Private communications with Signal

The internet is a global network. You would think that the fact that every message sent over it passes though many legal jurisdictions would make the need for encryption obvious and uncontroversial. Sadly that is not the case.

The need for something more than legal safeguards becomes obvious when you see that a request from a Toronto home to (whose server is in Toronto!) leaving Canadian legal jurisdiction on a path through both New York and Chicago before finally reaching it’s Toronto destination.

An example of a boomerang route from the project. About 25% of traffic with both a start and end point in Canada is routed this way.

Applications that deliver technical safeguards, like end-to-end encryption, offer that “something more” that protects my data beyond the border. One of these a applications is Signal, a project of Open Whisper Systems.

In an offline world, privacy was the default, a product of things you didn’t say or do, and probably also a byproduct of how hard it was to move information around. As things like chatting with friends and family or reading a newspaper all moved online, those activities suddenly involved sending data in plain text over public infrastructure. Privacy become something that existed only for those that found a way to avoid the default of sending plain text. Privacy became a product of action rather than inaction.

Signal and its predecessor projects Textsecure and Redphone are part of an effort to make privacy the default again by rolling high end encryption into apps polished for mainstream adoption.


Signal does two main things: sending text messages and making calls. What Signal actually does for secure communications is very different from what it does for insecure ones and is worth understanding.

Text messages

When sending a text message to someone who does not have Signal, the application sends a standard SMS message. The details of what constitutes an SMS message were hashed out in 1988 long before security was a thing and consequently, a related specification notes “SMS messages are transported without any provisions for privacy or integrity”, but importantly they are transported over the telephone network.

When sending secure text messages, Signal uses your mobile data to send the message using the Textsecure protocol v2.

The distinction between those two is worth making since coverage for data vs telephone can vary as can the costs, say if you are travelling and turn off mobile data.

The first time you send or receive a secure message with someone, behind the scenes you exchange cryptographic keys. Everything Signal does is focused on ensuring secure communication with the holder of that key. If the key for one of your contacts changes, it should be considered an event worth a quick phone call. This can happen innocently enough, say if they uninstall and then reinstall the app, but since all the other security measures a built on that, its worth asking about.

After the first text message has been sent or received, from then on Signal uses those keys to generate new keys for each individual message (Described in detail here.). This ensures that even if one message were to be decrypted, every other message is still safe.


Calling follows a similar pattern; for insecure calls Signal simply launches your phones standard phone app, while encrypted calls it handles itself. And like the secure text messages, this also uses your mobile data rather than routing through the phone network.

Secure calls are placed using the ZRTP protocol, the details of which are hidden from the user with one important exception.
On screen when you make a secure call you will notice two words displayed. These words are part of the ZRTP protocol and were generated based on the key that both parties used to encrypt the call.


Both parties should see the same two words. If you say one and ask your contact to read the other, and they don’t match up, they keys you have agreed upon are not the same. If the keys are not the same it suggests someone has tampered with connection information inflight and inserted themselves into your conversation.

Verifying keys

Part of whole key exchange process that allows users to identify each other involves pulling your contacts public key from a central key directory server. The use of a central server means that I now have to trust that server not to maliciously give me a key for someone else. Open Whisper Systems Trevor Perrin addressed the problem of trusting unauthenticated keys in a talk at the NSEC security conference. It’s just a few minutes but its an interesting insight into the balancing act involved in bringing private communications to the masses:

For the interested or the paranoid, Signal lets you verify a contacts key by touching your contacts name/number at the top of your conversation. This brings up the details for that contact which includes a “Verify identity” option.verifyWith that, and your own identity details, found under Settings (three vertical dots on the top right of the main screen) > “My Identity Key”, you are able to either read a key fingerprint or if you have a QR/Barcode scanner you can use that to verify your contacts key.


Open Source

Establishing that there is no secret behaviour or hidden flaws somewhere in the code is critical in this world where we put a significant amount of trust in computers (and where the definition of computer is expanding to encompass everything from voting machines to Volkswagens).

Signal establishes trust by developing the code in the open the code so that it can be reviewed (like this review of Signal’s predecessor Redphone by Cryptographer Matthew Green). Former Google security researcher Morgan Marquise-Boire has endorsed Signal as has Edward Snowden.

But even if you believe the experts that Signal works as advertised, its common for “free” apps to seem significantly less “free” once you realize what they do to turn a profit. With that in mind, another component of trust is understanding the business model behind the products you use.

When asked about the business model on a popular tech forum, Open Whisper Systems founder Moxie Marlinspike explained “in general, Open Whisper Systems is a project rather than a company, and the project’s objective is not financial profit.”

The project is funded by a combination of grants and donations from the Freedom of the Press Foundation and The Shuttlesworth Foundation among others. It is worked on by a core group of contributors led by and supporting cast of volunteers.

Final thoughts

Signal does a great job of making encrypting your communications a non-event. Encrypted as they travel the network, our messages are now secure against tampering and interception, no matter whose servers/routers they pass through. The result: privacy.

The fact that applying security measures result in privacy should tell you that the oft quoted choice between “security vs privacy” is a false one. As pointed out by Timothy Mitchener-Nissen, assuming a balance between these things only results in sacrificing increments of privacy in pursuit of the unachievable idea of total security. The ultimate result is reducing privacy to zero. Signal is just one way to grab back one of those increments of privacy.

All of that said my interest in privacy technologies and encryption is an interest for me as a technologist. If you are counting on these technologies like Signal to protect you from anyone serious (like a nation-state) the information above is barely a beginning. I would suggest reading this best practices for Tor and the grugq’s article on signals, intelligence. Actually anything/everything by the grugq.


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