Signed request query string with Elixir

SignedRequest is a simple Elixir module created to sign request query strings using HMAC.

Let’s say app-1 generates a page with a list of images dynamically generated by app-2. Something like:

    <img src="http://app-2.local/generate-image?type=png&size=200">

The URL of that image is viewable by users, this means that someone can maliciously open that link and change the size parameter to a big number, making app-2 blow up.

Instead of adding validations for each parameter we receive in the query string, we can just sign the request so that app-2 knows that the url has been generated by app-1.

Both apps share the same secret key:

config :signed_request, :secret_key, "SECRET-KEY-SHARED-BY-BOTH-APPS"

app-1 uses SignedRequest.SignedURI to sign the query string:

query = %{type: :png, size: 200}
query_string = SignedRequest.SignedURI.encode_query(query)

The generated query string contains now a sig parameter:


app-2 can now decode the query string and validate the signature:

case SignedRequest.SignedURI.decode_query(query) do
  {:ok, query} ->
  {:error, :invalid_hmac} ->