Building a Caesar cipher (a letter-shifting cipher)

This lesson covers implementing a Caesar cipher in VB.NET, with the aim of understanding how shifting letters encrypts text.

The Caesar cipher replaces each letter with another a fixed number of places along the alphabet — about as simple as a cipher gets. It is a common introductory subject for practising character-code manipulation.

The sample code converts a character to its code with Asc(c), shifts it by shift, keeps it inside the alphabet with Mod 26, and converts back with Chr(). Note Char.IsUpper(c), which checks for an upper-case letter.

A common early stumble is the case where the shifted result runs past the end of the alphabet (A–Z). Forget the Mod 26 that wraps it back into 26 letters and you get unintended symbols.

The Caesar cipher is far too simple for real security, but manipulating character codes is the basis of more advanced encryption and encoding work.

📖 Reference code

Function CaesarEncrypt(text As String, shift As Integer) As String
    Dim result As New System.Text.StringBuilder
    For Each c As Char In text
        If Char.IsUpper(c) Then
            Dim code As Integer = (Asc(c) - 65 + shift) Mod 26 + 65
            result.Append(Chr(code))
        Else
            result.Append(c)
        End If
    Next
    Return result.ToString()
End Function

Console.WriteLine(CaesarEncrypt("HELLO", 3))

✍️ Your code

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🧪 This site can't compile or run VB.NET directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).