Miller Rabin Primality Test? - TizzyT

''' Miller Rabin Primality Test '''
    Public Function MRtest(N As BigInteger, iterations As Integer) As Boolean
        If N < 2 then return false
        Dim a, y, t As BigInteger, r As New BigInteger((N - 1).ToByteArray())
        If N = 2 OrElse N = 3 Then Return True
        If N Mod 2 = 0 Then Return False
        While r Mod 2 = 0 : r = r / 2 : End While
        For i = 0 To iterations - 1
            t = r
            Do : a = RandBigInt(BitLen(N), New Random) : Loop While a < 2 OrElse a > N - 2
            y = BigInteger.ModPow(a, r, N)
            While t <> N - 1 AndAlso y <> 1 AndAlso y <> N - 1
                y = BigInteger.ModPow(y, 2, N)
                t = BigInteger.Multiply(t, 2)
            End While
            If y <> N - 1 AndAlso BigInteger.ModPow(t, 1, 2) = 0 Then Return False
        Next
        Return True
    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

    Private Function BitLen(ByVal TestingNbr As BigInteger) As BigInteger
        Dim Nbr As String = TestingNbr.ToString("X")
        BitLen = Convert.ToString(Convert.ToInt32(Nbr(0), 16), 2).TrimStart("0").Length
        BitLen = BigInteger.Add(BitLen, BigInteger.Multiply(New BigInteger(4), New BigInteger(Nbr.Length - 1)))
    End Function