What is wrong with £ symbol when using it in encryptor? - dart

Question

When I use £ symbol in password according with an AES encryptor I get the error
Key length must be 128/192/256 bits
String pass = 'my_cool_password_£..............';
var key = Key.fromUtf8(pass);
var encrypter = Encrypter(AES(key));
encrypter.encrypt(plainText, iv: iv); // error `Key length must be 128/192/256 bits`
Stack trace
Unhandled exception:
Invalid argument(s): Key length must be 128/192/256 bits
#0 AESFastEngine.init (package:pointycastle/block/aes_fast.dart:66:7)
#1 SICStreamCipher.init (package:pointycastle/stream/sic.dart:55:22)
#2 StreamCipherAsBlockCipher.init (package:pointycastle/adapters/stream_cipher_as_block_cipher.dart:27:18)
#3 PaddedBlockCipherImpl.init (package:pointycastle/padded_block_cipher/padded_block_cipher_impl.dart:43:12)
#4 AES.encrypt (package:encrypt/src/algorithms/aes.dart:19:9)
#5 Encrypter.encryptBytes (package:encrypt/src/encrypter.dart:12:19)
#6 Encrypter.encrypt (package:encrypt/src/encrypter.dart:20:12)
the package was used https://pub.dev/packages/encrypt
here is the package encrypt function
Encrypted encrypt(String input, {IV iv}) {
return encryptBytes(convert.utf8.encode(input), iv: iv);
}

Answer 1

Since you are using UTF-8 to represent your password, you need to take into account that not all letters can be represented with only 1 byte (8 bits).
E.g. the £ is represented by using two bytes (16 bits): c2 a3
This can be seen in the following example:
import 'dart:convert';
void main() {
print(utf8.encode('my_cool_password_£..............').length * 8); // 264
print(utf8.encode('my_cool_password_x..............').length * 8); // 256
print(utf8.encode('£').length * 8); // 16
print(utf8.encode('£').map((i) => i.toRadixString(16))); // (c2, a3)
}