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README.md

private-stream

Very simple encryption protocol for p2p systems.

Add privacy to a duplex pull-stream, but not integrity checking, or identity. Intended for use in p2p systems such as secure-scuttlebutt

*** WARNING - implemented as exercise, do not use ***

This does not have authentication or authenticated encryption. This protocol does not confirm the identity of the remote party, so it's vunerable to man-in-the-middle attacks, and it does not use an MAC/authenticated encryption so it's also vulnerable to attackers who may flip bits in the ciphertext, unless the application layer protocol is fully authenticated.

There is no cipher-suite negotiation handshake. This is by design, as it's often a source of complexity and insecurity. p2p systems usually have a peer lookup system - perhaps a DHT or gossip protocol - that is used to discover temporary peer addresses. Encryption protocols are also temporary, because weaknesses are found or better algorithms are developed. If the supported encryption configuration is also shared via the lookup system, then it is not necessary to have a negotiation phase.

Even better, it's possible to upgrade the protocol, without having to backwards-support the negotiation phase. When a peer upgrades and supports a better protocol, they update their public configuration, listening with the new protocol on a new port. Once sufficient members of the network have upgraded, peers can deprecate the old protocol.

Since a p2p system must be designed to accept nodes to become unavailable, the network should continue to function even if some peers deprecate a protocol before some peers upgrade, as long as most nodes support both protocols.

Without a cipher-suite negotiation phase, all bytes transmitted appear random. This forces attackers to use more difficult techniques to identify the type of network traffic (such as packet timing and size).

Finally, since there is no negotiation phase, the implementation can be very simple. The core part of the logic of the protocol is less than 40 lines of javascript, not counting the implementations of the primitives, or the libraries used to implement the streams, etc, which are all rigorously tested else where, so code is highly auditable.

Example

var private = require('private-stream')
var pull = require('pull-stream')

//aliceStream and bobStream must be DUPLEX pull streams
var alice = private(aliceStream)
var bob = private(bobStream)

pull(alice, bob, alice)

API

//initialize createPrivateStream
//the settings passed are the default settings.
var createPrivateStream = require('private-stream')

//wrap a plain duplex stream to encrypt it.
var cipherDuplexStream = createPrivateStream(plainDuplexStream)

to use your own primitives, require the inject submodule and pass it implementations of the 3 primitives.

var inject = require('private-stream/inject')

var private = inject(KeyExchange, StreamCipher, Hash)

private(stream) //encrypt a stream with your chosen primitives.

Threat Model

private-stream is intended to prevent passive eavesdroppers from being able to observe the traffic in a peer to peer network. It is likely that peers will need to share information somewhat freely with other peers, so this cannot prevent peers from maliciously participating in the network, but by forcing surveillers to participate in the network, at least defensive peers will have a chance to know they are being observed.

What this is NOT

private-stream does not ensure the integrety of the stream. It does not check whether what you received is actually what the remote sent, but p2p protocols are generally secure even in plain text, and can detect invalid data. Even if an attacker injected random noise into your connection, those would relibly produce errors at the next layer.

Protocol

private-stream takes a key-exchange algorithm (diffie-helman with modp14) a stream cipher (salsa20).

The protocol is symmetrical, and each side performs the same operations in parallel. I'll call one side the "local" side, and one "remote", but the caller and the answerer are both of local and remote to each other.

First the local side generates a key exchange and an nonce (their initialization vector), and sends it to the remote. who has also done the same, sending it to the local side. On receiving the remote's key exchange, they are combined to compute the secret.

Stream cipher keys, initialization vectors must not be reused. Each side initializes two stream ciphers, one for encrypting their data to the remote, and one for decrypting data the remote has sent to them.

The local nonce is used for the encrypting cipher, and the remote nonce is used to decrypt. Since the remote is the mirror image of the local, it means that the remote's decryption cipher is the same as the local's encryption cipher and vice versa.

Known Weaknesses

  • A man in the middle could access the plaintext (a highly active attack)

  • If either party had a random number generator that could be predicted by a third party, that party could access the plain text. (but if this is true they can probably guess their private key too)

  • Authenticated encryption is not used, so an attacker that can flip random bits can corrupt the plaintext.

Cryptographic Primitives

reference: [RFC3526 (sha256-42fcef9cabf127b9f1fc310489bc42401c0a4f74e8f804c3c3fac9818fe240ed)])(http://www.rfc-editor.org/rfc/rfc3526.txt)

A Diffie-Hellman group must be selected that is not weaker than the AES mode. RFC mentions that there where two disagreeing estimates of the relative strength of Diffie Hellman and AES, private-stream's defaults follow the advice of the more conservative estimate.

The conservative strength estimate for diffie-helman with modp15 is 130 bits.

Since secure-scuttlebutt requires streaming realtime data, and salsa20 is straightforward to implement securely, I have decided to use salsa20 instead of aes.

License

MIT