Click the “Start scan” link and choose where the encrypted .crypt file lies (that file, for which you have an unencrypted copy as well).
Then the tool will ask for the original file.
After that RannohDecryptor starts searching for all other files with “.crypt” extension and tries to decrypt all files, which weigh less than your original. The bigger file you’ve feed to the utility — the more files would be decrypted.