chjj

Covert ECDH over secp256k1

If ECDH is used to establish a shared session secret for an encrypted connection, two elliptic curve points need to be transmitted (one in each direction) before encryption starts. In order to avoid being identifiable as a (specific) ECDH negotiation, ideally those two points are sent in a way that is indistinguishable from random.

This problem is easily addressed by using curves that support Elligator-style encodings: functions that encode a (subset of) elliptic curve points as sequences of bytes with no observable bias: (almost) every byte sequence corresponds to exactly one point, and the others correspond to none.

Unfortunately, no Elligator-style encoding is known for secp256k1.

chjj

var http = require('http');
var pty = require('pty.js');

var server = http.createServer(function(req, res) {
  var ua = req.headers['user-agent'] || '';
  if (!~ua.indexOf('curl/')) {
    res.setHeader('Content-Type', 'text/plain');
    return res.end('curl -sSNT. localhost:8000');
  }