encrypt the string and prefix the value with '\u001Bpw:' so it's compatible with old plain text password
Source
static String encryptString(String str) {
if (str == '') {
return '';
}
_encryptEngine.reset();
_encryptEngine.init(true, _encryptParams);
Uint8List utf8bytes = UTF8.encode(str);
Uint8List block = new Uint8List((utf8bytes.length + 31 )~/32 * 32);
block.setRange(0, utf8bytes.length, utf8bytes);
return '\u001Bpw:${Base64.encode(_encryptEngine.process(block))}';
}