Secure Biometric Login

TL;DR

This guide shows you how to securely handle biometric authentication data against a server. We’ll focus on not storing the actual biometric data, but instead using it to create a secure key for verification.

Steps

1. Understand the Risks

• Storing raw biometric data (fingerprints, face scans) is a huge security risk. If your server is hacked, that data is compromised.
• Biometric data can’t be easily changed if stolen – unlike passwords.

Choose a Biometric Method

• Consider the user experience and security level needed. Fingerprint scanners are common, but facial recognition or voice biometrics might be suitable.
• The method will determine how you get an initial biometric reading from the device.

Client-Side Processing: Create a Biometric Template

This happens on the user’s device (phone, laptop). Never send raw biometric data to your server.

• Use a secure library or API provided by the operating system (e.g., Android BiometricPrompt, Apple LocalAuthentication). These libraries handle the complex biometric processing and security aspects.
• The library will create a template from the biometric data. This template is a mathematical representation of the unique features – not the actual image or scan.

Server-Side: Generate a Unique Key

When the user first registers, you need to link their biometric template to a secure key on your server.

• Generate a strong, random secret key for each user. This is used for verification later.
• Hash the biometric template using a strong cryptographic hash function (e.g., SHA-256) along with a unique salt specific to that user. This creates a secure identifier.
• Store the hashed template and the secret key in your database, associated with the user’s account.

# Example (Python - using hashlib for demonstration only)
import hashlib
salt = 'your_unique_salt'
hashed_template = hashlib.sha256((biometric_template + salt).encode()).hexdigest()

Authentication Process

1. Client-Side: Capture Biometric Data Again

• Use the same biometric method as registration.
• Create a new biometric template from the captured data using the secure library.

Send Hashed Template to Server

• Hash the new template on the client-side, using the same salt as during registration.
• Send only this hashed template to your server.

Server-Side: Verify the Hash

• Retrieve the stored hashed template for the user from the database.
• Compare the received hash with the stored hash. If they match, authentication is successful.

# Example (Python)
if hashlib.sha256((received_hash + salt).encode()).hexdigest() == stored_hashed_template:
  # Authentication successful!
else:
  # Authentication failed.

Use the Secret Key

• Once authentication is confirmed, use the user’s secret key to encrypt/decrypt sensitive data or grant access to resources. Do not expose the secret key directly to the client.

Security Considerations

• Salt Uniqueness: Each user must have a unique salt. This prevents rainbow table attacks.
• Key Storage: Protect your database containing the secret keys with strong encryption and access controls.
• Transport Layer Security (TLS): Always use HTTPS to encrypt communication between the client and server.
• Rate Limiting: Implement rate limiting on authentication attempts to prevent brute-force attacks.
• Regular Updates: Keep your biometric libraries and cryptographic functions up to date to address security vulnerabilities.