There is an utility which encrypts/decrypts files using a password. There is an encrypted text file, the password is unknown. The encrypted file is a text in the English language.
The utility uses a relatively strong cryptosystem, nevertheless, it was implemented with a serious blunder. With the mistake present, it is possible to decrypt the file with a little effort.
Try to find the mistake and decrypt the file.
The text file: files/text_encrypted.
More challenges: //challenges.re/; about solutions: //challenges.re/#Solutions.