cognito generates. You can then decrypt these secrets in the custom sender Lambda function to be able to send them to the end user in plaintext.

Step 3: Grant Amazon Cognito service principal cognito-idp.amazonaws.com access to invoke the Lambda function

Use the following command to grant access to the Lambda function:

        aws lambda add-permission --function-name lambda_arn --statement-id "CognitoLambdaInvokeAccess" --action lambda:InvokeFunction --principal cognito-idp.amazonaws.com
    

Step 4: Edit the code to use custom sender

Amazon Cognito uses AWS Encryption SDK to encrypt secrets (temporary passwords and authorization codes) before sending them to the custom sender Lambda function. You need to decrypt these secrets before sending them to end users using the custom provider of your choice. To use the AWS Encryption SDK with your Lambda function, you need to package the SDK with your function. For information, see Installing the AWS Encryption SDK for JavaScript. You can also update the Lambda package by completing the following steps.

1. Export the Lambda function package from the console

2. Unzip the package.

3. Add the AWS Encryption SDK to the package. For example, if you are using Node.js, then add the node_modules directory and include the libraries from @aws-crypto/client-node.

4. Recreate the package.

5. Update the Lambda function code from the modified directory.

Step 5: Update user pool to add custom sender Lambda triggers

Update the user pool to add the CustomSMSSender trigger.

Code examples

    #Send the parameter to update-user-pool along with any existing user pool configurations.
              
      --lambda-config "CustomSMSSender={LambdaVersion=V1_0,LambdaArn= lambda-arn },KMSKeyID= key-id"
               
 

The following Node.js example shows how to use the CustomSMSSender Lambda function.

            const AWS = require('aws-sdk');
            const b64 = require('base64-js');
            const encryptionSdk = require('@aws-crypto/client-node');
            
            #Configure the encryption SDK client with the KMS key from the environment variables.  
            
            const { encrypt, decrypt } = encryptionSdk.buildClient(encryptionSdk.CommitmentPolicy.REQUIRE_ENCRYPT_ALLOW_DECRYPT);
            const generatorKeyId = process.env.KEY_ALIAS;
            const keyIds = [ process.env.KEY_ID ];
            const keyring = new encryptionSdk.KmsKeyringNode({ generatorKeyId, keyIds })
            exports.handler = async (event) => {

            #Decrypt the secret code using encryption SDK.
          
            let plainTextCode;
            if(event.request.code){
            const { plaintext, messageHeader } = await decrypt(keyring, b64.toByteArray(event.request.code));
            plainTextCode = plaintext
            }

            #PlainTextCode now has the decrypted secret.
            
            if(event.triggerSource == 'CustomSMSSender_SignUp'){
            
           #Send sms to end-user using custom or 3rd party provider.
           #Include temporary password in the email.
            
            }else if(event.triggerSource == 'CustomSMSSender_ResendCode'){
            
            }else if(event.triggerSource == 'CustomSMSSender_ForgotPassword'){
            
            }else if(event.triggerSource == 'CustomSMSSender_UpdateUserAttribute'){
            
            }else if(event.triggerSource == 'CustomSMSSender_VerifyUserAttribute'){
            
            }else if(event.triggerSource == 'CustomSMSSender_AdminCreateUser'){
            
            }else if(event.triggerSource == 'CustomSMSSender_AccountTakeOverNotification'){
            
            }
            
            return;
            };
            
             

Custom SMS sender Lambda trigger sources

The following table shows the triggering event for custom SMS trigger sources in your Lambda code.

TriggerSource value	Triggering event
CustomSMSSender_SignUp	Custom message to send the confirmation code after sign-up.
CustomSMSSender_ResendCode	To send the temporary password to a new user.
CustomSMSSender_ForgotPassword	To resend the confirmation code to an existing user.
CustomSMSSender_UpdateUserAttribute	When a user's email or phone number is changed, this trigger sends a verification code automatically to the user. This can't be used for other attributes.
CustomSMSSender_VerifyUserAttribute	This trigger sends a verification code to the user when they manually request it for a new phone number.
CustomSMSSender_Authentication	To send MFA code during authentication.
CustomSMSSender_AdminCreateUser	To send the temporary password to a new user.