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Technology preview: Private contact discovery for Signal

signal.org

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Re: Technology preview: Private contact discovery for Signal

#51

Post by bascule »

zeveb wrote: post #50
bascule wrote: post #34

Earlier quoted context omitted.

Who are the two users in this case? How did they discover each other? From the OP: > "Very few people want to install a communication app, open the compose screen for the first time, and be met by an empty list of who they can communicate with." Signal needs to bootstrap itself automatically in order to solve this problem. Can you explain how two users doing a private set intersection of their contacts solves this bo…

The two users are two folks who wish to communicate who hold their phones back-to-back and let NFC validate who they each are. Both parties are now able to perform a private set intersection to find people whom they know in common who also use Signal, and are able to then use private set intersection with each of those people to find more. The idea is that users bootstrap via the social graph of folks they physically…

So you want to completely punt on the "open the app to an empty contact list" problem and force people to perform what's effectively a GPG keysigning party...

Call me crazy, but I think that'd be a bad user experience of the sort that held back encrypted messaging for decades.

Re: Technology preview: Private contact discovery for Signal

#52

Post by thinkloop »

mintplant wrote: post #36

In case someone from Open WhisperSystems is reading: please add stickers. Seriously, I know so many people who are on Telegram just because of its sticker system and won't consider anything without that feature. This comes up again and again in online discussions where Signal is mentioned, and it hurts to see all the care and effort the OWS team has put into providing real security rejected out of hand because of it.…

In case someone from Open WhisperSystems is reading: please do NOT add stickers and junk to your clean and pretty system. I know a lot of people that would leave the service if it turned into WeChat (including me). Stick to the fundamental engineering of privacy as you are doing, and forget the gimmicks.

Re: Technology preview: Private contact discovery for Signal

#53

Post by Pyxl101 »

DINKDINK wrote: post #5

The security assumptions that SGX is built on are flawed. They assume that no side channel attacks will be mounted to determine the processes that occur in side the chip. You can even build software to do side channel analysis on the physical chip: 7:16:26 Daniel Gruss / Michael Schwarz - Cache Attacks on SGX https://youtu.be/eCE2OzKIab8?t=26193

> The security assumptions that SGX is built on are flawed. They assume that no side channel attacks will be mounted to determine the processes that occur in side the chip.

Protecting against side channel attacks and reverse engineering is the responsibility of developers who use SGX, according to Intel's website and user manual:

> Intel designs the Intel Software Guard Extensions to protect against known hardware attacks [...] Intel Software Guard Extensions is not designed to handle side channel attacks or reverse engineering. It is up to the Intel SGX developers to build enclaves that are protected against these types of attack.

Intel's assumptions appear to correctly account for the fact that side channels can occur. They have assessed whether their product is designed to protect against that class of attack, and assigned responsibility for defending against it. What do you think they should do differently?

Re: Technology preview: Private contact discovery for Signal

#54

Post by thinkloop »

> The obvious problem with this method is that the hash of a user identifier can almost always be inverted. Regardless of whether the identifier is a phone number, a user name, or an email address, the “keyspace” of all possible identifiers is too small.

What does "inverted" mean here? And why is the keyspace too small? And how do these problems relate to not trusting the server? The problem definition and proposed solution don't seem to match to me, even though what they have solved is cool.

Re: Technology preview: Private contact discovery for Signal

#55

Post by dublinben »

mintplant wrote: post #36

In case someone from Open WhisperSystems is reading: please add stickers. Seriously, I know so many people who are on Telegram just because of its sticker system and won't consider anything without that feature. This comes up again and again in online discussions where Signal is mentioned, and it hurts to see all the care and effort the OWS team has put into providing real security rejected out of hand because of it.…

You can already send arbitrary images and attachments in Signal messages.

