Extract subdata from NSData without copying

Question

I have the following situation: there is a NSData that i need to decrypt. The data consists of:

• fixed length file header
• encrypted content

I'm using CCCrypt for decryption, but it probably doesn't matter, because this is more of an NSData related question. This is how i'm separating things now (pseudocode):

int hdrsize; // this contains the size of the header
NSData *data; // this contains full encrypted data with a header

// this gives me information, stored in the header + some additional stuff
NSDictionary *hdr = [self _headerInfoFromData:data];

// THIS IS THE PROBLEM AREA
data = [data subdataWithRange:NSMakeRange(hdrsize, [data length] - hdrsize)];

// And the decryption part
CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt, MS_SEC_ENC_ALGORITHM, kCCOptionPKCS7Padding,
                                      [key bytes], MS_SEC_ENC_KEY_SIZE,
                                      [[hdrdict objectForKey:@"iv"] bytes],
                                      [data bytes], dataLength,
                                      buffer, bufferSize,

As you can see, my problem here is that for decryption i need to extract the part of NSData without the header. But is there a way to simply somehow "reuse" the bytes that are already there instead of making the copy? Maybe there's some sort of way to create a no-copy byte buffer out of it, skipping first X bytes and passing that into CCCrypt instead?

Thanks for your help

Answer 1

Have you verified that -subdataWithRange: does copy the bytes? If it does, you can always use +dataWithBytesNoCopy:length:, just make sure to handle ownership properly.

EDIT

I'm such a fool. Just do this:

int hdrsize; // this contains the size of the header
NSData *data; // this contains full encrypted data with a header

// this gives me information, stored in the header + some additional stuff
NSDictionary *hdr = [self _headerInfoFromData:data];

// And the decryption part
CCCryptorStatus cryptStatus = CCCrypt(
    kCCDecrypt,
    MS_SEC_ENC_ALGORITHM,
    kCCOptionPKCS7Padding,
    [key bytes],
    MS_SEC_ENC_KEY_SIZE,
    [[hdrdict objectForKey:@"iv"] bytes],
    data.bytes + hdrsize,
    data.length - hdrsize,
    buffer,
    bufferSize,