Constant Time Analysis

Trail of BitsOfficialPublished by Trail of Bits

Skill
Review and audit
Security
CC-BY-SA-4.0
Low risk

From the author

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.

— the description by Trail of Bits

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

Constant-Time Analysis is a static analysis tool designed to identify potential timing side-channel vulnerabilities in cryptographic implementations. By inspecting compiled assembly or bytecode, it flags operations that may exhibit variable-time execution, such as secret-dependent branches, divisions, or early-exit comparisons. The tool is intended for developers and security reviewers working on sensitive routines like encryption, signing, or key derivation across a wide range of languages including C, Rust, Go, Java, and JavaScript. It emphasizes a triage-based workflow where users must verify whether flagged instructions actually process secret data, distinguishing between true vulnerabilities and false positives.

What it does

Scans compiled assembly or bytecode for variable-time instructions like division and modulo.
Detects secret-dependent branches and early-exit comparisons that leak information.
Provides a triage framework to help users verify if flagged operations involve secret data.
Supports multiple languages including C, C++, Go, Rust, Swift, Java, Kotlin, C#, PHP, JS, TS, Python, and Ruby.
Allows filtering by function name and provides machine-readable output for CI integration.
Offers configuration options for different architectures and optimization levels to ensure comprehensive coverage.

When to use it

When implementing or reviewing cryptographic primitives like encryption, signing, or KEMs.
When code performs division or modulo operations on values derived from secrets.
When you need to ensure code is resistant to timing attacks.
When reviewing functions that handle sensitive data like keys, nonces, or authentication tags.
When you want to verify that comparisons (e.g., MAC verification) are constant-time.

Advantages

Enables proactive detection of timing vulnerabilities before deployment.
Provides a structured approach to triaging findings to minimize false positives.
Supports cross-architecture and cross-optimization level testing to catch configuration-specific bugs.
Integrates into CI pipelines for automated security checks.
Helps developers identify idiomatic constant-time replacements for dangerous operations.

Usage examples

“Analyze my C implementation of a signature routine for timing leaks.”: The tool will compile the code and report any variable-time instructions found in the signature function, which you then triage to see if they touch secret key material.
“Check my TypeScript crypto code for early-exit comparisons.”: The tool will analyze the transpiled bytecode to identify potential early-exit patterns in comparisons, prompting you to replace them with constant-time alternatives.
“Run a constant-time analysis on my Go crypto package across multiple architectures.”: The tool will perform the analysis for each specified architecture, helping you identify if compiler optimizations introduce timing variances on specific platforms.
“How do I filter the analysis to only check my verify function?”: You can use the --func flag to restrict the output to only the 'verify' function, reducing noise in large files.

Install

Download the skill as a ZIP:
https://mp-staging.autogineer.com/agent-skills/trailofbits/constant-time-analysis/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip constant-time-analysis.zip -d .agents/skills

Documentation by Trail of Bits

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	C, C++, Go, Rust
references/swift.md	Swift
references/vm-compiled.md	Java, C#
references/kotlin.md	Kotlin
references/php.md	PHP
references/javascript.md	JavaScript, TypeScript
references/python.md	Python
references/ruby.md	Ruby

Running the Analyzer

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

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 #defined 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:

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 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:

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

Triaging Findings

The analyzer has no data flow analysis. It flags every dangerous instruction regardless of whether a secret reaches it, so a FAILED report is a worklist, not a verdict. Reporting the raw output as a set of vulnerabilities is the primary failure mode of this skill.

For each flagged instruction, read the source and answer one question: does an operand depend on secret data? Trace from the instruction's function back to the caller's inputs, then classify:

// FALSE POSITIVE: operands are a buffer length, already public from the ciphertext size
int num_blocks = data_len / 16;

// TRUE POSITIVE: dividend is a private-key coefficient; IDIV/SDIV leaks its magnitude
int32_t q = secret_coef / GAMMA2;

Question	If yes
Is the operand a compile-time constant?	Likely false positive
Is the operand a public parameter — length, count, index bound?	Likely false positive
Is the operand derived from a key, plaintext, nonce, or token?	True positive
Can an attacker influence the operand's value?	True positive

State the verdict and the data flow that justifies it for every flagged item. A finding you cannot trace to a secret is not a finding; say so explicitly rather than dropping it silently.

