openssl encryption in c++ and php

Question

I need to encode some data in c++ and decode it in php, but php is not decoding properly. I checked encoding this same message with the same key and iv, and there is a diference betwen results. This is my code:

struct ctr_state
{
    unsigned char ivec[AES_BLOCK_SIZE];
    unsigned int num;
    unsigned char ecount[AES_BLOCK_SIZE];
};
unsigned char indata[AES_BLOCK_SIZE];
unsigned char outdata[AES_BLOCK_SIZE];
unsigned char iv[AES_BLOCK_SIZE];

struct ctr_state state;

int init_ctr(struct ctr_state *state, const byte iv[16])
{
    /* aes_ctr128_encrypt requires 'num' and 'ecount' set to zero on the
     * first call. */
    state->num = 0;
    memset(state->ecount, 0, AES_BLOCK_SIZE);

    /* Initialise counter in 'ivec' to 0 */
    memset(state->ivec + 8, 0, 8);

    /* Copy IV into 'ivec' */
    memcpy(state->ivec, iv, 8);
}

void aes_encoder(byte *read, byte *write, int size, byte *enc_key, byte *iv)
{
    AES_KEY key;
    if (AES_set_encrypt_key(enc_key, 128, &key) < 0)
    {
       Logger::getInstance()->Error("problem with setting encrypt key");
    }
    init_ctr(&state, iv);

    AES_ctr128_encrypt(read, write, size, &key, state.ivec, state.ecount, &state.num);
}

byte *key = (byte*)"2123456789012345";
byte *iv = (byte*)"2asdasdasdasdasd";

QByteArray message = "this is message";
byte *data = reinterpret_cast<byte *>(message.data());

aes_encoder(data, data,  message.size(), key, iv);
qDebug() << message.toBase64();

the result is: "hF/nlW4e+FmuF8Bfny9M"

and php code:

<?php
$message = "this is message";
$key = "2123456789012345";
$iv = "2asdasdasdasdasd";

$encrypted = openssl_encrypt($message, 'aes-128-ctr', $key, true, $iv);

echo base64_encode($encrypted);

result: "RLLUkP54El9FCeWpO/bI"

Why the results are not the same?

Answer 1

Your problem is that you are not using CTR mode in the standard way. In init_ctr you are only copying 8 bytes of the provided IV and setting the rest to zero. If instead you use the whole IV, you will get the same result as the PHP code:

//don't do this:
//memset(state->ivec + 8, 0, 8);
//memcpy(state->ivec, iv, 8);  

//do this:
memcpy(state->ivec, iv, AES_BLOCK_SIZE);  

The lesson is that just because you found some code somewhere, doesn't mean you can copy-n-paste it without understanding what it is doing. This is especially true with crypto code. If you knew even the basics of what a block cipher is and how one works in CTR mode, you would have realized the problem with your code straightaway.

Oh, important safety tip: When using CTR mode, never encrypt more than one message with the same IV. Or you will die.