Please, poke holes in my cryptographic function...

Saul Johnson

Hello, I'm a complete newbie to the field of cryptographic algorithms themselves, always having relied on third-party libraries and code in the past. Now I'm starting to poke around with them by myself I finally managed to throw together some sort of tiny cryptographic library in VB.NET. However, I'm concerned that... quite frankly... it's a bit rubbish. Sorry about the long code block here:

Imports System.Numerics

Public Class BlumBlumShub

Private \_x As BigInteger
Private \_p As BigInteger
Private \_q As BigInteger
Private \_m As BigInteger

Private \_pow As New BigInteger(2)

Public Function NextNumber() As BigInteger
    \_x = BigInteger.ModPow(\_x, \_pow, \_m)
    Return \_x
End Function

Public Sub New(ByVal seed As BigInteger)
    \_p = BigInteger.Parse("32416190071")
    \_q = BigInteger.Parse("32416185031")
    \_x = seed
    \_m = BigInteger.Multiply(\_p, \_q)
End Sub

End Class

Public Class RandomBitStream

Private \_b As BlumBlumShub

Public Function ReadByte() As Byte
    Dim num As Byte = 0
    For i = 1 To 8
        num += Math.Pow(i, 2) \* If(\_b.NextNumber().IsEven, 1, 0)
    Next
    Return num
End Function

Public Sub New(ByVal seed As BigInteger)
    \_b = New BlumBlumShub(seed)
End Sub

End Class

Public Class BlumXor

Private \_bitSrc As RandomBitStream
Private \_key As BigInteger

Public Sub Cipher(ByRef message As Byte())
    \_bitSrc = New RandomBitStream(\_key)
    For i = 0 To message.Length - 1
        message(i) = \_bitSrc.ReadByte() Xor message(i)
    Next
End Sub

Public Function ByteToStr(ByVal inByte As Byte) As String
    Return inByte.ToString().PadLeft(3, "0")
End Function

Public Sub New(ByVal keyStr As String)
    Dim encoder As New System.Text.ASCIIEncoding()
    Dim keyBytes As Byte() = encoder.GetBytes(keyStr)
    \_key = BigInteger.Parse(String.Join("", Array.ConvertAll(Of Byte, String)(keyBytes, New System.Converter(Of Byte, String)(AddressOf ByteToStr))))
End Sub

End Class

And that is that. Firstly, I think my implementation of the BlumBlumShub PRNG is off, secondly I'm not entirely sure that I should be using the parity of numbers generated to give me random bits and thirdly I'm not so sure about my use of Xor or how I'm generating a seed integer for the PRNG from a string. I welcome the input and criticism of any cryptographers or ma