Sometimes it is necessary to implement some cryptographic functions for security reasons within your iOS apps. In one of our projects I was faced with the problem of implementing some kind of a two-step registration/confirmation process with a iOS app and a corresponding server-side interface. We decided to implement this by using a generated confirmation code based on HMAC and MD5. Since there is no native support of the iOS framework to realize this, I needed to implement it on my own.

Due to this post Implementing HMAC encryption algorithm in iPhone application it was quite easy to implement a simple category for NSString  to provide this functionality. The following code creates a HMAC+MD5 encrypted string based on a given secret and returns it in a hexadecimal string representation.

#include <CommonCrypto/CommonDigest.h>
#include <CommonCrypto/CommonHMAC.h>

#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

@implementation NSString (HMAC)

- (NSString*) HMACWithSecret:(NSString*) secret
{
    CCHmacContext    ctx;
    const char       *key = [secret UTF8String];
    const char       *str = [self UTF8String];
    unsigned char    mac[CC_MD5_DIGEST_LENGTH];
    char             hexmac[2 * CC_MD5_DIGEST_LENGTH + 1];
    char             *p;

    CCHmacInit( &amp;ctx, kCCHmacAlgMD5, key, strlen( key ));
    CCHmacUpdate( &amp;ctx, str, strlen(str) );
    CCHmacFinal( &amp;ctx, mac );

    p = hexmac;
    for (int i = 0; i &lt; CC_MD5_DIGEST_LENGTH; i++ ) {
        snprintf( p, 3, "%02x", mac[ i ] );
        p += 2;
    }

    return [NSString stringWithUTF8String:hexmac];
}

@end

You can now simply use this like the following:

NSString *strToEncrypt  = @"mytexttoencyrpt";
NSString *secret        = @"SECRET";
NSString *hexHmac       = [strToEncrypt HMACWithSecret:secret];

NSLog(@"HMAC in hex is %@", hexHmac);

//Outputs: HMAC in hex is 3849b37cf06d014a13839a9062b5723d

Find the source code for the NSString  category at HMAC category.