-s[password]   Encrypt files

  By specifying this option you may encrypt data as it is compressed by
  PKZIP.  This feature of PKZIP causes the information contained within a
  .ZIP file to be scrambled.  A file which has been scrambled will be
  unreadable until extracted with the same encryption key.

  If a password is not specified the user will be prompted for one.

  This feature should be used with caution.  For a complete explanation of
  the use of this option please see Using Data Encryption.