Fix RSA modulus bit size calculation and OTP 20 compatibility

lib/json_web_token/algorithm/rsa.ex

@@ -43,17 +43,25 @@ defmodule JsonWebToken.Algorithm.Rsa do
   end
 
   @doc "RSA key modulus, n"
-  def modulus(key), do: :crypto.mpint(Enum.at key, 1)
+  def modulus([_exponent, modulus | _rest]) do
+    bytes_list = Integer.digits(modulus, 256)
+    :erlang.list_to_binary(bytes_list)
+  end
 
   defp validate_params(sha_bits, key) do
     Common.validate_bits(sha_bits)
     validate_key_size(key)
   end
 
   # http://tools.ietf.org/html/rfc7518#section-3.3
-  defp validate_key_size(a_key) do
-    key = Util.validate_present(a_key)
-    weak_key(bit_size(modulus key) < @key_bits_min)
+  defp validate_key_size(key) when is_list(key) do
+    key = Util.validate_present(key) |> modulus()
+    weak_key(bit_size(key) < @key_bits_min)
+  end
+
+  defp validate_key_size(key) do
+    key = Util.validate_present(key)
+    weak_key(bit_size(key) < @key_bits_min)
   end
 
   defp weak_key(true), do: raise "RSA modulus too short"

lib/json_web_token/util.ex

@@ -26,15 +26,13 @@ defmodule JsonWebToken.Util do
   defp arithmetic_compare("", "", acc), do: acc
 
   @doc """
-  Return the string passed in, unless it is nil or an empty string
+  Return the parameter passed in, unless it is nil or an empty string
 
   ## Example
       iex> JsonWebToken.Util.validate_present("a")
       "a"
   """
-  def validate_present(param), do: validate_present(param, param == "")
-
-  defp validate_present(nil, _), do: raise "Param nil"
-  defp validate_present(_, true), do: raise "Param blank"
-  defp validate_present(param, _), do: param
+  def validate_present(nil), do: raise "Param nil"
+  def validate_present(""), do: raise "Param blank"
+  def validate_present(param), do: param
 end

test/json_web_token/algorithm/rsa_test.exs

@@ -59,8 +59,9 @@ defmodule JsonWebToken.Algorithm.RsaTest do
   end
 
   test "sign/3 w private_key size < key_bits_min raises" do
+    # private_key_weak.pem holds a 2000-bit key
     private_key = RsaUtil.private_key(@path_to_keys, "private_key_weak.pem")
-    assert byte_size(Rsa.modulus private_key) == 255
+    assert byte_size(Rsa.modulus private_key) == 250
     invalid_key(private_key, "RSA modulus too short")
   end
 

test/json_web_token/algorithm/rsa_util_test.exs

@@ -9,19 +9,19 @@ defmodule JsonWebToken.Algorithm.RsaUtilTest do
   test "private_key" do
     key = RsaUtil.private_key(@path_to_keys, "private_key.pem")
     assert length(key) == 3
-    assert byte_size(Rsa.modulus key) == 261
+    assert byte_size(Rsa.modulus key) == 256
   end
 
   test "public_key" do
     key = RsaUtil.public_key(@path_to_keys, "public_key.pem")
     assert length(key) == 2
-    assert byte_size(Rsa.modulus key) == 261
+    assert byte_size(Rsa.modulus key) == 256
   end
 
   test "private key with ASN.1 header" do
     key = RsaUtil.private_key(@path_to_keys, "private_key_asn1_header.pem")
     assert length(key) == 3
-    assert byte_size(Rsa.modulus key) == 261
+    assert byte_size(Rsa.modulus key) == 256
   end
 
   test "private key passed in directly" do