WhatsappFlowCrypto

If you want to implement a WhatsApp Flow-Endpoint you need to implement the specified workflow to decrypt the request and encrypt the response.

For many tasks you can use the Erlang crypto module, but unfortunately this is not possible at the time of writing in this case because the necessary configuration of the block cipher is not supported:

To decrypt the AES key you need

RSA/ECB/OAEPWithSHA-256AndMGF1Padding algorithm with SHA256 as a hash function for MGF1;

Once the AES key is decrypted we can do the rest of the workflow using the crypto module.

This package contains a Rust implementation to decrypt the encrypted AES key. With the help of Rustler a safe NIF binding is created and the missing function can be compensated quickly and efficiently.

Installation

If available in Hex, the package can be installed by adding whatsapp_flow_crypto to your list of dependencies in mix.exs:

def deps do
  [
    {:whatsapp_flow_crypto, "~> 0.1.0"}
  ]
end

Private key

The package provides a function to extract the private key (with or without a password) from a pem string.

To create the private/public key you can use then openssl tool. In the following example we will create an encrypted private key using the AES-128 cipher (DES3 cipher is not supported anymore). The last command exports the private key without encryption.

openssl genrsa -aes128 -out private.pem 2048
openssl rsa -in private.pem -outform PEM -pubout -out public.pem
openssl pkcs8 -topk8 -inform PEM -outform PEM -nocrypt -in private.pem -out unencrypted-private.pem

Basic usage

Loading the private key:

{:ok, private_key_pem} = File.read(private_pem_path);
WhatsappFlowCrypto.fetch_private_key(private_key_pem, "test")
{:ok, #Reference<0.1652491651.443678740.227250>}

Decrypt the request and encrypt the response:

WhatsappFlowCrypto.decrypt_request(private_key_ref, encrypted_aes_key, initial_vector, encrypted_flow_data)
{:ok, {decrypted_body, aes_key, initial_vector}}

WhatsappFlowCrypto.encrypt_response(aes_key, initial_vector, %{"Hello" => "World"})
"KUkRnUDAUKqhiovnQ9RRwmdBjcg87/wh+ZrMtbh8xlx3"

For the detail see the WhatsappFlowCrypto.

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/whatsapp_flow_crypto.