AES does not support key with length 64 about encrypt

[leocavalcante]

Length 64 you mean 64 bytes, I guess, right?
AES specification limits key sizes on 128, 192, or 256 bits. A 64 bytes means a 512 bits key, which isn't supported by the algorithm itself.

Any way, the output of the HMAC256() you posted resembles a Hexdecimal string, to use this output, you should create a Key using the fromBase16 constructor. This hexdec hash itself you posted (1a991c1e2984f20af91361f916a708b1fc7e247f8f4e6cc090b89fe0280d39ed) is 32 bytes (or 256 bits) which is supported by the AES algorithm, look:

import 'package:encrypt/encrypt.dart';

void main() {
  final key = Key.fromBase16('1a991c1e2984f20af91361f916a708b1fc7e247f8f4e6cc090b89fe0280d39ed');

  assert(key.bytes.length == 32);

  final iv = IV.fromLength(16);

  final encrypter = Encrypter(AES(key));
  final encrypted = encrypter.encrypt('Lorem ipsum', iv: iv);

  print(encrypted.base16); // cbd1b4ea592191d9d9b498303e29bfe0
}

[satheler]

Sorry, I didn't know that specification. Thank you very much.