Securing your callback

Once your server is configured to receive callbacks, it'll listen for any request sent to the endpoint you configured.

In order to secure your data, you will want to make sure the callbacks are coming from Xfers in order to avoid attackers sending malformed data. The easiest way to verify the callbacks is to validate the signature that is sent along with the request.

1. Retrieve the signing secret from your dashboard

Once you have saved your application's callback end-point, we will provide you with a "Signing Secret" for each end-point.

2. Calculate and compare HMAC Signature

a. Xfers gives you signature in the header of each callback, i.e. “Xfers-Signature”
b. You can verify the signature to see if it did indeed came from Xfers
c. Calculate the signature by using signing secret (see screenshot above) and the callback payload body with HMAC-SHA256 algorithm
d. Use the calculated signature in (c) to match with (a)

Example:

# NOTE: It's recommended for you to put the secret in a secret file or environment variable.
# We are showing it directly in the code for illustration purposes only.
SIGNING_SECRET = "ss_5572cf13d099"

post '/callback' do
  request.body.rewind
  payload_body = request.body.read
  
  verify_signature!(request.headers["Xfers-Signature"], payload_body)

  puts "Got a valid request"
end

def verify_signature!(signature, payload_body)
  generated_signature = OpenSSL::HMAC.hexdigest(
    "SHA256", 
    SIGNING_SECRET,
    payload_body,
  )
  
  raise "Invalid signature!" unless Rack::Utils.secure_compare(signature, generated_signature)
end

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