Decrypting using openssl c/c++ API fails - c++

Question

I create a file using openssl from the commandline. Let's say:
echo 'foobar' | openssl enc -aes-128-cbc -e -pass pass:testing >
sample
I can decrypt this doing
cat sample | openssl enc -aes-128-cbc -d -pass pass:testing
That works fine.
Now I want to decrypt that file using the openssl c/c++ API but I can't get this quite right. I am trying to decrypt like so
#include <fstream>
#include <memory>
#include <string>
#include <openssl/err.h>
#include <openssl/evp.h>
using EVP_CIPHER_CTX_free_ptr = std::unique_ptr<EVP_CIPHER_CTX, decltype(&::EVP_CIPHER_CTX_free)>;
int main(int argc, char* argv[])
{
ERR_load_crypto_strings();
EVP_add_cipher(EVP_aes_128_cbc());
std::string l_key{"testing"};
std::string l_ctext{};
std::ifstream l_file("sample", std::ios::binary | std::ios::in | std::ios::ate);
if(l_file.is_open())
{
std::streampos l_size = l_file.tellg();
char * lp_buffer = new char[l_size];
l_file.seekg(0, std::ios::beg);
l_file.read(lp_buffer, l_size);
l_ctext.append(lp_buffer, l_size);
delete lp_buffer;
l_file.close();
}
std::string l_rtext;
EVP_CIPHER_CTX_free_ptr ctx(EVP_CIPHER_CTX_new(), ::EVP_CIPHER_CTX_free);
if(1 != EVP_DecryptInit_ex(ctx.get(), EVP_aes_128_cbc(), NULL,
(const unsigned char*)l_key.c_str(), NULL));
ERR_print_errors_fp(stdout);
int l_len;
if(1 != EVP_DecryptUpdate(ctx.get(), (unsigned char*)&l_rtext[0], &l_len,
(const unsigned char*)l_ctext.data(),
l_ctext.size()))
ERR_print_errors_fp(stdout);
if(1 != EVP_DecryptFinal_ex(ctx.get(), (unsigned char*)&l_rtext[0] + l_len,
&l_len))
ERR_print_errors_fp(stdout);
ERR_free_strings();
exit(0);
}
However that last call to EVP_DecryptFinal_ex fails with
140559955420968:error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt:evp_enc.c:596:
Most of this code is based on the examples that come with the openssl API. What am I doing wrong? I set the IV to NULL in EVP_DecryptInit_ex because as far as I found out, this is acceptable as long as I don't use the key multiple times.
All this on a Fedora machine with GCC

Answer 1

I kind of figured out what I did wrong.
The commandline 'openssl enc' command uses a default salt, my code above didn't look for that. So (rather than try to work with the '-nosalt' option) I remove the first 16 bytes from the encrypted string and use the last 8 of that as the salt.
I was feeding the password to the EVP methods as a 'key'. It seems 'openssl enc' doesn't do this so one needs to use the salt and the password to create a key and IV (rather than using NULL which is what I did).
There was an error in the above code in that l_rtext needs a resize() to reflect the fact that characters were added to it.