AES_encrypt : initialization of one of the variables - c++

Question

I have this function that in the first call, is giving me back the Correct Encrypted Value
120692dbcdca656394fc10147e2418f2
But all that comes after are incorrect :
764e1a39b43c42f30da2e9e327d4ed22
b93b46dbc936ae3b06f571ffe1a59cac
b93b46dbc936ae3b06f571ffe1a59cac
a71787b35326e282f8c1bf3a0a034620
I'm new with C++ and I think it is a matter of initialization of one of the variables.
I did many Tests but I'm not able to point out where is the Error.
Could someone help me please ?
Thank You -
#include <openssl/aes.h>
#include <iostream>
#include <iomanip>
using namespace std;
char* pxEnAndDeCrypt(char* pStr )
{
static const unsigned char key[] = {0x00, 0x11, 0x22, 0x33, 0x44, 0x55};
char *ptr = NULL;
unsigned char enc_out[80]= {};
unsigned char dec_out[80]= {};
int i,j,lenHexa ;
char enc_out_HEXA[200]= {};
unsigned char enc_out_TRANSF[200]= {};
unsigned char enc_out_BACK[200]= {};
AES_KEY enc_key, dec_key;
// ENCRYPT : Input =*pStr Output = enc_out
AES_set_encrypt_key(key, 128, &enc_key);
AES_encrypt((const unsigned char *)pStr, enc_out, &enc_key);
// TRANSFORM OUTPUT OF ENCRYPT TO HEXA : Input =enc_out Output = enc_out_HEXA
int len = strlen((char*)enc_out);
for (i = 0, j = 0; i < len; ++i, j += 2)
{
sprintf(enc_out_HEXA + j, "%02x", enc_out[i] & 0xff);
}
ptr = (char *) enc_out_HEXA;
// OUTPUT
return ptr;
}

Answer 1

The problem with your code is that your function returns a pointer to enc_out_HEXA.
ptr = (char *) enc_out_HEXA;
return ptr;
The issue here is that enc_out_HEXA is declared inside pxEnAndDeCrypt so it no longer exists once you have exitted pxEnAndDeCrypt, so your function is returning a pointer to an object which no longer exists. This results in the strange behaviour you see.
Since you are programming C++, the simple solution is to use C++ (your current code is pure C). Instead of returning a pointer, return a std::string.
#include <string>
std::string pxEnAndDeCrypt(char* pStr )
{
...
return ptr;
}
There are many other places in the above code where you could replace the C code with C++. But this simple change should be enough to get over the current problem.
Of course you will also have to change the code that calls pxEnAndDeCrypt, but since you didn't post that I can't really help with that.
EDIT
Here's an alternative solution that doesn't require std::string.
The basic problem is that enc_out_HEXA has been declared inside the pxEnAndDeCrypt function and so you can't use it (or a pointer to it) outside the function. So one solution is to move the enc_out_HEXA to the calling function and pass a pointer to that array to the function. Like this
void pxEnAndDeCrypt(char* pStr, char* result)
{
...
for (i = 0, j = 0; i < len; ++i, j += 2)
{
sprintf(result + j, "%02x", enc_out[i] & 0xff);
}
}
Then somewhere else in your code you will have
char enc_out_HEXA[200];
pxEnAndDeCrypt(some_string, enc_out_HEXA);
That's the solution that would be used in a C program.