Achieving secure communication between an Implantable Medical Device (IMD) inside the body and a gateway outside the body has showed its criticality with recent reports of hackings. The use of asymmetric cryptography is not a practical solution for IMDs due to the scarce computational and power resources, symmetric key cryptography is preferred. One of the factors in security of a symmetric cryptographic system is to use a strong key for encryption. A solution without using extensive resources in an IMD, is to extract it from the body physiological signals. To have a strong enough key, the physiological signal must be a strong source of randomness and InterPulse Interval (IPI) has been advised to be such that. A strong randomness source should have five conditions: Universality, Liveness, Robustness Permanence and Uniqueness. Nevertheless, for current proposed random extraction methods from IPI these conditions (mainly last three conditions) were not examined. In this study, firstly, we proposed a methodology to measure the last three conditions. Then, using a huge dataset of IPI values, we showed that IPI does not have conditions of Robustness and Permanence. Thus, extraction of a strong uniform random number from IPI value, mathematically, is impossible. Thirdly, rather than using the value of IPI, we proposed the trend of IPI as a source for a new randomness extraction method named as Martingale Randomness Extraction from IPI (MRE-IPI). MRE-IPI satisfies the Robustness condition completely and Permanence to some level. We, also, used randomness test suites and showed that MRE-IPI is able to outperform all recent randomness extraction methods from IPIs and its quality is half of the AES random number. To the best of our knowledge, this is the first work in this area which uses such a comprehensive method and large dataset to examine the randomness of a physiological signal.

中文翻译：

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从植入式医疗器械中密码算子的 IPI 趋势中提取随机性

最近关于黑客攻击的报道表明，在体内的植入式医疗设备 (IMD) 与体外的网关之间实现安全通信已显示出其重要性。由于计算和电力资源稀缺，非对称加密的使用对于 IMD 来说不是一个实用的解决方案，对称密钥加密是首选。对称密码系统的安全性因素之一是使用强密钥进行加密。在 IMD 中不使用大量资源的解决方案是从身体生理信号中提取它。要获得足够强的密钥，生理信号必须是随机性的强来源，并且建议 InterPulse Interval (IPI) 是这样的。一个强随机源应该具备五个条件：普遍性、活性、鲁棒性、持久性和唯一性。尽管如此，对于当前从 IPI 提出的随机提取方法，这些条件（主要是最后三个条件）没有被检查。在这项研究中，首先，我们提出了一种方法来衡量最后三个条件。然后，使用庞大的 IPI 值数据集，我们表明 IPI 不具备稳健性和持久性的条件。因此，从数学上讲，从 IPI 值中提取强均匀随机数是不可能的。第三，我们没有使用 IPI 的值，而是提出了 IPI 趋势作为一种新的随机性提取方法的来源，称为 Martingale Randomness Extraction from IPI (MRE-IPI)。MRE-IPI 完全满足鲁棒性条件和一定程度的持久性。我们也，使用随机性测试套件并表明 MRE-IPI 能够胜过所有最近的 IPI 随机性提取方法，其质量是 AES 随机数的一半。据我们所知，这是该领域的第一项工作，它使用如此全面的方法和大型数据集来检查生理信号的随机性。