Caesar cipher

It is possible to use the Caesar cipher to obscure text in Go programs. By shifting letters, we can encode a message. This deters casual snooping.

Implementation details

In Go we can implement the Caesar cipher with the strings.Map method. We avoid complex for-loops and bug-prone code this way.

Method

Let us begin. We introduce the caesar() method. This receives a rune and returns a modified rune. It shifts characters, and then shifts characters to a valid range.

Start In the main() method control flow begins. To call caesar() within the strings.Map method, we use funcs.

Note Each func we define calls the caesar() method with a special shift argument. This shifts characters by that number of places.

Note 2 The strings.Map method can only call a method that receives and returns a rune. The shift argument is specified in a wrapper func.

Warning A global variable could be used to control the shift number, but this is not accepted as good programming practice.

package main

import (
    "fmt"
    "strings"
)

func caesar(r rune, shift int) rune {
    // Shift character by specified number of places.
    // ... If beyond range, shift backward or forward.
    s := int(r) + shift
    if s > 'z' {
        return rune(s - 26)
    } else if s < 'a' {
        return rune(s + 26)
    }
    return rune(s)
}

func main() {
    value := "test"
    fmt.Println(value)

    // Test the caesar method in a func argument to strings.Map.
    value2 := strings.Map(func(r rune) rune {
        return caesar(r, 18)
    }, value)
    value3 := strings.Map(func(r rune) rune {
        return caesar(r, -18)
    }, value2)
    fmt.Println(value2, value3)

    value4 := strings.Map(func(r rune) rune {
        return caesar(r, 1)
    }, value)
    value5 := strings.Map(func(r rune) rune {
        return caesar(r, -1)
    }, value4)
    fmt.Println(value4, value5)

    value = "exxegoexsrgi"
    result := strings.Map(func(r rune) rune {
        return caesar(r, -4)
    }, value)
    fmt.Println(value, result)
}test
lwkl test
uftu test
exxegoexsrgi attackatonce

Some notes

The strings.Map is a good way to translate characters in strings. But its first argument is a func that only receives a rune (and no shift value).

So We can modify how a method is called by adding another "wrapper" method that specifies the shift in an internal method calls.

Note In some ways, this implementation is less clear than a custom looping method, but using strings.Map in this way is reliable.

Uppercase letters

This method will not work correctly on uppercase letters. To add support for uppercase letters, please try adding another pair of if-else statements.

Summary

The Caesar cipher is similar to the ROT13 cipher, but it accommodates any shift value. It is not useful for battles anymore. But it can help us learn more about how to use Go.