The Internet of Broken Protocols: Showcase #8
(complete list of showcases: https://vnhacker.blogspot.com/search/label/The%20Internet%20of%20Broken%20Protocols ) In cryptography, a construction consisting of multiple encryption algorithms in a sequential order is called a cascade. For example, TripleSec by Keybase encrypts data with XSalsa20 then AES in counter mode. Contrary to popular belief, cascading may not always improve security. Matthew Green wrote a nice blog post explaining why. In this showcase, you're asked to analyze a cascade of MAC and digital signature algorithms. Please send your solutions to thaidn@gmail.com. -- The Hooli Photos app wanted to protect the integrity of media files stored in SSD card. Initially, they used GMAC (the MAC of AES-GCM) with a secret key. Later on, the requirement was changed. They wanted other apps to be able to verify the integrity of the media files produced by the Photos app. Toward that end, they applied a digital signature on top of the existing MAC, and settled on t