Signal's New Usernames Help Keep Cops Out of Your Data

Longtime Slashdot reader SonicSpike shares a report from The Intercept: With the new version of Signal, you will no longer broadcast your phone number to everyone you send messages to by default, though you can choose to if you want. Your phone number will still be displayed to contacts who already have it stored in their phones. Going forward, however, when you start a new conversation on Signal, your number won't be shared at all: Contacts will just see the name you use when you set up your Signal profile. So even if your contact is using a custom Signal client, for example, they still won't be able to discover your phone number since the service will never tell it to them. You also now have the option to set a username, which Signal lets you change whenever you want and delete when you don't want it anymore. Rather than directly storing your username as part of your account details, Signal stores a cryptographic hash of your username instead; Signal uses the Ristretto 25519 hashing algorithm, essentially storing a random block of data instead of usernames themselves. This is like how online services can confirm a user's password is valid without storing a copy of the actual password itself. "As far as we're aware, we're the only messaging platform that now has support for usernames that doesn't know everyone's usernames by default," said Josh Lund, a senior technologist at Signal. The move is yet another piece of the Signal ethos to keep as little data on hand as it can, lest the authorities try to intrude on the company. Whittaker explained, "We don't want to be forced to enumerate a directory of usernames." [...] If Signal receives a subpoena demanding that they hand over all account data related to a user with a specific username that is currently active at the time that Signal looks it up, they would be able to link it to an account. That means Signal would turn over that user's phone number, along with the account creation date and the last connection date. Whittaker stressed that this is "a pretty narrow pipeline that is guarded viciously by ACLU lawyers," just to obtain a phone number based on a username. Signal, though, can't confirm how long a given username has been in use, how many other accounts have used it in the past, or anything else about it. If the Signal user briefly used a username and then deleted it, Signal wouldn't even be able to confirm that it was ever in use to begin with, much less which accounts had used it before. In short, if you're worried about Signal handing over your phone number to law enforcement based on your username, you should only set a username when you want someone to contact you, and then delete it afterward. And each time, always set a different username. Likewise, if you want someone to contact you securely, you can send them your Signal link, and, as soon as they make contact, you can reset the link. If Signal receives a subpoena based on a link that was already reset, it will be impossible for them to look up which account it was associated with. If the subpoena demands that Signal turn over account information based on a phone number, rather than a username, Signal could be forced to hand over the cryptographic hash of the account's username, if a username is set. It would be difficult, however, for law enforcement to learn the actual username itself based on its hash. If they already suspect a username, they could use the hash to confirm that it's real. Otherwise, they would have to guess the username using password cracking techniques like dictionary attacks or rainbow tables.

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