Post-quantum additions to PRIds, public keys, and encrypted User Data

[wesbiggs]

Problem

Current DSNP cryptography relies on elliptic-curve algorithms (X25519, Ed25519) that are broken by Shor's algorithm on a cryptographically-relevant quantum computer. NIST completed standardization of post-quantum algorithms in 2024 (FIPS 203 ML-KEM, FIPS 204 ML-DSA). This proposal enables implementations to begin publishing PQ keys and using PQ-protected data types while preserving full interoperability with classical-only implementations during a transition period.

Solution

This proposal adds optional post-quantum cryptography (PQC) support to DSNP, addressing the long-term vulnerability of the current X25519/Ed25519/XSalsa20-Poly1305 constructions to quantum attacks. It introduces:

1. Optional ML-KEM-768 (mlkem768-pub) keys for key agreement, alongside the existing X25519 keys
2. Optional ML-DSA-65 (mldsa65-pub) keys for digital signatures, alongside the existing Ed25519 keys
3. A post-quantum PRId scheme using a HMAC-SHA2-256 Merkle accumulator with ML-KEM-768-encrypted off-chain witnesses
4. New privateFollowsPQ and privateConnectionsPQ User Data types encrypted with the X-Wing hybrid KEM (X25519 + ML-KEM-768) and ChaCha20-Poly1305

All changes are backward-compatible; existing classical types and implementations are unaffected.

Change summary:

• Added PQ types for keyAgreementPublicKeys and assertionMethodPublicKeys
• Defined hybrid signature algorithm
• Defined new PQ PRId algorithm and off-chain witness format
• Defined PQ encryption algorithm and applicability to User Data types