We present KEMTLS, an alternative to the TLS 1.3 handshake that uses key-encapsulation mechanisms (KEMs) instead of signatures for server authentication. Among existing post-quantum candidates, signature schemes generally have larger public key/signature sizes compared to the public key/ciphertext sizes of KEMs: by using an IND-CCA-secure KEM for server authentication in post-quantum TLS, we obtain multiple benefits. A size-optimized post-quantum instantiation of KEMTLS requires less than half the bandwidth of a size-optimized post-quantum instantiation of TLS 1.3. In a speedoptimized instantiation, KEMTLS reduces the amount of server CPU cycles by almost 90% compared to TLS 1.3, while at the same time reducing communication size, reducing the time until the client can start sending encrypted application data, and eliminating code for signatures from the server's trusted code base.

While server-only authentication with certificates is the most widely used mode of operation for the Transport Layer Security (TLS) protocol on the world wide web, there are many applications where TLS is used in a different way or with different constraints. For example, embedded Internet-of-Things clients may have a server certificate preprogrammed and be highly constrained in terms of communication bandwidth or computation power. As post-quantum algorithms have a wider range of performance trade-offs, designs other than traditional "signedkey-exchange" may be worthwhile. The KEMTLS protocol, presented at ACM CCS 2020, uses key encapsulation mechanisms (KEMs) rather than signatures for authentication in the TLS 1.3 handshake, a benefit since most post-quantum KEMs are more efficient than PQ signatures. However, KEMTLS has some drawbacks, especially in the client authentication scenario which requires a full additional roundtrip.We explore how the situation changes with pre-distributed public keys, which may be viable in many scenarios, for example pre-installed public keys in apps, on embedded devices, cached public keys, or keys distributed out of band. Our variant of KEMTLS with pre-distributed keys, called KEMTLS-PDK, is more efficient in terms of both bandwidth and computation compared to post-quantum signed-KEM TLS (even cached public keys), and has a smaller trusted code base. When client authentication is used, KEMTLS-PDK is more bandwidth efficient than KEMTLS yet can complete client authentication in one fewer round trips, and has stronger authentication properties. Interestingly, using pre-distributed keys in KEMTLS-PDK changes the landscape on suitability of PQ algorithms: schemes where public keys are larger than ciphertexts/signatures (such as Classic McEliece and Rainbow) can be viable, and the differences between some lattice-based schemes is reduced. We also discuss how using pre-distributed public keys provides privacy benefits compared to pre-shared symmetric keys in TLS.

The NIST post-quantum cryptography (PQC) standardization project is probably the largest and most ambitious cryptography standardization effort to date, and as such it makes an excellent case study of cryptography standardization projects. It is expected that with the end of round 3 in early 2022, NIST will announce the first set of primitives to advance to standardization, so it seems like a good time to look back and see what lessons can be learned from this effort. In this paper, we take a look at one specific aspect of the NIST PQC project: software implementations.We observe that many implementations included as a mandatory part of the submission packages were of poor quality and ignored decades-old standard techniques from software engineering to guarantee a certain baseline quality level. As a consequence, it was not possible to readily use those implementations in experiments for post-quantum protocol migration and software optimization efforts without first spending a significant amount of time to clean up the submitted reference implementations.We do not mean to criticize cryptographers who submitted proposals, including software implementations, to NIST PQC: after all, it cannot reasonably be expected from every cryptographer to also have expertise in software engineering. Instead, we suggest how standardization bodies like NIST can improve the software-submission process in future efforts to avoid such issues with submitted software. More specifically, we present PQClean, an extensive (continuousintegration) testing framework for PQC software, which now also contains "clean" implementations of the NIST round 3 candidate schemes. We argue that the availability of such a framework-either in an online continuous-integration setup, or just as an offline testing system-long before the submission deadline would have resulted in much better implementations included in NIST PQC submissions and overall would have saved the community and probably also NIST a lot of time and effort.

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