HMAC Generator & Calculator

How HMAC Works

HMAC (Hash-based Message Authentication Code) is defined in **RFC 2104**. It's more than just `hash(key + message)`, which is vulnerable to "length extension attacks". Instead, HMAC uses a nested hashing structure:

• H: The cryptographic hash function (e.g., SHA-256).
• K': The secret key K, padded to the block size of the hash function.
• m: The message to be authenticated.
• opad: Outer padding (0x5c repeating).
• ipad: Inner padding (0x36 repeating).
• ⊕: Bitwise XOR operation.
• ||: Concatenation.

HMAC vs. Hash vs. Digital Signature

Code Examples

How to generate a SHA-256 HMAC in popular programming languages:

const crypto = require('crypto');

const secret = 'my-secret-key';
const message = 'Hello World';

const hmac = crypto.createHmac('sha256', secret)
                   .update(message)
                   .digest('hex');

console.log(hmac);

import hmac
import hashlib

secret = b'my-secret-key'
message = b'Hello World'

hash = hmac.new(secret, message, hashlib.sha256).hexdigest()
print(hash)

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

String secret = "my-secret-key";
String message = "Hello World";

Mac sha256_HMAC = Mac.getInstance("HmacSHA256");
SecretKeySpec secret_key = new SecretKeySpec(secret.getBytes(), "HmacSHA256");
sha256_HMAC.init(secret_key);

byte[] hash = sha256_HMAC.doFinal(message.getBytes());

$secret = 'my-secret-key';
$message = 'Hello World';

$hash = hash_hmac('sha256', $message, $secret);
echo $hash;

Security Best Practices

• Use Strong KeysYour secret key should be random and at least as long as the hash output length (e.g., 32 bytes for SHA-256).
• Constant-Time ComparisonWhen verifying HMACs on your server, always use constant-time comparison (like `crypto.timingSafeEqual`) to prevent timing attacks.
• Rotate KeysChange your secret keys periodically. Support multiple keys (current and previous) to allow smooth rotation without downtime.
• CanonicalizationEnsure the message is formatted consistently (e.g., sorting JSON keys) before hashing, or the signatures won't match.