Caesor Cipher by Golang

cipher.go

package main

import (
    "flag"
    "fmt"
    "os"
    "strings"
    "unicode"
)

var (
    isEncrypt bool
    isDecrypt bool
    rotNum int
    text string
    help bool
    start = int('a')
    end = int('z') + 1
)

func init () {
    flag.BoolVar(&isEncrypt, "e", false, "Encrypt the plain text")
    flag.BoolVar(&isDecrypt, "d", false, "Decrypt the encrypted text")
    flag.IntVar(&rotNum, "n", 13, "Number of places to rotate by")
    flag.StringVar(&text, "t", "", "Text to encrypt/decrypt")
    flag.BoolVar(&help, "h", false, "Echo the usage and exit")
    flag.Parse()

    flag.Usage = func() {
        fmt.Fprintln(os.Stderr, `
CaesarCipher - version: 1.0.0
Usage: caesar [-ed] [-n num] [-t text] [-h]

Options:
`)
        flag.PrintDefaults()
        os.Exit(1)
    }
}

func encrypt(plainText string, key int) string {
    encryptedChars := make([]rune, len(plainText))
    chars := []rune(plainText)
    for i, c := range chars {
        if unicode.IsLetter(c) {
            encrypted := int(c) + key
            if encrypted >= end {
                c = rune(encrypted - end + start)
            } else {
                c = rune(encrypted)
            }
        }
        encryptedChars[i] = c
    }
    return string(encryptedChars)
}

func decrypt(cipherText string, key int) string {
    decryptedChars := make([]rune, len(cipherText))
    chars := []rune(cipherText)
    for i, c := range chars {
        if unicode.IsLetter(c) {
            decrypted := int(c) - key
            if decrypted < start {
                c = rune(end - (start - decrypted))
            } else {
                c = rune(decrypted)
            }
        }
        decryptedChars[i] = c
    }
    return string(decryptedChars)
}

func main() {
    if help {
        flag.Usage()
    }
    if (isEncrypt && isDecrypt) || !(isEncrypt || isDecrypt) {
        fmt.Println("Please specify either encryption mode (-e) or decryption mode (-d).")
        flag.Usage()
    }
    if len(text) == 0 {
        if isEncrypt {
            fmt.Println("Please enter the text to be encrypted")
        } else {
            fmt.Println("Please enter the text to be decrypted")
        }
        flag.Usage()
    }
    text = strings.ToLower(strings.TrimSpace(text))
    if isEncrypt {
        fmt.Printf("Encrypted: %s\n", encrypt(text, rotNum))
    } else {
        fmt.Printf("Decrypted: %s\n", decrypt(text, rotNum))
    }
}