“End-to-end encryption”

1st August 2017

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The question of regulating encrypted communication has come up again. I was going to write again about how the politicians don’t understand the technologies, and they probably don’t, but if they did, what would they do about it?  The details are too complex to debate on TV news. What percentage of the viewing public even knows what public-key encryption is?

Politicians often talk as if “end-to-end encryption” is a technology, and one which is rare and might practically be banned. There are then huge arguments about whether such banning would be good or bad, which leave me somewhat bemused.

Of course, “end-to-end encryption” is no more a technology than “driving to a friend’s house” is a technology. Cars and roads and driving are technologies, driving to a friend’s house, or to a restaurant, or to work, are social or economic practices that make use of the technology.

Similarly, sending encrypted messages is a technology. sending “end-to-end” encrypted messages is not a technology, it’s just sending encrypted messages to an intended end recipient. Whether a particular message is “end-to-end” encrypted depends on who the end is.

The soundbites talk about one kind of messaging: messages sent person-to-person from a sender to a recipient via a service provider like Whatsapp, Microsoft or Google.

In 2017, most data sent over the internet that is at all personal is encrypted. Huge efforts have been made over the last five or so years to get to this stage, yet the debates about encryption have not even touched on the fact. Data in motion seems to be invisible. The encryption used to send the messages is very strong; again, a few years ago, there were quite a few bugs in commonly used implementations, but efforts have been made to find and fix such bugs, and while there are likely to be some left, it is plausible that nearly all such encrypted messages are unbreakable even by the most powerful national security organisations.

However, the way most of these services work today is that the sender makes a connection to the service provider and authenticates himself with a password. The Service Provider also authenticates itself to the sender with a certificate, though that’s mostly invisible. The sender then sends their message encrypted to the Service Provider, which decrypts it and stores it. Later (or simultaneously) the recipient makes a connection to the Service Provider the same way, and the Service Provider encrypts the message and sends it to the recipient. This is fundamentally the same whether we are talking about messaging apps, chat, or email, and whether the devices used are computers, phones or tablets.

Anyway, call this method 1. Service Provider Mediated

A few of these services now have an extra feature. The sender’s app first encrypts the message in a way that con only be decrypted by the recipient, then encrypts it again to send to the Service Provider. The Service Provider decrypts one level of encryption, but not the second. When the recipient connects, the Service Provider re-encrypts the already encrypted message and sends to the recipient. The recipient decrypts the message twice, once to get what the Service Provider had stored, and then again to get what the sender originally wrote.

That is why the politicians are talking about Whatsapp, Telegram and so on.

This is method 2. Service Provider Mediated, with provided end-to-end encryption

An important question here is who keeps track of the encryption keys. If the Service Provider has that responsibility, then it can support interception by giving the sender the wrong encryption key; one that it or the government can reverse. If the sender keeps the recipient’s encryption key, that is not possible, the Service Provider receives no messages that it is able to decrypt.

Going back to method 1, if the Service Provider doesn’t guide the end-to-end encryption, it’s still possible to add it with special software for the sender and recipient. This is awkward for the users and has never caught on in a big way, but it’s the method that the authorities used to worry about, decades back.

Method 3. Service Provider Mediated with independent end-to-end encryption

There are plenty more. The sender connects to the Service Provider and indicates, via an encrypted message, what recipient they want to message. The Service Provider replies with an endpoint that the sender can connect to. The sender then directly connects to the recipient and transmits an encrypted message, which the recipient decrypts.

This peer-to-peer messaging isn’t fundamentally different in technology from the end-to-end encrypted scenario. In both cases the actual networking is “store-and-forward”: An intermediary receives data, stores it, and then transmits it to either another intermediary or the recipient. The only difference is how long the data is stored from; a typical router will store the data for only a fraction of a second before transmitting and deleting it, whereas a Service Provider’s application server will store it at least until the recipient connects to retrieve it, and quite likely will archive it permanently. (Note there are regulations in some jurisdictions that require Service Providers to archive it permanently, but that applies to their application servers and not to routers, which handle orders of magnitude more data, most of which is transient).

It’s not always obvious to the user whether a real-time connection is mediated or not. Skype calls were originally peer-to-peer, and Microsoft changed it to mediated after they bought Skype. The general assumption is that this was at the behest of the NSA to enable interception, though I’ve not seen any definitive evidence.

Another thing about this kind of service is that the Service Provider does not need nearly as much resource as one that’s actually receiving all the messages their users send. There could be a thousand different P2P services, in any jurisdiction. With WebRTC now built into browsers, it’s easy to set one up.

Method 4. Service Provider directed peer-to-peer.

