David Wong | Cryptologie | Markdown http://www.cryptologie.net/ About my studies in Cryptography. en-us Wed, 03 Oct 2018 10:51:20 +0200 Real World Crypto 2019 registrations open David Wong Wed, 03 Oct 2018 10:51:20 +0200 http://www.cryptologie.net/article/456/real-world-crypto-2019-registrations-open/ http://www.cryptologie.net/article/456/real-world-crypto-2019-registrations-open/#comments ]]> Encrypting a file David Wong Thu, 27 Sep 2018 13:03:05 +0200 http://www.cryptologie.net/article/455/encrypting-a-file/ http://www.cryptologie.net/article/455/encrypting-a-file/#comments
Encrypting a file is hard. I often need to do it to protect confidential data before sending it to someone. Besides PGP (yerk) there doesn't seem to be any light tools to do that easily. The next best option is often to have a common messaging app like Signal. So I made my own. It's called [Eureka](https://github.com/mimoo/eureka) and it's available in [binaries](https://github.com/mimoo/eureka/releases) or if you have Golang installed on your device, directly by doing this:

$ go get github.com/mimoo/eureka

It's also 100 LOC. It's just doing a simple job that seems to be missing from most default tooling. ]]>
Want to implement something fun today? David Wong Wed, 05 Sep 2018 12:44:34 +0200 http://www.cryptologie.net/article/454/want-to-implement-something-fun-today/ http://www.cryptologie.net/article/454/want-to-implement-something-fun-today/#comments
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](https://strobe.sourceforge.io/) implementation (Disco uses Strobe for all the **symmetric crypto**). Reference implementations of Strobe exist in C and Python, unofficial ones exist in [Golang](https://github.com/mimoo/StrobeGo) (from yours truly) and in [Rust](https://github.com/rozbb/strobe-rs) (from Michael Rosenberg). but if you're dealing with another language, you'll have to implement the [Strobe specification](https://strobe.sourceforge.io/specs/) first!

**2. Noise.** Read the "*How to Read This Document and Implement Disco*" section of the [Disco specification](http://discocrypto.com/disco.html). What it tells you is to implement the [Noise specification](http://noiseprotocol.org/noise.html) 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](http://discocrypto.com/disco.html).

**4. Tada!**

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](https://github.com/mimoo/disco/issues/4). ]]>
Tamuro meetup in London tonight David Wong Fri, 31 Aug 2018 13:48:45 +0200 http://www.cryptologie.net/article/453/tamuro-meetup-in-london-tonight/ http://www.cryptologie.net/article/453/tamuro-meetup-in-london-tonight/#comments
To know the location you need to solve one of the challenges [there](https://twitter.com/TamuroLondon/status/1032983065591721986) summarized here as well:

* **EASY**: Decrypt the cipertext (single byte XOR) to find the GPS coordinates of the next Tamuro London meetup:
6d69766d686f69747568766861686e (hex string)
* **MEDIUM**: Solve the easy challenge without a brute-force method.
* **HARD**: https://gist.github.com/tkeetch/b1b21f621813ff11a75930f80f1c9e5b ]]>
Advances in permutation-based cryptography David Wong Tue, 28 Aug 2018 11:51:49 +0200 http://www.cryptologie.net/article/452/advances-in-permutation-based-cryptography/ http://www.cryptologie.net/article/452/advances-in-permutation-based-cryptography/#comments
I'll personally introduce sponge constructions, Strobe and [Disco](https://www.discocrypto.com):

> 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. ]]>
Smart Contract Security @ Black Hat Asia David Wong Mon, 27 Aug 2018 23:35:17 +0200 http://www.cryptologie.net/article/451/smart-contract-security-black-hat-asia/ http://www.cryptologie.net/article/451/smart-contract-security-black-hat-asia/#comments
<iframe width="560" height="315" src="https://www.youtube.com/embed/95-pdnUQQSo" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>

