PowerArchiver can quickly encrypt already created archives when they are opened in PowerArchiver. The archives are encrypted using the most secure form of encryption, AES.
When an archive is open in the main PowerArchiver window, go to Actions and select Encrypt Archive. You will then be presented with the Encryption window that will offer two choices for encryption:
After you select whether to encrypt the same archive or create a new one with the same files, Same or New Archive, click OK and the encryption process will begin.