HMAC

Learn message authentication with Hmac.

HMAC

HMAC (Hash-based Message Authentication Code) is a keyed hash. It provides not just a hash but message integrity and authentication together. It is used as Hmac(sha256, key).convert(data).

Example: final key = utf8.encode('anahtar'); final data = utf8.encode('mesaj'); final hmac = Hmac(sha256, key); final digest = hmac.convert(data);. Here the key and data are turned into bytes and HMAC-SHA256 is computed. The result is again 64 hex characters. The receiver computes the same HMAC with the same key and compares; if equal the message is unchanged and the sender is authenticated.

HMAC verification

HMAC works with a key shared by two parties. The sender produces an HMAC from the message and the key and sends it with the message. The receiver computes its own HMAC with the same key and compares. If equal the message is unchanged and from the correct sender.

This is a common method to sign API requests. For example a web service gives each user a key; requests are signed with this key. The server verifies the signature and confirms the request comes from an authorized source. In this lesson we learn HMAC computation.

Exercise Compute an HMAC-SHA256

Add import 'dart:convert'; and import 'package:crypto/crypto.dart';. In main add final key = utf8.encode('anahtar'); and final data = utf8.encode('mesaj');. Compute the HMAC with final hmac = Hmac(sha256, key); and final digest = hmac.convert(data);. Print with print(digest).

Show example solution

import 'dart:convert';
import 'package:crypto/crypto.dart';

void main() {
  final key = utf8.encode('anahtar');
  final data = utf8.encode('mesaj');
  final hmac = Hmac(sha256, key);
  final digest = hmac.convert(data);
  print(digest);
}

Test yourself

Creating an Hmac

final hmac = ____(sha256, key);

Show answer

final hmac = Hmac(sha256, key);

What does HMAC add?
- Storage
- Compression
- Encryption
- Authentication with a key
Show answer

Authentication with a key — HMAC adds authentication with a key.
Match the HMAC parts

Hmac

Signed hash
sha256

Hash algorithm
key

Key
convert

Computes

Next lesson: encrypt

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