Ars Technica has a really good article with details:

Ultimately, the architects settled on a creative solution. Rather than bolt KEM onto the existing double ratchet, they allowed it to remain more or less the same as it had been. Then they used the new quantum-safe ratchet to implement a parallel secure messaging system.

Now, when the protocol encrypts a message, it sources encryption keys from both the classic Double Ratchet and the new ratchet. It then mixes the two keys together (using a cryptographic key derivation function) to get a new encryption key that has all of the security of the classical Double Ratchet but now has quantum security, too...

"I didn’t see this loser in the group," Waltz told Fox News about Atlantic editor in chief Jeffrey Goldberg, whom Waltz invited to the chat. "Whether he did it deliberately or it happened in some other technical mean, is something we’re trying to figure out."

Waltz’s implication that Goldberg may have hacked his way in was followed by a ...

Onstage at TechCrunch Disrupt 2023, Meredith Whittaker, the president of the Signal Foundation, which maintains the nonprofit Signal messaging app, reaffirmed that Signal would leave the U.K. if the country’s recently passed Online Safety Bill forced Signal to build “backdoors” into its end-to-end encryption.

“We would leave the U.K. or any jurisdiction if it came down to the choice between backdooring our encryption and betraying the people who count on us for privacy, or leaving,” Whittaker said. “And that’s never not true.”...

An app with the name Signal Plus Messenger was available on Play for nine months and had been downloaded from Play roughly 100 times before Google took it down last April after being tipped off by security firm ESET. It was also available in the Samsung app store and on signalplus[.]org, a dedicated website mimicking the official Signal.org. An app calling itself FlyGram, meanwhile, was created by the same threat actor and was available through the same three channels. Google removed it from Play in 2021. Both apps remain available in the Samsung store...

As currently drafted, the Bill could break end-to-end encryption,opening the door to routine, general and indiscriminate surveillance of personal messages of friends, family members, employees, executives, journalists, human rights activists and even politicians themselves, which would fundamentally undermine everyone’s ability to communicate securely.

The Bill provides no explicit protection for encryption, and if implemented as written, could empower OFCOM to try to force the proactive scanning of private messages on end-to-end encrypted communication services—nullifying the purpose of end-to-end encryption as a result and compromising the privacy of all users...

WhatsApp uses the Signal encryption protocol to provide encryption for its messages. That was absolutely a visionary choice that Brian and his team led back in the day ­- and big props to them for doing that. But you can’t just look at that and then stop at message protection. WhatsApp does not protect metadata the way that Signal does. Signal knows nothing about who you are. It doesn’t have your profile information and it has introduced group encryption protections. We don’t know who you are talking to or who is in the membership of a group. It has gone above and beyond to minimize the collection of metadata...

Here’s what our users need to know:

• All users can rest assured that their message history, contact lists, profile information, whom they’d blocked, and other personal data remain private and secure and were not affected.
• For about 1,900 users, an attacker could have attempted to re-register their number to another device or learned that their number was registered to Signal. This attack has since been shut down by Twilio. 1,900 users is a very small percentage of Signal’s total users, meaning that most were not affected...

News article. Slashdot post.

The whole story is puzzling. Cellebrite’s details will make it easier for the Signal developers to patch the vulnerability. So either Cellebrite believes it is so good that it can break whatever Signal does, or the original blog post was a mistake.

EDITED TO ADD (12/22): Signal’s Moxie Marlinspike takes serious issue with Cellebrite’s announcement. I have urged him to write it up, and will link to it when he does...