/* 
 RC4 Pen & Paper Cipher as propose by Shevek
 at https://crypto.anarres.info/2016/rc4_pencilandpaper
*/

#include <stdlib.h>
#include <stdio.h> 
#include <stdint.h>
#include <string.h>


#define swap(X,Y) { uint8_t T = X; X = Y; Y = T; }



//-----------------------------------------
// 
// rm -f rc4_PP_keystream && gcc -O3 -std=c99 rc4_PP_keystream.c -o rc4_PP_keystream
// 
// Usage:
// ./rc4_PP_keystream key out_size-in-64bit > rc4_PP_keystream_out.bin
//
//
// ./rc4_PP_keystream 47bce5c74f589f4867dbd57e9ca9f808 32768 > rc4_PP_keystream_out.bin
//



//-----------------------------------------
// Main 
//
int main (int argc, char *argv[]) 
{ 
  // initialisation
  char* PrgName;

	uint8_t  a, b, j, k, t, out_8bit;
	uint8_t SBox[256];
	uint64_t output_size, i;

	// Check if a Seed is passed through
 	if (argc < 3) {
 		fprintf(stderr,"\nError: key (hex in format) > 0x0");
 		fprintf(stderr,"\n       and size of output must be byte value >0 and max. 64bit\n\n");
 		return 1;
 	}	
		
 	output_size = (int)strtoul(argv[2], NULL, 0);
 	
 	if (output_size == 0) {
		fprintf(stderr,"\nError: size of output must be byte value >0 and max. 64bit\n\n");
	 	exit(1);
	}

	// We have a key
	uint8_t key[256];
	uint8_t keyLen;
	uint8_t hexKey[256];
	uint8_t hex[3]; 

	// Make sure the arrays get initialised properly
	// and wipe out anything which might be there
	memset(key,    0, 256);
	memset(hexKey, 0, 256);
	memset(hex,    0, 3);

	// Rebuild key[] from KeyHash
	strcpy(hexKey, argv[1]);
	keyLen = strlen(hexKey);
	hexKey[keyLen] = 0;

	keyLen = keyLen/2;
	for (uint8_t i=0; i<keyLen; i++) {
		strncpy(hex, hexKey +(i*2), 2);
		key[i] = strtol(hex, NULL, 16);
	}


	fprintf(stderr, "========= Start Values (rc4 pen and paper 8-bit) ============\n\n");
	fprintf(stderr, "             key = %s\n\n", argv[1]); 
	fprintf(stderr, "  Output size (byte) = %s\n\n", argv[2]); 
	fprintf(stderr, "===================================================\n\n");

		
  //-----------------------------------------
  // Initialisation of rc4
  //
  // perform the Key Schedule
  for (int i=0; i<256; i++) {
    SBox[i] = i;
  }

	k=0; j=0;
	
	for (int i=0; i<256; i++) {
		k = (i % keyLen);
		j = (j + SBox[i] + key[k]);
		swap(SBox[i], SBox[j]);
	}

	a=0; b=0;

  fflush(stdout);

	// Output the keystream
  for (uint64_t i=0; i<output_size; i++) {
  	k = b;
		a = a + 1;
		b = b + SBox[a];
	
		swap(SBox[a], SBox[b]);
	
		t = (SBox[a] + SBox[b] + k);

		out_8bit = SBox[t];
    
    // write keystream to stdout
    fwrite(&out_8bit, sizeof(out_8bit), 1, stdout);
	}

  fflush(stdout);

  return 0;
}