Post-quantum Hybrid ECDHE-SCloud+ Key Exchange for TLS 1.3

This draft specifies how to enable hybrid key exchange with ECDHE and SCloud+ in Transport Layer Security protocol version 1.3 (TLS 1.3) to mitigate quantum threats. SCloud+ is an unstructured lattice based KEM (key encapsulation mechanism) with post-quantum security. This draft follows the post-quantum hybrid key exchange framework specified by [TLS.Hybrid], by concatenating the public keys and ciphertexts of ECDHE and SCloud+. Three concrete hybrid key exchange schemes are specified in this draft, which are X25519SCloud+128, SecP256r1SCloud+192 and SecP384r1SCloud+256. [EDNOTE: ... ]