It's a summary of what is available on [dasp.co](https://www.dasp.co) + what we've learned auditing smart contracts. ]]>
Facebook's TLS 1.3 library David Wong Thu, 16 Aug 2018 11:09:29 +0200 http://www.cryptologie.net/article/450/facebooks-tls-13-library/ http://www.cryptologie.net/article/450/facebooks-tls-13-library/#comments
> 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](https://github.com/facebookincubator/fizz/blob/09ba244f83592ae89b1f9137f7b99ba58ae4f2a9/fizz/client/ClientProtocol.cpp#L32):




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](https://gist.github.com/mimoo/779dcf8c44d80a2a34a1a2f2ed620711) but it's probably outdated. ]]>
Problems that UDP and only UDP has David Wong Mon, 13 Aug 2018 16:30:46 +0200 http://www.cryptologie.net/article/449/problems-that-udp-and-only-udp-has/ http://www.cryptologie.net/article/449/problems-that-udp-and-only-udp-has/#comments
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](https://www.chromium.org/quic), the very first message from a client needs to be padded with `0`s 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](https://tools.ietf.org/html/rfc6347) (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. ]]>
TLS 1.3 is out! David Wong Sat, 11 Aug 2018 13:55:45 +0200 http://www.cryptologie.net/article/448/tls-13-is-out/ http://www.cryptologie.net/article/448/tls-13-is-out/#comments
* 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](https://bearssl.org/) going to implement TLS 1.3? It sounds like it.
WhatsApp, Secure Messaging, Transcript Consistency and Trust in a group chat David Wong Fri, 10 Aug 2018 11:54:10 +0200 http://www.cryptologie.net/article/447/whatsapp-secure-messaging-transcript-consistency-and-trust-in-a-group-chat/ http://www.cryptologie.net/article/447/whatsapp-secure-messaging-transcript-consistency-and-trust-in-a-group-chat/#comments
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](https://www.whatsapp.com/security/WhatsApp-Security-Whitepaper.pdf) 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 root@WhatsApp in order to perform this kind of shenanigans. In the [blog post's attack](https://github.com/romanzaikin/BurpExtension-WhatsApp-Decryption-CheckPoint) 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](https://news.ycombinator.com/item?id=17723973) 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**. ]]>
QUIC Crypto and simple state machines David Wong Thu, 09 Aug 2018 12:39:32 +0200 http://www.cryptologie.net/article/446/quic-crypto-and-simple-state-machines/ http://www.cryptologie.net/article/446/quic-crypto-and-simple-state-machines/#comments
* Google wanted to improve TCP (2.0™️)
* but TCP can't really be changed
* so they built it on top of UDP (which is just IP with ports, check the [2 page RFC for UDP](https://tools.ietf.org/html/rfc768) if you don't believe me)
* they made it with **encryption by default**
* and they called it **QUIC**, because it's quick, you know

There is more to it, it makes HTTP blazing fast with multiplexed streams and all, but I'm only interested about the crypto here.

Google QUIC's (or gQUIC) default encryption was provided by a home-made crypto protocol called QUIC Crypto. The thing is documented in a 14-page [doc file](https://docs.google.com/document/d/1g5nIXAIkN_Y-7XJW5K45IblHd_L2f5LTaDUDwvZ5L6g/edit) and is more or less up-to-date. It was at some point agreed that things needed to get standardized, and thus [the process of making QUIC an RFC (or RFCs) began](https://datatracker.ietf.org/wg/quic/about/).

Unfortunately QUIC Crypto did not make it and the IETF decided to replace it with TLS 1.3 for diverse reasons.

**Why "Unfortunately" do you ask?**

Well, as Adam Langley puts it in some of [his slides](https://www.ietf.org/proceedings/92/slides/slides-92-saag-5.pdf). The protocol was dead simple:

![quic crypto](/upload/quiccrypto.jpg)

While the protocol had some flaws, in the end, it was still a beautiful and elegant protocol. At its core was an extremely straight forward and linear state machine summed up by this diagram:

![quic crypto diagram](/upload/quiccryptodiagram.jpg)

A few things to help you read it:

* a server config is just a blob that contains the server current semi-ephemeral keys. The server config is rotated every X days.
* an inchoate client hello is just an empty client hello, which prompts the server to send a REJ(ect) message containing its latest config (after that the client can try again with a full client hello)
* SHLO is a (encrypted) server hello which contains ephemeral keys

As you can see **there isn't much going on**, if you know the keys of the server you can do some 0-RTT magic, if you don't then request the keys and start the handshake again.

