What is the proper way to send an SMS to US users?

To implement SMS verification for US users in your AWS-based app, you can use AWS Simple Notification Service (SNS) to send SMS messages for verification. Start by generating a unique one-time password (OTP) for each user during registration, storing it temporarily in a database, and sending the OTP to the user’s phone using SNS. Ensure the phone number is in the correct format and validated to avoid sending messages to invalid or fraudulent numbers.

To prevent abuse, set rate limits on how often users can request OTPs and consider using CAPTCHA to prevent bots from triggering SMS requests. You should also track failed OTP verification attempts and implement temporary blocking if abuse is detected. It's important to ensure compliance with US regulations, including 10-digit long code (10DLC) registration for high-volume SMS traffic and offering opt-out options for users.

Additionally, you can monitor SMS delivery with AWS CloudWatch to track successful or failed messages. If AWS SNS doesn’t meet your requirements, third-party services like Twilio or Nexmo offer advanced features such as phone number validation and built-in verification APIs. Finally, ensure a smooth user experience by clearly communicating the OTP process and providing alternatives like voice call verification if SMS fails.

Hi Gerard

Solution: AWS SNS and Amazon Pinpoint with Lambda for SMS Verification

Step-by-Step Guide

1. User Phone Number Submission:

• When a user enters their phone number in the app, store it temporarily in a database (e.g., DynamoDB) and generate a unique verification code (typically 4-6 digits).
• Optionally, use Amazon Cognito for user management and verification workflows, as it natively supports MFA with SMS verification.

2. Generate Verification Code:

• Use a secure random number generator (AWS Lambda or directly in your backend) to create a unique verification code for each phone number submission.

3. Send SMS with AWS SNS (Simple Notification Service):

• Use AWS SNS to send the verification code via SMS to the user. Amazon SNS can handle SMS delivery globally, and it's simple to integrate.

• Key Points:

• Ensure that SNS is configured for sending SMS in the US region.

• Set the correct SNS topic or direct-to-phone messaging.

Code Sample:

import boto3

sns = boto3.client('sns')
phone_number = "+1XXXXXXXXXX"  # User's phone number
message = f"Your verification code is: {verification_code}"

response = sns.publish(
    PhoneNumber=phone_number,
    Message=message
)

4. Handle Abuse Prevention (Throttling & Rate Limiting):

• Implement rate limiting and throttling mechanisms to prevent abuse.

  • Limit the number of SMS verification requests per phone number (e.g., 3 attempts in 10 minutes).
  • Use AWS API Gateway and Lambda to handle this logic, or implement rate-limiting directly in your backend service (e.g., by storing request timestamps in DynamoDB and checking before sending SMS).

5. Store Verification State:

• Store the verification code along with a timestamp and phone number in a database such as DynamoDB.
• Set a TTL (Time to Live) for the verification code to expire after a defined period (e.g., 5-10 minutes). This prevents old codes from being reused.

6. User Verification:

• When the user enters the code sent via SMS, validate it against the stored verification code in the database.
• If the code matches and hasn’t expired, mark the phone number as verified in the database or user profile.

7. Monitor & Adjust Delivery Success:

• Use Amazon Pinpoint for detailed SMS analytics. Amazon Pinpoint offers:

    * SMS delivery success rate monitoring.
   * Support for promotional and transactional messages.
   * Compliance features to manage opt-out lists, especially in the US where certain regulations apply to SMS.
  

• Pinpoint can be used for more complex scenarios if you want to track engagement, delivery rates, and user behavior for the SMS campaigns.

Additional Security Measures:

Phone Number Validation:

• Before sending the SMS, validate that the phone number follows the E.164 format and is a valid US-based number. This can be done using third-party APIs or AWS Pinpoint's built-in phone number validation.

CAPTCHA Integration:

• Integrate Google reCAPTCHA or hCaptcha to prevent bots from registering phone numbers and abusing the SMS verification system.

MFA (Multi-Factor Authentication):

• As an extra security layer, consider integrating multi-factor authentication using AWS Cognito to enforce SMS-based MFA during user registration.

Cost Considerations:

• Sending SMS in the US via SNS is cost-effective, but the pricing varies based on the message type (transactional vs. promotional) and volume. Using Amazon Pinpoint for tracking and advanced SMS management may incur additional costs but provides more control over the SMS campaigns.