So, I’m interested how the implementation of “Perfect Forward Secrecy” in Signal looks like, like does every messages has a different encryption key? or does it change over time like #whatsapp does? I tried to find any official documention of this, sadly did not find anything.
Thats why I’m asking, does anyone of you know smth about this and maybe can provide a link to a official source?
#signal #signalapp #privacy #encryption @signalapp @SignalUpdateInfo @privacy
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?
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
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.
Double Ratchet Messaging Encryption - Computerphile
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WhatsApp uses the encryption protocol developed for Signal, so I’d not expect them to be significantly different.
@Rose True, but there are still differences, for example Signal encrypts everything, while WA only encrypts chats and calls, so the implementation is a bit different, which maybe could be also in Signal with PFS.
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.
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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.
Asked GPT about that after skimming through the wiki to confirm if it’s also comes to the same conclusion that the keys are periodically changed and yeah it did confirm it:
The “Double Ratchet Algorithm” used by Signal provides both forward and future secrecy for messages. Here’s a basic breakdown of how this works:
The Double Ratchet Algorithm:
Initialization Phase: The two communicating parties (let’s call them Alice and Bob) establish a shared secret using the Diffie-Hellman key exchange. Ratcheting Phase: After the shared secret is established, every message is encrypted using a new symmetric key. Keys are derived in a chain where the next key is a function of the previous key. This is the “symmetric-key ratchet”. After sending or receiving a message, Alice and Bob independently update their chain keys, which helps ensure forward secrecy. Diffie-Hellman Ratchet: Periodically, Alice and Bob also update their Diffie-Hellman key pairs, exchange the public components, and derive a new shared secret. This helps provide future secrecy. Example:
Let’s consider a simple conversation between Alice and Bob.
Initialization: Alice and Bob meet in person and exchange public keys. They generate a shared secret that only they know using these keys. First Message (from Alice to Bob): Alice derives an encryption key from the shared secret. Alice sends “Hello, Bob!” encrypted with this key. After sending the message, Alice updates her chain key. Bob’s Reply: Bob derives the same encryption key from the shared secret, decrypts Alice’s message, and reads “Hello, Bob!”. Bob then derives a new encryption key from his updated chain and sends “Hi, Alice!” encrypted with this new key. After sending the message, Bob updates his chain key. Diffie-Hellman Ratchet: After a few messages or after a certain period, Alice decides to perform a Diffie-Hellman ratchet. She generates a new key pair. Alice sends her new public key to Bob along with her next message. Both derive a new shared secret using Alice’s new public key and Bob’s current private key. This new shared secret is used to start a new chain of symmetric keys. Continued Conversation: Alice sends another message: “How are you?” with a key derived from the new shared secret. After every message, sender and receiver update their chain keys. Periodically, they will also perform the Diffie-Hellman ratchet to refresh the shared secret. The benefit of this mechanism is that even if an attacker compromises a single message key, they cannot decrypt past messages (forward secrecy) or future messages (future secrecy). Even if they compromise a current Diffie-Hellman private key, they can’t decrypt messages that were sent before the compromise (because of the symmetric ratchet) or messages sent after the next Diffie-Hellman ratchet (because a new shared secret will be established).
