In the past I’ve had to recover passwords from certain applications that customers don’t have or have simply forgotten.
Here is a simple class I’ve written to generate key combinations used in brute force password cracking.

Public Class cComboGenerator
    Dim Positions() As Byte
    Dim MaxLength As Integer = 10

    Dim bCharSet() As Byte
    Dim bCombo() As Byte

    Public Sub New()
        Dim CharSet As String = " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
        bCharSet = System.Text.Encoding.Default.GetBytes(CharSet)

        ReDim Positions(MaxLength)
        ReDim bCombo(MaxLength)
    End Sub

    Function NextCombo(Optional Offset As Integer = 1) As String

                If (Positions(MaxLength) + Offset) > bCharSet.Length - 1 Then
                    Positions(MaxLength) = (Positions(MaxLength) + Offset) Mod bCharSet.Length

                    For x = (MaxLength - 1) To 0 Step -1
                        If Positions(x) >= bCharSet.Length - 1 Then
                            Positions(x) = 1
                        Else
                            Positions(x) = Positions(x) + 1
                            Exit For
                        End If
                    Next

                Else
                    Positions(MaxLength) = Positions(MaxLength) + Offset
                End If

                For x = 0 To MaxLength
                    bCombo(x) = bCharSet(Positions(x))
                Next

                Return System.Text.Encoding.Default.GetString(bCombo).Replace(vbNullChar, String.Empty).Trim 'Dirty Hack
    End Function
End Class

There are some optimisations that can be made, but chances are the bottleneck will be the encryption routine or waiting for a response and not the combination generation itself.