Problem Statement

Implement a Caesar cipher, both encoding and decoding. The key is an integer from 1 to 25. This cipher rotates (either towards left or right) the letters of the alphabet (A to Z). The encoding replaces each letter with the 1st to 25th next letter in the alphabet (wrapping Z to A). So key 2 encrypts "HI" to "JK", but key 20 encrypts "HI" to "BC". This simple "mono-alphabetic substitution cipher" provides almost no security, because an attacker who has the encoded message can either use frequency analysis to guess the key, or just try all 25 keys. Caesar cipher is identical to Vigenère cipher with a key of length 1. Also, Rot-13 is identical to Caesar cipher with key 13.

Let's start with the solution:

PROGRAM CAESAR

!$INCLUDE="PC.LIB"

PROCEDURE CAESAR(TEXT$,KY%->CY$)
    LOCAL I%,C%
    FOR I%=1 TO LEN(TEXT$) DO
      C%=ASC(MID$(TEXT$,I%))
      IF (C% AND $1F)>=1 AND (C% AND $1F)<=26 THEN
        C%=(C% AND $E0) OR (((C% AND $1F)+KY%-1) MOD 26+1)
        CHANGE(TEXT$,I%,CHR$(C%)->TEXT$)
      END IF
    END FOR
    CY$=TEXT$
END PROCEDURE

BEGIN
    RANDOMIZE(TIMER)
    PLAINTEXT$="Pack my box with five dozen liquor jugs"
    PRINT(PLAINTEXT$)

    KY%=1+INT(25*RND(1))  ! generates random between 1 and 25
    CAESAR(PLAINTEXT$,KY%->CYPHERTEXT$)
    PRINT(CYPHERTEXT$)

    CAESAR(CYPHERTEXT$,26-KY%->DECYPHERED$)
    PRINT(DECYPHERED$)

END PROGRAM