Constant-Time Analysis

Analyze cryptographic code to detect operations that leak secret data through execution timing variations.

When to Use User writing crypto code? ──yes──> Use this skill │ no │ v User asking about timing attacks? ──yes──> Use this skill │ no │ v Code handles secret keys/tokens? ──yes──> Use this skill │ no │ v Skip this skill

Concrete triggers:

User implements signature, encryption, or key derivation Code contains / or % operators on secret-derived values User mentions "constant-time", "timing attack", "side-channel", "KyberSlash" Reviewing functions named sign, verify, encrypt, decrypt, derive_key When NOT to Use Non-cryptographic code (business logic, UI, etc.) Public data processing where timing leaks don't matter Code that doesn't handle secrets, keys, or authentication tokens High-level API usage where timing is handled by the library Language Selection

Based on the file extension or language context, refer to the appropriate guide:

Language File Extensions Guide C, C++ .c, .h, .cpp, .cc, .hpp references/compiled.md Go .go references/compiled.md Rust .rs references/compiled.md Swift .swift references/swift.md Java .java references/vm-compiled.md Kotlin .kt, .kts references/kotlin.md C# .cs references/vm-compiled.md PHP .php references/php.md JavaScript .js, .mjs, .cjs references/javascript.md TypeScript .ts, .tsx references/javascript.md Python .py references/python.md Ruby .rb references/ruby.md Quick Start

Analyze any supported file type

uv run {baseDir}/ct_analyzer/analyzer.py

Include conditional branch warnings

uv run {baseDir}/ct_analyzer/analyzer.py --warnings

Filter to specific functions

uv run {baseDir}/ct_analyzer/analyzer.py --func 'sign|verify'

JSON output for CI

uv run {baseDir}/ct_analyzer/analyzer.py --json

Native Compiled Languages Only (C, C++, Go, Rust)

Cross-architecture testing (RECOMMENDED)

uv run {baseDir}/ct_analyzer/analyzer.py --arch x86_64 crypto.c uv run {baseDir}/ct_analyzer/analyzer.py --arch arm64 crypto.c

Multiple optimization levels

uv run {baseDir}/ct_analyzer/analyzer.py --opt-level O0 crypto.c uv run {baseDir}/ct_analyzer/analyzer.py --opt-level O3 crypto.c

VM-Compiled Languages (Java, Kotlin, C#)

Analyze Java bytecode

uv run {baseDir}/ct_analyzer/analyzer.py CryptoUtils.java

Analyze Kotlin bytecode (Android/JVM)

uv run {baseDir}/ct_analyzer/analyzer.py CryptoUtils.kt

Analyze C# IL

uv run {baseDir}/ct_analyzer/analyzer.py CryptoUtils.cs

Note: Java, Kotlin, and C# compile to bytecode (JVM/CIL) that runs on a virtual machine with JIT compilation. The analyzer examines the bytecode directly, not the JIT-compiled native code. The --arch and --opt-level flags do not apply to these languages.

Swift (iOS/macOS)

Analyze Swift for native architecture

uv run {baseDir}/ct_analyzer/analyzer.py crypto.swift

Analyze for specific architecture (iOS devices)

uv run {baseDir}/ct_analyzer/analyzer.py --arch arm64 crypto.swift

Analyze with different optimization levels

uv run {baseDir}/ct_analyzer/analyzer.py --opt-level O0 crypto.swift

Note: Swift compiles to native code like C/C++/Go/Rust, so it uses assembly-level analysis and supports --arch and --opt-level flags.

Prerequisites Language Requirements C, C++, Go, Rust Compiler in PATH (gcc/clang, go, rustc) Swift Xcode or Swift toolchain (swiftc in PATH) Java JDK with javac and javap in PATH Kotlin Kotlin compiler (kotlinc) + JDK (javap) in PATH C# .NET SDK + ilspycmd (dotnet tool install -g ilspycmd) PHP PHP with VLD extension or OPcache JavaScript/TypeScript Node.js in PATH Python Python 3.x in PATH Ruby Ruby with --dump=insns support

macOS users: Homebrew installs Java and .NET as "keg-only". You must add them to your PATH:

For Java (add to ~/.zshrc)

export PATH="/opt/homebrew/opt/openjdk@21/bin:$PATH"

For .NET tools (add to ~/.zshrc)

export PATH="$HOME/.dotnet/tools:$PATH"

See references/vm-compiled.md for detailed setup instructions and troubleshooting.

Quick Reference Problem Detection Fix Division on secrets DIV, IDIV, SDIV, UDIV Barrett reduction or multiply-by-inverse Branch on secrets JE, JNE, BEQ, BNE Constant-time selection (cmov, bit masking) Secret comparison Early-exit memcmp Use crypto/subtle or constant-time compare Weak RNG rand(), mt_rand, Math.random Use crypto-secure RNG Table lookup by secret Array subscript on secret index Bit-sliced lookups Interpreting Results

PASSED - No variable-time operations detected.

FAILED - Dangerous instructions found. Example:

[ERROR] SDIV Function: decompose_vulnerable Reason: SDIV has early termination optimization; execution time depends on operand values

Verifying Results (Avoiding False Positives)

CRITICAL: Not every flagged operation is a vulnerability. The tool has no data flow analysis - it flags ALL potentially dangerous operations regardless of whether they involve secrets.

For each flagged violation, ask: Does this operation's input depend on secret data?

Identify the secret inputs to the function (private keys, plaintext, signatures, tokens)

Trace data flow from the flagged instruction back to inputs

Common false positive patterns:

// FALSE POSITIVE: Division uses public constant, not secret int num_blocks = data_len / 16; // data_len is length, not content

// TRUE POSITIVE: Division involves secret-derived value int32_t q = secret_coef / GAMMA2; // secret_coef from private key

Document your analysis for each flagged item

Quick Triage Questions Question If Yes If No Is the operand a compile-time constant? Likely false positive Continue Is the operand a public parameter (length, count)? Likely false positive Continue Is the operand derived from key/plaintext/secret? TRUE POSITIVE Likely false positive Can an attacker influence the operand value? TRUE POSITIVE Likely false positive Limitations

Static Analysis Only: Analyzes assembly/bytecode, not runtime behavior. Cannot detect cache timing or microarchitectural side-channels.

No Data Flow Analysis: Flags all dangerous operations regardless of whether they process secrets. Manual review required.

Compiler/Runtime Variations: Different compilers, optimization levels, and runtime versions may produce different output.

Real-World Impact KyberSlash (2023): Division instructions in post-quantum ML-KEM implementations allowed key recovery Lucky Thirteen (2013): Timing differences in CBC padding validation enabled plaintext recovery RSA Timing Attacks: Early implementations leaked private key bits through division timing References Cryptocoding Guidelines - Defensive coding for crypto KyberSlash - Division timing in post-quantum crypto BearSSL Constant-Time - Practical constant-time techniques