On Thu, Sep 6, 2018 at 11:33 PM Tim Ruffing via bitcoin-dev <firstname.lastname@example.org> wrote: > Now you can argue that the attacker is storing encrypted traffic today to > decrypt it later.
That is the argument. We know for that state level parties are storing unimaginable amounts of data for future decryption, that threat isn't theoretical. > Sure, > but if that's your threat model then Bitcoin is probably not the right tool > for you. (And if Why not? > you insist that Bitcoin is the right tool, then you can and probably should > use it over Tor > anyway.) Currently, Tor provides _no confidentiality at all_ in that threat model. Part of why I think this enhancement is interesting is because without it BIP151 doesn't actually add anything for those p2p connections running over Tor, but with it -- it at least adds some long term confidentiality hedge. > It's not worth the hassle, would hinder adoption, Why do you say this? > impression of "bulletproof" security. Even worse, there will be too many > people that will suddenly > assume that Bitcoin is post-quantum secure. People already make that claim with respect to public key hashing. I don't think "we shouldn't improve security because someone will mistake an improvement for perfection" is an an especially interesting argument. > Key exchange indistinguishable from random > ========================================== > I would rather love to see a simple ECDH key exchange as currently used but > with an encoding of > public key that provides indistinguishability from random bitstrings. > "Elligator" does not work > but "Elligator Squared"  does the job for secp256k1 -- it just doubles the > size of the public Here is where I turn the argument around on you: This requires writing a non-trivial amount of moderately complex new cryptographic code (which is not the case for PQ schemes-- that merely requires dropping in pre-existing code) and yet I am not aware of any attack model that this which would any improvement in actually delivered security: Bitcoin traffic is _trivially_ identifiable by its traffic patterns. (Blockstream previously wrote the SW forward transform for asset generation, but this requires the inverse too, as well as glue code. It also isn't clear to me if it's possible to make this construction constant time, which would be okay for BIP151 purposes but if we wanted to have a generic uniform encoder in libsecp256k1 I think we'd prefer it be constant time? maybe?) The scheme in the BIP thus far achieves the property that there are no fixed bytes for brain-dead byte matching DPI traffic filtering or anti-virus to match on (or accidentally false positive on). AV false positives are already an existing problem with the current protocol and any fixed bytes in the communication are at risk for false positives or targeted filtering. And achieving that property requires basically nothing: a test for the first byte of a generated serialized pubkey and a negate on the private key if it was wrong. > key. Together with the encrypted packet lengths, the entire data stream looks > like random then, No, it doesn't-- due to traffic analysis. Including, for example, the pattern that 64-bytes must be sent in each direction, before further data continues, bursts of traffic coinciding with blocks newly found on the network, etc. I don't believe that indistinguishable keys are actually useful outside of the context of things like stegnographic embedding-- cases where protocol 'metadata' doesn't tell you that a key is there regardless. I suppose if the code already existed to do it I might as well go "okay, sure why not", it's not going to harm anything (the added computation time to generate the uniform encoding would probably be no more than a 10% slowdown). I wouldn't argue against it on the basis that someone might believe it resulted in anti-censorship properties that it doesn't have ... even though it's clearly the case... because I categorically reject that form of argument. :) I think your view on the two distinctive proposals is askew: PQ agreement has a clear benefit under a plausible threat model and is quite easy to implement... while uniform encoding is somewhat harder to implement (though admittedly not very hard) but doesn't appear to provide a concrete benefit under any threat model that I'm currently aware of... > The key derivation can be improved. It should include each peer's > understanding of its role, > i.e., requester (or "initiator" is the more common term) or responder. At the > moment, an attacker > can create a situation where two peers think they're in the same session > (with the same session > id) but they're actually not. Also, it's possible for an attacker to > rerandomize the public keys. > That's nothing bad by itself but anything which restricts the flexibility of > the attacker without > adding complexity is a good idea. Something like > "salt = BitcoinSharedSecret||INITIATOR_PUBKEY||RESPONDER_PUBKEY" should > just avoid this issue. I also prefer the contributory key model, but Pieter proved me on IRC last week that the attack form that I was concerned about was not possible. Do you actually have an attack in mind that you can spell out here? I don't see a harm in changing that, but given that I'd already twice talked myself out of proposing the same thing, I'd like to understand if I'm missing something. :) > Re-keying > ========= > The problem with signalling re-keying in the length field is that the length > field is not covered > by the MAC. It's AAD data in the mac, unless I misunderstand the protocol. > Deterministic rekeying rules may be better. Otherwise there will be > implementations that rekey > every 10 seconds That would be pretty harmless, since the rekeying operation costs similar to one message decryption. > and implementations that just don't rekey at all (rendering the 10 s rekeying > interval in the opposite direction useless). The protocol requires rekeying at least after a given amount of data is transmitted. Peers that violate that can be disconnected. But it could be unfortunately infrequent. > Different policies also make it possible to > fingerprint implementations. I agree that is a good point. Personally I'd prefer that we used a ciphersuite that effectively "rekeyed" every message-- along the lines of the constructions described https://blog.cr.yp.to/20170723-random.html Unfortunately I was unable to find _any_ well analyized authenticated encryption mode that has the fast erasure property. It's too bad because it would be trivial to adhoc one (e.g. use the extra 32 bytes from the poly1305 chacha run to update the keys for the next message). > What's better: 5 min or 30 min? I don't know, but both are reasonable > choices. (Thats's very much It doesn't much matter, except for fingerprinting reasons. > like discussions about ciphers... What's better AES-GCM or ChaCha20/Poly1305? > I don't know, but > again both are reasonable choices.) Here we have a very clear motiviation. On devices without hardware AES/clmul constant time AES-GCM is _terribly slow_ compared to ChaCha20/Poly1305. Performance on the slowest devices is where the the ones where the ciphersuite choice likely matters at all (because it could actually make a meaningful difference in their system's ability to keep up), and the slowest devices that I'm aware of users commonly using are also devices without good AES-GCM performance. Unfortunately. On fast desktop hardware the performance of AES-GCM and ChaCha20/Poly1305 is also fairly close. So when it matters, chacha20/poly1305 is higher performance by a wide margin. (Too bad, because otherwise I'd much rather use AES-GCM) > I didn't think about this in detail: maybe there are a few meaningful cases > where padding could > hide the message length without too much overhead. (I'm not convinced, just a > random thought.) This can be done at the message level. E.g. new TX messages that round tx sizes up to the next multiple. I don't think useful low overhead padding is otherwise possible. > written in stone, again to avoid complexity and to avoid fingerprinting. Writing things in stone is a great way to never finish a protocol. Right now we know we have new upcoming proposals for messages where the overhead matters, e.g. we need a replacement addr message ASAP, and there is ongoing work for transaction relay that would also benefit from low overhead. The norm in Bitcoin is to ignore messages you don't know how to parse anyways, so there is no complexity that arises from "may negotiate" itself-- only from actually making use of that possibility in the future, so the merits of any particular usage could be decided when something wants to actually use it. The purpose of pointing out "may negotiate" is, I think, primarily to avoid a debate about who would assign numbers from this limited space in the future-- and the answer just is that they're implementation defined (e.g. by the BIPs using them). > necessary anyway, so maybe just use IDs for anything? ASCII is nice if you > want to debug your code > or some random network failure but that's hard anyway when encryption is used. Right, encryption kills external analysers in any case. It's also easy to just logprintf traffic (this is open source software after all), which doesn't have a decoding problem. > - "The Re-Keying must be done after every 1GB of data sent or received" Hm, > every peer updates its > own sending key, so this should just read "sent" instead of "sent or > received"? I think it says 'received there' mostly because it's implicitly telling you that you can hang up on someone who violates it. I agree it would be more consistent to express it sending side there. _______________________________________________ bitcoin-dev mailing list email@example.com https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev