[josh2600]

This is why signal’s encrypted phone number lookup system is so cool. The server uses a bitwise xor when querying for numbers using hardware encrypted ram. The result is that even if you’re examining the machine at the most basic levels you can’t tell the difference between a negative or positive hit for the phone number unless you’re the phone requesting the api.

Obviously ratelimiting is a separate and important issue in api management.

The thing about building secure systems is that there are a lot of edges to cover.

[heavyset_go]

I don't think it's cool at all, a secure messaging app should not require personal/tracking identifiers like phone numbers in the first place.

[maqp]

The sad part is, that's what's keeping Signal safe from spam.

Also, average Joe is not using proxy to hide the IP-address of their device so they leak their identity to the server anyway. Signal is not keeping those logs so that helps.

Messaging apps cater to different needs, sometimes you need only content-privacy. It's not a secret you're married to your partner and you talk daily, but the topics of the conversation aren't public information.

When you need to hide who you are and who you talk to (say Russian dissident group, or sexual minorities in fundamentalist countries), you might want to use Tor-exclusive messaging tools like Cwtch. But that comes at a near-unavoidable issue of no offline-messaging, meaning you'll have to have a schedule when to meet online.

Signal's centralized architecture has upsides and downsides, but what matters ultimately is, (a) are you doing what you can in the architectural limitations of the platform (strong privacy-by-design provides more features at same security level), and (b), are you communicating the threat model to the users so they can make informed decision whether the applications fits their threat model.

[coppsilgold]

If you intend to use SMS (phone numbers) as a resource constraint (sign up requires 'locking up' a resource that is worth at least a few cents) then at least you can offer a ZKP system where the 'consumed' phone number is not tied to an account. You could also offer to accept cryptocurrency for this function - call it a donation.

That Signal did none of those things implies that privacy was not their objective. Only secure communications was.

It's possible that the reason behind their anti-privacy stand is strategic, to discourage criminal use which could be used as a vector of attack against them. Doesn't change the fact that Signal is demonstrably anti-privacy by design.

[ethersteeds]

Your first formulation I agree with:

> privacy was not their objective. Only secure communications was.

> Signal is demonstrably anti-privacy by design.

But your second is uncharitable and misses Signal's historical context.

The value of a phone number for spam prevention has been mentioned, but that's not the original reason why phone numbers were central to Signal. People forget that Signal was initially designed around using SMS as transport, as with Twitter.

Signal began as an SMS client for Android that transparently applied encryption on top of SMS messages when communicating with other Signal users. They added servers and IP backhaul as it grew. Then it got an iOS app, where 3rd party SMS clients aren't allowed. The two clients coexisted awkwardly for years, with Signal iOS as a pure modern messenger and Signal Android as a hybrid SMS client. Finally they ripped out SMS support. Still later they added usernames and communicating without exposing phone numbers to the other party.

You can reasonably disdain still having to expose a phone number to Signal, but calling it "anti-privacy by design" elides the origins of that design. It took a lot of refactoring to get out from under the initial design, just like Twitter in transcending the 140-character limit.

[coppsilgold]

> Signal is demonstrably anti-privacy by design.

> You can reasonably disdain still having to expose a phone number to Signal, but calling it "anti-privacy by design" elides the origins of that design.

They introduced usernames without removing the requirement for phone numbers.

I rest my case.

[hnarn]

Not a very good case made since you obviously didn’t read the parent discussion.

[illiac786]

You’re blatantly trolling, it’s boring man. What do you call SMS if signal is “anti-privacy by design”?

[lukeschlather]

If privacy wasn't their objective they would just have a database of all the phone numbers.

Perfect privacy would mean not sending any messages at all, because you can never prove the message is going to the intended recipient. Any actual system is going to have tradeoffs, calling Signal anti-privacy is not serious, especially when you're suggesting cryptocurrency as a solution.

A ZKP system where you make a public record of your zero-knowledge proof sounds anti-privacy to me. Even if you're using something obfuscated like Monero, it's still public. I see where you're coming from, but I think I would prefer Signal just keep a database of all their users and promise to try and keep it safe rather than rely on something like Monero.

[coppsilgold]

> have a database of all the phone numbers

They have exactly that. They rely on TPMs for "privacy" which is not serious.

> Perfect privacy would mean not sending any messages at all

Not sending messages is incompatible with secure messaging which is the subject of the discussion...

> ZKP system where you make a public record of your zero-knowledge proof sounds anti-privacy to me.

A zero-knowledge proof provably contains zero information. Even if you use a type of ZKP vulnerable to a potential CRQC it's still zero information and can never be cracked to reveal information (a CRQC could forge proofs however).

> especially when you're suggesting cryptocurrency as a solution

Would you elaborate on why cryptocurrencies are not a solution? Especially if combined with ZKPs to sever the connection between the payment and the account. When combined with ZKPs, they could even accept Paypal for donations in exchange for private accounts.

[lucyjojo]

signal was intended for the general public. crypto defeats the purpose.

[vintermann]

It's also possible that a lot of the criticism for Signal setting a practical/realistic level of what security they will try to provide, is from people who would rather that people either

1. were unable to communicate effectively, or

2. used no security at all.

Do you really use a communication system where you have all exchanged private keys in person and where even the fact that you use it is hidden from your government and phone operator?

[wkat4242]

I get lots of spam on WhatsApp which also requires a number. And some on signal too for that matter.

Signal is just much smaller in terms of users so the potential value is lower.

[rendaw]

I get zero spam on Line... I don't allow people to add me by phone number. Line isn't known for their cryptography/security skills.

[kragen]

If you wanted to keep it safe from spam, you'd use a proof-of-work scheme using a memory-hard hash function like scrypt, or a Captcha, or an invite-code system like lobste.rs or early Gmail. Signal's architects already knew that when they started designng it.

[maqp]

>proof-of-work scheme using a memory-hard hash function like scrypt

So who's doing the computation? The spammer can't afford to run 3 second key derivation time per spam device? Or how long do you think normal user will wait while you burn their battery power before saying "Screw it, I'll just use WA"? Or is this something the server should be doing?

>Captcha

LLMs are getting quite good at getting around captchas.

>invite-code system

That works in lobste.rs when everyone can talk together, and recruit interesting people to join the public conversation. Try doing that with limited invites to recruit your peers to build a useful local network of peers and relatives. "I'm sorry Adam, I'm out of invites can you invite my mom's step-cousin, my mom needs to talk to them?"

>Signal's architects already knew that when they started designng it.

I think they really did, and they did what the industry had already established as the best practice for a hard problem.

The only reasonable alternative would've been email with heavy temp-mail hardening, or looking into the opposite end of Zooko's triangle and having long, random, hard-to-enumerate usernames like Cwtch and other Tor-based messengers do. But even that's not removing the spam-list problem of any publicly listed address ending up in a list that gets spammed with contact requests or opening messages with spam.

[kragen]

Those are reasonable questions, but they suggest that you don't understand the landscape very well.

The user's device has to do the computation for it to be effective. How long does it normally take to sign up for a new messaging service like WhatsApp? Five minutes? You should burn the user's cellphone battery for about half that long, 150 seconds, 50 times more than you were thinking. Plus another half-minute every time you add a new contact. Times two for every time someone blocks you, up to a limit of 150 seconds. Minus one second for each day you've been signed up. Or something like that.

The value of signing up for Signal is much higher to a real user than it is to a spammer, so you just have to put the signup cost somewhere in the wide range in between.

LLMs didn't exist when Signal was designed, and Captchas still seem to be getting a lot of use today.

Invite codes worked fine for Gmail, and would work even better for any kind of closed messaging system like Signal; people who don't know any users of a particular messaging system almost never try to use it. The diameter of the world's social graph is maybe ten or twelve, so invite codes can cover the world's social graph with only small, transitory "out of invites" problems.

The "industry" had "established" that they "should" gather as much PII as possible in order to sell ads and get investments from In-Q-Tel.

[cyphar]

> Invite codes worked fine for Gmail

Back in 2004, sure. Today, Gmail asks you for a phone number when signing up because of the spam problem.

[lukeschlather]

> How long does it normally take to sign up for a new messaging service like WhatsApp? Five minutes? You should burn the user's cellphone battery for about half that long, 150 seconds

If you actually do that you're going to crash a lot of cellphones and people will rightly blame your app for being badly coded.

[maqp]

>but they suggest that you don't understand the landscape very well.

Yeah, what could I possibly know about secure messaging.

>Plus another half-minute every time you add a new contact.

Can you point to some instant messaging app that has you wait 30 seconds before talking to them? Now niché is it?

You want proper uptake and accessibility to everyone, you need something like Samsung A16 to run the work in 150 seconds. Some non-amateur spammer throws ten RTX 5090s to unlock access to random accounts at 80x parallelism (capped by memory cost), with the reasonable time cost of whatever iterations that is, with quite a bit shorter time than 150 seconds. 121.5GFLOPs vs 10x104.8 TFLOPs leads to overall performance difference of 8,800x. And that account is then free to spam at decent pace for a long time before it gets flagged and removed.

The accounts are not generated in five minutes per random sweat shop worker: https://www.youtube.com/watch?v=CHU4kWQY3E8 has tap actions synced across sixty devices. And that's just to deal with human-like captchas that need to show human-like randomness. Proof-of-work is not a captcha, so you can automate it. Signal's client is open source for myriad of reasons, the most pressing of which is verifiable cryptographic implementations. So you can just patch your copy of the source to dump the challenge and forward it to the brute force rig.

Either the enumeration itself has to be computationally infeasible, or it has to be seriously cost limited (one registration per 5 dollar prepaid SIM or whatever).

>Invite codes worked fine for Gmail

Yeah and back in ~2004 when Hotmail had 2MB of free storage, GMail's 1,000MB of free storage may have also "helped".

[immibis]

If the PoW cost is a low-end cellphone CPU for 2.5 minutes, then it's nothing to the spammer with the 200-core hourly AWS server. If each spammer can create 10000 identities (not connections, identities) per hour, then you might as well not have a limit at all. If they could even create only 2 identities per day that would be enough to spam with (yet still unacceptable to actual users). 250000 identities per day is way too many.

[vel0city]

Invite codes worked fine for Gmail, but you weren't limited to only the people on Gmail to talk to. It was a full, regular email service. You could email anyone and receive mail from anyone. I doubt it would have been very successful if it was invite only and you could only email other Gmail users for the first few years.

Waze was also invite-only, G+ was initially invite only. Did that model help or hurt them?

[codedokode]

Or a small payment in cryptocurrency.

[kragen]

Yes, that would also work, but you should probably offer alternatives.

[creata]

> you'd use a proof-of-work scheme

I thought the general belief (e.g., '“Proof-of-Work” Proves Not to Work') was that proof-of-work isn't very good anti-spam.

> or a Captcha

Aren't bots better at those than humans by now?

And making people do captchas in an instant messenger is a great way to make people not use that instant messenger.

> or an invite-code system like lobste.rs or early Gmail.

That's not a long-term option if you want to make something mainstream.

[kragen]

There are people who believe that proof-of-work isn't very effective, but none of them have succeeded in spamming the Bitcoin network with blocks they've mined, driving the other miners out of business, nor (for the last several years) with spamming the Bitcoin network with dust transactions they've signed, so I don't think we should take their opinions very seriously.

Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.

I don't see why invite codes would be a problem for mainstream use.

[Ieghaehia9]

> There are people who believe that proof-of-work isn't very effective, but none of them have succeeded in spamming the Bitcoin network with blocks they've mined, driving the other miners out of business, nor (for the last several years) with spamming the Bitcoin network with dust transactions they've signed, so I don't think we should take their opinions very seriously.

Different system. The parent and GP are talking about proof-of-work being used directly for account creation. If a chat service required mining-levels of PoW (and hence any prospective new users to have an ASIC), it would not be very popular. Nor would it be very popular if it used a relative difficulty system and the spammers used dedicated servers while the legitimate users had to compete using only their phones.

[creata]

> none of them have succeeded in spamming the Bitcoin network with blocks they've mined

I'm not saying you're wrong, but I have no idea what you're getting at, because the sentence sounds kind of absurd. As a result, I'm not sure if it addresses your point, but just to throw it out there: Bitcoin and anti-spam are different applications of proof of work. Anti-spam has to strike a compromise between being cheap for the user (who is often on relatively low-powered mobile hardware), and yet annoying enough to deter the spammer. It's not unreasonable to believe that such a compromise does not exist.

> Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.

Fair point, but again, even in 2014, an instant messenger with captchas would have much more friction than every other messenger. And captchas aren't just bad because they introduce enough friction to drive away pretty much everybody: they also make users feel like they're being treated as potential criminals.

> I don't see why invite codes would be a problem for mainstream use.

Can you elaborate? Invite codes blocking access to the service itself "like lobste.rs" mean that no one can use your service unless they've been transitively blessed by you. That's obviously going to limit its reach...

