Blowfish Encrypt and Decrypt char buffers - c

Question

i'm try to get a bowfish function working for simple char buffers. The program fails when I try to decrypt an encrypted buffer and fails on the EVP_CipherFinal_ex() call.
#include <string.h>
#include <openssl/evp.h>
#include <openssl/buffer.h>
#include <openssl/blowfish.h>
#include <openssl/evp.h>
int do_crypt(unsigned char *inbuf, int inlen, unsigned char *outbuf, int *outlen, int do_encrypt) {
outbuf=(unsigned char*) malloc(inlen+EVP_MAX_BLOCK_LENGTH);
int tmplen=0;
unsigned char key[] = "0123456789";
unsigned char iv[] = "12345678";
EVP_CIPHER_CTX ctx;
EVP_CIPHER_CTX_init(&ctx);
EVP_CipherInit_ex(&ctx, EVP_bf_cbc(), NULL, NULL, NULL, do_encrypt);
EVP_CIPHER_CTX_set_key_length(&ctx, 10);
EVP_CipherInit_ex(&ctx, NULL, NULL, key, iv, do_encrypt);
if(!EVP_CipherUpdate(&ctx, outbuf, outlen, inbuf, inlen)) {
/* Error */
printf("* update failed *\n");
EVP_CIPHER_CTX_cleanup(&ctx);
return 0;
}
int db=*outlen;
if(!EVP_CipherFinal_ex(&ctx, outbuf+db, &tmplen)) {
/* Error */
ERR_print_errors_fp(stderr);
printf("* finalise failed *\n");
EVP_CIPHER_CTX_cleanup(&ctx);
return 0;
}
(*outlen)=db+tmplen;
EVP_CIPHER_CTX_cleanup(&ctx);
return 1;
}
int main(int argc, char **argv) {
char *plain="ENCRYPT DECRYPT this string";
int plain_len=strlen(plain);
unsigned char *cipher;
int cipher_len;
printf("***** ENCRYPT *****\n");
if (!do_crypt((unsigned char*) plain, strlen(plain), cipher, &cipher_len, 1)) {
printf("failed to encrypt\n");
return 1;
}
char *decrypt;
int decrypt_len;
printf("***** DECRYPT *****\n");
if(!do_crypt( cipher ,cipher_len , decrypt, &decrypt_len, 0)) {
printf("failed to decrypt\n");
return 1;
}
printf("decrypt=\"%s\"\n",decrypt);
printf("decrypt_len=%d\n",decrypt_len);
return 0;
}
any help would be much appreciated.

Answer 1

You have two problems: The first is that you should not cast the result of malloc in C. This is the most likely reason EVP_CipherUpdate crashes.
The second error is that arguments in C are passed by value, meaning they are copied and the functions only have copies of the arguments passed by the caller. This means that in the do_crypt function when you assign to the argument output you are only assigning to the local copy inside the function, the variable cipher in the main function will not be changed.
The last problem can be solved by imitating pass by reference by passing a pointer to the pointer, and use the address-of operator & and the dereference * operator:
/* Note extra indirection */
/* | */
/* v */
int do_crypt(unsigned char *inbuf, int inlen, unsigned char **outbuf, int *outlen, int do_encrypt) {
...
*output = malloc(...);
...
if(!EVP_CipherUpdate(&ctx, *outbuf, outlen, inbuf, inlen)) { ... }
...
}
You then call it like
do_crypt((unsigned char*) plain, strlen(plain), &cipher, &cipher_len, 1)