Dart / Flutter : How Do I Sign a String Using an ES 256 Algorithm and Private Key

Clifton Labrum 0
Viewed 41

Since Flutter doesn't support any map APIs across all platforms (mobile and desktop), I'm trying to fetch map snapshots with Apple's Web Snapshots API. This involves constructing a URL with various options then signing the URL. I append the signature to the end of my request URL so Apple can verify that it's from me.

Apple's instructions state:

To generate a signature, sign the string with your private key using a ES256 algorithm (also known as ECDSA using P-256 curve and SHA-256 hash algorithm). The signature must be Base64 URL-encoded.

I don't need to decrypt anything, I just need to sign the string and add it to the end of my request URL. So I don't think I need anything beyond the crypto library included with Flutter.

Here's what I've tried:

import 'package:crypto/crypto.dart';

//Private Key
var key = utf8.encode('''
-----BEGIN PRIVATE KEY-----
abcdef...
-----END PRIVATE KEY-----
''');

var bytes = utf8.encode('My URL String to Sign...');

var hmacSha256 = Hmac(sha256, key);
var sig = hmacSha256.convert(bytes);
    
var signature = base64UrlEncode(sig.bytes);

I get an unintelligible string as signature and add it to my request URL, but I still get a 401 Not Authorized error, so my signature must be incorrect.

How can I properly sign my URL string with my private key?

digital-signature dart flutter
1 Answers

Using pointycastle, you need a suitable random number generator instance and a signer initialized with the relevant digest. Then just call generateSignature. That only gets you the r and s values which you need to encode.

Here's an example:

import 'dart:convert';
import 'dart:math';
import 'dart:typed_data';

import 'package:pointycastle/asn1.dart';
import 'package:pointycastle/export.dart';

  // the private key
  ECPrivateKey? privateKey;
  
  // some bytes to sign
  final bytes = Uint8List(0);

  // a suitable random number generator - create it just once and reuse
  final rand = Random.secure();
  final fortunaPrng = FortunaRandom()
    ..seed(KeyParameter(Uint8List.fromList(List<int>.generate(
      32,
      (_) => rand.nextInt(256),
    ))));

  // the ECDSA signer using SHA-256
  final signer = ECDSASigner(SHA256Digest())
    ..init(
      true,
      ParametersWithRandom(
        PrivateKeyParameter(privateKey!),
        fortunaPrng,
      ),
    );

  // sign the bytes
  final ecSignature = signer.generateSignature(bytes) as ECSignature;

  // encode the two signature values in a common format
  // hopefully this is what the server expects
  final encoded = ASN1Sequence(elements: [
    ASN1Integer(ecSignature.r),
    ASN1Integer(ecSignature.s),
  ]).encode();

  // and finally base 64 encode it
  final signature = base64UrlEncode(encoded);
Richard Heap0
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