Question

So finally figured out how to do an hmac sha 256 hashing. I will be using this for a wcf service api i made. My problem is that the NSData output that my method is sending out have spaces.

eg. This is how it looks like what my API sends out

“2efb00aba01a3f5b674fba3063b43fee7a9356947118……”

And this is how my iphone app shows it

<2efb00ab a01a3f5b 674fba30…..>

This is how my code in objective c looks like:

NSData *hmacSHA256(NSString *key, NSString *data)
{
    const char *cKey  = [key cStringUsingEncoding:NSASCIIStringEncoding];
    const char *cData = [data cStringUsingEncoding:NSASCIIStringEncoding];
    unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
    CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);
    return [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
}

This came from this answer:

https://stackoverflow.com/a/8459123/639713

Anyway, my issue is, how do I deal with this. How do I convert the NSdata output to string? And if does get converted to string I’m guessing the output will be different from what the WCF Service API sends out. Do I change how the API processes it’s hmacsha256 output?

Thanks!

Answer 1

You could modify your method slightly so that instead of creating an NSData containing the digest bytes, you could create a string formatting the bytes as hexadecimal.

NSString *hmacSHA256(NSString *key, NSString *data)
{
    const char *cKey  = [key cStringUsingEncoding:NSASCIIStringEncoding];
    const char *cData = [data cStringUsingEncoding:NSASCIIStringEncoding];
    unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
    CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);

    NSMutableString *result = [NSMutableString string];
    for (int i = 0; i < sizeof cHMAC, i++)
    {
        [result appendFormat:@"%02hhx", cHMAC[i]];
    }

    return result;
}