2004 Symantec Corporation, All Rights Reserved Principles and Practice of X-raying Frédéric Perriot Peter Ferrie Symantec Security Response

2 – 2004 Symantec Corporation, All Rights Reserved What is x-raying?  A detection method based on breaking the encryption of the virus  Works for weak encryption methods –Recent real-world examples among win32 viruses –Applicable to worms as well  Similar to a ‘known plaintext attack’

3 – 2004 Symantec Corporation, All Rights Reserved Example of a ‘known plaintext attack’ From: Peter ? KEY is rot13! Known plaintext From: Peter Subject: Hello VB2004 Decrypted message Corresponding ciphertext Sebz: Crgre Fhowrpg: Uryyb IOZZVI Message encrypted with unknown Caesar cipher

4 – 2004 Symantec Corporation, All Rights Reserved Differences between x-raying and ‘known plaintext attacks’  X-raying has lower complexity –Simpler ciphers –Simpler breaking  More constraints for AV than cryptanalysis –Time constraints –Space (memory usage) constraints  Some specific x-raying techniques –Sliding: consider several ciphertexts –Hybrid approaches (using decryptor parsing) –Encryption algorithm not fixed (XOR or ADD or ROL…)

5 – 2004 Symantec Corporation, All Rights Reserved Analogous to hidden patterns in pictures  Inverted colors  Stereograms  Images d’Épinal

6 – 2004 Symantec Corporation, All Rights Reserved X-raying ‘xor 0xFF’

7 – 2004 Symantec Corporation, All Rights Reserved Typical encryption methods  Fixed op and fixed key  A few ops among a set and fixed keys  Multiple layers  Running keys  No key (RDA)  Strong crypto (IDEA virus) –No x-ray but the crypto itself may be detectable!   x  x x

8 – 2004 Symantec Corporation, All Rights Reserved A more complex encryption: stereograms cheep, cheep

9 – 2004 Symantec Corporation, All Rights Reserved Equivalent to X-raying for stereograms  The encryption method is a special projection of a 3D object onto a 2D image  The decryption key is the divergence angle between the direction of the eyes of the observer  Infinite number of keys (!)  Seeing a stereogram is hard the first time

10 – 2004 Symantec Corporation, All Rights Reserved Sliding x-ray  Multiple potential ciphertexts distinguishes x-raying from a regular known plaintext attack  Virus hidden somewhere in the host program –Exact position might not be known because the decryptor is inaccessible (too much I/O)  Often need to x-ray more than one spot –Determine an x-ray region based on geometry of the virus infection method

11 – 2004 Symantec Corporation, All Rights Reserved Arriving to the enchanted forest, Feared retreat of two dark giants, A valiant knight provokes them in combat : But the hidden giants do not answer him Practice your sliding x-ray on this Image d’Épinal

12 – 2004 Symantec Corporation, All Rights Reserved Approaches to X-raying (theory) 42 = 6 * ?  Key recovery –Attempts to recover the encryption key –May be necessary for host repair  Key validation –Attempts to prove that a valid (sub)key exists  Invariant scanning –Reduces the ciphertext to patterns independent from the encryption key is prime? which is divisible by 3: 29369, 117, 3514?

13 – 2004 Symantec Corporation, All Rights Reserved Approaches to X-raying (real-world uses)  Key recovery –W32/Magistr –W32/Perenast (aka W32/Stepar)  Key validation –W32/Bagif (useful for variants detection)  Invariant scanning –W32/Efish –W32/Perenast

14 – 2004 Symantec Corporation, All Rights Reserved Anatomy of a sample x-ray  Substitution cipher  Used by W32/Efish  Simple and homophonic

15 – 2004 Symantec Corporation, All Rights Reserved Can you catch Efish?

16 – 2004 Symantec Corporation, All Rights Reserved What about variable plaintext?  So far we assumed plaintext was fixed  Wildcards are possible (see Bagif)  What if the majority of the plaintext varies? I am a bad virus, boo I am a mad virus, boo I am a sad virus, boo I am a bad virus, boo I, virus am a bad boo Bad am I a boo, virus

17 – 2004 Symantec Corporation, All Rights Reserved Anamorphosis (‘catoptric’) What would metamorphism look like?

18 – 2004 Symantec Corporation, All Rights Reserved DIY catoptric anamorphosis (no assembly required)

19 – 2004 Symantec Corporation, All Rights Reserved Anamorphosis without a complex optical system (‘oblique’) “The Ambassadors” Hans Holbein the younger, 1533

20 – 2004 Symantec Corporation, All Rights Reserved What to do about metamorphism?  X-raying a metamorphic virus is a little like looking at a stereogram of an anamorphosis  You need to close one eye  You need to diverge your eyes  It’s hard to do both at the same time!  Open question to the audience

2004 Symantec Corporation, All Rights Reserved Gunax lbh! Frédéric Perriot Peter Ferrie