Question

In the code below, the decrypted text does not match the original plaintext. The first 12 bytes are messed up. Note that block cipher padding has been disabled. I have tried different values for BUF_SIZE, all multiples of 16 – every time the first 12 bytes of the decrypted data is wrong. Here’s the output:

    plain buf[32]:
    11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
    11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
    outlen=32
    outlen=32
    dec buf[32]:
    0C 08 01 46 6D 3D FC E9 98 0A 2D E1 AF A3 95 3A
    0B 31 1B 9D 11 11 11 11 11 11 11 11 11 11 11 11

Here’s the code:

#include <stdio.h>
#include <string.h>
#include <CommonCrypto/CommonCryptor.h>

static void
dumpbuf(const char* label, const unsigned char* pkt, unsigned int len)
{
    const int bytesPerLine = 16;

    if (label) {
        printf("%s[%d]:\n", label, len);
    }

    for (int i = 0; i < int(len); i++) {
        if (i && ((i % bytesPerLine) == 0)) {
            printf("\n");
        }
        unsigned int c = (unsigned int)pkt[i] & 0xFFu;
        printf("%02X ", c);
    }
    printf("\n");
}

int main(int argc, char* argv[])
{
    unsigned char key[16];
    unsigned char iv[16];

    memset(key, 0x22, sizeof(key));
    memset(iv, 0x33, sizeof(iv));

#define BUF_SIZE  32
    unsigned char plainBuf[BUF_SIZE];
    unsigned char encBuf[BUF_SIZE];
    memset(plainBuf, 0x11, sizeof(plainBuf));
    dumpbuf("plain buf", plainBuf, sizeof(plainBuf));

    int outlen;
    CCCryptorStatus status;
    status = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, 0,
            key, kCCKeySizeAES128, iv, plainBuf, sizeof(plainBuf),
            encBuf, sizeof(encBuf), (size_t*)&outlen);
    if (kCCSuccess != status) {
        fprintf(stderr, "FEcipher: CCCrypt failure\n");
        return -1;
    }
    printf("outlen=%d\n", outlen);

    status = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, 0,
            key, kCCKeySizeAES128, iv, encBuf, sizeof(encBuf),
            plainBuf, sizeof(plainBuf), (size_t*)&outlen);
    if (kCCSuccess != status) {
        fprintf(stderr, "FEcipher: CCCrypt failure\n");
        return -1;
    }
    printf("outlen=%d\n", outlen);
    dumpbuf("dec buf", plainBuf, sizeof(plainBuf));
    return 0;
}

Thanks,
Hari

Answer 1

@owlstead, thanks for your response. CBC is the default – you don’t need to specify anything special in the options to enable it.

The same code using CCCrypt() was working before. I don’t know what changed – may be a new library was installed during an update. Instead of using the convenience function CCCrypt() I’m now using the Create/Update/Final API – it works, so I have a workaround.