Key transparency allows communicating parties to verify public keys used in end-to-end encryption. In many end-to-end encryption services, to initiate communication a user will reach out to a central server and request the public keys of the user with which they wish to communicate.Melara, Marcela S., Aaron Blankstein, Joseph Bonneau, Edward W. Felten, and Michael J. Freedman. "{CONIKS}: Bringing key transparency to end users." In 24th USENIX Security Symposium (USENIX Security 15), pp. 383-398. 2015. If the central server is malicious or becomes compromised, a man-in-the-middle attack can be launched through the issuance of incorrect public keys. The communications can then be intercepted and manipulated.Bonneau, Joseph. "EthIKS: Using Ethereum to audit a CONIKS key transparency log." In International Conference on Financial Cryptography and Data Security, pp. 95-105. Berlin, Heidelberg: Springer Berlin Heidelberg, 2016. Additionally, legal pressure could be applied by surveillance agencies to manipulate public keys and read messages.
With key transparency, public keys are posted to a public log that can be universally audited.Chen, Brian, Yevgeniy Dodis, Esha Ghosh, Eli Goldin, Balachandar Kesavan, Antonio Marcedone, and Merry Ember Mou. "Rotatable zero knowledge sets: Post compromise secure auditable dictionaries with application to key transparency." In International Conference on the Theory and Application of Cryptology and Information Security, pp. 547-580. Cham: Springer Nature Switzerland, 2022. Communicating parties can verify public keys used are accurate.
See also Certificate Transparency
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