[alkindiffie]

> That's not a long-term option if you want to make something mainstream.

Groups in messaging apps rarely contain more than 100 users. So invite codes can work well for messaging apps.

[robot-wrangler]

Signal blasted my whole contacts list the day I signed up so that I was surprised to see lots of people saying "finally you got signal". That was also the moment I uninstalled the app. Leaking contact info appears to be part of the design.

Should have deleted my account instead of just removing the app, because it turns out the difference between using signal and using SMS is obscured for most phones, and when people thought they were texting me they weren't. I was just out of contact for a long time as people kept sending me the wrong kind of messages. I suppose one could argue protecting contact/identity is not a real goal for e2e encryption, but what I see is a "privacy oriented" service that's clearly way too interested in bootstrapping a user base with network effects and shouldn't be trusted.

[mmooss]

> Leaking contact info appears to be part of the design.

Those people already had your contact info, probably.

Also, I think there is a setting in Signal to prevent that - and via the OS you can block Signal's access to your contacts, of course.

[robot-wrangler]

> Those people already had your contact info, probably.

What leaked was that I was a signal user, and that the person on the other side was a signal user. The security implications are obvious, and by itself, that's already enough to get someone who really needs to care about privacy killed.

> Also, I think there is a setting in Signal to prevent that

False. It happened without my permission as soon as the app was installed, and there was no way to opt out. Maybe they changed it since then, but the fact remains they obviously cared more about network-effects and user-counts than user privacy.

Sigh, there's just no need for this kind of apologism. You could just admit that a) it's bad behavior, b) they did it on purpose, and c) it's not possible to trust someone who does something like this. I'm aware they are nonprofit, so I don't know why it's like this, but the answer is probably somewhere in the list of donors.

[lukeschlather]

How would you suggest Signal allow you to communicate with your contacts without leaking the fact that both of you are Signal users? Should it just blackhole the message if the other number doesn't have an account?

I understand the unease about the notifications, but there are some hard tradeoffs between how you can store as little information as possible, remain as decentralized as possible, while getting the same benefits as centralized systems like Facebook.

I'm really of the opinion that a messenger similar to Signal but more centralized in the fashion of WhatsApp or even Facebook Messenger should exist, but I also understand why Signal works the way it does.

[notfed]

Suggestion: it should, at very least, not show a UI a notification "_ is now on signal!" As a nice to have, yes, it should blackhole a message until at least one reply happens.

[mmooss]

That's a lot to pile on people who disagree with you. Maybe other people have perspectives that are both 1) different from yours and, 2) valid?

[robot-wrangler]

> Maybe other people have perspectives

Yeah, no. The whole "every perspective has some validity" thing won't really apply to most safety/security issues. The most charitable thing to say here is that the workflow is completely broken. Less charitable but also valid is pointing out that it's actively harmful, and deliberate. I would be really surprised if this hadn't ever caused serious consequences whether a whistle blower was fired, an abused spouse got extra abused, or an informant was killed. If you think you've got a "valid perspective" that prioritizes mere user-discovery over user-safety, then you should not be attempting work that's close to safety and security, full stop.

[minitech]

> What leaked was that I was a signal user, and that the person on the other side was a signal user.

Clearly, either this was before Signal had its username-lookup-only feature, or you opted into letting people find you by your phone number. At that point, the information is already effectively leaked in the same way (it’s easy for anyone to enumerate all phone numbers, let alone for you to enumerate your own contacts or vice versa), and if the notification surprised you then the absence of the notification would simply have been giving you a false sense of security.

Communication by non-phone-number identifiers is critically important, and I’m glad for recent Signal developments in that direction and hopeful for more in the future, but opting into phone-number-based communication and complaining that your contacts were merely notified about the communication option they would have been able to access anyway on a security or privacy basis is silly. The fact that this information (your contacts) passes through Signal is much more objectionable to me, even though they do the SGX thing, and I would never recommend allowing it access to your contacts for that reason.

[immibis]

When someone on your contacts list gets Signal, Signal displays this in its UI. I don't think this is a privacy violation. Signal aims to hide your messages, but it does not have its own contacts system, and piggybacks on your existing phone number and phone number contacts. Nor does it attempt to hide the fact you have Signal.

[guizadillas]

The people that already had your contact info in their devices were notified that you joined Signal via that contact info?

