Constant Time Testing
Measures timing side channels in cryptographic implementations by running them, using...
Install
npx skills add trailofbits/skills --skill constant-time-testing -g -a claude-code -y- Repository
- trailofbits/skills7.9k
- Category
- Testing
- Updated
- 7d ago
- License
- CC-BY-SA-4.0
- Context cost
- Mediumabout 3,000 tokens
- Runs scripts
- No, instructions only
What it does
Measures timing side channels in cryptographic implementations by running them, using dudect for statistical analysis and Timecop over Valgrind for dynamic tracing. Covers the formal, symbolic, dynamic, and statistical tool categories and how to read a result. Use when testing whether a running implementation is constant-time, measuring timing variance on a compiled binary, or investigating a suspected timing attack. Not for statically inspecting compiler output, the constant-time-analysis plugin covers that.
Constant-Time Testing
Timing attacks exploit variations in execution time to extract secret information from cryptographic implementations. Unlike cryptanalysis that targets theoretical weaknesses, timing attacks leverage implementation flaws - and they can affect any cryptographic code.
Background
Timing attacks were introduced by Kocher in 1996. Since then, researchers have demonstrated practical attacks on RSA (Schindler), OpenSSL (Brumley and Boneh), AES implementations, and even post-quantum algorithms like Kyber.
Key Concepts
| Concept | Description |
|---|---|
| Constant-time | Code path and memory accesses independent of secret data |
| Timing leakage | Observable execution time differences correlated with secrets |
| Side channel | Information extracted from implementation rather than algorithm |
| Microarchitecture | CPU-level timing differences (cache, division, shifts) |
Why This Matters
Timing vulnerabilities can:
- Expose private keys - Extract secret exponents in RSA/ECDH
- Enable remote attacks - Network-observable timing differences
- Bypass cryptographic security - Undermine theoretical guarantees
- Persist silently - Often undetected without specialized analysis
Two prerequisites enable exploitation:
- Access to oracle - Sufficient queries to the vulnerable implementation
- Timing dependency - Correlation between execution time and secret data
Common Constant-Time Violation Patterns
Four patterns account for most timing vulnerabilities:
// 1. Conditional jumps - most severe timing differences
if(secret == 1) { ... }
while(secret > 0) { ... }
// 2. Array access - cache-timing attacks
lookup_table[secret];
