Implement nip/04: Encrypted Direct Message

[niamtokik]

Summary

• [ ] Ensure the generate secp256k1 is correct
• [ ] Find or implement the new functions required to generate encrypted messages
• [ ] Create the interfaces required to encrypt, decrypt, sign and validate encrypted messages

What we need

1. A simple and light way to create encrypted message
2. A function to get a shared secret between a private and a public key
3. A comprehensible way to do it

Notes

The following javascript code is coming from nip/04 standard.

import crypto from 'crypto'

//--- 8< This part of the code is probably not supported by Erlang
import * as secp from 'noble-secp256k1'
let sharedPoint = secp.getSharedSecret(ourPrivateKey, '02' + theirPublicKey)
let sharedX = sharedPoint.substr(2, 64)
//--- 8< This part of the code is probably not supported by Erlang

let iv = crypto.randomFillSync(new Uint8Array(16))
var cipher = crypto.createCipheriv(
  'aes-256-cbc',
  Buffer.from(sharedX, 'hex'),
  iv
)
let encryptedMessage = cipher.update(text, 'utf8', 'base64')
encryptedMessage += cipher.final('base64')
let ivBase64 = Buffer.from(iv.buffer).toString('base64')

let event = {
  pubkey: ourPubKey,
  created_at: Math.floor(Date.now() / 1000),
  kind: 4,
  tags: [['p', theirPublicKey]],
  content: encryptedMessage + '?iv=' + ivBase64
}

It seems some of the functions used in this snippet are not available in Erlang (or at least in default libraries).

getSharedSecret Method

getSharedSecret method create a common secret between a public and a private key. This function is defined in index.ts in the noble-secp256k1 project.

In pure Elixir, the function get_shared_secret/3 defined in the Curvey module can do the same action.

In both case, others non-implemented functions will be required, like extracting the coordinate of the public/private key and implement Jacobian multiplication (noble-secp256k1 implementation.

The functions used are very small subset of all the feature available on secp256k1 bitcoin library. It could be interesting to create a function called nostrlib_secp256k1:get_shared_secret/3 functions. The same module could also integrated all other functions related to secp256k1 encryption like:

• nostrlib_secp256k1:generate_key/0: generate a new key with default options based on crypto:generate_key/2 module.
• nostrlib_secp256k1:generate_key/1: generate a new key with extra options.
• nostrlib_secp256k1:get_private_key/1: extract the private key from the value returned by generate_key/0 function
• nostrlib_secp256k1:get_public_key/1: extract the public key from the value returned generate_key/0 function
• nostrlib_secp256k1:get_coordinates/1: extract the coordinates of the key
• ...

Using External Implementation

Using our own secp256k1 partial implementation in Erlang

pros:

• no dependencies at all
• only what we need
• could be a good introduction to elliptic curve encryption

cons:

• less secure
• require a lot of work (it should be secure enough to be used)
• require external audit
• using Erlang will be less performant than a C lib
• we should avoid re-implement cryptographic libraries

Using secp256k1 C NIF Elixir Implementation

pros:

• NIF: performance
• Use official bitcoin library
• Used by other projects

cons:

• Elixir: this project wants to use only pure Erlang code with only few external deps
• NIF: if not well implemented can do nasty thing
• NIF: requires to maintain an external C library and compile it for every releases
• old version of the library used by default: a really old version from 2018, based on the build_deps.sh script.
• seems to be not maintained since January 2020 (latest commit)

References and Resources

• Official NIP/04
• Bitcoin secp256k1 C89 Implementation
• secp256k1 Elixir implementation
• secp256k1 C NIF Elixir implementation
• Curvy secp256k1 pure Elixir implementation
• secp256k1 Python implementation using C code
• Secp256k1 on Bitcoin Wiki
• SEC 2: Recommended Elliptic Curve Domain Parameters
• Jacobian Coordinates on Wikibooks
• Elliptic Curve Cryptography in Practice
• Elliptic Curve Cryptography on Wikibooks