Ephemeral usernames safeguard privacy and make Signal harder to subpoena
theintercept.com
theintercept.com
1. Imagine a journalist publicizes a username for a long time, then changes it. The old username would persist in data stores online and in address books, and would be used in error. An attacker could acquire the old username and impersonate the journalist. Perhaps a solution is to make at least some usernames, possibly at the user's option, non-reusable.
2. One strength of Signal is not only do they not collect much user data, they can't. Under some court order, they could retain the hashed usernames.
Let’s say I create a username and then later delete it as they suggest.
Can someone else then create the username and continue the conversation?
Is there a means to know if I’m talking to the same “Bob.smith.123” I was a few weeks back?
1. You use the username to connect to Bob.smith.123. This creates a conversation with the user who currently has that username.
2. You can’t see that user’s username, just “Bob Smith” or their name in your contacts, so them changing or deleting their username isn’t even visible to you. Your conversation is unaffected.
3. If, in the future, you add username Bob.smith.123 again, it may be a different person.
I think the name is a mistake, I’d have called it an “add code” or something. Clearer to explain a new thing than to say a word that means a dozen wrong things and always be correcting everybody.
I’ll go ahead and repeat that I have said in previous comments:
> or their name in your contacts
This is a huge mistake on Signal’s part — even if both have saved each other’s numbers the username-to-username connection should not mix with any other “normal conversation” ever!!
They should also not try to show the ser “name” either - some other name or a named conversation.
Keep your phone number private with Signal usernames - https://news.ycombinator.com/item?id=39444500 - Feb 2024 (872 comments)
Maybe it's more complicated than that?
So, you tell someone my username is "ABC", their Signal client hashes it and looks up the account and messages you.
Signal can keep the mapping of hash to number, but doesn't care about the plaintext value of the username.
After the person has messaged you/added you, then you can delete the mapping, at that point they're connected to you, even if they don't see your metadata like phone number.
Then, from the article, others can reuse the username too.
So I come up with a username, on my client side it is hashed and Signal's kv store is checked to make sure no one claimed the hash yet.
Next I tell the username to my peer via some other secure channel or IRL and their client sends the hashed username to this kv store to do the usual things.
I guess the hash still points at the phone number behind the scenes though?
AFAIK, from the article, yes. But, you can create/delete that hash->phone mapping at will.
Hence the recommendation to just create when required, or frequently change them if you're concerned.
Or, use the QR code/link generation too. Those work the same way.
This sounds like a potential security hole. You have to be sure that your contact hasn't reset their username as you add them. Or maybe that's what the username registration timestamp is for? To show that this username has been in use by the same person for a while?
Imagine a journalist who publicizes a username, then changes it. The original username would linger on the Internet, in people's address books, etc. A persistant attacker might acquire it, and snap up contacts as they come in.
Maybe Signal should have made usernames non-reusable, at least as an option. They still could dissociate the username from the phone number when the user 'deletest' it. The deleted username would just be shifted to the null user.
Note that this is no longer the case and signal stores a lot of data in the cloud now (name, phone number, photo, and your contacts)
> It says the username values are hashed, so they can't actually see them.
Even if I didn't already believe that Signal was compromised this wouldn't inspire much confidence. When they're keeping your phone number in the cloud forever (along with your contacts) not having a username doesn't mean a whole lot. Phone numbers are trivial to match up to a name/address/credit card/billing address/location.
The only thing signal know is that this phone number uses signal … that’s not a lot.
https://community.signalusers.org/t/proper-secure-value-secu...
Even if we forgive them for keeping/using phone numbers, all of the other data collection is entirely unnecessary and it's also exactly the kind of data the feds have been asking them to hand over.
Do you have any links talking about that?
It's contrary to what I believe, but I'm not so confident in those beliefs that I'm not intensely curious about that claim.
I don't blame you for not knowing about it. They were extremely unclear about it at the time it was implemented, and to this day they've refused to update their privacy policy the very first sentence of which is a lie. Make of that what you will.
[1]: https://privacytools.seas.harvard.edu/courses-educational-ma...
I really don't like differential privacy.
Assuming of course that you can use Signal on the desktop with usernames without ever involving a mobile app. If they haven't fixed that then I'm leaving them in the untrusted bin.
> (...) Signal can't be trusted
Can't be trusted for what?
Keeping your data safe from yourself?
The usual excuse is that they need phone numbers to combat spam, but that is only because they allow arbitrary contact requests form random people. It would be easy to imagine accounts without arbitrary contact permission. Contact requests could still be exchanged by e.g. meeting offline in person or with time-limited friend request codes.
How does that make sense? Signal just made phone numbers for contact discovery optional, in which case this person still couldn't find their mother, even though Signal has their number on file.
What people are asking for is for phone numbers to be optional for account creation and identification. Everybody that wants to could still provide (and verify) their phone number for contact discovery, and this could even remain the default for non-sophisticated users as the one described above.
