> For the complete documentation index, see [llms.txt](https://polarischain-1.gitbook.io/polarischain/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://polarischain-1.gitbook.io/polarischain/4.-cryptographic-foundations-and-compliance-framework/4.5-cryptographic-agility-and-migration-choreography.md).

# 4.5 - Cryptographic Agility & Migration Choreography

The protocol is designed to migrate to alternative or higher-security schemes (Dilithium3/5, Falcon, hybrid constructions, or future standards) **without hard forks**.

**Flag-Byte Scheme Tagging**

Each signature, public key, and address representation is prefixed with a single **flag byte** indicating the cryptographic scheme:

| Flag        | Scheme                     | Status           |
| ----------- | -------------------------- | ---------------- |
| `0x01`      | Dilithium2 / ML-DSA-44     | Default in v1.0+ |
| `0x02`      | Dilithium3 / ML-DSA-65     | Reserved         |
| `0x03`      | Dilithium5 / ML-DSA-87     | Reserved         |
| `0x10–0x1F` | Hybrid (PQ + classical)    | Reserved         |
| `0x20–0x2F` | Falcon variants            | Reserved         |
| `0x80–0xFE` | Future PQ schemes          | Reserved         |
| `0xFF`      | Reserved (multisig prefix) | Reserved         |

This design enables:

* Phased migration without breaking existing keys
* Validators and users to operate on different schemes simultaneously
* Gradual rotation (e.g., Dilithium2 → ML-DSA-87 over 5–10 years)
* Deployment of hybrid schemes for defense-in-depth during transitions

The flag byte adds only 1 byte per signature/key — negligible relative to Dilithium2 sizes.

**Migration Choreography**

Scheme migration is a governance-coordinated event following this pattern:

1. **Binary Update:** New scheme added to validator binary; flag-byte handler routes to new verifier.
2. **Transition Epoch:** All validators sign with both old and new schemes for redundancy.
3. **Protocol Flip:** Consensus version update deprecates old scheme; new transactions must use new flag.
4. **Deprecation Window:** Existing balances under old scheme remain spendable for \~1 year during key rotation.
5. **Enforcement:** Transactions signed with deprecated flag are rejected.

This staged approach handles routine standard maturation and emergency response (§8.7 Disaster Recovery).


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