REBOL []

cesar: func
[   message
    /decript
    /local
        encription-table character characters letter
]
[   encription-table: copy {abcdefghijklmnopqrstuvwxyzaABCDEFGHIJKLMNOPQRSTUVWXYZA}

    if decript [reverse encription-table]

    character:
    [   copy letter message: skip
        (   if letter: select/case encription-table letter
            [   change message letter
            ]
        )
    ]

    characters: [any character]

    parse/all  copy message  characters

    head message
]

probe m: copy " Logo, c'est cool"
probe cesar m
probe m

print "---"

probe m: copy "B cjfoupu qpvs ef opvwfmmft bwfouvsft ;-)"
probe cesar/decript m

>> do %cesar.r
 Logo, c'est cool
 Mphp, d'ftu dppm
 Logo, c'est cool
---
B cjfoupu qpvs ef opvwfmmft bwfouvsft ;-)
A bientot pour de nouvelles aventures ;-)