What is Signal? The messaging app, explained.

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With the recent news that the Atlantic’s editor in chief was accidentally added to a group Signal chat for American leaders planning a bombing in Yemen, many people are wondering: What is Signal? Is it secure? If government officials aren’t supposed to use it for military planning, does that mean I shouldn’t use it either?

The answer is: Yes, you should use Signal, but government officials having top-secret conversations shouldn’t use Signal.

Read on to find out why.

What is Signal?

Signal is an app you can install on your iPhone or Android phone, or on your computer. It lets you send secure texts, images, and phone or video chats with other people or groups of people, just like iMessage, Google Messages, WhatsApp, and other chat apps.

Installing Signal is a two-minute process—again, it’s designed to work just like other popular texting apps.

Why is it a problem for government officials to use Signal?

Signal is very secure—as we’ll see below, it’s the best option out there for having private conversations with your friends on your cell phone.

But you shouldn’t use it if you have a legal obligation to preserve your messages, such as while doing government business, because Signal prioritizes privacy over ability to preserve data. It’s designed to securely delete data when you’re done with it, not to keep it. This makes it uniquely unsuited for following public record laws.

You also shouldn’t use it if your phone might be a target of sophisticated hackers, because Signal can only do its job if the phone it is running on is secure. If your phone has been hacked, then the hacker can read your messages regardless of what software you are running.

This is why you shouldn’t use Signal to discuss classified material or military plans. For military communication your civilian phone is always considered hacked by adversaries, so you should instead use communication equipment that is safer—equipment that is physically guarded and designed to do only one job, making it harder to hack.

What about everyone else?

Signal is designed from bottom to top as a very private space for conversation. Cryptographers are very sure that as long as your phone is otherwise secure, no one can read your messages.

Why should you want that? Because private spaces for conversation are very important. In the US, the First Amendment recognizes, in the right to freedom of assembly, that we all need private conversations among our own selected groups in order to function.

And you don’t need the First Amendment to tell you that. You know, just like everyone else, that you can have important conversations in your living room, bedroom, church coffee hour, or meeting hall that you could never have on a public stage. Signal gives us the digital equivalent of that—it’s a space where we can talk, among groups of our choice, about the private things that matter to us, free of corporate or government surveillance. Our mental health and social functioning require that.

So if you’re not legally required to record your conversations, and not planning secret military operations, go ahead and use Signal—you deserve the privacy.

How do we know Signal is secure?

People often give up on finding digital privacy and end up censoring themselves out of caution. So are there really private ways to talk on our phones, or should we just assume that everything is being read anyway?

The good news is: For most of us who aren’t individually targeted by hackers, we really can still have private conversations.

Signal is designed to ensure that if you know your phone and the phones of other people in your group haven’t been hacked (more on that later), you don’t have to trust anything else. It uses many techniques from the cryptography community to make that possible.

Most important and well-known is “end-to-end encryption,” which means that messages can be read only on the devices involved in the conversation and not by servers passing the messages back and forth.

But Signal uses other techniques to keep your messages private and safe as well. For example, it goes to great lengths to make it hard for the Signal server itself to know who else you are talking to (a feature known as “sealed sender”), or for an attacker who records traffic between phones to later decrypt the traffic by seizing one of the phones (“perfect forward secrecy”).

These are only a few of many security properties built into the protocol, which is well enough designed and vetted for other messaging apps, such as WhatsApp and Google Messages, to use the same one.

Signal is also designed so we don’t have to trust the people who make it. The source code for the app is available online and, because of its popularity as a security tool, is frequently audited by experts.

And even though its security does not rely on our trust in the publisher, it does come from a respected source: the Signal Technology Foundation, a nonprofit whose mission is to “protect free expression and enable secure global communication through open-source privacy technology.” The app itself, and the foundation, grew out of a community of prominent privacy advocates. The foundation was started by Moxie Marlinspike, a cryptographer and longtime advocate of secure private communication, and Brian Acton, a cofounder of WhatsApp.

Why do people use Signal over other text apps? Are other ones secure?

Many apps offer end-to-end encryption, and it’s not a bad idea to use them for a measure of privacy. But Signal is a gold standard for private communication because it is secure by default: Unless you add someone you didn’t mean to, it’s very hard for a chat to accidentally become less secure than you intended.

That’s not necessarily the case for other apps. For example, iMessage conversations are sometimes end-to-end encrypted, but only if your chat has “blue bubbles,” and they aren’t encrypted in iCloud backups by default. Google Messages are sometimes end-to-end encrypted, but only if the chat shows a lock icon. WhatsApp is end-to-end encrypted but logs your activity, including “how you interact with others using our Services.”

Signal is careful not to record who you are talking with, to offer ways to reliably delete messages, and to keep messages secure even in online phone backups. This focus demonstrates the benefits of an app coming from a nonprofit focused on privacy rather than a company that sees security as a “nice to have” feature alongside other goals.

(Conversely, and as a warning, using Signal makes it rather easier to accidentally lose messages! Again, it is not a good choice if you are legally required to record your communication.)

Applications like WhatsApp, iMessage, and Google Messages do offer end-to-end encryption and can offer much better security than nothing. The worst option of all is regular SMS text messages (“green bubbles” on iOS)—those are sent unencrypted and are likely collected by mass government surveillance.

Wait, how do I know that my phone is secure?

Signal is an excellent choice for privacy if you know that the phones of everyone you’re talking with are secure. But how do you know that? It’s easy to give up on a feeling of privacy if you never feel good about trusting your phone anyway.

One good place to start for most of us is simply to make sure your phone is up to date. Governments often do have ways of hacking phones, but hacking up-to-date phones is expensive and risky and reserved for high-value targets. For most people, simply having your software up to date will remove you from a category that hackers target.

If you’re a potential target of sophisticated hacking, then don’t stop there. You’ll need extra security measures, and guides from the Freedom of the Press Foundation and the Electronic Frontier Foundation are a good place to start.

But you don’t have to be a high-value target to value privacy. The rest of us can do our part to re-create that private living room, bedroom, church, or meeting hall simply by using an up-to-date phone with an app that respects our privacy.

Jack Cushman is a fellow of the Berkman Klein Center for Internet and Society and directs the Library Innovation Lab at Harvard Law School Library. He is an appellate lawyer, computer programmer, and former board member of the ACLU of Massachusetts.