随机字符串

  char data[16];

    for (int i=0;x<16;data[i++] = (char)('a' + (arc4random_uniform(26))));

//    for (int i=0;x<32;data[i++] = (char)('1' + (arc4random_uniform(9))));

    return [[NSString alloc] initWithBytes:data length:16 encoding:NSUTF8StringEncoding];