Constant-Time Analysis

# Constant-Time Analysis

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

When to Use

```text 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

```bash # Analyze any supported file type uv run {baseDir}/ct_analyzer/analyzer.py <source_file>

# Include conditional branch warnings uv run {baseDir}/ct_analyzer/analyzer.py --warnings <source_file>

# Filter to specific functions uv run {baseDir}/ct_analyzer/analyzer.py --func 'sign|verify' <source_file>

# JSON output for CI uv run {baseDir}/ct_analyzer/analyzer.py --json <source_file> ```

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

```bash # 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#)

```bash # 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)

```bash # 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:

```bash # 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:

```text [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?

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

1. Trace data flow from the flagged instruction back to inputs

1. Common false positive patterns:

```c // 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 ```

1. 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

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

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

1. 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