Building a Caesar Cipher (a Character-Shifting Cipher)

This lesson covers implementing a Caesar cipher in C, so you understand the basics of a character-shifting encryption scheme. It's for anyone searching "C Caesar cipher implementation."

A Caesar cipher is a simple cipher, dating back to ancient Rome, that shifts letters by a fixed number of positions. It's like a childhood game of shifting every letter of the alphabet by a few spots.

The example processes one character at a time until reaching the string's end, checked with text[i] != '0', computing the shifted character code while checking whether it's in the alphabet range. Notice a C string ends with a terminating character, '0'.

A common early mistake is the character-code math. You need to correctly handle the case where the shifted result goes past the end of the alphabet (A–Z), using % 26.

It's simple compared to modern cryptography, but it's a perfect introductory exercise for practicing character-code arithmetic.

📖 Reference code

#include <stdio.h>
#include <string.h>

void caesarEncrypt(char *text, int shift) {
    for (int i = 0; text[i] != '\0'; i++) {
        if (text[i] >= 'A' && text[i] <= 'Z') {
            text[i] = (text[i] - 'A' + shift) % 26 + 'A';
        }
    }
}

int main(void) {
    char text[] = "HELLO";
    caesarEncrypt(text, 3);
    printf("%s\n", text);
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).