For those interested, the underlying technology for this feature is transparency logs. Some technical details for iMessage's approach can be found here: https:

For those interested, the underlying technology for this feature is transparency logs. Some technical details for iMessage's approach can be found here: https://security.apple.com/blog/imessage-contact-key-verification/ https://security.apple.com/blog/imessage-contact-key-verific... The same technology powers WhatsApp's key transparency: https://engineering.fb.com/2023/04/13/security/whatsapp-key-transparency/ https://engineering.fb.com/2023/04/13/security/whatsapp-key-... Less than a month ago the first workshop on "transparency systems" was held at ACM CCS: https://catsworkshop.dev https://catsworkshop.dev Shameless plug: I'm one of the designers of the Sigsum public transparency log, as well as System Transparency - a security architecture intended to bring transparency to the reachable state space of a remote running system.