Decrypt AES in C++ Example - c++

Question

I need some help with decrypt a char array in C++ using AES decrypt with Open SSL library. I already done encryption mode and works fine, but decryption is not working.
This is the Encrypt Function:
string Encrypt(char *Key, char *Msg, int size)
{
static char* Res;
static const char* const lut = "0123456789ABCDEF";
string output;
AES_KEY enc_key;
Res = (char *)malloc(size);
AES_set_encrypt_key((unsigned char *)Key, 128, &enc_key);
for(int vuelta = 0; vuelta <= size; vuelta += 16)
{
AES_ecb_encrypt((unsigned char *)Msg + vuelta, (unsigned char *)Res + vuelta, &enc_key, AES_ENCRYPT);
}
output.reserve(2 * size);
for (size_t i = 0; i < size; ++i)
{
const unsigned char c = Res[i];
output.push_back(lut[c >> 4]);
output.push_back(lut[c & 15]);
}
free(Res);
return output;
}
This is the Decrypt Function (not working):
char * Decrypt( char *Key, char *Msg, int size)
{
static char* Res;
AES_KEY dec_key;
Res = ( char * ) malloc( size );
AES_set_decrypt_key(( unsigned char * ) Key, 128, &dec_key);
for(int vuelta= 0; vuelta<=size; vuelta+=16)
{
AES_ecb_encrypt(( unsigned char * ) Msg+vuelta, ( unsigned char * ) Res+vuelta, &dec_key, AES_DECRYPT);
}
return (Res);
}
This is an Example of the Main function that call the methods, the problem is thar no mather how i print the "Res" variable in the Decrypt function, it always show random ASCII values, and i like to show the result in a string like the Encrypt function:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include "openSSL/aes.h"
using namespace std;
int main(int argc, char const *argv[])
{
char key[16];
char message[128];
char enc_message[128];
string s_key = "THIS_IS_THE_KEY_";
string s_message = "Hello World !!!";
memset(key, 0, sizeof(key));
strcpy(key, s_key.c_str());
memset(message, 0, sizeof(message));
strcpy(message, s_message.c_str());
string response = Encrypt(key, message, sizeof(message));
cout<<"This is the Encrypted Message: "<<response<<endl;
memset(enc_message, 0, sizeof(enc_message));
strcpy(enc_message, response.c_str());
Decrypt(key, enc_message, sizeof(enc_message));
return 0;
}
Any improve in this methods?

Answer 1

I wanted to put the answer to how I solved it: The problem with my example was that I was trying to use the decrypt function with a HEXADECIMAL STRING and it should be done with an ASCII STRING with the values ​​as delivered by the encryption function.
That is, instead of trying to decrypt a string like this: 461D019896EFA3
It must be decrypted with a string like this: #(%_!#$
After that, the decryption will be delivered in ASCII values. They must be passed to Hexadecimal and finally to a String.
Here is the example that worked for me:
string Decrypt_string(char *Key, string HEX_Message, int size)
{
static const char* const lut = "0123456789ABCDEF";
int i = 0;
char* Res;
AES_KEY dec_key;
string auxString, output, newString;
for(i = 0; i < size; i += 2)
{
string byte = HEX_Message.substr(i, 2);
char chr = (char) (int)strtol(byte.c_str(), NULL, 16);
auxString.push_back(chr);
}
const char *Msg = auxString.c_str();
Res = (char *)malloc(size);
AES_set_decrypt_key((unsigned char *)Key, 128, &dec_key);
for(i = 0; i <= size; i += 16)
{
AES_ecb_encrypt((unsigned char *)Msg + i, (unsigned char *)Res + i, &dec_key, AES_DECRYPT);
}
output.reserve(2 * size);
for (size_t i = 0; i < size; ++i)
{
const unsigned char c = Res[i];
output.push_back(lut[c >> 4]);
output.push_back(lut[c & 15]);
}
int len = output.length();
for(int i = 0; i < len; i += 2)
{
string byte = output.substr(i, 2);
char chr = (char) (int)strtol(byte.c_str(), NULL, 16);
newString.push_back(chr);
}
free(Res);
return newString;
}