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/*
* The ARC4 stream cipher.
*
* Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
FILE_LICENCE ( GPL2_OR_LATER );
#include <gpxe/crypto.h>
#include <gpxe/arc4.h>
#define SWAP( ary, i, j ) \
({ u8 temp = ary[i]; ary[i] = ary[j]; ary[j] = temp; })
/**
* Set ARC4 key
*
* @v ctxv ARC4 encryption context
* @v keyv Key to set
* @v keylen Length of key
*
* If an initialisation vector is to be used, it should be prepended
* to the key; ARC4 does not implement the @c setiv function because
* there is no standard length for an initialisation vector in the
* cipher.
*/
static int arc4_setkey ( void *ctxv, const void *keyv, size_t keylen )
{
struct arc4_ctx *ctx = ctxv;
const u8 *key = keyv;
u8 *S = ctx->state;
int i, j;
for ( i = 0; i < 256; i++ ) {
S[i] = i;
}
for ( i = j = 0; i < 256; i++ ) {
j = ( j + S[i] + key[i % keylen] ) & 0xff;
SWAP ( S, i, j );
}
ctx->i = ctx->j = 0;
return 0;
}
/**
* Perform ARC4 encryption or decryption
*
* @v ctxv ARC4 encryption context
* @v srcv Data to encrypt or decrypt
* @v dstv Location to store encrypted or decrypted data
* @v len Length of data to operate on
*
* ARC4 is a stream cipher that works by generating a stream of PRNG
* data based on the key, and XOR'ing it with the data to be
* encrypted. Since XOR is symmetric, encryption and decryption in
* ARC4 are the same operation.
*
* If you pass a @c NULL source or destination pointer, @a len
* keystream bytes will be consumed without encrypting any data.
*/
static void arc4_xor ( void *ctxv, const void *srcv, void *dstv,
size_t len )
{
struct arc4_ctx *ctx = ctxv;
const u8 *src = srcv;
u8 *dst = dstv;
u8 *S = ctx->state;
int i = ctx->i, j = ctx->j;
while ( len-- ) {
i = ( i + 1 ) & 0xff;
j = ( j + S[i] ) & 0xff;
SWAP ( S, i, j );
if ( srcv && dstv )
*dst++ = *src++ ^ S[(S[i] + S[j]) & 0xff];
}
ctx->i = i;
ctx->j = j;
}
static void arc4_setiv ( void *ctx __unused, const void *iv __unused )
{
/* ARC4 does not use a fixed-length IV */
}
/**
* Perform ARC4 encryption or decryption, skipping initial keystream bytes
*
* @v key ARC4 encryption key
* @v keylen Key length
* @v skip Number of bytes of keystream to skip
* @v src Message to encrypt or decrypt
* @v msglen Length of message
* @ret dst Encrypted or decrypted message
*/
void arc4_skip ( const void *key, size_t keylen, size_t skip,
const void *src, void *dst, size_t msglen )
{
struct arc4_ctx ctx;
arc4_setkey ( &ctx, key, keylen );
arc4_xor ( &ctx, NULL, NULL, skip );
arc4_xor ( &ctx, src, dst, msglen );
}
struct cipher_algorithm arc4_algorithm = {
.name = "ARC4",
.ctxsize = ARC4_CTX_SIZE,
.blocksize = 1,
.setkey = arc4_setkey,
.setiv = arc4_setiv,
.encrypt = arc4_xor,
.decrypt = arc4_xor,
};
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