White-Box Cryptography

White-Box Cryptography

Outline• Motivation• White-Box Cryptography• White-Box Implementation• White-Box In Practice• Conclusion

MotivationCryptography is widely used nowadays, attack still exists.

• Black-Box Attack Model• White-Box Attack Model

Black-Box Attack Model• Tries to deduce the key from a list {(plaintext,

ciphertext)}

Black-Box Attack Model• Side-channel Attack• Executing time• Electromagnetic radiation• Power consumption

White-Box Attack Model• Attacker has full control over software execution• Full access to the implementation of cryptography

algorithm• Full access to the platform: CPU calls, memory, registers,

etc.• Binary completely visible• Can manipulate the execution

White-Box Attack Model• Target for attack• Implementation of cryptography• Secret key

White-Box Attack Example• Key Whitening Attack• Zero lookup tables(such as S-box) using hex editor• Getting output of penultimate operation• Original AES key easily be derived

White-Box Attack Example• Entropy Attack• Object: Computer Memory• Keys: usually chose by random generator• Code: contains structure

White-Box Attack Example• Format Analysis• Analyze binary code

White-Box Attack Example• Code Boot Attack• Applicable to Bitlocker, TrueCrypt, FileVault• TrueCrypt boot loader

• Password entered at boot time• Disk encryption key needs to be stored in memory

• Attack: exploit data remanency property of DRAM, cooling increase time• Removed & inserted into another hacked machine to

read data, such as crypto keys


Object• Hide a cryptography key in a white-box

implementation

A Naive Example• Implement a cipher as one big lookup table

• No more information ‘leaks’ from the set of {(plaintext, ciphertext)}• Lookup Table size: For n-bit block cipher, size would

be n*2n bit• 32 bit: 232*32 bit =237 bit=4 GBytes

• Using a network of lookup table instead

void encrypt (uint32_t* plaintext, uint32_t* ciphertext) {char S[] = { 0x9e37b8e9, 0xaf48c9fa, 0x8d26a7d8, … }; /* Sbox */ciphertext = S[plaintext];

}

What is White-Box Cryptography?• Definition

• Dwb(m): need ONE input• Dk(m): need TWO input• Essentially, Dwb(m) is the exclusive edition of Dk(m) with

specific cipher key.

What is White-Box Cryptography?• Main Idea• Embed both the fixed key & random data in a

composition.• Hard to derive the original key.

• Attacker knows which crypto algorithm• Attacker knows where in the memory• Attacker knows where in the application

What is White-Box Cryptography?• State of Art• Unfortunately, there is no white-box cryptography

proved to be secure• Current best method: hide keys according to

characteristics of the specific crypto algorithm• Only white-box DES & AES published

• Both have been broken• No academic paper on asymmetric primitives

What is White-Box Cryptography?• State of Art• Interesting:

• After some company buying white-box crypto solutions, they mix their own crypto, which is not recommended in crypto application.

• For white-box crypto, this is reasonable.• Security of white-box crypto depends on how hard the cipher

key is hidden, not the cipher primitives.


First White-Box Implementation• Chow et al. 2002. A White-Box DES Implementation

for DRM Applications• Chow et al. 2002. White-Box Cryptography and an

AES Implementation

Original DES

• Basic operations: Replacing, Changing places, XOR• Chow, et al.: Transform to randomized networked lookup

tables closely related to the crypto key

White-Box DES• Transform a cipher into a series of key-dependent

lookup tables.• Secret key is hard-code into the lookup tables• Protected by randomization techniques

Lookup Tables Example• Lookup Tables: define every input & output• Any finite function can transform to a lookup table

• Table A: Replacing Operation• Table B: XOR Operation• Table C: Negative Operation

A

输入输出

00 1001 0110 1111 00

B

输入输出

00 001 110 111 0

C

输入输出

0 11 0

Lookup Tables Example• All basic primitives in DES transform into lookup

tables:

Divide and Conquer• Attacker may recognize every lookup table and

analyze each basic operation.• Mix 3 tables into 1 big lookup table:

A

输入输出

00 1001 0110 1111 00

B

输入输出

00 001 110 111 0

C

输入输出

0 11 0

C ◦ B ◦ A

输入输出

00 001 010 111 1

Divide and Conquer• BUT, the lookup table will become very huge.• For n bits input & m bits output, 2n×m bits is required.• Solution: we need a series of networked lookup tables:

L1 ◦ L2 ◦ L3 ◦ …

Partial Evaluation• Chow, et al. adopted partial evaluation to mix crypto keys with

algorithm.• Dskey(m) Dwb(m)

• In DES:• Some operation is fixed (e.g. changing place)

Corresponding lookup tables are fixed -------- not affected by crypto keys• Some operation is NOT fixed (e.g. replacing using crypto key)

Corresponding lookup tables are NOT fixed -------- affected by crypto keys

• Attacker can distinguish the unfixed lookup tables by analyzing each table• We need to randomize every lookup table

• Making distinguishing more difficult

Internal Encodings• Considering 3 consecutive lookup tables in the

network: L3◦L2◦L1, L2 contains some key information.• e.g. L2(x)=x k⊕

• Every lookup table is available to the white-box attacker• The key information can be extracted directly• e.g. L2(0)

Internal Encodings• Countermeasure: Add internal encoding:

• b1, b2: randomization operations• b1

-1, b2-1: opposite operations

• L’3◦ L’

2◦ L’1= L3◦b2

-1◦b2◦ L2◦b1-1◦b1◦ L1= L3◦ L2◦ L1

• Now, L’2 does not leak any key information

• Attacker have to analyze all 3 encoded tables to gain information


Code Lifting• Attacker: No need to know internal details, just

need API.• Embed the white-box implementation into his App.• Still encrypt/decrypt data as having the key.

External Encodings• Same as Internal Encodings.

• But not between 2 blocks inside cryptography implementation

• But outside

• Annihilating encoding somewhere else• e.g. incorporate into the decryption functions

Traitor Tracing• Object: Detect who has been sharing code (pirate)• Use case: DRM

• Insert fingerprints into white-box implementation

• Can also be used in software tamper resistance• Malware instructions can be detected

• Any modification leads to lookup tables collapse

Conclusion• Being used in real-world application, mainly DRM

apps.• Although academic attacks have been published• No attacks on commercial white-box implementation

have been seen.

• White-box cryptography still in its early days• Requires further research before being widely adopted.

Documents

White-Box Cryptography