Vigenere Cipher: Help to show output in ONLY letters

Hello everybody. I'm making project for school and it includes a Vigenere Encipher. There are 3 textboxes. Enter the keyword in Textbox 1, Enter message in Textbox 2, Press button, Output displays in Textbox 3. The following code does the job.

Code:

---

Public Shared Function VIGEncrypt(ByVal cipherTxt As String, ByVal key As String)

        Dim encryptedText As String = ""
        For i As Integer = 1 To cipherTxt.Length
            Dim temp As Integer = Asc(GetChar(cipherTxt, i)) + Asc(GetChar(key, i Mod key.Length + 1))
            encryptedText += Chr(temp)
        Next
        Return encryptedText

    End Function

---

But, this code shows the output in ASCII beyond the letter Z such as / * & ^. How can this code be modified to only show characters between a - z & A - Z. If I didn't explain it well enough, please let me know. I also don't know whether the solution is simple, but any help would be greatly appreciated. Thanks in advance!

After you get the value for temp, you need to check it to see if it has exceeded the boundaries you've set... if so, then you need to decide what to do... does the value wrap back around so that it goes ...XYZabcd... or do you simply truncate the value at the specified ceiling (which may present a problem at decryption time)... or something else...

-tg

You should validate the data before you do the conversion. For example, you should check if the input in your first TextBox contains any non-letters and display a message if it does. While I cannot provide you with a Windows Form Application example, I can provide you with a Console Application equivalent.

Fiddle: https://dotnetfiddle.net/DFmevu

My guess is that even with pre-processed values all being alphas.... the end resulting cipher is what's containing the special chars....
Given that's he's getting the ASCII values and adding to them:
Asc(GetChar(cipherTxt, i)) + Asc(GetChar(key, i Mod key.Length + 1))

It's not too difficult to see that letters in the latter part of the alphabet get manipulated into a special char. The OP wants to prevent that, but doesn't say what they want it to do instead ... but your point is valid too because if the original ASCII value is already outside the bounds...

-tg

I only did a quick Google search of Vigenere Cipher. I found that you have a message followed by a keyword where the keyword will repeat its characters, in order, until it matches the length of the message. Then each letter in the keyword represents how many letters to move the keyword forward. So if I have a message of hello and the keyword is ab then the keyword get's transformed to ababa and the message get's transformed to:

h: h + a(0) = h
e: e + b(1) = f
l: l + a(0) = l
l: l + b(1) = m
o: o + a(0) = o

So the ciphered message results in hflmo. So non-letter characters do not make sense, at least to me in the short time I studied it, that is why I would not allow for non-letter characters.