Identifying the features mass audiences want isn't so hard. But building even simple-sounding enhancements within Signal's privacy constraints—including a lack of metadata that even WhatsApp doesn't promise–can require significant feats of security engineering, and in some cases actual new research in cryptography.
Take stickers, one of the simpler recent Signal upgrades. On a less secure platform, that sort of integration is fairly straightforward. For Signal, it required designing a system where every sticker "pack" is encrypted with a "pack key." That key is itself encrypted and shared from one user to another when someone wants to install new stickers on their phone, so that Signal's server can never see decrypted stickers or even identify the Signal user who created or sent them.
Signal's new group messaging, which will allow administrators to add and remove people from groups without a Signal server ever being aware of that group's members, required going further still. Signal partnered with Microsoft Research to invent a novel form of "anonymous credentials" that let a server gatekeep who belongs in a group, but without ever learning the members' identities. "It required coming up with some innovations in the world of cryptography," Marlinspike says. "And in the end, it’s just invisible. It’s just groups, and it works like we expect groups to work."
Signal is rethinking how it keeps track of its users' social graphs, too. Another new feature it's testing, called "secure value recovery," would let you create an address book of your Signal contacts and store them on a Signal server, rather than simply depend on the contact list from your phone. That server-stored contact list would be preserved even when you switch to a new phone. To prevent Signal's servers from seeing those contacts, it would
encrypt them with a key stored in the SGX secure enclave that's meant to hide certain data even from the rest of the server's operating system.
That feature might someday even allow Signal to ditch its current system of identifying users based on their phone numbers—a feature that many privacy advocates have criticized, since it forces anyone who wants to be contacted via Signal to hand out a cell phone number, often to strangers. Instead, it could store persistent identities for users securely on its servers. "I’ll just say, this is something we’re thinking about," says Marlinspike. Secure value recovery, he says, "would be the first step in resolving that."
With new features comes additional complexity, which may add more chances for security vulnerabilities to slip into Signal's engineering, warns Matthew Green, a cryptographer at Johns Hopkins University. Depending on Intel's SGX feature, for instance, could let hackers steal secrets the next time security researchers expose a vulnerability in Intel hardware. For that reason, he says that some of Signal's new features should ideally come with an opt-out switch. "I hope this isn't all or nothing, that Moxie gives me the option to not use this," Green says.
But overall, Green says he's impressed with the engineering that Signal has put into its evolution. And making Signal friendlier to normal people only becomes more important as Silicon Valley companies
come under increasing pressure from governments to create encryption backdoors for law enforcement, and as Facebook hints that its own ambitious end-to-end encryption plans
are still years away from coming to fruition.
"Signal is thinking hard about how to give people the functionality they want without compromising privacy too much, and that's really important," Green adds. "If you see Signal as important for secure communication in the future—and possibly you don't see Facebook or WhatsApp as being reliable—then you definitely need Signal to be usable by a larger group of people. That means having these features."
Brian Acton doesn't hide his ambition that Signal could, in fact, grow into a WhatsApp-sized service. After all, Acton not only founded WhatsApp and helped it grow to billions of users, but before that joined Yahoo in its early, explosive growth days of the mid-1990s. He thinks he can do it again. "I’d like for Signal to reach billions of users. I know what it takes to do that. I did that," says Acton. "I’d love to have it happen in the next five years or less."
That wild ambition, to get Signal installed onto a significant fraction of all the phones on the planet, represents a shift—if not for Acton, then for Marlinspike. Just three years ago, Signal's creator
mused in an interview with WIRED that he hoped Signal could someday "fade away," ideally after its encryption had been widely implemented in other billion-user networks like WhatsApp. Now, it seems, Signal hopes to not merely influence tech's behemoths, but to become one.
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