In the spring of 2021, the encrypted communications app Signal announced that it would add a payments feature in beta for its users in the UK, testing out an integration with a relatively new, privacy-focused cryptocurrency called MobileCoin. But a much broader phase of that experiment has quietly been underway since mid-November. That’s when Signal made the same feature accessible to all of its users without fanfare, offering the ability to send digital payments far more private than a credit card transaction—or a Bitcoin transfer—to many millions of phones.
MobileCoin founder Josh Goldbard confirmed the timing of the rollout, and says that it spurred massive adoption of the cryptocurrency, which now sees thousands of daily transactions versus just dozens before the global beta release. “There are over a hundred million devices on planet Earth right now that have the ability to turn on MobileCoin and send an end-to-end encrypted payment in five seconds or less,” Goldbard says, referencing reports of Signal’s total download numbers.
In fact, getting started using Signal’s payments feature still isn’t quite that simple. Anyone outside of sanctioned companies like North Korea and Syria can access their MobileCoin wallet within a message by tapping the “+” icon and then “payment.” But the challenge for many will be loading that wallet in the first place; the cryptocurrency is listed for sale on only a few smaller cryptocurrency exchanges—such as BitFinex and FTX—none of which yet offer it to US consumers.
Signal itself didn’t respond to WIRED’s requests for comment on the global rollout of the payments feature. But last April, Signal creator Moxie Marlinspike explained to WIRED that he wanted to add payments to the encrypted video-calling and texting app to match features from rivals like WhatsApp and Facebook Messenger—while also bringing Signal’s lauded privacy protections to monetary transactions. “I would like to get to a world where not only can you feel [a sense of privacy] when you talk to your therapist over Signal, but also when you pay your therapist for the session over Signal,” Marlinspike said at the time.
Marlinspike has argued that sort of monetary privacy requires integration with a cryptocurrency rather than traditional, surveillance-friendly banking and credit card systems. In 2017, Marlinspike helped launch MobileCoin with that potential integration in mind, serving as a paid technical advisor for the cryptocurrency. He and Goldbard say they designed MobileCoin to be both easier to use for small purchases on a mobile device, with fast transaction confirmations, and also far more private than Bitcoin, whose public blockchain can allow powerful forms of tracking.
To avoid that blockchain-based tracing of user finances, MobileCoin deploys techniques that have been pioneered in older so-called « privacy coins » such as Monero and Zcash. Those include a protocol called CryptoNote and a feature called Ring Confidential Transactions, which hides the amount of payments and makes them hard to trace by mixing them up. MobileCoin also uses a form of mathematical proof known as Bulletproofs that can guarantee a transaction has occurred without revealing its value. « I don’t think it’s reasonable to send transactions over a ledger where all actions can be linked, » Goldbard says of Bitcoin’s less-private blockchain. « There are so many different ways that this has problems. If I pay my bills, my barista now knows that I just paid my therapist or I went to the doctor. Every transaction that I make with that wallet is now visible to my barista, forever. »