Hey everyone!
I’m currently implementing sign in with apple on my app. I’m already at the point where I got the JWT that I need to decode to validate the user and the key hash from apple (includes the kid, the alg etc…
How can I use the information from the key hash to decode the JWT correctly?
In Ruby it seems pretty straight forward with
But I’m having a hard time finding the equivalent in Elixir. I took a look at JOSE.JWK but I’m not sure which method I should use to generate the JWK and fetch the public key afterward
Thanks!
With Assent it is very easy too (in my mix file: {:assent, "~> 0.1.13"}). You just have to pass the apple authorization code (ASAuthorizationAppleIDCredential → authorizationCode) and the apple_id is then verified. No need to manually pass the public key and algorithm. You don’t even need to pass the identityToken:
def check_apple_sign_in(_apple_jwt, apple_auth) do
[
client_id: "com.your_domain.your_app",
team_id: "YOUR_ID",
private_key_id: "KEY_ID",
private_key_path: "/Users/you/development/Keys/AuthKey_2YZ4LV4PYI.p8",
redirect_uri: nil
]
|> Assent.Config.put(:session_params, %{})
|> Assent.Strategy.Apple.callback(%{"code" => apple_auth})
|> case do
{:ok, %{user: user, token: token}} ->
%{"sub" => user_id} = user
Logger.info "apple sign in verification successful. user: #{Kernel.inspect(user)}; token: #{Kernel.inspect(token)}"
{:ok, user_id} #the user_id you are interested in
error ->
Logger.info "apple sign in verification not successful: #{Kernel.inspect(error)}"
{:error, :authentication}
end
end
Just in case: please note that jose does not validate any claims of a jwt. It only verifies the signature - so you have to check all other things by yourself - especially exp and aud. Just don’t forget that!
Joken provides sane default behavior - which is useful. I did jwt handling both jose only and jose/joken and both are pretty straightforward.
Probably harder part is to parse a key in PEM format haha.
defmodule Accounts.AppleAuth do
@apple_keys_url "https://appleid.apple.com/auth/keys"
@aud "com.blahblah"
def verify_identity_token(identity_token) do
# Fetch Apple's public keys
case get_apple_public_keys() do
{:ok, apple_keys} ->
with {:ok, token} <- decode_and_verify(identity_token, apple_keys) do
verify_claims(token)
end
{:error, reason} ->
{:error, reason}
end
end
defp get_apple_public_keys() do
case HTTPoison.get(@apple_keys_url) do
{:ok, %HTTPoison.Response{status_code: 200, body: body}} ->
case Jason.decode(body) do
{:ok, %{"keys" => keys}} ->
{:ok, keys}
{:error, _} ->
{:error, "Failed to decode Apple's public keys"}
end
{:ok, %HTTPoison.Response{status_code: status_code}} ->
{:error, "Failed to fetch Apple's public keys, status code: #{status_code}"}
{:error, %HTTPoison.Error{reason: reason}} ->
{:error, "HTTP request failed: #{reason}"}
end
end
defp decode_and_verify(token, apple_keys) do
# Find the appropriate key for the token
key = find_apple_key(token, apple_keys)
if key do
jwk = JOSE.JWK.from(key)
{verified, jwt, _} = JOSE.JWT.verify_strict(jwk, ["RS256"], token)
if verified, do: {:ok, jwt}, else: {:error, "Invalid token signature"}
else
{:error, "No matching key found"}
end
end
defp verify_claims(jwt) do
%JOSE.JWT{fields: claims} = jwt
case claims do
%{"aud" => @aud, "exp" => exp} ->
current_time = System.system_time(:second)
if exp > current_time do
{:ok, claims}
else
{:error, "Token has expired"}
end
_ ->
{:error, "Invalid claims"}
end
end
defp find_apple_key(token, keys) do
# Decode token header to find matching `kid`
%JOSE.JWS{fields: %{"kid" => kid}} = JOSE.JWT.peek_protected(token)
Enum.find(keys, fn key -> key["kid"] == kid end)
end
end
The above seems to be working for me! Hope it helps someone out one day.
This is more than enough and you shouldn’t need anything more, same goes for android, they have their verification key at a known URL, in the exact same format (at least it used to be about 4 years ago when I implemented the same thing).
