Mutual TLS Peer-Authentication Contract Design

Wrote docs/architecture/tasks/payment-mtls-001/mtls-design/mtls-contract-spec.md. Complete Mutual TLS peer-authentication specification establishing TLS 1.3 cipher restrictions, SPIFFE SAN validation, 24-hour certificate lifecycles, and secure XFCC header ingestion.

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skill: mtls-design
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# Mutual TLS Peer-Authentication Spec: Core Payment Settlement [MTLS-PAY-001]

## Summary

This specification establishes the Mutual TLS (mTLS) peer-authentication contract, cryptographic cipher suite constraints, client certificate validation sequence, and identity forwarding rules for `payment-settlement-service v3.0` under run ID `payment-mtls-001`. It secures inter-service communications across 40 microservices sustaining 5,200 handshakes/second on AWS EKS. It decisively eliminates the unauthorized access and cipher downgrade vulnerabilities demonstrated in incident SEC-4930 (where permissive mTLS and legacy TLS 1.2 RSA ciphers allowed an unauthenticated rogue pod to execute fraudulent settlement calls). The contract enforces strict **TLS 1.3 exclusive** transport, restricts cipher suites to modern AEAD ciphers (`TLS_AES_256_GCM_SHA384`, `TLS_CHACHA20_POLY1305_SHA256`), mandates SPIFFE URI Subject Alternative Name (SAN) identity matching, caps client certificate lifespan at **24 hours** via SPIRE automated rotation, and enforces strict `X-Forwarded-Client-Cert` (XFCC) sanitization.

## Detailed Description

Operating microservice architectures with permissive mTLS or optional client verification creates a false sense of security. When servers accept connections without validating peer certificates, attackers who breach a perimeter pod can initiate unauthenticated East-West lateral connections across the cluster. Enforcing strict TLS 1.3 with mandatory client certificate verification guarantees non-repudiation and cryptographic workload identity.

Caller Pod: payment-authorization
│
▼ (Client Hello: TLS 1.3, Curve25519 Key Share)
[ Envoy Ingress Sidecar: payment-settlement ]
├── 1. Requests Client Certificate: CertificateRequest
├── 2. Receives Peer Cert: Leaf signed by Bank-Intermediate-CA
├── 3. Enforces Cipher Match: TLS_AES_256_GCM_SHA384
└── 4. SAN Validation Gate:
├── Extracts URI SAN: spiffe://bank.internal/ns/pay/sa/pay-auth
└── Whitelist Match: Authorized Caller Verified
│
┌──────────┴──────────┐
▼ (Valid Peer Cert) ▼ (Missing / Unapproved SAN / Expired)
Pass to Port 8080 TCP Reset / HTTP 403: ERR_MTLS_PEER_UNAUTHORIZED
Sanitize & Inject XFCC

### Criteria and weights

| Criterion | Why it matters here | Weight | Source of the weight |
|---|---|---|---|
| Zero-Trust Bidirectional Peer Authentication | Unauthenticated microservices must be blocked from establishing network sockets to settlement engines (SEC-4930). | 0.40 | David O'Reilly (CISO SecOps) |
| Handshake Latency Overhead (p99 <= 4.0 ms) | Connection establishment must not degrade 5,200 req/sec high-throughput settlement execution. | 0.25 | Marcus Vance (Principal Architect) |
| Ephemeral Certificate Lifespan (24h TTL) | Short-lived certificates eliminate the need for complex, latency-inducing real-time CRL distribution. | 0.20 | Corporate Cryptographic Policy |
| Secure Ingress Header Sanitization (XFCC) | Upstream proxies must sanitize client certificate headers to prevent header spoofing attacks. | 0.15 | Application Security Standard |


### Comparison

| mTLS Architecture Candidate | Protocol Floor | Client Cert Enforcement | Peer Identity Standard | Evaluation |
|---|---|---|---|---|
| Option A: Permissive TLS 1.2 (Legacy) | TLS 1.2 (RSA ciphers) | Optional (Permissive mode) | Common Name (CN) string | Rejected: Caused SEC-4930 unauthorized settlement breach. |
| Option B: Strict TLS 1.2 with DNS SANs | TLS 1.2 (ECDHE only) | Mandatory | DNS SAN (`*.bank.internal`) | Rejected: DNS SANs lack workload role granularity and namespace isolation. |
| Option C: Strict TLS 1.3 + SPIFFE SANs (Chosen) | TLS 1.3 Exclusive | Mandatory (Strict mode) | SPIFFE URI SAN | Selected: Modern AEAD ciphers, sub-4ms handshakes, strict identity. |


### Result

Option C is selected. Exclusive TLS 1.3 eliminates handshake roundtrips and obsolete ciphers; SPIFFE URI SANs provide granular workload identity.

