Building a Caesar Cipher (a Character-Shifting Cipher)

In this lesson you will learn how to implement a Caesar cipher in Go, and understand the basic idea of encrypting by shifting characters. This is for anyone searching for "Go Caesar cipher implementation".

The Caesar cipher is a simple cipher from ancient Rome that replaces each letter with the one a fixed number of positions further along the alphabet. It is the same idea as a childhood game of shifting every letter by three.

The sample code converts the string into a slice of runes with []rune(text) and, checking whether each character is in the alphabet, computes the shifted letter with 'A' + (c-'A'+rune(shift))%26. In Go, using the rune type is the basic way to handle characters correctly, including non-ASCII text.

A common stumbling block for beginners is the difference between byte-oriented []byte and character-oriented []rune. Multi-byte characters break when handled as bytes, so runes are the foundation of correct character handling in Go.

The Caesar cipher itself is far too simple for real security, but the idea of manipulating character codes underpins more advanced encryption and encoding work.

📖 Reference code

package main

import "fmt"

func caesarEncrypt(text string, shift int) string {
    result := []rune(text)
    for i, c := range result {
        if c >= 'A' && c <= 'Z' {
            result[i] = 'A' + (c-'A'+rune(shift))%26
        }
    }
    return string(result)
}

func main() {
    fmt.Println(caesarEncrypt("HELLO", 3))
}

✍️ Your code

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