LargePrimeGen - TizzyT

''' Large Prime Generator '''
    Public Function PrimeGen(ByVal length As Integer, ByRef Rand As Random, _
                             Optional ByVal iteration As Integer = 0) As BigInteger
        If length < 2 Then length = 2
        If iteration = 0 Then iteration = Math.Sqrt(length)
        Dim temp, r, y, a(iteration - 1) As BigInteger
        PrimeGen = RandBigInt(length, Rand)
        While True
            If PrimeGen.IsEven Then PrimeGen -= 1
            If PrimeGen Mod 5 = 0 Then PrimeGen -= 2
            If PrimeGen <= 3 Then Return Rand.Next(2, 4)
            For i = 0 To a.Length - 1
                While a(i) <= 2 Or a(i) >= PrimeGen - 2 : a(i) = RandBigInt(length, Rand) : End While
            Next
            r = New BigInteger((PrimeGen - 1).ToByteArray())
            While r Mod 2 = 0 : r = r / 2 : End While
            For i = 0 To iteration - 1
                temp = r
                y = BigInteger.ModPow(a(i), r, PrimeGen)
                While temp <> PrimeGen - 1 AndAlso y <> 1 AndAlso y <> PrimeGen - 1
                    y = BigInteger.ModPow(y, 2, PrimeGen)
                    temp = BigInteger.Multiply(temp, 2)
                End While
                If y <> PrimeGen - 1 AndAlso BigInteger.ModPow(temp, 1, 2) = 0 Then : Exit For
                ElseIf i = iteration - 1 Then : Return PrimeGen : End If
            Next
            PrimeGen -= 2
        End While
    End Function

    Private Function RandBigInt(ByVal BitLength As BigInteger, ByRef Rnd As Random) As BigInteger
        Dim Rmdr, numParts(BigInteger.DivRem(BitLength, 8, Rmdr)) As Byte
        For i = 0 To numParts.Length - 2 : numParts(i) = Rnd.Next(0, 256) : Next
        numParts(numParts.Length - 1) = If(Rmdr = 0, 0, Rnd.Next(0, 2 ^ Rmdr - 1))
        RandBigInt = New BigInteger(numParts)
    End Function