blumshub.cpp

// blumshub.cpp - originally written and placed in the public domain by Wei Dai
 
#include "pch.h"
#include "blumshub.h"
#include "integer.h"
 
NAMESPACE_BEGIN(CryptoPP)
 
PublicBlumBlumShub::PublicBlumBlumShub(const Integer &n, const Integer &seed)
    : modn(n),
      current(modn.Square(modn.Square(seed))),
      maxBits(BitPrecision(n.BitCount())-1),
      bitsLeft(maxBits)
{
}
 
unsigned int PublicBlumBlumShub::GenerateBit()
{
    if (bitsLeft==0)
    {
        current = modn.Square(current);
        bitsLeft = maxBits;
    }
 
    return static_cast<unsigned int>(current.GetBit(--bitsLeft));
}
 
byte PublicBlumBlumShub::GenerateByte()
{
    byte b=0;
    for (int i=0; i<8; i++)
        b = byte((b << 1) | PublicBlumBlumShub::GenerateBit());
    return b;
}
 
void PublicBlumBlumShub::GenerateBlock(byte *output, size_t size)
{
    while (size--)
        *output++ = PublicBlumBlumShub::GenerateByte();
}
 
void PublicBlumBlumShub::ProcessData(byte *outString, const byte *inString, size_t length)
{
    while (length--)
        *outString++ = *inString++ ^ PublicBlumBlumShub::GenerateByte();
}
 
BlumBlumShub::BlumBlumShub(const Integer &p, const Integer &q, const Integer &seed)
    : PublicBlumBlumShub(p*q, seed),
      p(p), q(q),
      x0(modn.Square(seed))
{
}
 
void BlumBlumShub::Seek(lword index)
{
    Integer i(Integer::POSITIVE, index);
    i *= 8;
    Integer e = a_exp_b_mod_c (2, i / maxBits + 1, (p-1)*(q-1));
    current = modn.Exponentiate(x0, e);
    bitsLeft = maxBits - i % maxBits;
}
 
NAMESPACE_END