1 /* 2 * The ARC4 stream cipher. 3 * 4 * Copyright (c) 2009 Joshua Oreman <oremanj (at) 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 }; 132
