/*

 *  Jail Cell RC4 (Rivest Cipher #4)

 *  Source by Jeremy Lennert

 *  jcrc4.cpp

 *  Wednesday, February 20, 2002

 *

 *  Implements a modified version of the RC4 cipher.

 *

 *  Usage: encrypt with "jcrc4 -e", decrypt with "jcrc4 -d".

 *  Reads stdin, writes stdout.

 *

 *  Requires a key stored in the file key.txt in

 *  the current directory.

 */



/*

 *  CIPHER OPERATION

 *  

 *  Key Schedule:

 *

 *  A permutation table of the numeric values 0-36 is

 *  constructed using a key consisting of 13 letters, numbers,

 *  and periods.  They key values are converted to values in

 *  the range 0-36 with 0=0, 1=1,...A=10, B=11, C=12,...period=36.

 *

 *  The first value in the key (K[0]) becomes the first value inserted

 *  into the permutation table, and it is inserted in the location

 *  corresponding to the second value in the key (K[1]).  The second

 *  value inserted into the permutation table is K[0]+1 (mod 37), and

 *  is inserted into the position of the previous insertion, offset

 *  by the next value of the key (K[2]).  Insertion continues in this

 *  manner (reusing values of the key as necessary) until all 37 values

 *  have been inserted.  If the table position targeted for insertion is

 *  already occupied, the insertion position is offset again by the

 *  same value of the key.

 *

 *

 *  Keystream Generation:

 *

 *  Two counters, i and j, are both initialized to zero.

 *

 *  To output one keystream value (which is in the range 0-36),

 *	i = i + 1 mod 37

 *	j = j + S[i] mod 37

 *	swap the values in S[i] and S[j]

 *	z = S[i] + S[j] mod 37

 *	output S[j]

 *  Where S[x] is the xth value in the permutation table.

 *

 *  Each letter of the plaintext is then shifted by a different

 *  keystream output to encrypt or reverse-shifted to decrypt.

 *

 *  For security reasons, the first 11 keystream outputs are

 *  not used, because they may not be influenced by every

 *  part of the key.

 */



#include <stdio.h>

#include <fstream.h>



int main(int argc, char **argv) {

    FILE* kin;	    // Key in

    int key[13];    // Translated key

    int s[37];	    // Permutation S-Box used for generated keysteam

    int i, j;	    // i and j counters for generating keystream

    unsigned char c;

    int ic;

    int x, z;

    int encrypt;



    if (argc == 2 && !strcmp(argv[1], "-e"))

      encrypt = 1;

    else if (argc == 2 && !strcmp(argv[1], "-d"))

      encrypt = 0;

    else {

      fprintf(stderr, "usage: jcrc4 [-e|-d]\n");

      exit(1);

    }

    /*

     *	Get the key from key.txt

     *	Assume that the key is 13 characters long

     */

    kin = fopen("key.txt", "rb");  // Read untranslated text

    for (x = 0; x < 13; x++) {

	if ( (ic = getc(kin)) == EOF) {

	    fprintf(stderr, "illegal argument: key too short\n");

	    exit(1);

	}

	c = ic;

	if ((c > '0') && (c <= '9')) {

	    key[x] = c - '0';	    // Convert char digits to integers

	}

	else if ((c >= 'A') && (c <= 'Z')) {

	    key[x] = c - 'A' + 10;  // A=10, B=11, C=12, etc.

	}

	else if ((c >= 'a') && (c <= 'z')) {

	    key[x] = c - 'a' + 10;  // a=10, b=11, c=12, etc.

	}

	else if (c == '.') {

	    key[x] = 36;	    // period = 36

	}

	else {

	  fprintf(stderr, "illegal key: invalid key character %c\n", c);

	  exit(1);

	}

    }



    /*

     *	Initialize S-Box using key

     */

    for (x = 0; x < 37; x++) {

	s[x] = -1;	// Slot has not been filled yet

    }

    i = 1;

    j = key[0];

    x = 0;

    while (true) {

	do {

	    x = (x + key[i]) % 37;

	} while (s[x] != -1);	// Until s[x] is empty

	s[x] = j;



	i++;

	if (i > 12) i = 0;

	j++;

	if (j > 36) j = 0;

	if (j == key[0]) break;	// All values (0-36) assigned

    }



    /*

     *	Warm up keystream generator

     *	The first length-2 = 11 outputs need to be

     *	thrown away to ensure all parts of the key

     *	affect all keystream outputs used in encryption

     */

    i = j = 0;

    for (x = 0; x < 11; x++) {

	i++;

	j = (j + s[i]) % 37;



	z = s[i];

	s[i] = s[j];

	s[j] = z;

    }



    /*

     *	Encrypt Message

     */

    while ((ic = getc(stdin)) != EOF) {	// Until end of file

	if ((ic >= '0') && (ic <= '9')) {

	    ic = ic - '0';	// Convert char digits to integers

	}

	else if ((ic >= 'A') && (ic <= 'Z')) {

	    ic = ic - 'A' + 10;  // A=10, B=11, C=12, etc.

	}

	else if ((ic >= 'a') && (ic <= 'z')) {

	    ic = ic - 'a' + 10;  // a=10, b=11, c=12, etc.

	}

	else if (ic == '.') {

	    ic = 36;    // period = 36

	}

	else continue;  // Invalid character for encryption; drop it from message



	i++;

	j = (j + s[i]) % 37;



	z = s[i];

	s[i] = s[j];

	s[j] = z;



	z = (s[i] + s[j]) % 37;



	if (encrypt)

	    ic = (ic + s[z]) % 37;

	else

	    ic = (ic - s[z] + 37) % 37;

	if (ic <= 9) c = ic + '0';		// Numeric Dig