> 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/4.3-post-quantum-transport-and-symmetric-security.md).

# 4.3 Post-Quantum Transport & Symmetric Security

**Inter-Validator Session Security**

All P2P connections use **ML-KEM-768**, standardized as the Module-Lattice-Based Key-Encapsulation Mechanism in FIPS 203 (2024). ML-KEM provides post-quantum forward secrecy: even if an adversary records all encrypted traffic today and later acquires a quantum computer, past sessions remain undecryptable. ML-KEM-768 is paired with classical **X25519** in a hybrid handshake (**PQXDH — Post-Quantum Extended Diffie-Hellman**). This defense-in-depth construction ensures session keys remain secure as long as *either* the classical or post-quantum component remains unbroken, providing a secure transition pathway during cryptographic migration.

**Hashing & Data Integrity**

All hash operations use **BLAKE2b-256**, selected for:

* **128-bit post-quantum security** (Grover’s algorithm halves effective bits)
* **Higher throughput** than SHA-256/SHA-3 on modern 64-bit architectures
* **Alignment** with Dilithium2’s NIST Level 2 security target

BLAKE2b-256 is used for block digest computation, transaction pre-hashing, asset/token ID generation, and general-purpose data integrity checks.

**Encryption-at-Rest & Key Derivation**

| Primitive         | Purpose                                            | PQ Security                              |
| ----------------- | -------------------------------------------------- | ---------------------------------------- |
| AES-256-GCM       | Database encryption (RocksDB/LMDB column families) | 128-bit PQ (Grover mitigation)           |
| Argon2id          | Key derivation from operator passphrase            | Quantum-safe (memory-hard symmetric KDF) |
| BLAKE3 keyed-hash | Internal key derivation & MAC                      | Quantum-safe (symmetric construction)    |

All symmetric and KDF primitives are inherently quantum-safe, as quantum computers offer at most quadratic speedup against symmetric cryptography, neutralized by 256-bit key lengths.


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