linux/mppe/arcfour.c

   1 /*
   2  * arcfour.c
   3  * by Frank Cusack <frank@google.com>
   4  * 100% public domain
   5  *
   6  * Implemented from the description in _Applied Cryptography_, 2nd ed.
   7  *
   8  * ** Distribution ** of this software is unlimited and unrestricted.
   9  *
  10  * ** Use ** of this software is almost certainly legal; however, refer
  11  * to <http://theory.lcs.mit.edu/~rivest/faq.html>.
  12  */
  13
  14 #include "arcfour.h"
  15 #if defined(__linux__)
  16 #include <linux/string.h>
  17 #endif
  18
  19 #define swap(a, b)              \
  20 {                               \
  21     unsigned char t = b;        \
  22     b = a;                      \
  23     a = t;                      \
  24 }
  25
  26 /*
  27  * Initialize arcfour from a key.
  28  */
  29 void
  30 arcfour_setkey(arcfour_context *context, const unsigned char *key,
  31                unsigned keylen)
  32 {
  33     unsigned i, j;
  34     unsigned char K[256];
  35
  36     context->i = context->j = 0;
  37
  38     for (i = 0; i < 256; i++) {
  39         context->S[i] = i;
  40         K[i] = key[i % keylen];
  41     }
  42
  43     j = 0;
  44     for (i = 0; i < 256; i++) {
  45         j = (j + context->S[i] + K[i]) % 256;
  46         swap(context->S[i], context->S[j]);
  47     }
  48
  49     memset(K, 0, sizeof(K));
  50 }
  51
  52 /*
  53  * plaintext -> ciphertext (or vice versa)
  54  */
  55 void
  56 arcfour_encrypt(arcfour_context *context, const unsigned char *in, unsigned len,
  57                 unsigned char *out)
  58 {
  59     unsigned i = context->i;
  60     unsigned j = context->j;
  61     unsigned char *S = context->S;
  62     unsigned char K;
  63
  64     while (len--) {
  65         i = (i + 1) % 256;
  66         j = (j + S[i]) % 256;
  67         swap(S[i], S[j]);
  68         K = S[(S[i] + S[j]) % 256];
  69         *out++ = *in++ ^ K;
  70     }
  71
  72     context->i = i;
  73     context->j = j;
  74 }
  75