Implementation of PQC-Classical Hybrid Signatures for Self-Sovereign Identity

Quantum computers pose a threat to parts of cryptography in use today. Fortunately, post-quantum cryptography (PQC) provides replacements believed to be secure against quantum computers. Transitioning away from cryptography vulnerable to quantum algorithms is becoming urgent with the advancement of technology in quantum computing, and the release of standards for PQC algorithms. Hybrid cryptography uses both PQC and classical cryptography in parallel, ensuring security in case either of the two algorithms is compromised. Hybrid cryptography is especially useful during the transition to PQC, when the security of the relatively new algorithms has not been time-tested. We have implemented a hybrid signature scheme in the context of a specific application: Self-Sovereign Identity. There are multiple decisions during the implementation of hybrid signature schemes in this context: the combiner used, how keys are stored, and how the hybrid signatures are encapsulated in relevant web standards. Our implementation uses strong nesting for hybridisation, a specific pair of algorithms rather than a general combination, and encodes both the key and signature as a single entity, rather than as two separate components. In addition to an overview of the relevant concepts, we present arguments for the decisions made, as well as related proposals for standardization.