Input : Plaintext :   GEEKSFORGEEKS
             Keyword :  AYUSH
Output : Ciphertext :  GCYCZFMLYLEIM
For generating key, the given keyword is repeated
in a circular manner until it matches the length of 
the plain text.
The keyword "AYUSH" generates the key "AYUSHAYUSHAYU"
The plain text is then encrypted using the process 
explained below.

package main

import (
	"fmt"
	"strings"
)
func encodeString(cipher, key rune) rune {
	const asciiA rune = 65
	const numLetters = 26

	plainTextIndex := cipher + key
	asciiLetter := (plainTextIndex+numLetters)%numLetters + asciiA
	return asciiLetter
}
func encode(message, kw string) string {
	var plainText strings.Builder
	kwChars := []rune(kw)

	for i, cipherChar := range message {
		key := i % len(kwChars)
		plainText.WriteRune(encodeString(cipherChar, kwChars[key]))
	}

	return plainText.String()
}
func decipherString(cipher, key rune) rune {
	const asciiA rune = 65
	const numLetters = 26

	plainTextIndex := cipher - key
	asciiLetter := (plainTextIndex+numLetters)%numLetters + asciiA
	return asciiLetter
}

func decipher(message, kw string) string {
	var plainText strings.Builder
	kwChars := []rune(kw)

	for i, cipherChar := range message {
		key := i % len(kwChars)
		plainText.WriteRune(decipherString(cipherChar, kwChars[key]))
	}

	return plainText.String()
}

func main() {
	fmt.Println("Enter Your string: ")

	var first string

	fmt.Scanln(&first)
	fmt.Println("Enter your KEY: ")
	var second string
	fmt.Scanln(&second)
	cipherText := first
	keyword := second
	fmt.Print("Do you want to  1. Encrypt or 2. Decrypt")
	var option int
	fmt.Scanln(&option)
	if option == 1 {
		fmt.Println(encode(cipherText, keyword))
	} else if option == 2 {
		fmt.Println(decipher(cipherText, keyword))
	} else {
		fmt.Println("please choose the right option")
	}

}