Extract secret from otpauth uri

Author: samueltardieu

lib/nimble_totp.ex

@@ -157,6 +157,23 @@ defmodule NimbleTOTP do
     "otpauth://totp/#{URI.encode(label)}?#{query}"
   end
 
+  @doc """
+  Extract the secret from an otpauth URI.
+
+  ## Examples
+
+      iex> NimbleTOTP.secret_from_uri("otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme")
+      {:ok, "abcd"}
+
+  """
+  @spec secret_from_uri(String.t()) :: {:ok, <<>>} | {:error, any()}
+  def secret_from_uri(uri) when is_binary(uri) do
+    with {:ok, uri} <- URI.new(uri),
+         query = URI.decode_query(uri.query, %{}, :rfc3986),
+         {:ok, secret} <- Map.fetch(query, "secret"),
+         do: Base.decode32(secret, padding: false)
+  end
+
   @doc """
   Generate a binary composed of random bytes.