In December 2020, the European Telecommunications Standards Institute (ETSI) Technical Committee Cyber Security published Technical Specification TS 103 744, defining methods for quantum-safe hybrid key exchanges. The specification provided several methods for deriving cryptographic keys from multiple shared secrets established using both classical elliptic curve Diffie-Hellman (ECDH) and post-quantum key encapsulation mechanisms (KEMs).
TS 103 744 defined two combiner constructions for hybrid key exchange: a concatenate method and a cascade method. Both approaches ensured that the derived key remained at least as secure as the strongest individual key exchange component. The specification described hybrid key exchanges as a migration technique for moving to quantum-safe technology in advance of establishing full security assurance in the underlying post-quantum cryptographic schemes.
An informative reference implementation was published on ETSI’s open-source repository. The specification was later revised as V1.2.1 in March 2025 to incorporate fixed parameter sets aligned with NIST’s ML-KEM (FIPS 203) standard and to include test vectors for interoperability.