NSString category using CommonCrypto returns nil in RubyMotion

Question

I am trying to encrypt a string with HMAC-SHA1 using CCHmac(). I created an NSString category for this:

NSString+HMAC.h

#import <Foundation/Foundation.h>
#import <CommonCrypto/CommonDigest.h>
#import <CommonCrypto/CommonHMAC.h>

@interface NSString (HMAC)

- (NSString *)HMACSHA1WithKey:(NSString *)key;

@end

NSString+HMAC.m

#import "NSString+HMAC.h"

@implementation NSString (HMAC)

- (NSString *)HMACSHA1WithKey:(NSString *)key
{
    const char *cKey  = [key cStringUsingEncoding:NSUTF8StringEncoding];
    const char *cData = [self cStringUsingEncoding:NSUTF8StringEncoding];

    unsigned char cHMAC[CC_SHA1_DIGEST_LENGTH];

    CCHmac(kCCHmacAlgSHA1, cKey, strlen(cKey), cData, strlen(cData), cHMAC);

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

    return [[NSString alloc] initWithData:HMAC encoding:NSUTF8StringEncoding];
}

@end

I am using this category from RubyMotion like so:

key = "ZjiUOHkl5tllz2PwaoZYrDRMkg9b43k5CcIOUjSE&"
string = "POST&https%3A%2F%2Fapi.twitter.com%2Foauth%2Frequest_token&oauth_consumer_key=XXX&oauth_nonce=OWhJeFpZbzRoVU4xck1RTENyN0w4T1J0czNIa01rSVA&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1389974660&oauth_version=1.0"
string.HMACSHA1WithKey(key) # => nil

However, the HMACSHA1WithKey: method returns nil all the time...

What am I doing wrong please?

Answer 1

The result of the SHA-1 algorithm (which is stored in HMAC) is a sequence of bytes, but not a valid UTF-8 sequence. Therefore

[[NSString alloc] initWithData:HMAC encoding:NSUTF8StringEncoding]

fails and returns nil.

What you probably want is to convert the NSData *HMAC to a NSString containing the hexadecimal representation of the data. There are many solutions available for that, for example How to convert an NSData into an NSString Hex string?.