Hey! I'm David, the author of the Real-World Cryptography book. Previously I was the security lead for Diem (Libra) at Facebook, and a security consultant for the Cryptography Services of NCC Group. This is my blog about cryptography and security and other related topics that I find interesting.

Want to implement something fun today? posted September 2018

You should try to implement Disco!

Disco is a specification that once implemented allows you to encrypt sessions (like TLS) and encrypt, authenticate, hash, generate random numbers, derive keys, etc. (like a cryptographic library). All of that usually only needs less than a thousand lines of code.

Here's how you can do it:

1. Strobe. The first step is to find a Strobe implementation (Disco uses Strobe for all the symmetric crypto). Reference implementations of Strobe exist in C and Python, unofficial ones exist in Golang (from yours truly) and in Rust (from Michael Rosenberg). but if you're dealing with another language, you'll have to implement the Strobe specification first!

2. Noise. Read the "How to Read This Document and Implement Disco" section of the Disco specification. What it tells you is to implement the Noise specification but to ignore its SymmetricState and CipherState sections. (You can also ignore any symmetric crypto in there.) You can find Noise libraries in any languages, but implementing it yourself is usually pretty straight forward (here you only really have to implement the HandshakeState).

3. Disco. Once you have that (which should take 500 LOC top), implement the SymmetricState specified by Disco.

PS: if you want to write one in python, that'd be really great! You can take a look at the already existing implementations here.

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Tamuro meetup in London tonight posted August 2018

I will be at Tamuro tonight. It's a security/crypto meetup where we mostly drink beers and chat.

To know the location you need to solve one of the challenges there summarized here as well:

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Advances in permutation-based cryptography posted August 2018

I will be talking about modern session encryption at the Advances in permutation-based cryptography workshop in Milan in October. The program looks quite interesting with talks on Xoodoo, Gimli and others.

I'll personally introduce sponge constructions, Strobe and Disco:

Today, SSL/TLS is the de-facto standard for encrypting communication. While its last version (1.3) is soon to be released, new actors in the field are introducing more modern and better designed protocols. This talk is about the past, the present and the future of session encryption. We will see how TLS led the way, how the Noise protocol framework allowed the standardization of more modern and targeted protocols and how the duplex construction helped change the status quo.

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Smart Contract Security @ Black Hat Asia posted August 2018

Mason and I's talk from Black Hat Asia 2018 is now online

It's a summary of what is available on dasp.co + what we've learned auditing smart contracts.

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Facebook's TLS 1.3 library posted August 2018

Facebook has released their TLS 1.3 library Fizz in open source. In their post they mention early data (0-RTT):

Using early data in TLS 1.3 has several caveats, however. An attacker can easily replay the data, causing it to be processed twice by the server. To mitigate this risk, we send only specific whitelisted requests as early data, and we’ve deployed a replay cache alongside our load balancers to detect and reject replayed data. Fizz provides simple APIs to be able to determine when transports are replay safe and can be used to send non-replay safe data.

My guess is that either all GET requests are considered safe, or only GET requests on the / route are considered safe. I'm wondering why they use a replay cache on the other side as this overhead could nullify the benefits of 0-RTT.

They also mention every state transitions being stored in one place, this is true:

FIZZ_DECLARE_EVENT_HANDLER(
ClientTypes,
StateEnum::Uninitialized,
Event::Connect,
StateEnum::ExpectingServerHello);

FIZZ_DECLARE_EVENT_HANDLER(
ClientTypes,
StateEnum::ExpectingServerHello,
Event::HelloRetryRequest,
StateEnum::ExpectingServerHello);

FIZZ_DECLARE_EVENT_HANDLER(
ClientTypes,
StateEnum::ExpectingServerHello,
Event::ServerHello,
StateEnum::ExpectingEncryptedExtensions);

I think this is a great idea, which more TLS libraries should emulate. I had started a whitelist of transitions for TLS 1.3 draft 18 here but it's probably outdated.

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Problems that UDP and only UDP has posted August 2018

You sometimes don't want or can't use TCP, and you thus have to deal with UDP.

To Simplify, TCP is just a collection of algorithms that extend UDP to make it support in-order delivery of streams. UDP on the other hand does not care about such streams and instead sends blocks of messages (called datagrams) in whatever-order and provides no guarantee what-so-ever that you will ever receive them.

TCP also provides some security guarantees on top of IP by starting a session with a TCP handshake it allows both endpoints of a communication to provide a proof of IP ownership, or at least that they can read whatever is sent to their claimed IPs. This means that to mess up with TCP, you need to be an on-path attacker man-in-the-middle'ing the connection between the two endpoints. UDP has none of that, it has no notion of sessions. Whatever packets are received from an IP, it'll just accept them. This means that an off-the-path attacker can trivially send packets that look like they are coming from any IP, effectively spoofing the IP of either one of the endpoint or anyone on the network. If the protocol built on top of UDP does not do anything to detect and prevent this, then bad things might happen (from complex attacks to simple denial of services).

