[resolved]String Encrypter help

[qazwsx]

I am making a string encrypter. This is what i have so far:

    Public Function stringEncrypt(ByVal input As String, ByVal key As String, Optional ByVal encrypt As Boolean = True, Optional ByVal passes As Integer = 1)
        Dim inputNumber(input.Length - 1) As Integer
        Dim keyNumber(key.Length - 1) As Integer
        Dim outputNumber(input.Length - 1) As Integer
        Dim output As String = ""
        Dim letters() As Char = {"a", "b", "c", "d", "e", "f", "g", "h", _
                                "i", "j", "k", "l", "m", "n", "o", "p", _
                                "q", "r", "s", "t", "u", "v", "w", "x", _
                                "y", "z", " ", ".", ",", "!", ""} 'must have extra char
        Dim keyCount As Integer = 0
        Dim i As Integer, j As Integer

        For i = 0 To input.Length - 1 'converts input to number representation
            For j = 0 To letters.Length - 1
                If input.ElementAt(i) = letters(j) Then 'see below
                    inputNumber(i) = j
                    Continue For
                End If
                If j = letters.Length - 1 Then MsgBox(input.Length)
            Next
        Next

        For i = 0 To key.Length - 1 'converts key to number representation
            For j = 0 To letters.Length - 1
                If key.ElementAt(i) = letters(j) Then 'if equal replace letter with number
                    keyNumber(i) = j
                    Continue For
                End If
            Next
        Next

        Select Case encrypt
            Case True 'encrypt

                ' add keys together, if over max must subract
                For i = 0 To inputNumber.Length - 1
                    outputNumber(i) = inputNumber(i) + keyNumber(keyCount)

                    If outputNumber(i) > (letters.Length - 1) Then outputNumber(i) -= (letters.Length - 1)
                    keyCount += 1
                    If keyCount > (keyNumber.Length - 1) Then keyCount = 0
                Next


            Case False 'decrypt
                'subtract keys

                For i = 0 To inputNumber.Length - 1
                    outputNumber(i) = inputNumber(i) - keyNumber(keyCount)

                    If outputNumber(i) < 0 Then outputNumber(i) += (letters.Length - 1)

                    keyCount += 1
                    If keyCount > keyNumber.Length - 1 Then keyCount = 0
                Next
        End Select

        'convert back to letters
        For i = 0 To outputNumber.Length - 1
            output = output & letters(outputNumber(i))
        Next

        Dim rand As New Random(keyNumber(0))

        Return output
    End Function

I was wondering is there a much easier way to do this, and is that a fairly safe encryption method?

Also the above code only works sometimes, does anyone have an idea why. The encryptes strings are often cut short, and the last character in the array is replaced with the first when decrypting.