Imports System

Imports System.Text.RegularExpressions

Public Module Module1

    Public Sub Main()

        Dim keyword, message, output As String

        'Loop until the message variable only contains letters

        Do

            Console.Write("Enter a message that consists of only letters: ")

            message = Console.ReadLine()

        Loop Until Regex.IsMatch(message, "^[a-zA-Z]+$")

        Console.WriteLine()

        'Loop until the keyword variable only contains letters

        Do

            Console.Write("Enter a keyword that consists of only letters: ")

            keyword = Console.ReadLine()

        Loop Until Regex.IsMatch(keyword, "^[a-zA-Z]+$")

        Console.WriteLine()

        'Convert the sequence of characters using the Vigener Cipher

        output = VigenerCipher(keyword, message)

        Console.WriteLine(output)

    End Sub

    Private Function VigenerCipher(ByVal keyword As String, ByVal message As String) As String

        If keyword.Length < message.Length Then

            'If the keyword is shorter than the message then append itself up to the length of the message

            Dim counter As Integer = 0

            Do

                keyword &= keyword(counter)

                counter += 1

            Loop Until keyword.Length = message.Length          

        ElseIf keyword.Length > message.Length Then

            'If the keyword is larger than the message then shrink the keyword down the length of the message

            keyword = keyword.Substring(0, message.Length)

        End If

        Dim cipheredText As String = String.Empty

        Dim letters() As Char = "abcdefghijklmnopqrstuvwxyz".ToCharArray()

        Dim keywordIndex, messageIndex As Integer

        For x As Integer = 0 To message.Length - 1

            keywordIndex = Array.IndexOf(letters, keyword(x))

            messageIndex = Array.IndexOf(letters, message(x))

            messageIndex += keywordIndex

            If messageIndex > 26 Then

                messageIndex = messageIndex - 26

            End If

            cipheredText &= letters(messageIndex)

        Next

        Return cipheredText

    End Function

End Module