Mods and ASCII in VB Caesar Shift - vb.net

Question

I have this code which shifts the alphabet by a certain amount. The size of the alphabet is 26. When I enter a larger size shift (for example 22) I get some weird characters displaying. I think I need to mod the ASCII alphabet to 26 to get it working but Im not quite sure which bit to mod.
Basically I need to wrap around the alphabet (once it reaches Z it goes back to letter A) Do I have to create a dictionary for the mod to work (like A = 0... Z = 26) or can I stick with using the normal ASCII table? Here is the code below:
Public Function encrypt(ByVal input As String) 'input is a variable within the funcion
Dim n as Integer
Dim i As Integer
n = key.Text Mod 26 'gets what is in the text box of 'key' and sets it as n
' the key is a multiple of 26 so 26 will = 0
'need to remove white spaces
While input.Contains(" ") 'when the input text contains a space
input = input.Replace(" ", "") 'replaces it with no space.
End While
For i = 1 To Len(input) 'find the length of the input
Mid(input, i, 1) = Chr(Asc(Mid(input, i, 1)) + n) 'chr returns the character associated with the specified character code
'
Next
encrypt = input
End Function

Answer 1

Look at this code:
For i = 1 To Len(input) 'find the length of the input
Mid(input, i, 1) = Chr(Asc(Mid(input, i, 1)) + n) 'chr returns the character associated with the specified character code
'
Next
String indexes are 0-based. Your first index is 0, not 1! Also, you are assigning to the result of a function call. You need to instead construct a new string.
You didn't say, but the way you used the Replace and Contains methods indicates .Net, and if that's the case, I would do it like this:
Public Function encrypt(ByVal key As Integer, ByVal input As String) As String 'Don't forget the return type on the function
key = key Mod 26
Return New String(input.Replace(" ", "").ToUpper().Select(Function(c) Chr(((Asc(c) + key - Asc("A"c)) Mod 26) + Asc("A"c))).ToArray())
End Function
Just like that, and it's almost a one-liner. I can see this works now by calling it this way:
Encrypt("C"c, "the quick brown fox jumps over the lazy dog")
Encrypt("D"c, "the quick brown fox jumps over the lazy dog")
The results:
BPMYCQKSJZWEVNWFRCUXMLWDMZBPMTIHGLWOA
CQNZDRLTKAXFWOXGSDVYNMXENACQNUJIHMXPB
Look for the results mapped for the word "lazy", and you will see that the 'a' wraps to 'z' and 'y' correctly, and that the 'D' key results are one letter off of the 'C' results.