Compare that to [the state machine of TLS 1.3](https://tools.ietf.org/html/draft-ietf-tls-tls13-28#appendix-A.1):

![tls state machine](/upload/tlsstatemachine.jpg)

In the end, TLS 1.3 is a solid protocol, but I'd like to see more experimentation here instead of just relying on TLS. version 1.3 is built on top of numerous previous failed versions which means a great amount of complexity due to legacy and a multitude of use cases and extensions it needs to support. Simpler protocols should be better, simple state machines make for better analysis and more secure implementations. Just look at the [Noise protocol framework](http://noiseprotocol.org/) and its 1k LOC implementations and its symbolic proofs done with [ProVerif](https://noiseexplorer.com) and [Tamarin](https://www.wireguard.com/papers/wireguard-formal-verification.pdf). Actually, why haven't we started using Noise for everything? ]]>
About Bitcoin Transactions David Wong Wed, 08 Aug 2018 16:31:11 +0200 http://www.cryptologie.net/article/445/about-bitcoin-transactions/ http://www.cryptologie.net/article/445/about-bitcoin-transactions/#comments
That's right! when crafting a transaction to send money on the bitcoin network, you actually do not include `I am sending my BTC to _this address_`. Instead, you include a script called a **ScriptPubKey** which dictates a set of inputs that are allowed to redeem the monies. The **PubKey** in the name surely refers to the main use for this field: to actually let a unique public key redeem the money (the intended recipient). But that's not all you can do with it! There exist a multitude of ways to write ScriptPubKeys! You can for example:

* not allow anyone to redeem the BTCs, and even use the transaction to record arbitrary data on the blockchain (this is what a lot of applications built on top of bitcoin do, they "burn" bitcoins in order to create metadata transactions in their own blockchains)
* allow someone who has a password to use the BTCs (but to submit the password, you would need to include it in clear inside a transaction which would inevitably be advertised to the network before actually getting mined. This is dangerous)
* allow a subset of signatures from a fixed set of public keys to redeem the BTCs (this is what we call multi-sig transactions)
* allow someone who can break a hash function (SHA-1) to redeem the BTCs (This is what [Peter Todd did in 2013](https://en.bitcoin.it/wiki/Script#Incentivized_finding_of_hash_collisions))
* only allow the BTCs to be redeemed after some time in the future (via a timestamp)
* etc.

On the other hand, if you want to use the money you need to prove that you can use such a transaction's output. For that you include a **ScriptSig** in a new transaction, which is another script that runs and creates a number of inputs to be used by the **ScriptPubKey** I talked about. And you guessed it, in our prime use-case this will include a signature (the **Sig** in the name)!

Recap: when you send BTCs, you actually send it to whoever can give you a correct input (created by a ScriptSig) to your program (ScriptPubKey). In more details, a Bitcoin transaction includes a set of **input BTCs** to **spend** and a set of **output BTCs** that are now **redeemable** by whoever can provide a valid ScriptSig. That's right, **a transaction actually uses many previous transactions to collect money from, and spread them in possibly multiple pockets of money that other transactions can use**. Each input of a transaction is associated to a previous transaction output, along with the ScriptSig to redeem it. Each output is associated with a ScriptPubKey. By the way, an output that hasn't been spent yet is called an UTXO for unspent transaction output.

The scripting language of Bitcoin is actually quite limited and easy to learn. It uses a stack and must return `True` at the end. The limitations actually bothered some people who thought it might be interesting to create something more [turing-complete](https://en.wikipedia.org/wiki/Turing_completeness), and thus [Ethereum](https://en.wikipedia.org/wiki/Ethereum) was born. ]]>
CryptoMag is looking for articles David Wong Thu, 05 Jul 2018 14:16:06 +0200 http://www.cryptologie.net/article/444/cryptomag-is-looking-for-articles/ http://www.cryptologie.net/article/444/cryptomag-is-looking-for-articles/#comments
You want to teach someone about a crypto concept, something 101 that could be explained in 1-2 pages with a lot of diagrams? Look no more, we need you.