Seems like it was working as designed, if you don't want any app to get your contact info don't share your contact info to anyone ever. Eventually they will share that info with any app.

[overfeed]

Security and usability are frequently at odds. The ease with which users can discover and exchange messages with their contacts is a major usability issue. Phone number as a proxy for identity mostly works, at the cost of some privacy risks.

[soulofmischief]

This made sense when Signal/TextSecure allowed users to send regular SMS, making it easy to convince others to set it as their default messenger.

Now that this crucial adoption feature has been removed, it makes zero sense for Signal to continue to rely on phone numbers. Since that feature has been removed, the utility of Signal has been lost anyway and many in my groups returned to regular SMS. So the system is already compromised from that perspective. At least forks such as Session tried to solve this (too bad Session removed forward secrecy and became useless)

[stavros]

What's more secure? A moderately secure messaging app all your friends have installed, or a very secure messaging app nobody else has?

[K0balt]

I agree, but since a messaging apps utility is some fraction of the square of the # of users on the platform, a facile way to propagate virally is a de facto requirement for an app targeting wide spread adoption / discovery rather than targeted cells of individuals focused around a pre shared idea.

It’s a compromise meant to propagate the network, and it has a high degree of utility to most users. There are also plenty of apps that are de-facto anonymous and private. Signal is de facto non-anonymous but private, though using a personally identifiable token is not a hard requirement and is trivial to avoid. (A phone number of some kind is needed once for registration only)

[josh2600]

Signal requires a phone number for signup but you only have to share a username.

We know from subpoenas that signal only holds the user phone number, creation timestamp, and last login timestamp. That’s it.

[immibis]

Signal's security model does not include metadata, and this is a valid design.

[0x1ch]

There's no alternative to reduce spam and fake accounts, unless we collectively are fine with blocking Russia, India, China, and friends from the internet.

[codedokode]

Does Signal protect from the scheme when the government sends discovery requests for all existing phone numbers (< 1B) and gets a full mapping between user id and phone number?

While slightly unrelated, I thought, how we can fix this for truly secure and privacy-aware, non-commercial communication platforms like Matrix? Make it impossible to build such mapping. The core idea is that you should be able to find the user by number only if you are in their contact list - strangers not welcome. So every user, who wishes to be discovered, uploads hash(A, B) for every contact - a hash of user's phone number (A) and contact's phone number (B), swapped if B < A. Let's say user A uploaded hashes h(A,B) and h(A,C). Now, user B wishes to discover contacts and uploads hashes h(A, B) and h(B, D). The server sees matching hashes between A and B and lets them discover each other without knowing their numbers.

The advantages:

- as we hash a pair of 9-digit numbers, the hash function domain space is larger and it is more difficult to reverse the hashes (hash of a single phone number is reversed easily)

- each user can decide who may discover them

Disadvantages:

- a patient attacker can create hashes of A with all existing numbers and discover who are the contacts of A. Basically, extract anyone's phone book via discovery API. One way to protect against this would be to verify A's phone number before using discovery, but the government, probably, can intercept SMS codes and pass the verification anyway. However, the government can also see all the phone calls, so they know who is in whose phone book anyway.

- if the hash is reversed, you get pairs of phone numbers instead of just one number

[Arathorn]

There's some really interesting stuff we've been looking into on the Matrix side to solve this - e.g. https://github.com/asonnino/arke aka https://eprint.iacr.org/2023/1218 or https://martin.kleppmann.com/2024/07/05/pudding-user-discove....

Meanwhile, Matrix for now does support hashed contact lookup, although few clients implement it given the privacy considerations at https://spec.matrix.org/unstable/identity-service-api/#secur...

[wkat4242]

Yeah you're doing a lot better job on the privacy side than signal is IMO.

Especially just being able to run my own service will be priceless when something like chatcontrol eventually makes it through. Signal can only comply or leave, but they'll never manage to kill all the matrix servers around.

[godelski]

Signal publicly shares government requests AND the data that they send them

The data Signal has is: 1) registration time for a given phone number, 2) knowledge of daily login (24hr resolution). That's it. That's the metadata.

They do not have information on who is communicating with who, when messages are sent, if messages are sent, how many, the size, or any of that. Importantly, they do not have an identity (your name) associated with the account nor does that show for contacts (not even the phone number needs be shared).

Signal is designed to be safe from Signal itself.

