feat: post-quantum dual-signature (Ed25519 + Dilithium3)

[HaraldeRoessler]

Summary

Add quantum-safe signing to all Verifiable Credentials using ML-DSA-65 (Dilithium3) via liboqs (MIT licensed).

Every credential is now signed with both Ed25519 and Dilithium3. Old verifiers check Ed25519 and ignore Dilithium. New verifiers check both. This is Phase 1 of the PQC migration.

New files

File	Purpose
app/crypto/dilithium.py	Dilithium3 key management (KMS + env fallback), sign, verify
app/crypto/hybrid.py	dual_sign() and verify_proof() for single or dual proofs
scripts/generate_dilithium_keys.py	One-time keypair generation utility

Modified files

File	Change
app/credentials.py	issue_credential() uses dual_sign(), verify_credential() supports both formats
app/swarm/endorsement.py	Endorsement VCs use dual_sign()
app/main.py	DID document dynamically includes Dilithium key when configured

Graceful degradation

If DILITHIUM_PRIVATE_KEY_HEX (or KMS equivalent) is not set, the system issues Ed25519-only credentials exactly as before. No breaking change on deploy. Enable PQC by:

pip install liboqs-python
python scripts/generate_dilithium_keys.py
# Set the output as env vars
export DILITHIUM_PRIVATE_KEY_HEX=...
export DILITHIUM_PUBLIC_KEY_HEX=...

Credential format

Before (Ed25519 only):

{"proof": {"type": "Ed25519Signature2020", "proofValue": "64 bytes..."}}

After (dual-signature):

{"proof": [
  {"type": "Ed25519Signature2020", "verificationMethod": "...#key-ed25519", "proofValue": "64 bytes..."},
  {"type": "DilithiumSignature2026", "verificationMethod": "...#key-dilithium", "proofValue": "~2420 bytes..."}
]}

Migration path

• Phase 1 (this PR): Dual-signature, Ed25519 + Dilithium
• Phase 2 (future): Dilithium-first, Ed25519 deprecated
• Phase 3 (future): Ed25519 sunset

Test plan

• Deploy without Dilithium keys — verify Ed25519-only credentials still work
• Generate Dilithium keypair and set env vars
• Issue credential — verify proof is an array with both signatures
• Verify dual-signed credential via /credentials/verify
• Verify old Ed25519-only credential still passes verification
• Check /.well-known/did.json includes Dilithium key when configured

Generated with Claude Code

[HaraldeRoessler]

Superseded by #7 which combines JCS + PQC to avoid merge conflicts.