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WhatsApp has "recently" deployed end-to-end encryption using the Signal protocol, which is of course also being used by Signal itself. The related white paper (PDF).

Now this raises the question:
Is there still any security benefit to use Signal over the much more widely deployed WhatsApp, now that both have good end-to-end encryption?

The threat model in this case includes basically anyone not having access to the phones at the ends. It especially includes the service provider and law enforcement.

  • They share the same E2E protocol and the same implementation flaw, i.e. not asking the user to verify keys. WA even allows the user to change keys without any security warning for other users, which did not change the defaults. – allo May 24 '18 at 11:58
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There are still a couple of security functions, which may matter to you, which Signal does better than WhatsApp.

Client-Side Fanout

When you use a group chat in WhatsApp, you send your message to the server who in turn distributes it to all the group members. This way WhatsApp learns all the social structures and can in theory perform traffic analysis to deduce quite a bit of information from the message volume exchanged.
In Signal on the other hand, group chats are actually normal peer-to-peer chats with a special flag, which is set inside the end-to-end encrypted frame. So this way OpenWhisperSystems (the makers of Signal) doesn't learn your social group structures. However they can still see that three messages are going to three different people at once and can guess that this is due to a group chat.
The blog post for Signal. The server-side fan-out is state in the white paper (PDF).

In-App Encryption

Signal offers to encrypt the past communication at app level, which lacks WhatsApp completely. Obviously this can protect your messages in case of theft however you probably won't gain that much security because most people will probably not choose good passwords here for usability reasons.

No Read notifications and no typing notifications

WhatsApp notifies you when somebody is typing and it notifies you when somebody read your message. This however allows WhatsApp to deduce app usage behavior and your habits. Like "Do you check your WhatsApp messages at 1am?", combine that with the other meta data WhatsApp is harvesting and you can make some useful guesses about people's lifes. Additionally the "typing" notifications can be used to deduce potential contents based on context and default keyboard suggestions and other factors.
Signal doesn't do this. Here's the discussion on it on GitHub.
As a more recent development, Signal adopted read notifications, but they're default-off (for pre-existing installations) and aren't forced-on in Group conversations. For groups I think they work indidually with each member, that is if a member and the sender have them both enabled, the sender will get the notification, which is much more privacy-focused than WhatsApp's solution.

Backup Security

WhatsApp offers you to backup your messages so you can recover them when your phone is inacessible or destroyed. However due to the very nature of this, the backup (which must (also) be hosted on Google Drive) cannot be encrypted / secured other than with your username / password for that account (which WhatsApp doesn't know). So as soon as that Google Drive account is breached or some government demands access, all the end-to-end security is gone if either party of the communication had backups enabled.
Even though the backup feature of Signal isn't as convenient as the one of WhatsApp it doesn't automatically store plaintext copies of messages on Google servers, but rather allows you to create a local (plaintext) file and push this one manually around (text-only for now).

Auto-deleting Messages

Automatically deleting old messages is good from a security standpoint. It means that if an attacker manages to break into your phone / backup that (s)he can't access all messages but only the recent ones. Auto-deletion is especially nice if you consider that you won't read all the really old messages anyways and that it will save you some storage. As of now, WhatsApp does not implement this.
Signal on the other hand does.

No meta-data storage

Signal was recently hit by a Subpoena. They complied (of course) but could only contribute very little, which confirms that they're holding true to their privacy policy.
At the same time WhatsApp is sitting on a large(r) amount of meta data and would be much more useful if hit (and if it's being disclosed). This is especially obvious if you compare what WhatsApp logs and what Signal logs.

Private Contact Discovery

WhatsApp uploades your entire adress book to their servers to compare which of the listed users have WhatsApp accounts. Obviously during that process WhatsApp learns your social graph, that is who you know, including people who don't use WhatsApp.
Signal now on the other hand, has somewhat recently deployed a much smarter solution, using fancy modern cryptographic techniques paired with Intel's SGX technology so that OpenWhisperSystems actually doesn't learn your adress book (only the SGX enclave does and that doesn't leak it), but only needs to keep on-record who their users are and thus they also don't learn anything about which users you may know but don't chat with using Signal and which people you know but don't use Signal (yet). The details of this can be read in their blog post.

So TL;DR:
The remaining security differences (after the protocol update) are mainly that WhatsApp generates a lot of meta data to be convenient while Signal tries to avoid meta data.

  • If a whatsapp message to a group is "fanned out" at the Whatsapp server level, how does the end-to-end encryption work? Does the client include an encrypted copy of the payload for each receiver? – Johan Oct 12 '16 at 14:36
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    @Johan for the details you'd have to consult the whitepaper, but in a nutshell you encrypt a key for the encrypted message such that every targeted recipient can decrypt said key and in turn the message. Also see this Q. – SEJPM Oct 12 '16 at 14:40
  • @SEJPM What about the security of names of people who download secure apps, such as Signal? Is there some master list that Signal keeps of every subscriber, that they could be forced to share, or could be hacked? – Dr. Beeblebrox Nov 11 '16 at 15:35
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Disclaimer: this is a non-technical contribution (addition to already given answer). Some content may be subjective, possibly speculative.

I believe that when evaluating/comparing information security solutions one needs to go beyond the purely technical //current state// of the solution and consider what trajectory a given product will likely take in the future given the known or assumed-likely motivation of the controlling organization.

In 2014 Facebook bought WhatsApp

As argued by Marc Goodman in his book "Future Crimes", to Facebook users are its product while the advertisers are its customers, and to be viable Facebook monetizes its products and it does so by maximizing the volume and quality of the product it offers to its customers. Simplifying, volume translates to the time users spend looking at Facebook content (time available to show advertisement), and quality translates to how accurately Facebook can target adds at users based on what it knows about them. In other words Facebook want to know as much as possible about users (which can't be done when keeping data truly secure), and to use that to:

  1. maximize eye-ball time (get users to spend more time looking at Facebook)
  2. maximize accuracy and effectiveness of targeted advertisement

Since WhatsApp has been offering end-to-end-encryption with no access to user data, why would Facebook pay over US$19 billion in 2014 to buy it (See https://en.wikipedia.org/wiki/WhatsApp citation 13, 14) given how Facebook monetizes its products? That is unless Facebook can find a way to harvest WhatsApp data about users. Recent change to WhatsApp's privacy policy allowing for WhatsApp users' contacts' phone numbers to be shared with Facebook is, I believe, indicative of the answer.

Future Speculation

It is speculative to consider what future trajectory Facebook will take with WhatsApp. However, when considering information security solutions I think it is prudent to evaluate organization's track record and what direction their business model points to. Consider:

  • Given above and "Future Crime"'s argument, maximizing WhatsApp's information security is counter-productive to Facebook's business model
  • Facebook has a history of documented questionable privacy practises, for example: resetting of user's privacy settings upon policy update, or experimenting in controlling user moods by filtering their feeds - google for more examples.

When comparing information security solutions, especially with similar technical capabilities, I suggest choosing a solution that is more likely to keep its primary focus on privacy in future development. Here, Signal, seems a better choice.

  • @chefarov - this is not what comments are for. – Rory Alsop May 25 '18 at 7:45

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