## Concept

The idea is to have a recurrent benevolent e-magazine (like [POC||GTFO](https://github.com/tylert/pocorgtfo)) that focuses on:

* **cryptography**: *duh!* That being said, cryptography does include: implementations, cryptocurrencies, protocols, at scale, politics, etc. so there are more topics that we deem interesting than just theoretical cryptography.
* **pedagogy**: heaps of *diagrams* and a focus on teaching. Taking an original writing style is a plus. We're looking not to bore readers.
* **101**: we're looking for *introductions* to concepts, not deeply technical articles that require a lot of initial knowledge to grasp.
* **short**: articles should be similar to a blog post, not a full-fledged paper. With that in mind articles should be around 1, 2 or 3 pages. We are not looking for something dense though, so no posters, rather a submission should be a light read that can be part of a series or influence the reader to read more about the topic.

## Topics

Preferably, authors should write about something they are familiar with, but here is a list of topics that would likely be interesting for such a light magazine:

* what is SSH?
* what is SHA-3?
* what is functional encryption?
* what is TLS 1.3?
* what is a linear differential attack?
* what is a cache attack?
* how does LLL work?
* what are common crypto implementation tricks?
* what is R-LWE?
* what is a hash-based signature?
* what is an RFC?
* what is the IETF?
* what is the IACR?
* why are companies encrypting databases?
* what is x509, .pem, asn.1 and base64?
* etc...

## Format

LaTeX if possible.

## Deadline

No deadline at the moment.

## How to submit

send me a dropbox link or something on the [contact page](/contact), you can also send it to me via [twitter](https://www.twitter.com/cryptodavidw)

PS: I am going to annoy you if you don't use diagrams in your article ]]>
Tamuro meetup in London David Wong Fri, 29 Jun 2018 12:20:15 +0200 http://www.cryptologie.net/article/443/tamuro-meetup-in-london/ http://www.cryptologie.net/article/443/tamuro-meetup-in-london/#comments Smart Contract Security @ IT Camp David Wong Fri, 08 Jun 2018 22:25:19 +0200 http://www.cryptologie.net/article/442/smart-contract-security-it-camp/ http://www.cryptologie.net/article/442/smart-contract-security-it-camp/#comments
<iframe width="560" height="315" src="https://www.youtube.com/embed/yIQUnV3dTBs" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>

Yours truly. ]]>
Ethereum Smart Contract Security @ IT Camp - Cluj Napoca David Wong Sun, 03 Jun 2018 15:59:53 +0200 http://www.cryptologie.net/article/441/ethereum-smart-contract-security-it-camp-cluj-napoca/ http://www.cryptologie.net/article/441/ethereum-smart-contract-security-it-camp-cluj-napoca/#comments If anyone is there and wants to talk about crypto while drinking beer, contact me! ]]> Decentralized Application Security Project David Wong Wed, 11 Apr 2018 20:30:18 +0200 http://www.cryptologie.net/article/440/decentralized-application-security-project/ http://www.cryptologie.net/article/440/decentralized-application-security-project/#comments

The page is on [github](https://github.com/CryptoServices/dasp) as well and we welcome contributions to the [top 10](https://dasp.co/) and the [list of known exploits](https://dasp.co/timeline.html). In addition we're looking to host more projects related to the Ethereum space there, if you are looking for research projects or are looking to contribute on tools or anything that can make smart contracts development more secure, [file an issue on github](https://github.com/CryptoServices/dasp/issues)!

