├── .gitignore
├── Makefile
├── README
├── rc4.c
├── rc4.h
└── test.c


/.gitignore:
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1 | *~
2 | *.c~
3 | *.h~
4 | .*~
5 | *.o
6 | 


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/Makefile:
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 1 | OBJS=rc4.o test.o
 2 | CC=gcc
 3 | CFLAGS=-O -g
 4 | 
 5 | test:$(OBJS)
 6 | 	$(CC) $(OBJS) -o test
 7 | 
 8 | rc_4.o:rc4.c rc4.h
 9 | 	$(CC) $(CFLAGS) -c rc4.c -o rc4.o
10 | 
11 | test.o:test.c rc4.h
12 | 	$(CC) $(CFLAGS) -c test.c -o test.o
13 | 
14 | clean:
15 | 	rm -rf *.o test


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/README:
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1 | This is the codes for RC4, one of the software stream cipher.
2 | Since RC4 generates a pseudorandom stream of bits, it can be used for encryption by combining it with the plaintext using bit-wise exclusive-or; decryption is perform the same way.
3 | 


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/rc4.c:
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 1 | #include "rc4.h"
 2 | 
 3 | /*S is initialized to the identity permutation, mixes in bytes of the key.*/
 4 | void RC4_key(RC4_KEY *rc4_key, unsigned char *key, int *keylength){
 5 |     int i, j, temp;
 6 | 
 7 |     /*Initialize S*/
 8 |     for (i = 0; i < 256; i++)
 9 | 	rc4_key -> S[i] = i;
10 | 
11 |     j = 0;
12 |     for (i = 0; i < 256; i++){
13 | 	j = (j + rc4_key -> S[i] + *(key + i % *keylength)) % 256;
14 | 	/*Swap rc4_key -> S[i] and rc4_key -> S[j]*/
15 | 	temp = rc4_key -> S[i];
16 | 	rc4_key -> S[i] = rc4_key -> S[j];
17 | 	rc4_key -> S[j] = temp;
18 |     }
19 | }
20 | 
21 | /*Generate the key stream which length is the same as plaintext's and encrypt the plaintext and output the ciphertext.*/
22 | void RC4(RC4_KEY *rc4_key, unsigned char *plaintext, int *plaintext_length, unsigned char *ciphertext){
23 |     int i = 0, j = 0, k = 0, n, temp;
24 | 
25 |     for (k = 0; k < *plaintext_length; k++){
26 | 	i = (i + 1) % 256;
27 | 	j = (j + rc4_key -> S[i]) % 256;
28 | 	/*Swap rc4_key -> S[i] and rc4_key -> S[j]*/
29 | 	temp = rc4_key -> S[i];
30 | 	rc4_key -> S[i] = rc4_key -> S[j];
31 | 	rc4_key -> S[j] = temp;
32 | 
33 | 	n = rc4_key -> S[(rc4_key -> S[i] + rc4_key -> S[j]) % 256];
34 | 
35 | 	/*Encryption*/
36 | 	*(ciphertext + k) = *(plaintext + k) ^ n;
37 |     }
38 | }
39 | 


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/rc4.h:
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 1 | #ifndef RC4_H
 2 | #define RC4_H
 3 | 
 4 | #define KEY_LENGTH 6
 5 | #define TEXT_LENGTH 10
 6 | 
 7 | typedef struct RC4_KEY_S{
 8 |     unsigned char S[256];
 9 | }RC4_KEY;
10 | 
11 | /*S is initialized to the identity permutation, mixes in bytes of the key.*/
12 | void RC4_key(RC4_KEY *rc4_key, unsigned char *key, int *keylength);
13 | 
14 | /*Encrypt the plaintext and output the ciphertext.*/
15 | void RC4(RC4_KEY *rc4_key, unsigned char *plaintext, int *plaintext_length, unsigned char *ciphertext);
16 | 
17 | #endif
18 | 


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/test.c:
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 1 | #include <stdio.h>
 2 | #include "rc4.h"
 3 | 
 4 | int main(){
 5 |     int i, x;
 6 |     int *p;
 7 |     unsigned char plaintext[TEXT_LENGTH];
 8 |     unsigned char key[KEY_LENGTH] = "cipher";
 9 |     unsigned char ciphertext[TEXT_LENGTH];
10 |     
11 |     RC4_KEY rc4_key;
12 |     
13 |     printf("------------------------------\n----------RC4 CIPHER----------\n------------------------------\n");
14 |     printf("Please enter the plaintext(%d characters): ", TEXT_LENGTH);
15 |     for (i = 0; i < TEXT_LENGTH; i++)
16 | 	scanf("%c", &plaintext[i]);
17 |    
18 |     /*Encryption*/
19 |     x = KEY_LENGTH;
20 |     p = &x;
21 |     RC4_key(&rc4_key, &key[0], p);
22 |     x = TEXT_LENGTH;
23 |     RC4(&rc4_key, &plaintext[0], p, &ciphertext[0]);
24 |     
25 |     printf("The plaintext is: ");
26 |     for (i = 0; i < TEXT_LENGTH; i++)
27 | 	printf("%x ", plaintext[i]);
28 |     printf("\nThe key is: ");
29 |     for (i = 0; i < KEY_LENGTH; i++)
30 | 	printf("%x ", key[i]);
31 |     printf("\nThe ciphertext is: ");
32 |     for (i = 0; i <TEXT_LENGTH; i++)
33 | 	printf("%x ", ciphertext[i]);
34 |     printf("\n\n");
35 |     
36 |     return 0;
37 | }
38 | 


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