White-Box Cryptography

Outline

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

Motivation

Cryptography 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

Outline

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

Object

• Hide a cryptography key in a white-box implementation

A Naive Example

• Implement a cipher as one big lookup table } void encrypt (uint32_t* plaintext, uint32_t* ciphertext) { char S[] = { 0x9e37b8e9, 0xaf48c9fa, 0x8d26a7d8, … }; /* Sbox */ ciphertext = S[plaintext]; • No more information ‘leaks’ from the set of {(plaintext, ciphertext)} • Lookup Table size: For n-bit block cipher, size would be n*2 n bit • 32 bit: 2 32 *32 bit =2 37 bit=4 GBytes • Using a network of lookup table instead

What is White-Box Cryptography?

• Definition • • • D wb (m): need ONE input D k (m): need TWO input Essentially, D wb (m) is the exclusive edition of D k (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.

Outline

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

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 A 输入 输出 00 10 01 01 10 11 11 00 B 输入 输出 00 0 01 1 10 1 11 0 • • • Table A: Replacing Operation Table B: XOR Operation Table C: Negative Operation C 输入 输出 0 1 1 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 10 01 01 10 11 11 00 B 输入 输出 00 0 01 1 10 1 11 0 C 输入 输出 0 1 1 0 C ◦ B ◦ A 输入 输出 00 0 01 0 10 1 11 1

Divide and Conquer

• BUT, the lookup table will become very huge.

• • For n bits input & m bits output, 2 n × m bits is required.

Solution: we need a series of networked lookup tables: L 1 ◦ L 2 ◦ L 3 ◦ …

Partial Evaluation

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

• D skey (m)  D wb (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: L 3 ◦ L 2 ◦ L 1 , L 2 contains some key information.

• e.g. L 2 (x)=x ⊕ k Every lookup table is available to the white-box attacker • • The key information can be extracted directly e.g. L 2 (0)

Internal Encodings

• Countermeasure: Add internal encoding: • • • • b 1 , b 2 : randomization operations b 1 -1 , b 2 -1 : opposite operations L ’ 3 ◦ L ’ 2 ◦ L ’ 1 = L 3 ◦ b 2 -1 ◦ b 2 ◦ L 2 ◦ b 1 -1 ◦ b 1 ◦ L 1 = L 3 ◦ L 2 ◦ L 1 Now, L ’ 2 • does not leak any key information Attacker have to analyze all 3 encoded tables to gain information

Outline

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

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.