Get the latest tech news
Decoding the 90s: Cryptography in Early Software Development (2023)
• Intro • Getting started - QText • Reversing MS-DOS binaries • PKZip • int 3f - MS-DOS Overlays • Key expansion function • Tracing through passcode flow • Key derivation function • Reversing the key derivation • Reversing the first stage - 4 bytes to 4 printable characters • Reversing the second stage - 16 bytes to 𝟦×𝟦 byte string • Putting it all togetherIntroIn August 2020, we were commissioned by a client with a cache of locked QText documents from the mid 90s - to whic
QText was a DOS era Hebrew-English word processor written in Turbo Pascal, released 15 or so odd years before neither I nor@Elisha had laid hands on a reverse engineering tool. This is slightly arduous but for the fairly short task at hand it would work, and we had ample overlay-to-root calls to break on since the root segment hosted a lot of Turbo Pascal standard functions. Our initial idea was perform hot spot analysis by using DOSBOX’s CPU LOG command to trace execution while repeatedly failing the passcode check on a locked document.
Or read this on Hacker News