Yes, it sucks that there is the phone number connected to the account, but you can probably understand that there's a reason authorities don't frequently send Signal data requests; because the information isn't very useful. So even if you have a phone number associated with a government ID (not required in America) they really can only show that you have an account and potentially that the account is active.

Like the sibling comment says, there's always a trade-off. You can't have a system that has no metadata, but you can have one that minimizes it. Signal needs to balance usability and minimize bots while maximizing privacy and security. Phone numbers are a barrier to entry for bots, preventing unlimited or trivial account generation. It has downsides but upsides too. One big upside is that if Signal gets compromised then there's can be no reconstruction of the chat history or metadata. IMO, it's a good enough solution for 99.9% of people. If you need privacy and security from nation state actors who are directly targeting you then it's maybe not the best solution (at least not out of the box) but otherwise I can't see a situation where it is a problem.

FWIW, Signal does look to be moving away from phone numbers. They have usernames now. I'd expect it to take time to completely get away though considering they're a small team and need to move from the existing infrastructure to that new one. It's definitely not an easy task (and I think people frequently underestimate the difficulty of security, as quoted in the article lol. And as suggested by the op: it's all edge cases)

https://signal.org/bigbrother/

[codedokode]

> Phone numbers are a barrier to entry for bots, preventing unlimited or trivial account generation.

What's wrong with account generation? Nothing. The problem is if they start sending spam to random people. So we can make registration or adding contacts paid (in cryptocurrency) and the problem is gone.

[jfindper]

>So we can make registration or adding contacts paid (in cryptocurrency) and the problem is gone.

The majority of the user base would be gone, too.

I had a hard enough time convincing my friend group to use Signal as is. If they had to pay (especially if it had to be via cryptocurrency) none of them would have ever even considered it.

[codedokode]

I would rather pay $1 than with my phone number which is much much much more valuable. Telegram did an experiment with paid anonymous registration, but the prices were ridiculous and targeted for the riches.

[jfindper]

>I would rather pay $1 than with my phone number which is much much much more valuable.

Most people would not, though, and that's the issue.

[godelski]

  > What's wrong with account generation?

Your comment *literally* explains one issue...

[wtfwhateven]

>What's wrong with account generation?

What's right with it? Accounts being generated (i.e. many inauthentic accounts controlled by few people) are always used to send spam, there are no exceptions. The perpetrators should be in prison.

[0xCMP]

Ah yes, and convincing friends/family/partners to use Signal instead of Whatsapp clearly what will convince them is that they need to setup, acquire, and use cryptocurrency to register or connect with me on the encrypted messaging service. "No thanks, I just use Whatsapp/iMessage. I heard they're actually e2e encrypted too, so what's the problem?"

[mmooss]

That doesn't answer the GP question:

> Does Signal protect from the scheme when the government sends discovery requests for all existing phone numbers (< 1B) and gets a full mapping between user id and phone number?

Signal does have the phone numbers, as you say. Can they connect a number to a username?

[godelski]

  > That doesn't answer the GP question:

It does.

They asked

   >>> Does Signal protect from the scheme when the government sends discovery requests for all existing phone numbers (< 1B) and gets a full mapping between user id and phone number?

Which yes, this does protect that. There is no mapping between a user id and phone number. Go look at the reports. They only show that the phone number has a registered account but they do not show what the user id is. Signal doesn't have that information to give.

  > Can they connect a number to a username?

From Signal

  Usernames in Signal are protected using a custom Ristretto 25519 hashing algorithm and zero-knowledge proofs. Signal can’t easily see or produce the username if given the phone number of a Signal account. Note that if provided with the plaintext of a username known to be in use, Signal can connect that username to the Signal account that the username is currently associated with. However, once a username has been changed or deleted, it can no longer be associated with a Signal account. 

This is in the details on[0] right above the section "Set it, share it, change it"

So Signal cannot use phone numbers to identify usernames BUT Signal can use usernames to identify phone numbers IF AND ONLY IF that username is in active use. (Note that the usernames is not the Signal ID)

If you are worried about this issue I'd either disable usernames or continually rotate them. If the username is not connected with your account at the time the request is being made then no connection can be made by Signal. So this is pretty easy to thwart, though I wish Signal included a way to automate this (perhaps Molly has a way or someone can add it?) Either rotating after every use or on a timer would almost guarantee that this happens given that it takes time to get a search warrant and time for Signal to process them. You can see from the BigBrother link that Signal is not very quick to respond...

