Convert C# function to Elixir

danish · October 25, 2018, 2:08pm

this is the function that I am trying to convert to Elixir:

private static byte[] HashBody(string content)
   {
       var contentBytes = Encoding.UTF8.GetBytes(content);

       using (var provider = new SHA256Managed())
       {
          var hash = provider.ComputeHash(contentBytes);
          return hash;
        }
    }

My attempt:

content = %{
  "id" => "123",
  "name" => "Jaqob"
} |> Poison.encode!

digest = :crypto.hash(:sha256, content)

I want to convert content variable to bytes which is where I am stuck, also is the hashing function correct?

easco · October 25, 2018, 3:04pm

Your content variable is already a Binary (an Elixir string is a Binary) so it is already “bytes”.

Your code looks complete to me. Where are you having problems?

Eiji · October 25, 2018, 3:05pm

How about using :erlang.term_to_binary/1? With this you can work with any data you want unlike Poison.

jordan0day · October 25, 2018, 3:12pm

I think @easco’s answer is pretty much correct, but I’m guessing what you’re after is maybe a list of bytes? Since your C# code returns an array of bytes? You can use :erlang.binary_to_list/1 on digest to convert the binary to a list.

danish · October 29, 2018, 5:46pm

Thanks for your prompt response @jordan0day , @Eiji , @easco
I needed to convert the map/json to string in order to match the hashed value.
I am now unable to match the Signature, in the elixir code I have include the expected Base64 string:

This is the signature code in C#:

public static string GenerateDigitalSignature(string body, string privateKeyText)
{
var hash = HashBody(body);

byte[] signedHash;
using (var privateKey = ImportPrivateKey(privateKeyText))
{
signedHash = privateKey.SignHash(hash, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
Console.WriteLine("signedHash");
}

var encodedHash = Convert.ToBase64String(signedHash);
Console.WriteLine(encodedHash);

return encodedHash;
}

and this is my code in elixir

def test(conn, _params) do
{:ok, rawSkey} = File.read("/home/danish/www/apis/signtest/keys/private_key.pem")

[encSKey] = :public_key.pem_decode(rawSkey)
sKey = :public_key.pem_entry_decode(encSKey)

string = "{\"MachineName\":\"DESKTOP-04VG548\",\"Userame\":\"inder\",\"Timestamp\":\"2018-10-26T20:58:48.7840832Z\"}"

msg = :crypto.hash(:sha256, string)

signature = :public_key.sign(msg, :sha256, sKey,  [{:rsa_padding, :rsa_pkcs1_padding}])
           |> Base.encode64

#
IO.inspect("signature")
IO.inspect(signature)
IO.inspect("EXPECTED SIGNATURE")
IO.inspect("1FdzHD5FhYFEbV9NpbXpE/CfrvHcv8p/LDQxMd900iZQxrSNbNqwQ/3coWO2Pof3nCmRIPicURyvp3+UrRALp/4eEFprdzmITFh9ojVrOvFI0FYqDsvEDQYvw6G2oX6vVw0b640dg3a+J3A8KEUs3RwSuiilSiqi/CeUzRNFhI3+7aPYF5p0QrAnoNmgmK6GbIY6eu0WnMxyJcGsrkE2KYWCqxrfMq5AiKIPdZVdXfLM6+0AoVw2Q7EgUILNRA91v2QGkdpBwvpHX5Feau2NspS8sq8zdTz8wEVHXC0GjLMqK5M1VvWAW+evYi15qqPOtYHXrvN2KJXjEwSbFdRc7A==")
json conn, "ok"

end

private_key:

what do you reckon I am doing wrong?

voltone · October 29, 2018, 6:27pm

This line (above) is unnecessary: your call to :public_key.sign already calculates a SHA256 over the input, so right now the input string is hashed twice. Just pass string instead of msg to the sign function.

voltone · October 29, 2018, 6:35pm

BTW, just FYI and FWIW, a bit more idiomatic Elixir would be:

def test(conn, _params) do
  raw_skey = File.read!("/home/danish/www/apis/signtest/keys/private_key.pem")

  [enc_skey] = :public_key.pem_decode(raw_skey)
  skey = :public_key.pem_entry_decode(enc_skey)

  string = ~s("MachineName":"DESKTOP-04VG548","Userame":"inder","Timestamp":"2018-10-26T20:58:48.7840832Z"})

  signature =
    string
    |> :public_key.sign(:sha256, skey,  rsa_padding: :rsa_pkcs1_padding)
    |> Base.encode64

  IO.puts("signature")
  IO.inspect(signature)
  json conn, "ok"
end

danish · October 30, 2018, 9:24am

thank you @voltone, this resolves my issue.