AES Decryption with key derivation function

Question

I am working on porting a decryption function from .NET to elixir as a proof of concept.

Can anyone give me some guidance on if this is doable with the Erlang crypto module? I have played around the block_decrypt functions but am never able to get the correct result. I am thinking my problem is coming from an incorrect key and IV.

I am unsure how to derive byte data from the encrypted plain text value to pass into block_decrypt.

Here is the elixir code I'm using to attempt to decrypt:

defmodule TestApp.Decrypt do
  @iv <<30,64,180,159,172,197,92,10,197,3,39,75,53,92,93,37>>

  def unpad(data) do
    to_remove = :binary.last(data)
    :binary.part(data, 0, byte_size(data) - to_remove)
  end


  def decrypt(data, key) do
    IO.puts "WOrking to decrypt #{data} using #{key}"
    padded = :crypto.block_decrypt(:aes_cbc256, key, @iv, :base64.decode(data))
    unpad(padded)
  end
end

I have a 32 byte key I'm trying to pass in but am getting this error:

Erlang error: :notsup

The Crypto Library points that error to the fact that dirty scheduler wasn't enabled on my erlang build, but I don't know if I'm going about this in the right direction before I look into that.

It would be better if you post the code used to encrypt, sample unencrypted and encrypted text (hexdump), and the code you wrote to decrypt which didn't work. I successfully implemented decryption functions for a client where the data was being encrypted by some crypto module in Perl, so as long as your algorithm is implemented by Erlang's crypto, it should be possible. — Dogbert
@Dogbert, thanks for the input. Ill put something together and update my question. — Botonomous
In the crypto docs it says May throw exception notsup in case the chosen Type is not supported by the underlying OpenSSL implementation. — ryanwinchester
fwiw, I can run it without an exception if I remove unpad: s.ryanwinchester.ca/2W2n0L0H062v — ryanwinchester

Dan Draper Dan Draper · Accepted Answer · 2019-06-08T08:24:21

The erlang crypto errors are not very helpful. It looks like you might have the wrong cipher specified. Instead of aes_cbc256 you probably want aes_256_cbc. See http://erlang.org/doc/man/crypto.html#block_decrypt-4

You may also like to check-out a project I'm working on called Apoc which encapsulates this stuff into a more "Elixir like" API: https://github.com/coderdan/apoc

It supports AES in GCM mode so far but I would love to have some contributions for other ciphers!

AES Decryption with key derivation function

1 Answers