arc4.cpp

// arc4.cpp - originally 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"
#define CRYPTOPP_ENABLE_NAMESPACE_WEAK 1
#include "arc4.h"

NAMESPACE_BEGIN(CryptoPP)
namespace Weak1 {

#if defined(CRYPTOPP_DEBUG) && !defined(CRYPTOPP_DOXYGEN_PROCESSING)
void ARC4_TestInstantiations()
{
	ARC4 x;
}
#endif

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

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

	m_x = 1;
	m_y = 0;

	unsigned int i;
	for (i=0; i<256; i++)
		m_state[i] = byte(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] = byte(a);
		if (++keyIndex >= length)
			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 = byte((y+a) & 0xff);
	unsigned int b = s[y];
	s[x] = byte(b);
	s[y] = byte(a);
	x = byte((x+1) & 0xff);
	return s[(a+b) & 0xff];
}

void ARC4_Base::GenerateBlock(byte *output, size_t size)
{
	while (size--)
		*output++ = static_cast<byte>(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++ ^ byte(MakeByte(x, y, s));
		}
		while(--length);
	}

	m_x = byte(x);
	m_y = byte(y);
}

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

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

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

	m_x = byte(x);
	m_y = byte(y);
}

}
NAMESPACE_END