CHAPTER 4 Be Prepared 43 Well then, can we go all-symmetric? Can we address the three main roles of cryptography — confidentiality, integrity, and authentication — without relying on the likes of RSA and ECC? The answer is »yes.« Tried-and-true cryptography It is easy to see that confidentiality can be achieved through symmetric cryptography. AES is the workhorse of state-of-the-art encryption. Integrity can be achieved through message authentication codes (MACs). In the case of authenticated encryption (AE), these become an integral part of the symmetric crypto system. Authentication of users and devices is allegedly the most important security property in the Internet of Things (IoT). It prevents illegitimate nodes from taking part in network activities. Authentication is typically implemented through challenge-response schemes. In the case of symmetric cryptography, these involve a trusted third party; typically, a so-called key distribution center (KDC). KDC and the individual node — for instance, your cell phone or your laptop computer — share a secret known to no one else. Let us look at two real-world examples. Your phone does it In a GSM network, phones have a unique international mobile subscriber identity (IMSI). A secret key (Ki), corresponding to the IMSI, is stored both in the SIM card (subscriber identification module) and in the authentication center (AUC) of a mobile network operator’s (MNO) home location register (HLR). To establish a GSM connection to a phone, the AUC has to be contacted. Given the IMSI, it will provide the requester with a triplet that consists of a random number (RAND), a session key (KC), and a signed response (SRES). SRES = A3(RAND, Ki) KC = A8(RAND, Ki) A phone can now be authenticated using RAND as a challenge and SRES as the expected response. Thereafter, confidentiality can be achieved using KC as a session key. In reality, matters are a little more complex; in part, because the original KC,
PQC_for_Dummies
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