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hmac · sha-1/256/384/512 · web crypto · live

Generate HMAC signatures

Hash-based Message Authentication Codes for verifying message authenticity. SHA-1, SHA-256, SHA-384, or SHA-512. Runs entirely in your browser.

HMAC Generator
Message
43 B
43 chars
Algorithm
HMAC-SHA256
MAC
Digest
—
lower
Status
…
web crypto
input · message
output · hmac hex
—
Quick Start

How to compute HMAC

HMAC(K, m) = H((K' xor opad) ‖ H((K' xor ipad) ‖ m)). The Web Crypto SubtleCrypto.sign API handles the details for you.

  1. 01

    Pick the hash

    HMAC-SHA256 is the modern default. SHA-1 only for legacy interop.

    Bytes flow through TextEncoder/TextDecoder for predictable UTF-8 semantics — emoji, surrogate pairs, and combining marks all encode losslessly.

    algoUTF-8TextEncoderlosslesslive
    algo · raw bytesSHA-256 · UTF-8
    HMAC-SHA256
    loaded · UTF-8MIME · ready
  2. 02

    Enter the secret

    Any UTF-8 string. The secret is mixed into the hash twice.

    Hash and signature operations call crypto.subtle from the Web Crypto API, so output matches OpenSSL / Node bit-for-bit.

    keyWeb Cryptosubtledeterministiclive
    key · optionssecret · live
    my-secret-key
    options applied · livelive
  3. 03

    Provide the message

    The bytes you want to authenticate.

    Options re-run instantly through useEffect + useMemo — switching URL-safe mode or padding doesn’t re-encode from scratch.

    msgmemoisedoptionsinstantreactive
    msg · encodedtext · ready
    hello
    encoded · livelossless
  4. 04

    Copy or download

    One click to clipboard. Download as .txt.

    Copy uses the Async Clipboard API; downloads ship via Blob with the appropriate MIME (text/plain, image/png, …) so external tools recognise them.

    exportclipboardblobMIME-correctclient-side
    ok · copiedready · done
    Copied
    shipped · ready to shipsuccess
Pro tips

Encoding without surprises

Always UTF-8 first

Use TextEncoder / TextDecoder for bytes ↔ string conversion. btoa alone breaks on non-ASCII.

URL-safe vs standard Base64

Standard uses +, /, =; URL-safe swaps to -, _, no padding. Don’t mix them.

Hashes aren’t passwords

SHA-256 is fast; that’s good for fingerprints, bad for passwords. Use bcrypt / argon2 for credentials.

Watch padding rules

Some decoders are strict about = padding, others auto-pad. If round-trips fail at exactly 1 or 2 bytes, check padding.

Use Web Crypto for keys

Generate cryptographic keys via crypto.subtle, never Math.random. The latter is predictable.

JWT decode ≠ verify

Decoding a JWT just splits and base64-decodes. Verification needs the issuer’s public key / shared secret.

Related toolsSHA-256SHA-512JWT EncoderJWT DecoderPBKDF2Bcrypt
Use Cases

When to use HMAC

Verifying that a message came from someone holding the key.

Webhook signatures

Stripe, GitHub, and Slack sign payloads with HMAC. Output round-trips losslessly with the matching decoder — both run client-side, so end-to-end testing happens in a single tab. Encoding flows through TextEncoder for predictable UTF-8 semantics.

RFC-compliant

JWT HS256/HS512

HMAC backs the symmetric JWT signing algorithms. Web Crypto's subtle.digest produces output that matches OpenSSL, GPG, and Node's crypto module byte-for-byte. That makes the result trustworthy as a content fingerprint, ETag, or integrity check.

OpenSSL-matched

AWS Sig V4

AWS API requests use chained HMAC-SHA256. Pairs cleanly with HMAC, JWT, and signed-URL workflows where bit-level correctness matters. The browser's crypto stack is FIPS 140-validated in Chrome and Firefox, so it's production-grade.

lossless

Session tokens

Sign cookies/tokens server-side without storage. Output uses canonical RFC 4648 / 7515 alphabets so curl, openssl, and any standards-compliant decoder accept it as input. URL-safe variants drop padding for tokens that travel in query strings.

byte-identical

Replay protection

Combine HMAC with a nonce or timestamp. Heavy hashing runs through useMemo so large inputs don't block typing. PBKDF2, bcrypt, and Argon2 expose configurable cost factors — tune for your threat model, not the default.

spec-grade

Test fixtures

Match RFC 4231 test vectors exactly. Same input plus same options always yields identical bytes. That's the foundation for content-addressed storage (IPFS, Git, S3 ETags) and means the output is safe to commit to fixtures.

production-ready
FAQ

Frequently asked questions

Tampering by someone who does not hold the secret key. Without the key, an attacker cannot generate a valid HMAC for a chosen message.
Yes for HMAC purposes — even broken hashes still produce strong HMACs. Prefer SHA-256 for new designs.
A raw hash with a prepended key is vulnerable to length-extension. HMAC's nested construction avoids that flaw.
At least as long as the hash output (e.g. 256 bits for HMAC-SHA256). Longer keys provide no extra strength.
No. HMAC is computed in your browser via Web Crypto and runs locally.
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