- (NSString *)md5 {

    if (nil == self || self.length == 0) {

        return nil;

    }

    const char *cStr = [self UTF8String];

    unsigned char result[CC_MD5_DIGEST_LENGTH];

    CC_MD5(cStr, (unsigned int)strlen(cStr), result)

return [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",

            result[0], result[1], result[2], result[3], result[4], result[5], result[6], result[7],

            result[8], result[9], result[10], result[11], result[12], result[13], result[14], result[15]];

}