A question about how to avoid getting an overflow

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VB

Hi.. I have a question.. My teacher gave me an assignment to encrypt files using RSA.. I understand the system very well.. I converted the file to byte[].. And when I do the encryption, I get values larger than the values permitted for the type byte and the program stops.. Is there any way to avoid that?

Sub encrypt()
    For i = 0 To bytes.Length - 1
        bytes(i) = fast(bytes(i), ee)
    Next
End Sub

Function fast(ByVal aa As Object, ByVal zz As Integer) As Integer
    Dim xx As Integer
    xx = 1
    While zz <> 0
        While zz Mod 2 = 0
            zz = zz \ 2
            aa = (aa * aa) Mod n
        End While
        zz = zz - 1
        xx = (xx * aa) Mod n
    End While
    Return xx
End Function

It stops after the fast function ends.. Means after it returns the resulted value...