#!/usr/bin/perl -0777  --

# $0 key infile > outfile 
# symmetric works both ways 

@k = unpack( 'C*', pack( 'H*', shift ) );

for ( @t = @s = 0 .. 255 ) {
    $y = ( $k[ $_ % @k ] + $s[ $x = $_ ] + $y ) % 256;
    &S;
}

$x = $y = 0;

for ( unpack( 'C*', <> ) ) {
    $x++;
    $y = ( $s[ $x %= 256 ] + $y ) % 256;
    &S;
    print pack( C, $_ ^= $s[ ( $s[$x] + $s[$y] ) % 256 ] );
}

sub S { @s[ $x, $y ] = @s[ $y, $x ] }
__END__
<code>
<p>
<code>
/*  gcc -o rc4 rc4.c  */
#include <stdio.h> 

#define buf_size 1024 

typedef struct rc4_key
{
   unsigned char state[256];
   unsigned char x;
   unsigned char y;
} rc4_key;

#define swap_byte(x,y) t = *(x); *(x) = *(y); *(y) = t 

void prepare_key(unsigned char *key_data_ptr, int key_data_len, rc4_ke
+y *key)
{
  int i;
  unsigned char t;
  unsigned char swapByte;
  unsigned char index1;
  unsigned char index2;
  unsigned char* state;
  short counter;

  state = &key->state[0];
  for(counter = 0; counter < 256; counter++)
  state[counter] = counter;
  key->x = 0;
  key->y = 0;
  index1 = 0;
  index2 = 0;
  for(counter = 0; counter < 256; counter++)
  {
    index2 = (key_data_ptr[index1] + state[counter] + index2) % 256;
    swap_byte(&state[counter], &state[index2]);
    index1 = (index1 + 1) % key_data_len;
  }
}
void rc4(unsigned char *buffer_ptr, int buffer_len, rc4_key *key)
{
  unsigned char t;
  unsigned char x;
  unsigned char y;
  unsigned char* state;
  unsigned char xorIndex;
  short counter;

  x = key->x;
  y = key->y;
  state = &key->state[0];
  for(counter = 0; counter < buffer_len; counter++)
  {
    x = (x + 1) % 256;
    y = (state[x] + y) % 256;
    swap_byte(&state[x], &state[y]);
    xorIndex = (state[x] + state[y]) % 256;
    buffer_ptr[counter] ^= state[xorIndex];
  }
  key->x = x;
  key->y = y;
}

int main(int argc, char* argv[])
{
  char seed[256];
  char data[512];
  char buf[buf_size];
  char digit[5];
  int hex, rd,i;
  int n;
  rc4_key key;

  if (argc < 2)
  {
    fprintf(stderr,"%s key <in >out\n",argv[0]);
    exit(1);
  }
  strcpy(data,argv[1]);
  n = strlen(data);
  if (n&1)
  {
    strcat(data,"0");
    n++;
  }
  n/=2;
  strcpy(digit,"AA");
  for (i=0;i<n;i++)
  {
    digit[2] = data[i*2];
    digit[3] = data[i*2+1];
    sscanf(digit,"%x",&hex);
    seed[i] = hex;
  }

  prepare_key(seed,n,&key);
  rd = fread(buf,1,buf_size,stdin);
  while (rd>0)
  {
    rc4(buf,rd,&key);
    fwrite(buf,1,rd,stdout);
    rd = fread(buf,1,buf_size,stdin);
  }
}
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