Note that I will be giving the talk again at [IT Camp](https://itcamp.ro/agenda/) in Cluj-Napoca in a few months. ]]>
Fed Up Getting Shattered and Log Jammed? A New Generation of Crypto Is Coming David Wong Sun, 08 Apr 2018 14:09:17 +0200 http://www.cryptologie.net/article/439/fed-up-getting-shattered-and-log-jammed-a-new-generation-of-crypto-is-coming/ http://www.cryptologie.net/article/439/fed-up-getting-shattered-and-log-jammed-a-new-generation-of-crypto-is-coming/#comments
<iframe width="800" height="515" src="https://www.youtube.com/embed/bTGLO4obxco" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>

[I gave a summary of the talk here](https://www.cryptologie.net/article/432/introducing-disco/). You can also directly check out the specification of Disco or the libdisco library on [www.discocrypto.com](https://www.discocrypto.com). ]]>
Two Student Tickets for Black Hat Asia in March 2018 David Wong Mon, 22 Jan 2018 12:12:13 +0100 http://www.cryptologie.net/article/438/two-student-tickets-for-black-hat-asia-in-march-2018/ http://www.cryptologie.net/article/438/two-student-tickets-for-black-hat-asia-in-march-2018/#comments
Anyone interested?

EDIT: these have been given away. ]]>
On Real World Crypto and Secure Messaging David Wong Thu, 11 Jan 2018 00:49:42 +0100 http://www.cryptologie.net/article/437/on-real-world-crypto-and-secure-messaging/ http://www.cryptologie.net/article/437/on-real-world-crypto-and-secure-messaging/#comments
Today Paul Rösler came to **Real World Crypto** to talk about the results, which is a good thing.
Interestingly, in the middle of the talk Wired released a worrying article untitled [WhatsApp Security Flaws Could Allow Snoops to Slide Into Group Chats](https://www.wired.com/story/whatsapp-security-flaws-encryption-group-chats/).
Interestingly as well, at some point during the day Matthew Green also wrote about it in [Attack of the Week: Group Messaging in WhatsApp and Signal](https://blog.cryptographyengineering.com/2018/01/10/attack-of-the-week-group-messaging-in-whatsapp-and-signal/).

They make it seem really worrisome, but should we really be scared about the findings?

**Traceable delivery** is the first thing that came up in the presentation. What is it? It’s the check marks that appear when your recipient receives a message you sent. It's mostly a UI feature but the fact that no security is tied to it allows a server to fake them while dropping messages, making you think that your recipient has wrongly received the message. This was never a security feature to begin with, and nobody never claimed it was one.

**Closeness** is the fact that the WhatsApp servers can add a new participant into your private group chat without your consent (assuming you’re the admin). This could lead people to share messages to the group including to a rogue participant. The caveat is that:

* previous messages cannot be decrypted by the newcomer because a new key is generated when someone new joins the mix

* everybody is receiving a notification that somebody joined, at this point everyone can choose to willingly send messages to the group

Again, I do not see this as a security vulnerability. Maybe because I’ve understood how group chats can work (or miswork) from growing up with shady websites and applications. But I see this more as a UI/UX problem.

The paper is not bad though, and I think they’re right to point out these issues. Actually, they do something very interesting in it, they start it up with a nice **security model** that they use to analyse several messaging applications:

> Intuitively, a secure group communication protocol should provide a level of security comparable to when a group of people communicates in an isolated room: everyone in the room hears the communication (**traceable delivery**), everyone knows who spoke (**authenticity**) and how often words have been said (**no duplication**), nobody outside the room can either speak into the room (**no creation**) or hear the communication inside (**confidentiality**), and the door to the room is only opened for invited persons (**closeness**).

Following this security model, you could rightfully think that we haven’t reached the best state in secure messaging. But the fuss about it could also wrongfully make you think that these are worrisome attacks that need to be dealt with.

The facts are here though, this paper has been blown out of proportion. [Moxie (one of the creator of Signal) reacts on hackernews](https://news.ycombinator.com/item?id=16117487):

> To me, this article reads as a better example of the problems with the security industry and the way security research is done today, because I think the lesson to anyone watching is clear: don't build security into your products, because that makes you a target for researchers, even if you make the right decisions, and regardless of whether their research is practically important or not.

I'd say the problem is in the reaction, not in the published analysis. But it's a sad reaction indeed.

Good night.