---

### Required Mechanisms

#### 1. TLS Protocol & Cipher Suite Constraints [MC-TC-01]
- **Protocol Version**: TLS 1.3 strictly enforced (`min_version: TLS_1_3`, `max_version: TLS_1_3`). Connections offering TLS 1.2 or lower are terminated during ClientHello.
- **Approved Cipher Suites**:
  1. `TLS_AES_256_GCM_SHA384` (Primary)
  2. `TLS_CHACHA20_POLY1305_SHA256` (Secondary)
- **Key Exchange**: ECDHE with Curve25519 (`x25519`) or NIST P-256 (`secp256r1`).
- **Session Resumption**: 0-RTT early data resumption is **disabled** (`early_data: false`) to prevent replay attacks on mutating settlement operations.

#### 2. Client Certificate SAN Validation Pipeline [MC-VP-01]
- The verifying server extracts and evaluates Subject Alternative Names:
  - **Type**: Must contain `URI` type SAN matching format:
    `spiffe://bank.internal/ns/{namespace}/sa/{serviceaccount}`
  - **Whitelisted Callers for `payment-settlement-service`**:
    - `spiffe://bank.internal/ns/payments-prod/sa/payment-auth-sa`
    - `spiffe://bank.internal/ns/clearing-prod/sa/clearing-engine-sa`
- If the client presents a valid X.509 certificate whose SPIFFE ID is not in the approved caller allowlist, the connection is aborted immediately with `HTTP 403 Forbidden` and diagnostic `ERR_PEER_SAN_DISALLOWED`.

#### 3. Ephemeral Certificate Lifecycles & SPIRE Rotation [MC-LC-01]
- Workload certificates are issued by SPIRE (SPIFFE Runtime Environment) agents running on each EKS worker node.
- **Validity Duration**: Exactly **24 hours** (86,400 seconds).
- **Automated Rotation**: SPIRE automatically rotates certificates at **12 hours remaining** (50% lifespan), streaming new keys to Envoy via Secret Discovery Service (SDS) with zero TCP connection drops.

#### 4. Proxy Header Ingestion & XFCC Sanitization [MC-XC-01]
- Ingress proxies terminating TLS at edge must sanitize `X-Forwarded-Client-Cert` (XFCC) headers:
  - All client-supplied incoming XFCC headers are stripped unconditionally.
  - The proxy constructs a verified, signed XFCC header containing:
    `Hash={SHA256};Subject="";URI="spiffe://bank.internal/ns/payments-prod/sa/payment-auth-sa"`
  - Downstream applications trust XFCC headers if and only if received from the local trusted loopback proxy.

---

### Invariants and Contracts

    Exclusive TLS 1.3 Protocol Invariant [INV-MTLS-01]
      Production settlement interfaces must mandate TLS 1.3 exclusively.
      Negotiating TLS 1.2, TLS 1.1, or non-AEAD cipher suites is strictly blocked by proxy configuration.

    Mandatory SPIFFE URI SAN Validation [INV-MTLS-02]
      Peer identity must be asserted via SPIFFE URI Subject Alternative Names.
      Verifying peer identity based solely on certificate Subject Common Name (CN) is prohibited.

    Zero-RTT Early Data Prohibition [INV-MTLS-03]
      TLS 1.3 0-RTT early data is forbidden on financial settlement interfaces.
      All handshakes must complete the full 1-RTT cryptographic exchange to eliminate replay vectors.

## Explicit Unknowns

- SPIRE Agent CPU utilization overhead during cluster-wide node restarts triggering 18,000 simultaneous certificate issuances (G-1).
- Envoy SDS memory leak risks during continuous 12-hour certificate rotation loops over 30 days (G-2).

## Traceability

| Claim | Classification | Source | Freshness |
|---|---|---|---|
| 40 internal microservices on AWS EKS | provided | Infrastructure intake | Current |
| Peak 5,200 mTLS handshakes/sec | provided | Traffic intake | Current |
| Incident SEC-4930 unauthorized settlement breach | provided | Post-mortem evidence | Historical |
| Handshake latency budget p99 <= 4.0 ms | provided | Performance SLA | Current |
| TLS 1.3 exclusive enforcement | decided | David O'Reilly (CISO SecOps) | 2026-09-15 |
| SPIFFE URI SAN identity standard | decided | Marcus Vance (Principal Architect) | 2026-09-15 |


## Verification

No validator was supplied, so no command was run.

Reviewer self-check against mTLS architecture standards:
- **Protocol Safety**: PASS. TLS 1.3 exclusive; modern AEAD ciphers; 0-RTT disabled.
- **Identity Rigor**: PASS. SPIFFE URI SAN validation eliminates ambiguous Common Name assumptions.
- **Credential Hygiene**: PASS. 24-hour short-lived certificates eliminate complex CRL distribution.
- **Markdown Hygiene**: PASS. Native Markdown syntax strictly adheres to `rule_markdown.md`.

## Open Decisions

- `DEC-MTLS-01`: David O'Reilly to determine whether post-quantum Kyber key exchange should be enabled in TLS 1.3 client hellos as an experimental cipher suite (Owner: David O'Reilly).

## Next steps

1. Marcus Vance configures Envoy proxy filter enforcing `TLS_1_3` and SPIFFE SAN allowlists.
2. Security team deploys SPIRE Server and Node Agents across production EKS clusters.
3. Conduct staging penetration drill attempting to connect using TLS 1.2 and self-signed certificates to verify immediate TCP reset.