How to resolve the algorithm Caesar cipher step by step in the Modula-2 programming language

Published on 12 May 2024 09:40 PM
#Modula-2

How to resolve the algorithm Caesar cipher step by step in the Modula-2 programming language

Table of Contents

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:

Step by Step solution about How to resolve the algorithm Caesar cipher step by step in the Modula-2 programming language

Source code in the modula-2 programming language

MODULE CaesarCipher;
FROM Conversions IMPORT IntToStr;
FROM Terminal IMPORT WriteString, WriteLn, ReadChar;

TYPE String = ARRAY[0..64] OF CHAR;

PROCEDURE Encrypt(p : String; key : CARDINAL) : String;
VAR e : String;
VAR i,t : CARDINAL;
VAR c : CHAR;
BEGIN
    FOR i:=0 TO HIGH(p) DO
        IF p[i]=0C THEN BREAK; END;

        t := ORD(p[i]);
        IF (p[i]>='A') AND (p[i]<='Z') THEN
            t := t + key;
            IF t>ORD('Z') THEN
                t := t - 26;
            END;
        ELSIF (p[i]>='a') AND (p[i]<='z') THEN
            t := t + key;
            IF t>ORD('z') THEN
                t := t - 26;
            END;
        END;
        e[i] := CHR(t);
    END;
    RETURN e;
END Encrypt;

PROCEDURE Decrypt(p : String; key : CARDINAL) : String;
VAR e : String;
VAR i,t : CARDINAL;
VAR c : CHAR;
BEGIN
    FOR i:=0 TO HIGH(p) DO
        IF p[i]=0C THEN BREAK; END;

        t := ORD(p[i]);
        IF (p[i]>='A') AND (p[i]<='Z') THEN
            t := t - key;
            IF t
                t := t + 26;
            END;
        ELSIF (p[i]>='a') AND (p[i]<='z') THEN
            t := t - key;
            IF t
                t := t + 26;
            END;
        END;
        e[i] := CHR(t);
    END;
    RETURN e;
END Decrypt;

VAR txt,enc : String;
VAR key : CARDINAL;
BEGIN
    txt := "The five boxing wizards jump quickly";
    key := 3;

    WriteString("Original:  ");
    WriteString(txt);
    WriteLn;

    enc := Encrypt(txt, key);
    WriteString("Encrypted: ");
    WriteString(enc);
    WriteLn;

    WriteString("Decrypted: ");
    WriteString(Decrypt(enc, key));
    WriteLn;

    ReadChar;
END CaesarCipher.

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