PreBoot Authentication Password Cracking on a budget
H2HC conference, Sao Paulo, Brasil
Jonathan Brossard, Nov 2009
«A desobediência é uma virtude necessária à criatividade »
- Raul Seixas
Before we start...
•Thanks to the organizers, sponsors and volonteers for making this happen in Brasil :)
•Thank you for coming.
•I'm very happy to be here !
Brute forcer design
Conclusion & bonus !
Goals, contributions :
•Demonstrate the feasability of brute force attacks on preboot authentication passwords.
•Give a pessimist estimation of the cost of password cracking on full encryption software using a generic instrumentation methodology.
•Use this metric to adapt password length policy acording with the value of the protected assets.
•Cryptographic software is mostly legalized in both North and South America and Europe.
•Wikipedia : « In China, a license is still required to use cryptography. Many countries have tight restrictions on the use of cryptography. Among the more restrictive are laws in Belarus, Kazakhstan, Mongolia, Pakistan, Russia, Singapore, Tunisia, and Vietnam. »
•Users of cryptographic software must give either a copy of their keys or plain text equivalent of any text asked by authorities in case of trial, or face prison sentences in most countries.
Crypto software poor reviews
+ Governments interrests + global business communications
+ terrorism blah blah
= high risk of (cryptographic ?)
& privacy threats
Is such a thing credible?
•Quoting Wikipedia :
«DES was designed to be resistant to diferential cryptanalysis, a powerful and general cryptanalytic technique known to NSA and IBM, that became publicly known only when it was rediscovered in the late 1980s. According to Steven Levy, IBM rediscovered diferential cryptanalysis, but kept the technique secret at NSA's request. The technique became publicly known only when Biham and Shamir
•Even serious developpers don't test their crypto software enough, if at all (Debian SSL bug : ~32k keys).
•Vendors (in particular Truecypt) have adopted policies where they do not cover certain attacks (eg: Plain text password leakage as we presented at Defcon 0x16, or Joanna Rutowska's evilmaid attack) leaving the
•Non tech people will say :
«if it fails just go for bruteforce ».
•Sure.. but how do you do it ?
I couldn't fnd a public tool myself. And then I started to wonder...
Interruption 0x16 invoked via functions
ah=0x00 , “Get keystroke” : returns the keystroke scancode in AH and its ASCII code in AL.
ah=0x01 , “Check for keystroke” : idem, but the Zero Flag is set if no keystroke is available in the Bios keyboard bufer.
eg : lilo password reading routine :
•Filling the BIOS keyboard bufer (with the keyboard) :
•Reading the BIOS keyboard bufer (using int 0x16, ah=0x00 or 0x01) :
Simulating keystrokes by
PIC programming (from real mode)
Simulating keystrokes by
(from protected mode under x86 GNU/Linux)
(aka: brute force any GUI)
Exemple of application :
Rebooting a computer protected with a password (assuming you know that password - for now ;), by simulating keystrokes at boot time...