This is not the only bad thing that can happen though. Sometimes (meaning for some protocols) a well-crafted message to an endpoint will trigger a large and disproportionate response. Malicious actors on the internet can use this to perform amplification attacks, which are denial-of-service attacks. To do that, the actor can send these special type of messages pretending to be a victim IP and then observe the endpoint respond with a large amount of data to the victim IP.

Intuitively, it sounds like both of these issues can be tackled by doing some sort of TCP handshake, but in practice it is rarely the case as the very first message of your protocol (which hasn't been able to provide a proof of IP ownership yet) can still trigger large messages. This is why in QUIC, the very first message from a client needs to be padded with 0s in order to make it as large as the server's response. Meaning that an attacker would have to at least spend as much resources that is provided by the attack, nullifying its benefits.

Looking at another protocol built on top of UDP, DTLS (TLS for UDP) has a notion of "cookie" which is really some kind of bearer token that the client will have to keep providing to the server in relevant messages, this in order to prove that it is indeed the same endpoint talking to the server.

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TLS 1.3 is out! posted August 2018

TLS 1.3 has been released as RFC 8446. It took 28 drafts and more than 4 years since draft 0 to come out. Cloudflare has a long blog post about it. Some questions about the deployment of 1.3:

• Will we see a fast deployment of the protocol? It seems like browsers are ready, but web servers will have to follow.
• Who will use 0-RTT? I'm expecting the big players to use it (largely because they've been requesting it) but what about the small ones?
• Are we going to see vulnerabilities in the protocol? It seems highly unlikely, TLS 1.2 itself (with AES-GCM) has remained solid for more than 10 years.
• Are we going to see vulnerabilities in the implementations? We will see about that. If anything happens, I'm expecting it to happen around 0-RTT, PSKs and key exports. But let's hope that libraries have learned their lessons.
• Is BearSSL going to implement TLS 1.3? It sounds like it.
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WhatsApp, Secure Messaging, Transcript Consistency and Trust in a group chat posted August 2018

Someone wrote a blogpost about man-in-the-middling WhatsApp.

First, there is nothing new in being able to man-in-the-middle and decrypt your own TLS sessions (+ a simple protocol on top). Sure the tool is neat, but it is not breaking WhatsApp in this regard, it is merely allowing you to look at (and to modify) what you're sending to the WhatsApp server.

The blog post goes through some interesting ways to mess with a WhatsApp group chat, as it seems that the application relies in some parts on metadata that you are in control of. This is bad hygiene, but for me the interesting attack is attack number 3: you can send messages to SOME members of the group, and send different messages to OTHER members of the group.

At first I thought: this is nothing new. If you read the WhatsApp whitepaper it is a clear limitation of the protocol: you do not have transcript consistency. And by that I mean, nothing is cryptographically enforcing that all members of a group chat are seeing the exact same thing.

It is always hard to ensure that the last messages have been seen by everyone of course (some people might be offline), but transcript consistency really only cares about ordering, dropping, and tampering of the messages.

Let's talk about WhatsApp some more. Its protocol is very different from what Signal does and in group chats, each member shares their unique symmetric key with the other members of the group (separately). This means that when you join a group with Alice and Bob, you first create some random symmetric key. After that, you encrypt it under Alice's public key and you send it to her. You then do the same thing with Bob. Once all the members have knowledge of your random symmetric key, you can encrypt all of your messages with it (perhaps using a ratchet). When a member leaves, you have to go through this dance again in order to provide forward secrecy to the group (leavers won't be able to read messages anymore). If you understood what I said, the protocol does not really gives you way to enforce transcript consistency, you are in control of the keys so you choose who you encrypt what messages to.

But wait! Normally, the server should distribute the messages in a fan-out way (the server distributes one encrypted message to X participants), forcing you to collude with a [email protected] in order to perform this kind of shenanigans. In the blog post's attack it seems like you are able to bypass this and do not need the help of WhatsApp's servers. This is bad and I'm still trying to figure out what really happened.

By the way, to my knowledge no end-to-end encrypted protocol has this property of transcript consistency for group chats. Interestingly, the Messaging Layer Security (MLS) which is the latest community effort to standardize a messaging protocol does not have a solution for this either. I'll probably talk about MLS in a different blog post because it is still very interesting.

The last thing I wanted to mention is trust inside of a group chat. We've been trying to solve trust in a one-to-one conversation for many many years, and between PGP being broken and the many wars between the secure messaging applications, it seems like this is still something we're struggling with. Just yesterday, a post titled I don't trust Signal made the front page on hackernews. So is there hope for trust in a group chat anytime soon?

First, there are three kinds of group chat:

• large group chats
• medium-sized group chats
• small group chats

I'll argue that large group chats have given up on trust, as it is next to impossible to figure out who is who. Unless of course we're dealing with a PKI and a company enforcing onboarding with a CA. And even this is has issues (beyond the traitors and snoops).

I'll also argue that small group chats are fine with the current protocols, because you're probably trusting people not to run this kind of attacks.

The problem is in medium-sized group chats.

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