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The goal of ExCrypto and ExPublicKey is to expose a subset of the functionality from the Erlang modules crypto and public_key so that writing secure Elixir applications is a little bit easier without being overwhelming. In many functions some sane defaults are provided to decrease the complexity of implementing those functions in your own code.

Using ExPublicKey

The ExPublicKey module provides functions for working with RSA public/private key operations. There are a couple common uses for public-key cryptography:

Authenticate a message

The goal of ExPublicKey.sign and ExPublicKey.verify is that the recipient of a message can identify the sender. This can be accomplished as follows:

Note: assume the message is a JSON payload.

  • Sender
    • serialize the JSON to a string
    • hash, time-stamp, and sign with your private-key
  • Receiver
    • ensure the time-stamp is within the current window
    • verify the signature with the sender's public-key

Here are the steps in order:

# load the RSA keys from a file on disk
rsa_priv_key = ExPublicKey.load!("/path/to/private_key.pem")
rsa_pub_key = ExPublicKey.load!("/path/to/public_key.pem")

# create the message JSON
msg = %{"name_first"=>"Chuck","name_last"=>"Norris"}

# serialize the JSON
msg_serialized = Poison.encode!(msg)

# generate time-stamp
ts = DateTime.utc_now |> DateTime.to_unix

# add a time-stamp
ts_msg_serialized = "#{ts}|#{msg_serialized}"

# generate a secure hash using SHA256 and sign the message with the private key
{:ok, signature} = ExPublicKey.sign(ts_msg_serialized, rsa_priv_key)

# combine payload
payload = "#{ts}|#{msg_serialized}|#{Base.url_encode64 signature}"
IO.puts payload

# pretend transmit the message...
# pretend receive the message...

# break up the payload
parts = String.split(payload, "|")
recv_ts = Enum.fetch!(parts, 0)
recv_msg_serialized = Enum.fetch!(parts, 1)
{:ok, recv_sig} = Enum.fetch!(parts, 2) |> Base.url_decode64

# pretend ensure the time-stamp is not too old (or from the future)...
# it should probably no more than 5 minutes old, and no more than 15 minutes in the future

# verify the signature
{:ok, sig_valid} = ExPublicKey.verify("#{recv_ts}|#{recv_msg_serialized}", recv_sig, rsa_pub_key)

# un-serialize the JSON
recv_msg_unserialized = Poison.Parser.parse!(recv_msg_serialized)
assert(msg == recv_msg_unserialized)

Note: this example is similar to the test "sign and verify a JSON payload" in test/ex_public_key_test.exs.

Transmit a shared secret

(in progress)

Load the keys from PEM format files

First load the public/private RSA keys from disk:

iex(1)> {:ok, rsa_private_key} = ExPublicKey.load("/tmp/test_rsa_private_key.pem")
{:ok, %ExPublicKey.RSAPrivateKey{...}}

iex(2)> {:ok, rsa_public_key} = ExPublicKey.load("/tmp/test_rsa_public_key.pem")
{:ok, %ExPublicKey.RSAPublicKey{...}}

Sign with RSA private key

To create a signature with the RSAPrivateKey like this:

iex(3)> message = "A very important message."
"A very important message."

ex(4)> {:ok, signature} = ExPublicKey.sign(message, rsa_private_key)
{:ok, <<...>>}

Verify signature with RSA public key

iex(5)> {:ok, valid} = ExPublicKey.verify(message, signature, rsa_public_key)
{:ok, true}

Encrypt with RSA public key

iex(6)> clear_text = "A super important message"
"A super important message"
iex(7)> {:ok, cipher_text} = ExPublicKey.encrypt_public(clear_text, rsa_public_key)
{:ok, "Lmbv...HQ=="}

Decrypt with RSA private key

iex(8)> {:ok, decrypted_clear_text} = ExPublicKey.decrypt_private(cipher_text, rsa_private_key)
{:ok, "A super important message"}

Using ExCrypto

The ExCrypto module provides relatively functions for AES cryptography operations.

Generate AES keys

Generate a new 128 bit AES key like this:

iex(1)> {:ok, aes_128_key} = ExCrypto.generate_aes_key(:aes_128, :bytes)
{:ok, <<...>>}

Often it's more convenient to handle the key as a base64 encoded string and you can generate a new key, already encoded as a base64 Unicode string like this:

iex(2)> {:ok, aes_128_key} = ExCrypto.generate_aes_key(:aes_128, :base64)

You can also generate 192/256 bit AES keys like this:

iex(3)> {:ok, aes_192_key} = ExCrypto.generate_aes_key(:aes_192, :base64)

iex(4)> {:ok, aes_256_key} = ExCrypto.generate_aes_key(:aes_256, :base64)

In both examples the :bytes atom can be substituted for :base64 if you wish to receive your key as a bitstring rather than as a base64 encoded Unicode string.

As you can see the keys grow longer in order of bit length. A 128 bit key is more than sufficient for most applications but if you are slightly more paranoid than average use a 192 bit key.

If your paranoia knows no bounds or you are protecting state secrets from nation-state owned quantum computers use a 256 bit key.

If you are concerned about hyper-advanced aliens with quantum computers you might need a longer key. Enterprise grade keys such as this can be generated upon request in the context of a consulting agreement. For this application we recommend at least a 612 bit key.

Copyright and License

Copyright (c) 2015 Josh Austin

This work is free. You can redistribute it and/or modify it under the terms of the MIT License. See the file for more details.