[0] https://signal.org/blog/phone-number-privacy-usernames/

[heavyset_go]

The hash space for phone numbers is so small that you can enumerate them all.

[ruined]

yes. users can disable phone number discovery

[Groxx]

can they disable it before or after it tells other people that they joined, if those other people had their number in their synced contacts list?

(I would be thrilled to learn that this changed, but it has been in place for many years and it's kinda hard to personally test)

[ruined]

yes before.

discoverability does default to "on", but there is an opportunity to disable it during registration, which prevents those notifications.

[wizzwizz4]

And it's trivial to reverse a hash in such a scenario. This scheme is completely broken.

[ronsor]

> The server uses a bitwise xor when querying for numbers using hardware encrypted ram. The result is that even if you’re examining the machine at the most basic levels you can’t tell the difference between a negative or positive hit for the phone number unless you’re the phone requesting the api.

Do you have further reading on this?

[dathinab]

This article https://signal.org/blog/building-faster-oram/ has some details but is more focused on improving their solution other blogs from the are "we want to build this soon" kind of blogs. It seems that most articles about this topic either have too little content to be of interest or are technology previews/"we maybe will do that" articles about things Signal wants to implement, where it's unclear if they did do that or something similar.

To cut it short they use Intel SGX to create a "trusted environment" (trusted by the app/user) in which the run the contact discovery.

In that trusted environment you then run algorithms similar to other messengers (i.e. you still need to rate limit them as it's possible to iterate _all_ phone numbers which exist).

If working as intended, this is better then what alternatives provide as it doesn't just protect phone numbers from 3rd parties but also from the data center operator and to some degree even signal itself.

But it's not perfect. You can use side channel attacks against Intel SGX and Signal most likely can sneak in ways for them to access things by changing the code, sure people might find this but it's still viable.

In the end what matters is driving up the cost of attacks to a point where they aren't worth in all cases (as in either not worth in general or in there being easier attack vectors e.g. against your phone which also gives them what they want, either way it should be suited for systematic mass surveillance of everyone or even just sub groups like politicians, journalists and similar).

[rajnathani]

Can someone please clarify: For the phone number to reach the enclave for use during search via XOR, won't it need to come in as regular RAM via the backend's API call?

[tapoxi]

https://signal.org/blog/private-contact-discovery/

[LunaSea]

I believe that the search term you can look for is constant time equality.

[jazzyjackson]

Still lame that they require phone number at all, it took them a long time to add usernames so you don't have to expose your phone number to a new contact. Still skeeves me out that the account is associated with a SIM at all.

[HNisCIS]

We need an established secure anonymous/subpoena-resistant chat app at this point. Signal is great for a minimal threat model but we're kinda past that now given everything going on.

Simplex was a decent option but they're going down the crypto rabbit hole and their project lead is...not someone who should be trusted by anyone in the crosshairs right now.

[integralid]

Can you explain more about simplex? I remember reading about it a while ago and being really impressed. Sad to hear the project is going downhill.

[heavyset_go]

Check out the developer/owner's social media, the chats they're in, and their responses to others and you'll see. They're much more interesting in crypto and politics than they are acting professional in public and towards others while representing their project and company.

It's not hard to do so, so if they're having difficulty doing that, what other simple things are they having difficulty with? Why would anyone hinge their safety and well being on the whims of such a person?

I say this as a person who bought into the initial concept, and who has used it myself.

[akimbostrawman]

>They're much more interesting in crypto and politics

I have yet to see any of that while just using the app. Do you think people owning a project should not be allowed to have and share there options about anything but the project?

[heavyset_go]

Do you think I'm allowed to criticize and not use an app based on my own judgment of the developer's behavior?

[maqp]

SimpleX front page lied by omission about it having no identifiers. The fine print threat model did not mention the server has access to your IP addresses, and the mitigation to create "decentralized" system of users talking via separate servers ran into the problem of there being two VPS companies hosting the entire public server infrastructure. These issues were major as SimpleX advertised itself as an improvement over Cwtch, which should've meant superset of metadata had been protected. But that obviously wasn't the case.

The CEO vanished from the discussion (again) so my proposals to improve ease of use of Tor never reached them. You can catch up on the discussion at https://discuss.privacyguides.net/t/simplex-vs-cwtch-who-is-...

[miroljub]

What do you use now? Catch? Briar? Tox?