Re: Technology preview: Private contact discovery for Signal

#56

Post by chrissnell »

thinkloop wrote: post #54

> The obvious problem with this method is that the hash of a user identifier can almost always be inverted. Regardless of whether the identifier is a phone number, a user name, or an email address, the “keyspace” of all possible identifiers is too small. What does "inverted" mean here? And why is the keyspace too small? And how do these problems relate to not trusting the server? The problem definition and proposed s…

Take every possible 10-digit U.S. phone number, for example, and perform a SHA-256 hash on each. Put those phone numbers and their hashes in a lookup table. Now, when a hashed phone number comes in from an anonymous user, look it up in that table and you now know the actual phone number.

Re: Technology preview: Private contact discovery for Signal

#57

Post by abovemypaygrade »

chrissnell wrote: post #56
thinkloop wrote: post #54

> The obvious problem with this method is that the hash of a user identifier can almost always be inverted. Regardless of whether the identifier is a phone number, a user name, or an email address, the “keyspace” of all possible identifiers is too small. What does "inverted" mean here? And why is the keyspace too small? And how do these problems relate to not trusting the server? The problem definition and proposed s…

Take every possible 10-digit U.S. phone number, for example, and perform a SHA-256 hash on each. Put those phone numbers and their hashes in a lookup table. Now, when a hashed phone number comes in from an anonymous user, look it up in that table and you now know the actual phone number.

I'm not an expert, so bear with me. Could this be thwarted by, say, having Signal add a random salt to every registering number and then advertising the hash of (number)+(random salt) as the final contact, invisible to the end user? Or would that involve Signal maintaining too much information on their users? If that's the case, I could see that being a problem.

Re: Technology preview: Private contact discovery for Signal

#58

Post by MichaelGG »

scienhack wrote: post #37

It is actually really neat how DRM tech is being reversed to benefit the consumer and privacy for once! Almost daily you read about yet another new "innocent" tech being exploited and used for some corporate gain, taking advantage of the end-users. It's refreshing to see it go the other way!

Trusted computing, TPMs etc have always been about this scenario. It's just the anti DRM crowd has some people that think the end user is always the person sitting in front of the terminal, and there should never be a way to reduce that person's access.

Re: Technology preview: Private contact discovery for Signal

#59

Post by mintplant »

thinkloop wrote: post #52
mintplant wrote: post #36

In case someone from Open WhisperSystems is reading: please add stickers. Seriously, I know so many people who are on Telegram just because of its sticker system and won't consider anything without that feature. This comes up again and again in online discussions where Signal is mentioned, and it hurts to see all the care and effort the OWS team has put into providing real security rejected out of hand because of it.…

In case someone from Open WhisperSystems is reading: please do NOT add stickers and junk to your clean and pretty system. I know a lot of people that would leave the service if it turned into WeChat (including me). Stick to the fundamental engineering of privacy as you are doing, and forget the gimmicks.

I don't understand your motivation here. Every time I advocate using Signal I run into a wall of "but it doesn't have stickers"; for those users, all the privacy engineering is moot if the app doesn't fill their needs. Adding another attachment type would do nothing to degrade your experience, nor would it turn Signal into WeChat.

Re: Technology preview: Private contact discovery for Signal

#60

Post by DINKDINK »

Pyxl101 wrote: post #53
DINKDINK wrote: post #5

The security assumptions that SGX is built on are flawed. They assume that no side channel attacks will be mounted to determine the processes that occur in side the chip. You can even build software to do side channel analysis on the physical chip: 7:16:26 Daniel Gruss / Michael Schwarz - Cache Attacks on SGX https://youtu.be/eCE2OzKIab8?t=26193

> The security assumptions that SGX is built on are flawed. They assume that no side channel attacks will be mounted to determine the processes that occur in side the chip. Protecting against side channel attacks and reverse engineering is the responsibility of developers who use SGX, according to Intel's website and user manual: > Intel designs the Intel Software Guard Extensions to protect against known hardware at…

You're correct. My original statement was wrong regarding Intel's assumption. The principle issue is that developers shouldn't assume that processes that occur in SGX are obfuscated in any sense because you can build SGX sidechannel detection within software that runs inside the enclave

The lecture I pointed to shows an example of this attack.

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