AmazonFPS IPN Signature.rb

def valid_request?
  query = params.except('action', 'controller')
  
  supplied_signature = query.delete('signature')
  return false unless supplied_signature

  query = query.sort_by { |k,v| k.downcase }
  
  digest = OpenSSL::Digest::Digest.new('sha1')
  hmac = OpenSSL::HMAC.digest(digest, AMAZON_SECRET_KEY, query.to_s)
  encoded = Base64.encode64(hmac).chomp
  
  encoded == supplied_signature
end