So that seems more like a retroactive justification for an early design choice (Signal was originally TextSecure and used SMS as a transport layer, so making numbers the primary key made total sense back then). The only thing that still makes sense to me today is spam prevention:
> Requiring phone numbers also makes it considerably harder for spammers to abuse Signal. “The existence of a handful of small apps that don’t really have a large scale of users, that don’t require phone numbers, I don’t think is proof that it’s actually workable for a large-scale app,” Whittaker said.
One possible solution could be to tie numberless account creation to a nominal donation payment: Still not great, but spam prevention is unfortunately not free to Signal either.
SMS is afaik the only real barrier they have to preventing that.
[0]: As in, AGPL and effectively a source dump with no instructions.
* For example, see https://news.ycombinator.com/item?id=10905937
* Mobile-phone baseband chipsets are proprietary and secret a.f. and part of that is down to the carrier's insistence.
* Baseband chipsets run software that the carrier ships OTA to the phone.
* While baseband chipsets are ostensibly part of the wireless modem and meant to simply provide a service to the rest of the phone it looks like they generally have some form of access to the phone's main memory bus (just like any other PCIe device in a PC) and so could read the framebuffer (assuming it's backed in RAM at all) - or at least the back-buffers of the screens of running applications.
* Even 6-7 years ago, there existed definite causes for concern in (at least) the 32-bit version of iOS - but I can't find any hard evidence that the baseband chip in Apple Silicon-era phones wouldn't have at least some access. See https://github.com/userlandkernel/baseband-research
walled-off from the rest of the phone (somehow) from what I can tell it looks like
A useful search term here is IOMMU, the major phone platforms have readily available documentation describing the architecture and its security goals.
We have way, way, way more than that. Both the GP and you are arguing about the security deficiencies of modern phones as you've imagined them, rather than as they are but that gap is trivial to close with relatively little reading.
I appreciate the strength of your conviction - but I'm not an phone industry insider, and have no access to the kinds of reading-material I assume you're pointing to - for example, Qualcomm put their docs behind a verify-your-employer-wall (which is outrageous): https://www.qualcomm.com/products/technology/modems/snapdrag...
...if Qualcomm's attitude towards openness and transparency is representative of the mobile comms industry in general then they have little hope of correcting any misinformation or misconceptions other technology folk like ourselves might have, let alone the general public.
The baseband parts here are not, as message board C.W. would have it, top secret unknowable wizard hardware. You can get the part numbers and look them up.
There's a lot of weird mythology about these modem parts. The thread you linked to included someone claiming that basebands were DMA'ing into host memory --- you couldn't even do DMA over the HSIC USB the parts were using. Like, it wasn't even physically possible.
(I have no idea what a 5G Snapdragon Xwhatever can do today, but I assure you that Apple's security team does).
Given the amount of spying that has been revealed (a lot of it seeming to be superficially illegal) it seems reasonable to assume that phones are compromised in all manner of ways unless proven otherwise. I'd prefer to be pleasantly surprised.
Anything that makes it more expensive for the government to read someone's communications is a bonus. Ideally panopticon states will remain uneconomic.
A GPS transponder with microphone and camera under the control of billionaires seems like a mistake
Likewise, you can highlight text on screenshots.
If your adversary has access to your phone, directly, then encryption will not help (practically speaking - got to keep saying that because heroic efforts can be expended). If that adversary is the phone manufacturer (a) you are screwed and (b) the manufacturer is taking a huge business risk
Point is using Signal your messages and secrets cannot be found on a server over which you have no control. You do have control over your phone. You can switch to a more reputable manufacturer, you can keep it away from your adversary
One way to express it: The phone isn't much use unless you have access to the data. If you can access it, so can adversaries with access to your phone.
"Basically, you’re either dealing with Mossad or not-Mossad. If your adversary is not-Mossad, then you’ll probably be fine if you pick a good password and don’t respond to emails from ChEaPestPAiNPi11s@ virus-basket.biz.ru. If your adversary is the Mossad, YOU’RE GONNA DIE AND THERE’S NOTHING THAT YOU CAN DO ABOUT IT." -- https://www.usenix.org/system/files/1401_08-12_mickens.pdf
There's a lot of humour in that article, but some cold hard truth as well.
While this advice suggests I should just give up, that's not a practical option.
I'm fortunate enough to have never had that knock on my door, but I'm certain I'm on several lists.
You don't need to "give up", you need to work out what stuff you do or talk about that "Mossad" isn't really interested in, and be much more circumspect about where and who you talk with about anything that they might be interested in.
Sadly, for anybody without my kind of middle aged white dude privilege, that's almost certainly a "chilling effect".
What's the most subversive thing you've ever posted, repeatedly, with a large number of people seeing it? Chances are that it's not being "fortunate" that keeps you save but you simply aren't of interest to them.
In Communist China, you're perfectly safe as long as you aren't the "wrong" ethnicity and never prominently criticize the Party. In democratic country X, despite people getting to vote, it's not fundamentally different.
PS: Are there any other articles that he's written that are similarly entertaining?
try here for a great start https://mickens.seas.harvard.edu/wisdom-james-mickens
https://hackaday.com/2020/09/29/bunnies-betrusted-makes-firs...