''' <summary>
    ''' 字符串加密
    ''' </summary>
    ''' <param name="s">要加密的字符串</param>
    ''' <param name="n">加密移n位，正为右移，负为左移</param>
    ''' <returns>加密后的字符串</returns>
    Function Encryption(s As String, n As Integer) As String
        Dim c As String
        Dim strL, i As Integer
        '’处理n移位控制在 -25到25之间
        n = n Mod 26
        '’当n为负时处理成对应的正值 如-4，处理成 22
        If n < 0 Then n += 26
        '’初始值
        Encryption = ""
        '’原字符串的长度
        strL = s.Length
        i = 1
        Do While i <= strL
            '’取第i个位置的字符
            c = Mid(s, i, 1)

            ''如果c在 大写字符范围，并且右移n位超出大写字符时
            If (c >= "A" AndAlso c <= "Z") AndAlso Asc(c) + n > Asc("Z") Then
                c = Chr(Asc(c) + n - 26)
            ElseIf (c >= "A" AndAlso c <= "Z") Then ''在大写范围内时，并且右移n位不超出大写字符时
                c = Chr(Asc(c) + n)
            End If

            If (c >= "a" AndAlso c <= "z") AndAlso Asc(c) + n > Asc("z") Then
                c = Chr(Asc(c) + n - 26)
            ElseIf (c >= "a" AndAlso c <= "z") Then
                c = Chr(Asc(c) + n)
            End If

            Encryption &= c
            i += 1
        Loop
    End Function

 Sub Main()

        Dim s As String = "world" ''原内容
        Dim s1 As String = Encryption(s, 4) ''加密后的内容
        Dim s2 As String = Encryption(s1, -4) '’解密后的内容

        Console.WriteLine(s) '’显示原内容
        Console.WriteLine(s1) '’显示加密后的内容
        Console.WriteLine(s2) '’显示解密后的内容
        Console.Read()

 End Sub

world
asvph
world