objective c hmac sha 256 gives wrong nsdata output - objective-c

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;
}

Answer 2

<2efb00ab a01a3f5b 674fba30.....> looks like the result of calling -[NSData description], like NSLog would do for any %# format strings. The NSData itself represents a sequence of bytes. The output you're after appears to be the byte sequence as a hexidecimal string. See Best way to serialize an NSData into a hexadeximal string for how to serialize the NSData to that format.