{baseDir}/ct_analyzer/tests/triage_samples/ holds a known-answer case per language: each fixture pairs a true positive with a false positive that the analyzer reports identically, and expectations.json records which is which and why. triage_c.c is the shortest example — the analyzer flags the division in both ct_high_bits and ct_block_count, and correct triage confirms the first and clears the second.

Weak-RNG and encoding findings ask a different question. For Math.random, mt_rand, random.randint, System.Random and base64_encode, no operand is secret, so "does an operand depend on a secret?" does not resolve them. Ask instead what the result is used for: seeding a nonce or key is a true positive, jittering a retry delay is not. These are reported by a regex scan over the source rather than from bytecode, so they are attributed to <source> with a line number instead of to the enclosing function — except in PHP, where they carry the function.

Comparison and lookup findings have their own question, and their own fix. For an early-exit comparison, ask whether either side is secret: comparing an authentication tag, MAC, or password hash is a true positive, comparing a public protocol header is not. For a table lookup, ask whether the index is secret — the array's contents do not matter, only what selects the element. Both are exploitable as written, so a confirmed one needs the language's constant-time primitive rather than a rewrite of the loop:

Language	Constant-time comparison
C, C++	`CRYPTO_memcmp` (OpenSSL) or `sodium_memcmp`
Go	`crypto/subtle.ConstantTimeCompare`
Rust	the `subtle` crate's `ConstantTimeEq`
Java, Kotlin	`MessageDigest.isEqual`
C#	`CryptographicOperations.FixedTimeEquals`
PHP	`hash_equals`
Python	`hmac.compare_digest`
Ruby	`OpenSSL.secure_compare`
JavaScript, TypeScript	`crypto.timingSafeEqual`

A secret-indexed lookup has no drop-in replacement: it needs a bit-sliced or arithmetic formulation that touches every element, which is why AES S-box tables are the classic case. Encoding a secret through a table — base64_encode, bin2hex, chr/ord — is the same problem in a library, and paragonie/constant_time_encoding is the reference fix for PHP.

Limitations

Static only — reads assembly and bytecode, never runtime behavior. Cache timing and other microarchitectural channels are invisible.
No data flow analysis — see triage above.
Configuration-specific — a different compiler, optimization level, architecture, or runtime version can emit different instructions from identical source.

Real-World Impact

KyberSlash (2023) — division instructions in 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

Files

ATTRIBUTION.mdAdded by AutoGineer960 B
LICENSELicence file added by AutoGineer19.7 KiB
README.md4.8 KiB
SKILL.md12.9 KiB
agents/openai.yaml240 B
assets/trail-of-bits-mark.svg3.0 KiB
references/compiled.md4.0 KiB
references/javascript.md4.3 KiB
references/kotlin.md6.6 KiB
references/php.md5.1 KiB
references/python.md5.1 KiB
references/ruby.md5.3 KiB
references/swift.md8.0 KiB
references/vm-compiled.md11.3 KiB

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Requirements and compatibility

Written for: Claude Code

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Declares allowed tools, Claude Code frontmatter, Needs Python, Needs Node.js

Security

What our scanner noticed· Low risk

Instructions that use the network

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Published by

Trail of Bits

Publisher

Trail of Bits (Official)

Source

trailofbits/skills · plugins/constant-time-analysis/skills/constant-time-analysis · commit e4dd0bb

Licence

Creative Commons Attribution-ShareAlike 4.0 International (redistribution allowed)

LICENSE (added by AutoGineer from LICENSE)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-08

AutoGineer is not affiliated with or endorsed by Trail of Bits.

Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download constant-time-analysis.zip