Air Combat Replanning #3: Deterministic JCS & Ed25519 Frag Fix

This is the third installment in the engineering series on the Air Combat Dynamic Mission Replanning System (SDC). In the previous episode, Air Combat Mission Replanning #2: Stack Spike & Doctrine, we validated the technology stack spike against 6 criteria and structured the operational threat matrix (T1T12) and system bottleneck hypotheses (H1H9) grounded in the ROK 2022 Defense White Paper.

This article documents the engineering work conducted on October 24, 2026, focusing on the M6 messaging and cryptographic security core engine. We address the challenges of ensuring reliable, tamper-proof state synchronization between ground Command & Control (C2) nodes and airborne fighters (F-16C, F-15K) over tactical data links (Link 16). We detail our zero-dependency implementation of RFC 8785 JSON Canonicalization Scheme (JCS) to eliminate cross-runtime (.NET/C# vs CPython) floating-point drift, our raw wire-byte Ed25519 multi-signature envelope architecture, and our resolutions for Defect A-01 (0.7 KB small envelope fragmentation failure) and Defect B-05 (signed delta benchmark distortion).

Figure 1: RFC 8785 deterministic JCS canonical hashing pipeline, Ed25519 raw wire-byte cryptographic envelope, and Link 16 MTU fragmentation architecture.

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1. Target & Architecture

1.1 State Synchronization Under Tactical Data Link Constraints

In our dynamic mission replanning system (SDC), Airspace Control Orders (ACO), threat circular zones (THREAT), flight mission plans (sdc.plan/0), and delta replan packages (sdc.change/0) must be exchanged between ground C2 and airborne strike packages within strict sub-second latency bounds. Over lossy, bandwidth-constrained tactical radio links (Link 16, MTU 1,024 Bytes), the architecture had to satisfy four core engineering requirements:

1. Zero-Drift State Verification (H2 / I4 Invariant):
   • Ground and airborne nodes must verify that flight plans match not only by integer revision (version), but through a cryptographically strict content hash: plan_sha256 = SHA-256(JCS(plan)).
2. Cross-Runtime Determinism:
   • The system must yield identical byte-level canonical outputs across research prototypes (CPython 3.12) and operational embedded avionics targets (.NET 8 / C# or C++), completely eliminating floating-point serialization discrepancies and dictionary key ordering mismatches.
3. Multi-Role Cryptographic Verification (I5 Delegation):
   • Replan changes require multi-signature envelopes (sdc.envelope/0) matching change severity grades (Grade A: target coordinate updates, Grade B: waypoint modifications, Grade C: stores/weapons reassignment).
   • A single Ed25519 key signing as both planner and approver (self-approval) must be mechanically rejected (MISSING_APPROVAL).
4. Link 16 MTU Budgeting:
   • High-urgency Grade A updates (~0.7 KB) must fit within a single 1,024-byte Link 16 packet without chunking overhead.
   • Large multi-waypoint replans (Grade B, ~1.7 KB) must be deterministically partitioned into ~600-byte payload chunks (sdc.frag/0) with out-of-order reassembly and 60-second assembly timeouts.

         [Ground C2 Planner & Approver]                   [F-16C / F-15K Cockpit]
  +-----------------------------------------+       +-----------------------------------------+
  |  sdc.change/0 (Grade A/B/C)             |       |  sdc.envelope/0 Reception               |
  |  - JSON Patch delta                     |       |  - Ed25519 multi-signature verification |
  |  - Raw Wire Bytes Serialization         |       |  - Delegation table key & role check    |
  |  - Ed25519 Multi-Signatures             |       |  - plan_sha256 JCS state comparison     |
  +--------------------+--------------------+       +--------------------+--------------------+
                       |                                                 ^
                       v                                                 |
  +--------------------+--------------------+       +--------------------+--------------------+
  |  MTU Boundary Guard (1,024 Bytes)       |       |  Fragment Reassembly (from_fragments)   |
  |  - <= 1024 B: Atomic Single Packet      |       |  - set_sha256 integrity verification    |
  |  - > 1024 B : Chunk to sdc.frag/0       |       |  - 60s timeout reassembly buffer        |
  +--------------------+--------------------+       +--------------------+--------------------+
                       |                                                 ^
                       +-----------[ Link 16 Tactical Radio ]------------+

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2. Real Errors & Pitfalls

During the implementation and Monte Carlo testing of module M6 (src/sdc/m6.py), we encountered two critical engineering pitfalls.

