blumshub.cpp

// blumshub.cpp - written and placed in the public domain by Wei Dai

#include "pch.h"
#include "blumshub.h"

NAMESPACE_BEGIN(CryptoPP)

PublicBlumBlumShub::PublicBlumBlumShub(const Integer &n, const Integer &seed)
        : modn(n),
          maxBits(BitPrecision(n.BitCount())-1)
{
        current = modn.Square(modn.Square(seed));
        bitsLeft = maxBits;
}

unsigned int PublicBlumBlumShub::GenerateBit()
{
        if (bitsLeft==0)
        {
                current = modn.Square(current);
                bitsLeft = maxBits;
        }

        return current.GetBit(--bitsLeft);
}

byte PublicBlumBlumShub::GenerateByte()
{
        byte b=0;
        for (int i=0; i<8; i++)
                b = (b << 1) | PublicBlumBlumShub::GenerateBit();
        return b;
}

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

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