It’s not actually hard to be your own Service Provider. The sender can put the message on his own server, and the recipient can connect to the sender’s server to receive it. Or, the sender can connect to the recipient’s server, and send the message to that. In either case, the transmission of the messages (and it’s only one transmission over the public internet, not two as in the previous cases) will be encrypted.

As with method 2,  the Service Provider might manage the encryption keys for the user, or the user’s app might retain encryption keys for the correspondents it has in its directory.

The software is all free and common. Creating a service requires a little knowledge, but not real expertise. I estimate it would take me 90 minutes and cost £10 to set up a publicly-accessible email, forum and/or instant messaging service, using software that has been widespread for many years, and that uses the same secure encryption that everything else on the internet uses. Whether this counts as “end to end encryption” depends entirely on what you count as an “end”.  If I want the server to be in my house instead of a cloud data centre in the country of my choice, it might cost me £50 instead of £10, and it’s likely to have a bit more downtime. That surely would make it “end-to-end”, at least for messages for which I am either the sender or the recipient.

This is getting easier and more common, as internet speeds improve, connected devices proliferate, and distrust of the online giants’ commercial surveillance practices grows. There have been one or two “server in a box” products offered which you can just buy and plug in to get this kind of service — so far they have been dodgy, but there is no technical barrier to making them much better. Even if such a server is intended and marketed simply as a personal backup/archive solution, it is nevertheless in practice a completely functional messaging platform. The difference between an application that saves your phone photos to your backup drive and a full chat application is just a little bit of UI decoration, and so software like owncloud designed to do the first just throws in the second because it’s trivial.

That is Method 5. Owned server

There are several variants covered there. The user’s own server might be on their own premises, or might be rented from a cloud provider. If rented, it might be a physical machine or a virtual machine. The messages might be encrypted with a key owned by the recipient, or encrypted with a key configured for the service, or both, or neither. Whether owned or rented, the server might be in the same country as the user, or a different country. Each of these makes a significant difference from the point of view of an investigating agency wanting to read the messages.

Investigating authorities aren’t only concerned with encryption, though, they also want to know who is sending or receiving a message, even if they can’t read it. This could make the politicians’ opposition to mediated end-to-end encryption more reasonable: the Service Providers allow users to connect to their servers more or less anonymously. Using peer-to-peer or personal cloud services, the data is secure but the identity of the recipients of messages is generally easier to trace. The Service Providers give the users that the authorities are interested in a crowd of ordinary people to hide among.

It’s easy to sneer at Amber Rudd, but can you imagine trying to describe a policy on this in a TV interview, or in the House of Commons? Note I’ve skipped over some subtle questions.

Even if you could, you probably wouldn’t want to. Why spell out, “We want to get cooperation from Facebook to give us messages, but we’re not stupid, we know that if the terrorists buy a £100 off-the-shelf NAS box and use that to handle their messages, that won’t help us”?

Summary: kinds of messaging practice

Service Provider mediated non-end-to-end

Data accessible to authorities: with co-operation of Service Provider
Identity accessible to authorities: IP addresses obtainable with co-operation of Service Provider but can be obscured by onion routing / using public wifi etc
User convenience: very convenient

Service Provider mediated end-to-end

Data accessible to authorities: No
Identity accessible to authorities: IP addresses obtainable with co-operation of Service Provider but can be obscured by onion routing / using public wifi etc
User convenience: very convenient

End-to-end layered over Service Provider (e.g. PGP mail)

Data accessible to authorities: No
Identity accessible to authorities: IP addresses obtainable with co-operation of Service Provider but can be obscured by onion routing / using public wifi etc
User convenience: very inconvenient, all users must use special software, do key management

Peer-to-peer
Data accessible to authorities: No
Identity accessible to authorities: IP addresses directly accessible by surveillance at either endpoint or at ISP
User convenience: fiddly to use, need to manage directories of some kind

Personal Internet Service (Hosted)


Data accessible to authorities: With the cooperation of the host, which could be in any country
Identity accessible to authorities: IP addresses directly accessible by surveillance at either endpoint or at ISP
User convenience: Significant up-front work required by one party, but very easy to use by all others. Getting more convenient.

Personal Internet Service (on-site)

Data accessible to authorities: If they physically seize the computer
Identity accessible to authorities: IP addresses directly accessible by surveillance at either endpoint or at ISP
User convenience: Significant up-front work required by one party, but very easy to use by all others. Getting more convenient.
Appendix: Things I can think of but have skipped over to simplify
  • Disk encryption — keys stored or provided from outside at boot
  • Certificate spoofing, certificate pinning
  • Client applications versus web applications 
  • Hostile software updates
  • Accessing data on virtual servers through hypervisor





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