> whatsApp seems to force you and your contacts to meet at a Starbucks so you can distribute and sign each other's public keys. The out-of-band meeting compare

> whatsApp seems to force you and your contacts to meet at a Starbucks so you can distribute and sign each other's public keys. The out-of-band meeting compares a QR code or a 60-digit public key-derived number, both of which are generated by WhatsApp. There is no ad hoc in-person signing involved. From what I can determine from their whitepaper, there is no private key involvement at all. Just concatenation or hashing using the public keys. Furthermore: When either user scans the other’s QR code, the keys are compared to ensure that what is in the QR code matches the Identity Key as retrieved from the server. Note 'as retrieved from the server'. The public keys are not distributed directly from Alice to Bob but always through Mallory. You can say for certain that the public key that Mallory's app tells you that you hold for Alice is the same as the one that Mallory presents for Alice, but that's it. That wouldn't be a problem if the subsequent communication was in a different channel, like sending an e-mail after retrieving a PGP key from a key-server; the distributing party isn't in the communication channel and can't interfere. But WhatsApp is centralised for both key distribution AND communication.