2.1 Defect A-01: Single-Packet Fragmentation Trap (< 0.7 KB Envelope Rejection)

• Date Recorded: 2026-10-24
• Component: M6 Fragmentation Engine (to_fragments)
• Symptoms:
  • A Grade A change envelope (1 target coordinate update + 1 Ed25519 signature) serializes to approximately 655~700 Bytes, well below the 1,024-byte Link 16 limit.
  • However, when passed to the fragmentation module, the envelope was either erroneously partitioned into a single-item fragment set (sdc.frag/0), or aborted with a runtime exception: ValueError: Invalid chunk count.
• Raw Root-Cause Code:

  # [DEFECTIVE IMPLEMENTATION]
  def to_fragments(envelope_bytes: bytes, set_id: str, max_msg_bytes: int) -> list[dict]:
      parts = [envelope_bytes[i:i + chunk] for i in range(0, len(envelope_bytes), chunk)]
      # Defect: Artificially rejecting chunk count == 1 as an error
      if len(parts) <= 1:
          raise ValueError("Invalid fragmentation: chunk count must be greater than 1")
      return [...]

• Consequences:
  • Any upper-layer pipeline invoking to_fragments() without a pre-check failed immediately on all high-priority Grade A coordinate updates.
  • Furthermore, wrapping single-message payloads into sdc.frag/0 imposed unnecessary Base64 encoding and header overheads (set_id, index, count, set_sha256), triggering avoidable reassembly timeout risks under packet loss.

2.2 Defect B-05: Delta vs Full-Plan Benchmark Distortion (H9 Collapse Risk)

• Date Recorded: 2026-10-24 (Discovered) $\rightarrow$ 2026-10-27 (Corrected in System Requirements)
• Component: System Bottleneck Hypothesis H9 (Bandwidth conservation via delta updates)
• Symptoms:
  • Preliminary empirical measurements in technical feasibility study F1/F3 indicated that JSON Patch deltas yielded no size benefit over full plans: Unsigned Full Plan = 526 Bytes vs Delta Envelope = 811 Bytes.
  • This counter-intuitive measurement threatened to invalidate our architectural justification for delta-based in-flight replanning.
• Root Cause Analysis:
  • The test harness had conducted an asymmetrical, unfair comparison: an unsigned raw flight plan (526 Bytes) was benchmarked against a cryptographically signed delta envelope containing two 64-byte Ed25519 signatures, Base64 strings, and envelope metadata (811 Bytes).
  • In military avionics, full flight plans transmitted in-flight must also be sealed within signed envelopes. Benchmarking them under identical security preconditions reversed the conclusion.

[Distorted Comparison (Root of B-05)]
  - Raw Unsigned Flight Plan : 526 Bytes  <-- Unfair basis
  - Signed Delta Envelope    : 811 Bytes  [False Inversion!]

[Apples-to-Apples Cryptographic Comparison (Corrected)]
  - Signed Full Plan Envelope : 920 Bytes
  - Signed Grade A Delta      : 811 Bytes  (109 Bytes smaller, 1 single packet)
  - Signed Grade B Delta (10 WPs): 1,740 Bytes (3 fragments)

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3. Deep Root Cause & Clean Fix

3.1 Zero-Dependency RFC 8785 JCS Engine Implementation

To achieve deterministic hashing across disparate runtimes without external dependencies, we implemented RFC 8785 directly within src/sdc/m6.py.

Figure 2: Standard CPython vs RFC 8785 ECMAScript float canonicalization, Appendix B 24 test vector verification, and UTF-16 code-unit property sorting.

