constant-time-analysis

Files (12)

• README.md
• SKILL.md
• openai.yaml · agents/
• trail-of-bits-mark.svg · assets/
• compiled.md · references/
• javascript.md · references/
• kotlin.md · references/
• php.md · references/
• python.md · references/
• ruby.md · references/
• swift.md · references/
• vm-compiled.md · references/

SKILL.md (first lines)

# Constant-Time Analysis

Compile the code, inspect the emitted assembly or bytecode for variable-time instructions, then decide which of the flagged operations actually touch secrets. The compilation step is mechanical; the triage step is the work.

## When to Use

- Implementing or reviewing a signature, encryption, KEM, or key derivation routine
- Code applies `/` or `%` to a value derived from a key, plaintext, nonce, or token
- The user mentions "constant-time", "timing attack", "side-channel", or "KyberSlash"
- Reviewing functions named `sign`, `verify`, `encrypt`, `decrypt`, `derive_key`

## When NOT to Use

- **Measuring** timing variance on a running binary — use the `constant-time-testing` skill from the `testing-handbook-skills` plugin, which covers dudect and statistical approaches and may not be installed. This skill inspects compiler output statically and never executes the code under test.
- Non-cryptographic code, or crypto code where every input is public
- High-level API usage where a vetted library owns the constant-time guarantees
- Cache and other microarchitectural side channels — the assembly view cannot see them

## Language Routing

Read the guide for the target language before interpreting any findings; each one lists that language's dangerous instructions and the idiomatic constant-time replacements.

| Guide | Languages |
| ----- | --------- |
| [references/compiled.md](references/compiled.md) | C, C++, Go, Rust |
| [references/swift.md](references/swift.md) | Swift |
| [references/vm-compiled.md](references/vm-compiled.md) | Java, C# |
| [references/kotlin.md](references/kotlin.md) | Kotlin |
| [references/php.md](references/php.md) | PHP |
| [references/javascript.md](references/javascript.md) | JavaScript, TypeScript |
| [references/python.md](references/python.md) | Python |
| [references/ruby.md](references/ruby.md) | Ruby |

## Running the Analyzer

The analyzer takes one file and detects the language from its extension. **Always pass `--warnings`:**

```bash
uv run {baseDir}/ct_analyzer/analyzer.py --warnings <source_file>
```

Without it the analyzer reports only error-severity findings, which means division, modulo and weak RNG. Four detector families are warning severity and stay silent: secret-dependent branches, early-exit comparison (`memcmp`, `strcmp`, `.equals`, `==`), table lookups indexed by a secret, and variable-time encoding. Early-exit comparison of an authentication tag is the most common timing bug in real code — Lucky Thirteen was exactly that — so a default run is quiet about the finding you are most likely to have.

| Flag | Effect |
| ---- | ------ |
| `--warnings` | Add the four warning-severity families above. Pass it every time |
| `--func <regex>` | Restrict output to function names matching the regex |
| `--json` | Machine-readable output |
| `--github` | GitHub Actions annotations |
| `--arch <target>` | Target architecture (`x86_64`, `arm64`, `riscv64`, ...) — native languages only |
| `--opt-level <level>` | Optimization level (`O0` through `O3`, `Os`, `Oz`) — native languages only |
| `--compiler <name>` | Override compiler choice (`gcc`, `clang`, `go`, `rustc`, `swiftc`) |

Narrow a large file to the routines that handle secrets with a regex, for example `--func 'sign|verify'`.

**Run natively compiled code (C, C++, Go, Rust, Swift) at more than one `--arch` and `--opt-level`.** Division timing and branch lowering are architecture- and optimization-dependent: x86_64 `IDIV` and arm64 `SDIV` differ, and a `cmov` at `-O2` can become a branch at `-O0`. A single clean run proves one configuration safe, not the code.

**How `--arch` crosses depends on the toolchain.** clang crosses with `--target` and needs no second compiler, but any source that includes libc headers also needs that target's C library headers — `libc6-dev-riscv64-cross` and friends — or it fails with `bits/libc-header-start.h file not found`. Go cross-builds through `GOARCH`, though `go tool objdump` has no riscv64 disassembler. A GNU cross toolchain is a *separate binary*, so gcc needs it named explicitly — `--compiler x86_64-linux-gnu-gcc`, `--compiler riscv64-linux-gnu-gcc` — and nothing is substituted for you, so the report always names the binary that ran. rustc needs the target's standard library (`rustup target add`), and Swift on Linux targets only the host. Compare against the toolchain that builds your product, not whichever cross build a distribution packages.

**Re-run the whole sweep on the fix, across compilers, targets and every level including `Os` and `Oz`.** Any fix that works by handing the compiler a constant divisor to strength-reduce is a fix only where the compiler chooses to cooperate, and that choice varies more than it looks. Replacing `key_coef / (2 * gamma2)` with a `#define`d divisor still emits a real divide here:

| Toolchain | Levels that emit a division |
| --------- | --------------------------- |
| gcc riscv64 | `O0` through `Oz` — every level |
| gcc arm64, gcc x86_64 | `Os`, `Oz` |
| clang arm64 | `O0`, `Oz` |

Strength reduction is an optimizer courtesy, not a language guarantee. Prefer an explicit multiply-shift, and verify it against the original expression over the full input range rather than on sampled values — an off-by-a-power-of-two reciprocal matches for millions of inputs before it diverges.

Java, Kotlin, and C# compile to JVM/CIL bytecode. The analyzer reads that bytecode, so `--arch` and `--opt-level` do not apply and the JIT may still introduce variable-time native code the analyzer cannot see.

### Per-language coverage limits

Coverage is not uniform, and the gaps change what a clean report means:

| Language | What the report does not cover |
| -------- | ------------------------------ |
| Go | Only symbols from the analyzed file. `go build` links the runtime in, and its divisions — all on public data — would otherwise dominate the findings |
| JavaScript, TypeScript | Bytecode findings are restricted to functions the file declares by name, because V8 dumps node's internals the same way it dumps yours. Anonymous callbacks fall to the source scan. For TypeScript, bytecode findings name the function but carry no line, since V8's positions index the transpiled output |
| Python, Ruby, PHP | Bytecode reflects the interpreter that ran, not a JIT'd or alternative runtime |
| Rust | Analyzed as a library unless the file declares `fn main`; private functions with no caller may be optimized away before analysis |
| Swift | Targets the host platform on Linux; iOS and macOS triples need an Apple toolchain |

Since findings and silence both depend on the configuration, say which compiler, architecture, and optimization level produced a result when reporting it.

To sweep a directory, loop in the shell — the analyzer is a deterministic script, one invocation per file:

```bash
for f in src/crypto/*.c; do uv run {baseDir}/ct_analyzer/analyzer.py --warnings --json "$f"; done
```

### Prerequisites

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

On a "toolchain not found" error, see [references/vm-compiled.md](references/vm-compiled.md) for JVM and .NET installation, macOS keg-only PATH configuration, and troubleshooting.

## Interpreting Results

**PASSED** — no *error*-severity finding for the configuration you ran. Warnings do not affect it, so `Result: PASSED` alongside `Warnings: 6` is normal and is not a clean result. Read the warning list before concluding anything.

**FAILED** — dangerous instructions found, reported per function:

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

## Triaging Findings

From trailofbits/skills, CC-BY-SA-4.0.

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