I liked the SimpleX concept, but would prefer its relay server were replaced by Tor or i2p network.

And if they used Signal instead of NIH protocol.

Actually, the only unique SimpleX feature I really like is that it uses separate ids for every connection and group.

[maqp]

>What do you use now?

Signal mostly.

>separate ids for every connection and group

The thing is, there's Akamai and Runonflux, two companies hosting the entire public SimpleX infrastructure. If you're not using Tor and SimpleX Onion Services with your buddies, these two companies can perform end-to-end correlation attacks to spy on which IPs are conversing, and TelCos know which IPs belong to which customers at any given time. Mandatory data retention laws about the assigned IPs aren't rare.

[akimbostrawman]

>so my proposals to improve ease of use of Tor never reached

Probably because it has always been trivial to proxy Tor with build in and supported socks5

[01HNNWZ0MV43FF]

Maybe DeltaChat?

[maqp]

No. https://delta.chat/en/help#pfs

[nanomonkey]

I agree, but you can mitigate that to some extent by using a phone number that is not linked to your identity.

Phreeli [https://www.phreeli.com/] allows you to get a cell number with just a zip code. They use ZKP (Zero Knowledge Proofs) for payment tracking.

[codedokode]

In my country, you cannot legally get a phone number not linked to the identity, and the prices are relatively high on the black market. Also, the phone discloses your location with pretty good precision, especially in US where everyone is living in their own house.

[sneak]

Signal accounts do not require a SIM. There is no requirement that the phone you use for running the app Signal has the phone number you use for Signal login.

My Signal number is a Google Voice number that has nothing to do with any mobile phone. The Google account has advanced protection turned on so you can’t port it or get the SMSes without a hardware login token.

[HNisCIS]

It's still associated with a credit card and your google account requires another phone number to create.

[the_gipsy]

In my country I cannot buy a SIM card / phone number without giving my full identification.

[extraduder_ire]

Can you buy a phone number from a different country? (genuinely curious, I live somewhere I can buy a sim card with cash, and saw some in the impulse-purchase section of a store earlier today)

[sneak]

So buy a number from a different country.

[codedokode]

But has something to do with a bank card you used to pay for it?

[jazzyjackson]

That's cool that there are phonenumbers without SIMs, my concern was more about SIM swap takeover. (Signal only guards this with a 4 digit PIN iirc)

[Zak]

The PIN can be longer than four digits. Signal also guards against this with safety numbers; if someone takes over an account, every contact will see that the safety number has changed and should consider that the account may be compromised until verifying out of band.

[codedokode]

Google Voice doesn't look like a safe option, your number can be taken away if you forget to pay or you can be banned for arbitrary reason without a way to appeal.

[theamk]

Such a nice design on server-side.. and yet on the client side, it uses system address book - something that Google backs up on the server, and many carriers back up too, and many apps (like Whatsapp) save it too.

"Hey, _we_ don't store your contacts, we are good! Instead you have to manage them yourself, and in process share your presumably "secure" Signal contact list with Apple, Google, Facebook phone carriers and everyone else. But it's not on our servers, so we don't care"

[m4rtink]

Do we relly know the server actually does this when you can't run your own Signal server instances you have compiled yourself from source code ?

[maqp]

Short answer is no.

Signal provides content-privacy by design with E2EE. Signal provide metadata-privacy by policy, i.e. they choose to not collect data or mine information from it. If you need metadata-privacy by design, you're better off with purpose-built tools like Cwtch, Ricochet Refresh, OnionShare, or perhaps Briar.

[master-lincoln]

I thought you could compile from source and run Signal server instances, but there is no federation, so you would need a client that points to your server and you could only talk to other people using that client.

https://github.com/signalapp/Signal-Server

[GranPC]

They use remote attestation based on SGX. So, assuming SGX can be trusted, yes. See https://signal.org/blog/private-contact-discovery/

[dathinab]

and assuming you have a practical way to

- verify the attestation

- make sure it means the code they have published is the attested code

- make sure the published code does what it should

- and catch any divergence to this *fast enough* to not cause much damage

....

it's without question better then doing nothing

but it's fundamentally not a perfect solution

but it's very unclear if there even is a perfect solution, I would guess due to the characteristics of phone numbers there isn't a perfect solution

[mjg59]

Well, no - as long as someone you trust is able to do that verification, that's good enough.