1) ECMAScript Number Formatting (_jcs_number)

Standard Python json.dumps() preserves trailing zeros (6000.0 $\rightarrow$ "6000.0") and produces negative zero (-0.0). RFC 8785 §3.2.2.3 strictly mandates ECMAScript Number.prototype.toString() conventions:

• Integer-valued floats (6000.0) must strip decimal components ("6000").
• Negative zero (-0.0) must normalize to "0".
• Exponential notation is restricted: fixed point is required when exponent $n$ satisfies $k \le n \le 21$ and leading zero padding when $-6 < n \le 0$; scientific notation is applied only outside these boundaries (1e+21).

def _jcs_number(x) -> str:
    """ECMAScript Number.prototype.toString canonicalization (RFC 8785 §3.2.2.3)."""
    x = float(x)
    if not math.isfinite(x):
        raise ValueError("JCS: NaN and Infinity are not allowed")
    if x == 0:
        return "0"
    mant, _, exp = repr(abs(x)).partition("e")
    ip, _, fp = mant.partition(".")
    s, point = ip + fp, len(ip) + (int(exp) if exp else 0)
    stripped = s.lstrip("0")
    point -= len(s) - len(stripped)
    s = stripped.rstrip("0")
    k, n = len(s), point  # Value = 0.s × 10^n
    if k <= n <= 21:
        out = s + "0" * (n - k)
    elif 0 < n <= 21:
        out = s[:n] + "." + s[n:]
    elif -6 < n <= 0:
        out = "0." + "0" * (-n) + s
    else:
        e = n - 1
        out = s[0] + ("." + s[1:] if k > 1 else "") + "e" + ("+" if e >= 0 else "-") + str(abs(e))
    return ("-" if x < 0 else "") + out

2) UTF-16 Code-Unit Key Sorting (Surrogate Pair Trap Avoidance)

Sorting dictionary keys by standard Unicode code points violates RFC 8785 §3.2.3 because code-point sorting places Astral Plane characters (such as emojis or extended symbols with surrogate pairs) after Basic Multilingual Plane (BMP) characters. We resolved this by encoding keys to UTF-16 BE:

# RFC 8785 §3.2.3: Sort keys by UTF-16 code units
keys = sorted(value, key=lambda k: k.encode("utf-16-be"))
return "{" + ",".join(_jcs_string(k) + ":" + _jcs(value[k]) for k in keys) + "}"

• Verification: Sorting {"｡": 1, "\U0001f600": 2, "a": 3}: standard Python sort puts ｡ (U+FF61) before \U0001f600 (U+1F600). However, under UTF-16 code-unit sorting, the high surrogate 0xD83D of \U0001f600 precedes 0xFF61, deterministically producing {"a":3,"\U0001f600":2,"｡":1}.

3) RFC 8785 Appendix B IEEE-754 Full Vector Verification

We extracted all 24 IEEE-754 test vectors from RFC 8785 Appendix B—including subnormals (5e-324, -5e-324), maximum finite doubles (1.7976931348623157e+308), precision boundaries (9007199254740992), and fractions (9.999999999999997e-7). Our test suite (test_jcs_numbers_rfc8785_appendix_b) passed 24 out of 24 vectors (100% bit-exact conformance).

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3.2 Raw Wire-Byte Signing Envelope Architecture

To eliminate signature verification breakage across varying runtime platforms, we established the rule that signatures are generated directly over the exact transmitted UTF-8 byte stream of the body string, rather than re-canonicalized runtime objects:

def dumps(obj) -> str:
    """Strict serialization: no whitespace, preserved UTF-8."""
    return json.dumps(obj, ensure_ascii=False, separators=(",", ":"), allow_nan=False)

def seal(body: dict, signers: list[tuple[str, Ed25519PrivateKey]]) -> dict:
    """Serializes body once; all signers sign the identical raw byte sequence."""
    raw = dumps(body)
    data = raw.encode("utf-8")
    sigs = [{"key_id": kid, "sig": base64.b64encode(key.sign(data)).decode()} for kid, key in signers]
    return {"schema": "sdc.envelope/0", "body": raw, "sigs": sigs}

Because receiving nodes extract envelope["body"].encode("utf-8") prior to JSON parsing, the Ed25519 signature remains valid regardless of whether the receiving runtime parses numbers as 6000 or 6000.0.

