Cryptomancy-oblivion

This module implements primitives for oblivious transfer, an interactive protocol which allows one party to choose strictly one out of some number of messages without revealing to the sender which message was chosen.

Based on kevinejohn's 'oblivious-transfer' but with different dependencies and major changes to the API.

DISCLAIMER

I have no idea how safe this is for actual use. I'm personally using it for implementing peer-to-peer games where long-term security is not a concern.

I strongly recommend against using this module for anything other than experimentation.

Use

npm install --save cryptomancy-oblivion

var OT = require("cryptomancy-oblivion");

// Alice has two messages, which can be anything
// To make it interesting, suppose they are codes which unlock prizes of different values
// Bob would like to have both messages, but Alice is forcing him to choose
var Messages = [
    'The key to door number 1',
    'The key to door number 2'
];

var Choice = 0; // 0 or 1

// the protocol uses some public parameters
// an exponent and a modulus as in RSA
var Public = OT.genkeys();

// Alice and Bob each need a 32-byte secret, generated however you like
// 'cryptomancy-source' implements various flavours of randomness, so you can use that
// it returns Uint8Arrays
var Source = require("cryptomancy-source");

var makeSecret = function () {
    return Source.bytes.secure()(32);
};

var alice_secret = makeSecret();
var bob_secret = makeSecret();

// Alice can use the public parameters to mask her secret such that she can share it with Bob
// without him learning what it is
var masked = OT.mask(Public, alice_secret);

// Bob wants what's behind Door number 1
// he can 'blind' his choice by using his secret and combining it with Alice's masked value
// Alice will use this blinded value, but she won't be able to learn which of the two doors Bob chose
var blinded = OT.blind(Public, bob_secret, masked, Choice);

// given Bob's blinded value and using her own secrets, Alice can derive two hashes
// you can derive encryption keys from these hashes. How you do so will depend on the cipher you want to use
var unmasked = OT.unmask(Public, alice_secret, masked, blinded); // an array of two hashes

// Alice can now use these two keys to encrypt the two messages
// there are a lot of ciphers you could use here, but one is provided which should work just fine
var Crypto = require("cryptomancy-oblivion/crypto");

// this cipher assumes a key made of 32 Uint8s
var Encrypted = unmasked.map(function (hash, i) {
    return Crypto.encrypt(hash.subarray(0, 32), Messages[i]);
});

// The trick is that Bob will be able to reproduce only one of the two keys
// corresponding to the choice he made.
var bob_key = OT.unblind(Public, bob_secret, masked).subarray(0, 32);

// Now he can try to decrypt both values, but only the first will work
var decrypted = Crypto.decrypt(bob_key, Encrypted[0]);
assert.equal(decrypted, Messages[0]);

Tests

npm test