HmacSHA256 objective-c encryptation

Question

I wanna encpryt a string with a key, using HmacSHA256. The code everyone use is the one below, but there is one thing that doesn´t make sense. Why would we use base64 at the end if all we want is the HmacSHA256 hash?

I tried seeing the hash generated after the method CCHmac is called with

NSString *str = [[NSString alloc] initWithData:HMAC encoding:NSASCIIStringEncoding];
NSLog(@"%@", str);

But i don´t get the hash generated, i get null, or garbage, like this:

2011-10-11 09:38:05.082 Hash_HmacSHA256[368:207] (null) 2011-10-11 09:38:05.085 Hash_HmacSHA256[368:207] Rwªb7iså{yyþ§Ù(&oá÷ÛËÚ¥M`f

import < CommonCrypto/CommonHMAC.h>

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);

NSData *HMAC = [[NSData alloc] initWithBytes:cHMAC
                                  length:sizeof(cHMAC)];

NSString *hash = [HMAC base64Encoding]; //This line doesn´t make sense

[key release];
[data release];

score 8 · Accepted Answer · edited May 23 '17 at 11:59

First of all, for those wondering, this is in reference to my answer to this question: Objective-C sample code for HMAC-SHA1

The HMAC you generate is a 256-bit binary value that may or may not start with a 0 byte.

To be able to print it, you need a string representation (binary, hex, decimal, base64, etc.). Base64 is one of the most efficient among these, that's why I used a Base64 encoding there.

The reason you get garbage is that most (if not all) of the octets in the HMAC value are outside the range of printable ASCII characters. If the first octet is 0 (0x00), you get nil. This is why you need an encoding that supports arbitrary values. ASCII doesn't.

Of course, if you don't want to print the HMAC value, then may not need such an encoding, and can keep the HMAC as is (binary NSData).

Also, in a lot of cases, you might pass something like this over the wire. Encoding it as Base64 ensures it makes it to the other side intact. — Timothy Lee Russell, Feb 08 '13 at 16:22

score 2 · Answer 2 · edited Feb 08 '13 at 16:24

I spend a whole day, trying to convert the generated hash (bytes) into readable data. I used the base64 encoded you said and it didn´t work at all for me .

So what i did was this:

CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);

// Now convert to NSData structure to make it usable again
NSData *out = [NSData dataWithBytes:cHMAC length:CC_SHA256_DIGEST_LENGTH];

// description converts to hex but puts <> around it and spaces every 4 bytes
NSString *hash = [out description];
hash = [hash stringByReplacingOccurrencesOfString:@" " withString:@""];
hash = [hash stringByReplacingOccurrencesOfString:@"<" withString:@""];
hash = [hash stringByReplacingOccurrencesOfString:@">" withString:@""];
// hash is now a string with just the 40char hash value in it
NSLog(@"%@",hash);

CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC); <-- on this line i am getting EXC_BAD_ACESS . Any idea ? — Muhammad Saad Ansari, Nov 10 '13 at 08:17

score 1 · Answer 3 · answered Jul 13 '12 at 20:24

Don't do "[out description]" to get the hash as a string.

Do [hash base64Encoding] to get the base64 encoding of it. Use http://cybersam.com/ios-dev/http-basic-access-authentication-with-objective-c-and-ios/attachment/nsdataadditions to get the base64Encoding function. The additions class is a category that will add the function base64Encoding to NSData's implementation.

Or you can do [[NSString alloc]initWithData:out encoding:NSUTF8StringEncoding].

HmacSHA256 objective-c encryptation

3 Answers3