How can I make CCCrypt compute AES in another mode?

Question

The signature of one-shot crypto method CCCrypt is this (from CommonCryptor.h):

CCCryptorStatus CCCrypt(
    CCOperation op,         /* kCCEncrypt, etc. */
    CCAlgorithm alg,        /* kCCAlgorithmAES128, etc. */
    CCOptions options,      /* kCCOptionPKCS7Padding, etc. */
    const void *key,
    size_t keyLength,
    const void *iv,         /* optional initialization vector */
    const void *dataIn,     /* optional per op and alg */
    size_t dataInLength,
    void *dataOut,          /* data RETURNED here */
    size_t dataOutAvailable,
    size_t *dataOutMoved)

None of the parameter seems to accept a CCMode value (maybe sneakily, since all the enums are integers?). I have tried around combining it with the CCOptions parameter, but to no avail; the two enums are not options, and don't combine unambiguously.

It's not explicitly documented there, but I surmise from what I find around the web that the mode used with kCCAlgorithmAES is CBC.

How can I change the AES mode CCCrypt uses?

It's pretty easy to implement other modes if you have access to ECB or CBC. The only challenge is to correctly implement the counter which is encrypted for each block. — Artjom B., Jul 12 '17 at 16:37
@ArtjomB. Yea, no, I won't go around and implement crypto methods myself. That way only doom and madness lie. — Raphael, Jul 12 '17 at 17:51
@zaph - I expected to see you answer this one. Don't you have a crypto library based on Apple Common Crypto? — jww, Jul 14 '17 at 19:40

score 1 · Accepted Answer · answered Jul 12 '17 at 14:41

The documentation says:

CCCrypt Stateless, one-shot encrypt or decrypt operation. This basically performs a sequence of CCCrytorCreate() [sic], CCCryptorUpdate(), CCCryptorFinal(), and CCCryptorRelease().

Now, CCCryptorCreate does not seem to accept a mode parameter, either; in fact, we see that CBC is indeed hardcoded (aside from the one, somewhat arbitrary that allows us to use ECB):

/* Determine mode from options - old call only supported ECB and CBC 
   we treat RC4 as a "mode" in that it's the only streaming cipher
   currently supported 
*/
if(alg == kCCAlgorithmRC4) mode = kCCModeRC4;
else if(options & kCCOptionECBMode) mode = kCCModeECB;
else mode = kCCModeCBC;

If we want to use another mode we have to use CCCryptorCreateWithMode. So, unfortunately, it does not appear to be possible to change the mode of CCCrypt.

How can I make CCCrypt compute AES in another mode?

1 Answers1