Signal partners with Microsoft to bring end-to-end encryption to Skype

Re:Signal partners with Microsoft to bring end-to-end encryp... by aeorgnoieang on Thursday January 11, 2018 @03:02PM (#16127278)

> if I have a group with 10 users and it's encrypted and then another person joins and can see the old messages, was it actually securely encrypted in the first place?

Sure, why not? Assume, just because it's the first thing that came to mind, that message history is encrypted using GPG and every message is encrypted with everyone's individual public keys. When a new user joins, one user just encrypts the message history with the new user's public key.

Or am I missing something? Do the existing users in the group not have access to their own message history?

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Re:Signal partners with Microsoft to bring end-to-end encryp... by jarcoal on Thursday January 11, 2018 @10:51PM (#16130271)

You’re not missing anything, this is definitely possible to do.

That said, encrypting every message with everyone’s public key doesn’t scale, so instead you’d likely want to generate a symmetric key for the group and as new users are invited the inviter would encrypt the group key with the invitee’s public key. That makes it trivial for new users to view history, and as new chats are posted they only need to be encrypted with one key.

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