rebol [
  title: "rot13"
  author: "vpavlu"
  date: 9-Dec-2002/13:12
  home: http://plain.at/vpavlu
]

filename: to-file ask "file to scramble? "
string: read filename

foreach char string [
  either ((char >= #"A") and (char <= #"Z")) [
    char: char + 13
    if char > #"Z" [ char: char - 26]
  ][
    if ((char >= #"a") and (char <= #"z")) [
      char: char + 13
      if char > #"z" [ char: char - 26 ]
    ]
  ]
  prin char
]