Hash-based message authentication code Ref:http://en.wikipedia.org/wiki/HMAC

hash256_hmac Objective-C

- (NSString*) HMACWithSecret:(NSString*)secret andData:(NSString *)data
{
    CCHmacContext    ctx;
    const char       *key = [secret UTF8String];
    const char       *str = [data UTF8String];
    unsigned char    mac[CC_SHA256_DIGEST_LENGTH];
    char             hexmac[CC_SHA256_DIGEST_LENGTH];
    char             *p;
    
    CCHmacInit( &ctx, kCCHmacAlgSHA256, key, strlen( key ));
    CCHmacUpdate( &ctx, str, strlen(str) );
    CCHmacFinal( &ctx, mac );
    
    p = hexmac;
    for (int i = 0; i < CC_SHA256_DIGEST_LENGTH; i++ ) {
        snprintf( p, 3, "%02x", mac[ i ] );
        p += 2;
    }
    
    return [NSString stringWithUTF8String:hexmac];
}

2013-11-26 17:16:07.704 ChatBotsMessager[547:60b] API request response str:{"success":0,"errorType":"Missing parameter","errorMessage":"The hash parameter must be passed in the url."}