Friday, May 20, 2016

ICMC16: Unboxing the White-Box: Practical Attacks Against Obfuscated Ciphers

Jasper van Woudenberg, CTO North America, Riscure

Jasper has been doing white boxing for a long time - hacking assembly in a video game to get passwords for higher levels as a kid :-)

It's important to protect the keys. Is it possible to do it with just software? White-box cryptography -> secure software crypto in an untrusted environment. This is used today in Pay TV DRMs, mobile payments... How to apply this to software environments?

Protection against key extracton in the white-box security model. A technique that allows merging a key into a given crypto algorithm: described for the first time in 2002 by S. Chow, et al. Available for DES and AES. Lookup tables are used for applying mathematical transforms to data. A known weakness is cloning/lifting.

Once you start applying these, you will have a huge amount of lookup tables. Attaks for all academic WBC proposals focus on key extractions, types of transformations assumed known and concrete transformation and key unknown.  In real life, we do not know much about the design. 

You can do an attack on DES using fault injection. There is a challenge online for you to try yourself at whiteboxcrypto.com . 

Then we got a demo of the tool retrieving a DES key by using the fault injection.

Have been able to break all that they've tried with fewer than a 100 faults, except  one that uses output encoding.

If you can perform measurement of the crypto target, you have a good chance of getting the key.

For side channel attacks, no detailed knowledge is required. the only protection is a secret random input/output encoding.

to protect against side channel attacks: must prevent statistical dependence between intermediates and key. Typical countermeasures based on randomness difficult in white-box scenario. 

Make sure you obfuscate control-flow and data, add anti-analysis and anti-tamper countermeasures.