Introduction

The Symmetric Authenticated Tunneling Protocol (SATP) is a next-generation secure communications framework designed to establish cryptographically authenticated and confidential tunnels using only symmetric primitives. SATP eliminates the reliance on online public-key infrastructures (PKI) and ephemeral asymmetric exchanges, achieving equivalent levels of confidentiality, integrity, and forward secrecy through pre-provisioned hierarchical symmetric keys and authenticated encryption.

SATP is optimized for environments where centralized trust authorities exist, such as industrial control, secure embedded systems, field communications, and closed network infrastructures. Its lightweight design enables high-speed, low-latency encryption without the computational cost of traditional hybrid or asymmetric protocols.

Statement of the Problem

Traditional secure tunneling protocols such as TLS, SSH, and IKE rely on asymmetric key exchange mechanisms and certificate-based authentication. These systems present challenges in environments where:

Certificate issuance and revocation infrastructure is unavailable.
Devices must operate offline or under intermittent connectivity.
Long-term cryptographic independence from public-key systems is required.

In such cases, asymmetric operations increase computational cost and complexity, while their key lifecycles remain vulnerable to compromise or certificate mismanagement. SATP was designed to provide a provably secure alternative in these constrained or regulated environments.

The SATP Solution

SATP uses a hierarchical symmetric key structure combined with ephemeral per-session derivations to establish a tunnel with full AEAD protection. The protocol consists of two phases:

Handshake Phase: The client and server exchange nonces and derive session transmit and receive keys (Kt, Kr) using SHAKE256-based expansion from a pre-provisioned device key (Kc,i). A session hash confirms mutual possession of the shared key material.
Encrypted Channel Phase: All communication occurs through an AEAD stream cipher (RCS or AES-GCM). Packet headers are serialized and included as associated data to bind sequence numbers and timestamps into the authentication domain.

The server then performs optional client authentication using a hardened SCB (SHAKE Cost-Based) passphrase verification. This process occurs after the encrypted channel is active, ensuring that credentials are never exposed in plaintext.

Hierarchical Key Structure

SATP employs a secure hierarchical key derivation system:

A master domain key (K₀) generates branch keys (Kb).
Each branch key derives device keys (Kc,i), each unique to a device.
Device keys are single-use: they are burned after one session to ensure perfect forward secrecy.

Session keys (Kt and Kr) are further derived from Kc,i and a mixed nonce (Nh = SHAKE256(Nh_c ‖ Nh_s)), ensuring bidirectional key independence. This structure isolates compromise to a single device and session, maintaining network-wide containment.

Advantages of SATP

Post-Quantum Symmetric Security: Uses SHAKE256 and RCS, offering resilience against quantum adversaries within symmetric bounds.
Forward Secrecy: Each session uses a unique one-time device key and nonce, preventing retrospective decryption.
Low Computational Cost: No elliptic-curve or modular exponentiation operations; handshake completes in two messages.
Replay Resistance: Timestamp and sequence enforcement validated before decryption.
Implementation Simplicity: Compact and MISRA-compliant codebase suitable for embedded and industrial systems.

Applications

SATP is applicable to:

Industrial and SCADA networks requiring secure but deterministic links.
Field-deployed or mobile systems operating offline from a central authority.
Embedded devices with hardware-enrolled symmetric credentials.
High-performance servers or IoT gateways needing rapid, low-overhead tunnels.
Environments where PKI management or asymmetric cryptography is impractical.

Conclusion

SATP provides a fully symmetric, authenticated encryption framework with forward secrecy and high operational efficiency. Its reliance on hierarchical symmetric derivations instead of online public-key exchanges allows deployment in closed or offline systems while preserving the cryptographic strength and integrity of traditional secure tunnels. By integrating SCB-based authentication, timestamp-bound replay protection, and deterministic AEAD operations, SATP achieves strong, scalable, and verifiable tunnel security within a lightweight implementation footprint.

License

Author: John G. Underhill

Date: 2025-11-04