src/crypto/arc4.c

   1 /*
   2  * The ARC4 stream cipher.
   3  *
   4  * Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
   5  *
   6  * This program is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU General Public License as
   8  * published by the Free Software Foundation; either version 2 of the
   9  * License, or any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful, but
  12  * WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  19  */
  20
  21 FILE_LICENCE ( GPL2_OR_LATER );
  22
  23 #include <gpxe/crypto.h>
  24 #include <gpxe/arc4.h>
  25
  26 #define SWAP( ary, i, j )       \
  27         ({ u8 temp = ary[i]; ary[i] = ary[j]; ary[j] = temp; })
  28
  29 /**
  30  * Set ARC4 key
  31  *
  32  * @v ctxv      ARC4 encryption context
  33  * @v keyv      Key to set
  34  * @v keylen    Length of key
  35  *
  36  * If an initialisation vector is to be used, it should be prepended
  37  * to the key; ARC4 does not implement the @c setiv function because
  38  * there is no standard length for an initialisation vector in the
  39  * cipher.
  40  */
  41 static int arc4_setkey ( void *ctxv, const void *keyv, size_t keylen )
  42 {
  43         struct arc4_ctx *ctx = ctxv;
  44         const u8 *key = keyv;
  45         u8 *S = ctx->state;
  46         int i, j;
  47
  48         for ( i = 0; i < 256; i++ ) {
  49                 S[i] = i;
  50         }
  51
  52         for ( i = j = 0; i < 256; i++ ) {
  53                 j = ( j + S[i] + key[i % keylen] ) & 0xff;
  54                 SWAP ( S, i, j );
  55         }
  56
  57         ctx->i = ctx->j = 0;
  58         return 0;
  59 }
  60
  61 /**
  62  * Perform ARC4 encryption or decryption
  63  *
  64  * @v ctxv      ARC4 encryption context
  65  * @v srcv      Data to encrypt or decrypt
  66  * @v dstv      Location to store encrypted or decrypted data
  67  * @v len       Length of data to operate on
  68  *
  69  * ARC4 is a stream cipher that works by generating a stream of PRNG
  70  * data based on the key, and XOR'ing it with the data to be
  71  * encrypted. Since XOR is symmetric, encryption and decryption in
  72  * ARC4 are the same operation.
  73  *
  74  * If you pass a @c NULL source or destination pointer, @a len
  75  * keystream bytes will be consumed without encrypting any data.
  76  */
  77 static void arc4_xor ( void *ctxv, const void *srcv, void *dstv,
  78                        size_t len )
  79 {
  80         struct arc4_ctx *ctx = ctxv;
  81         const u8 *src = srcv;
  82         u8 *dst = dstv;
  83         u8 *S = ctx->state;
  84         int i = ctx->i, j = ctx->j;
  85
  86         while ( len-- ) {
  87                 i = ( i + 1 ) & 0xff;
  88                 j = ( j + S[i] ) & 0xff;
  89                 SWAP ( S, i, j );
  90                 if ( srcv && dstv )
  91                         *dst++ = *src++ ^ S[(S[i] + S[j]) & 0xff];
  92         }
  93
  94         ctx->i = i;
  95         ctx->j = j;
  96 }
  97
  98 static void arc4_setiv ( void *ctx __unused, const void *iv __unused )
  99 {
 100         /* ARC4 does not use a fixed-length IV */
 101 }
 102
 103
 104 /**
 105  * Perform ARC4 encryption or decryption, skipping initial keystream bytes
 106  *
 107  * @v key       ARC4 encryption key
 108  * @v keylen    Key length
 109  * @v skip      Number of bytes of keystream to skip
 110  * @v src       Message to encrypt or decrypt
 111  * @v msglen    Length of message
 112  * @ret dst     Encrypted or decrypted message
 113  */
 114 void arc4_skip ( const void *key, size_t keylen, size_t skip,
 115                  const void *src, void *dst, size_t msglen )
 116 {
 117         struct arc4_ctx ctx;
 118         arc4_setkey ( &ctx, key, keylen );
 119         arc4_xor ( &ctx, NULL, NULL, skip );
 120         arc4_xor ( &ctx, src, dst, msglen );
 121 }
 122
 123 struct cipher_algorithm arc4_algorithm = {
 124         .name = "ARC4",
 125         .ctxsize = ARC4_CTX_SIZE,
 126         .blocksize = 1,
 127         .setkey = arc4_setkey,
 128         .setiv = arc4_setiv,
 129         .encrypt = arc4_xor,
 130         .decrypt = arc4_xor,
 131 };