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A place to discuss privacy and freedom in the digital world.
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Don’t know about Signal but the way PFS usually works is there is something like a Diffie-Hellman (DH) key exchange. Each person generates a random (private) number, remembers it, crunches it mathematically into a public number, and sends the public number to the other person. Each then combines their private number with the public number that they got from the other person, and this (because of how DH works) cleverly gives both people the same secret number they use for the encryption, but the secret can’t be reconstructed without knowing at least one of the private numbers. Finally, the PFS part is simply that each person permanently deletes both the shared secret and the private number they generated for that exchange (they will create new ones next time they want to communicate). That means there is no way to reconstruct the secret and re-decrypt the message.
Of course, authentication also has to be added to all this.
For more info, probably easiest to look up Diffie-Hellman key exchange online.
Diffie-Hellman is only a key exchange protocol and does not provide forward secrecy. Imagine that my and your client figure out our shared secred (key) with DH, then encrypt our chat with that. If someone breaks only a bit of our traffic, then they can read newer messages as well.
https://en.wikipedia.org/wiki/Forward_secrecy
Forward secrecy means that at any point messages are encrypted with keys that aren’t reused forever, which means if an attacker can intercept traffic and read a bit, or crack the encryption, that they don’t automatically get access to our future conversation as well.
The PFS comes from deleting the secret DH parameters after you are done using them.
@voxel @signalapp @SignalUpdateInfo @privacy https://signal.org/docs/specifications/doubleratchet/
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https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm
This is the encryption that they developed. Whatsapp and OMEMO (XMPP) use it as well. It has forward secrecy because if an attacker has the key for a message they can’t calculate the key for the key for the next message etc.
@sexy_peach So when I understood it right (just skimmed the text), the encryption keys changes per message?
Double Ratchet Messaging Encryption - Computerphile
https://youtu.be/9sO2qdTci-s?si=qPbDZ2-x_VSiEZ-1
Here is an alternative Piped link(s): https://piped.video/9sO2qdTci-s?si=qPbDZ2-x_VSiEZ-1
Piped is a privacy-respecting open-source alternative frontend to YouTube.
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essentially, yes. There are two levels of encryption keys and they “ratchet” along (change). Only the lower level is used to encrypt the text so it’s much harder to figure out the other one.
@sexy_peach Thank you very much! Just now wondering, handle WA this differently? Since Ig we all know this message which appears when the “keys” ig, changed
I don’t know if I fully understand you here.
To my understanding whatsapp does it like this too. The key changing must be the high level key, since the message means that it’s an unknown key (thus not yet trusted). From new install or new device.
@sexy_peach Okay, got it, ig.
WhatsApp uses the encryption protocol developed for Signal, so I’d not expect them to be significantly different.
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The main advantages of Signal over WA is that Signal minimises the amount of metadata Signal has access to and promises to not store or analyse the little remaining metadata.
The data (as in: the actual message content) should be similarly secure in both.