trailofbits / constant-time-analysis

Skill Content

---
name: constant-time-analysis
description: Detects timing side-channel vulnerabilities in cryptographic code. Use when implementing or reviewing crypto code, encountering division on secrets, secret-dependent branches, or constant-time programming questions in C, C++, Go, Rust, Swift, Java, Kotlin, C#, PHP, JavaScript, TypeScript, Python, or Ruby.
---

# 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](references/compiled.md)         |
| Go         | `.go`                             | [references/compiled.md](references/compiled.md)         |
| Rust       | `.rs`                             | [references/compiled.md](references/compiled.md)         |
| Swift      | `.swift`                          | [references/swift.md](references/swift.md)               |
| Java       | `.java`                           | [references/vm-compiled.md](references/vm-compiled.md)   |
| Kotlin     | `.kt`, `.kts`                     | [references/kotlin.md](references/kotlin.md)             |
| C#         | `.cs`                             | [references/vm-compiled.md](references/vm-compiled.md)   |
| PHP        | `.php`                            | [references/php.md](references/php.md)                   |
| JavaScript | `.js`, `.mjs`, `.cjs`             | [references/javascript.md](references/javascript.md)     |
| TypeScript | `.ts`, `.tsx`                     | [references/javascript.md](references/javascript.md)     |
| Python     | `.py`                             | [references/python.md](references/python.md)             |
| Ruby       | `.rb`                             | [references/ruby.md](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](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)

2. **Trace data flow** from the flagged instruction back to inputs

3. **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
   ```

4. **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.

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

3. **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](https://github.com/veorq/cryptocoding) - Defensive coding for crypto
- [KyberSlash](https://kyberslash.cr.yp.to/) - Division timing in post-quantum crypto
- [BearSSL Constant-Time](https://www.bearssl.org/constanttime.html) - Practical constant-time techniques