arc4.cpp

// arc4.cpp - written and placed in the public domain by Wei Dai

// The ARC4 algorithm was first revealed in an anonymous email to the
// cypherpunks mailing list. This file originally contained some
// code copied from this email. The code has since been rewritten in order
// to clarify the copyright status of this file. It should now be
// completely in the public domain.

#include "pch.h"
#include "arc4.h"

NAMESPACE_BEGIN(CryptoPP)

void ARC4_TestInstantiations()
{
        ARC4 x;
}

ARC4_Base::~ARC4_Base()
{
        m_x = m_y = 0;
}

void ARC4_Base::UncheckedSetKey(const byte *key, unsigned int keyLen, const NameValuePairs &params)
{
        AssertValidKeyLength(keyLen);

        m_x = 1;
        m_y = 0;

        unsigned int i;
        for (i=0; i<256; i++)
                m_state[i] = i;

        unsigned int keyIndex = 0, stateIndex = 0;
        for (i=0; i<256; i++)
        {
                unsigned int a = m_state[i];
                stateIndex += key[keyIndex] + a;
                stateIndex &= 0xff;
                m_state[i] = m_state[stateIndex];
                m_state[stateIndex] = a;
                if (++keyIndex >= keyLen)
                        keyIndex = 0;
        }

        int discardBytes = params.GetIntValueWithDefault("DiscardBytes", GetDefaultDiscardBytes());
        DiscardBytes(discardBytes);
}

template <class T>
static inline unsigned int MakeByte(T &x, T &y, byte *s)
{
        unsigned int a = s[x];
        y = (y+a) & 0xff;
        unsigned int b = s[y];
        s[x] = b;
        s[y] = a;
        x = (x+1) & 0xff;
        return s[(a+b) & 0xff];
}

byte ARC4_Base::GenerateByte()
{
        return MakeByte(m_x, m_y, m_state);
}

void ARC4_Base::ProcessData(byte *outString, const byte *inString, size_t length)
{
        if (length == 0)
                return;

        byte *const s = m_state;
        unsigned int x = m_x;
        unsigned int y = m_y;

        if (inString == outString)
        {
                do
                {
                        *outString++ ^= MakeByte(x, y, s);
                } while (--length);
        }
        else
        {
                do
                {
                        *outString++ = *inString++ ^ MakeByte(x, y, s);
                }
                while(--length);
        }

        m_x = x;
        m_y = y;
}

void ARC4_Base::DiscardBytes(size_t length)
{
        if (length == 0)
                return;

        byte *const s = m_state;
        unsigned int x = m_x;
        unsigned int y = m_y;

        do
        {
                MakeByte(x, y, s);
        }
        while(--length);

        m_x = x;
        m_y = y;
}

NAMESPACE_END

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