In the case of signal, it is provable that it cannot. They do not hold the keys to decrypt. The closest risk is the server injecting a new public key into the conversation, which the Signal app will warn about.
Signal does hold the public keys for every user. But having the public key doesn’t let you decrypt anything. You need the private key to decrypt data encrypted with the public key. So in a chat example, if you and I exchange public keys, I can encrypt the message using your public key, but only you can decrypt it, using your private key.
Signal does run the key exchange, which means they could hand a user the wrong public key, a public key which they have the private key for, instead of the other person’s. That is a threat model for this type of communications, however, signal users can see the key thumbprints of their fellow chat participants and verify them manually. And once a chat has begun, any changes to that key alerts all parties in the chat so they know a change has happened. The new key wont have access to any previous or pending messages, only new ones after the change took place.
I mean I still don’t see how it can be encrypted for a private key with the deryption at some point running through the server unless the members devices at some point communicate with each other without the server as an intermediary. Is that what happens at some point?
I think we are more talking about can the server decrypt the data. Not that the data is encrypted.
In the case of signal, it is provable that it cannot. They do not hold the keys to decrypt. The closest risk is the server injecting a new public key into the conversation, which the Signal app will warn about.
Yeah I just don’t get this. How does a person added to a chat get keys then?
Signal does hold the public keys for every user. But having the public key doesn’t let you decrypt anything. You need the private key to decrypt data encrypted with the public key. So in a chat example, if you and I exchange public keys, I can encrypt the message using your public key, but only you can decrypt it, using your private key.
Signal does run the key exchange, which means they could hand a user the wrong public key, a public key which they have the private key for, instead of the other person’s. That is a threat model for this type of communications, however, signal users can see the key thumbprints of their fellow chat participants and verify them manually. And once a chat has begun, any changes to that key alerts all parties in the chat so they know a change has happened. The new key wont have access to any previous or pending messages, only new ones after the change took place.
I mean I still don’t see how it can be encrypted for a private key with the deryption at some point running through the server unless the members devices at some point communicate with each other without the server as an intermediary. Is that what happens at some point?