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3.3 Defect Resolution: A-01 and B-05

Figure 3: Clean guard fix for Defect A-01 (single-packet MTU passthrough) and fair cryptographic envelope comparison for Defect B-05.

1) A-01 Fragmentation Guard Patch (src/sdc/m6.py)

We re-architected to_fragments() with an explicit guard clause:

def to_fragments(envelope_bytes: bytes, set_id: str, max_msg_bytes: int) -> list[dict]:
    """Splits envelope bytes into chunks. Each serialized chunk <= max_msg_bytes."""
    # [Clean Fix] If envelope fits in a single message, notify caller to send directly
    if len(envelope_bytes) <= max_msg_bytes:
        raise ValueError("fits in one message; send the envelope as is")
    
    digest = hashlib.sha256(envelope_bytes).hexdigest()
    head = {"schema": "sdc.frag/0", "set_id": set_id, "index": 9999, "count": 9999, "set_sha256": digest, "data": ""}
    chunk = (max_msg_bytes - len(dumps(head).encode("utf-8"))) // 4 * 3
    if chunk <= 0:
        raise ValueError("max_msg_bytes too small")
    parts = [envelope_bytes[i:i + chunk] for i in range(0, len(envelope_bytes), chunk)]
    return [{"schema": "sdc.frag/0", "set_id": set_id, "index": i, "count": len(parts), "set_sha256": digest,
             "data": base64.b64encode(p).decode()} for i, p in enumerate(parts)]

The dispatch layer verifies len(env_bytes) ≤ 1024 and transmits sdc.envelope/0 atomically without calling to_fragments(). High-urgency Grade A coordinate updates are guaranteed single-packet delivery.

2) B-05 Envelope Parity & Break-Even Boundary

Under identical dual-signed Ed25519 envelopes:

• Grade A Target Update: Signed Delta Envelope (811 Bytes, 1 packet) vs Signed Full Plan (920 Bytes, 1 packet) $\rightarrow$ 109 Bytes saved (11.8% bandwidth reduction).
• Break-Even Analysis: Delta updates retain bandwidth superiority as long as the modified waypoints do not exceed 63% to 70% of total route waypoints.
• S4 Lossy Environment (30% Packet Drop Rate) Simulation:
  • Full plan transmission (3 chunks): 18.6% unconfirmed rate due to packet fragmentation loss.
  • JSON Patch delta transmission: 0.8% unconfirmed rate (23x reliability improvement).

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4. Verification & Quality Metrics

Following implementation and defect resolution:

1. 87/87 M6 Unit Tests Passing:
   • tests/test_m6.py: Verified multi-signature signing, tampered body detection, unknown key rejection, RFC 8785 24-sample bit matching, UTF-16 code unit ordering, and out-of-order fragment reassembly.
2. 100% Branch Coverage:
   • coverage run -m pytest tests/test_m6.py confirmed 100.0% branch coverage (42/42 branches) for src/sdc/m6.py.
3. Cross-Platform Conformance Vectors (vectors_v0.json):
   • Staged a 25 KB canonical test suite (vectors_v0.json) covering JCS edge cases, Ed25519 signatures, and out-of-order chunk assembly to guarantee cross-language parity for future .NET implementations.

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5. Next Steps

With M6 messaging and cryptographic determinism established, our tactical communication pipeline is hardened against serialization drifts and packet loss. In the next episode:

• [EP. 04] 2026-10-24 (2): 5 Operational Scenarios (S1S5) and I1I5 Safety Invariant Verification Engine
• Implementing 27 safety rejection reason codes across operational, airspace, and terrain boundaries,
• Enforcing terrain elevation clearance (DEM) and radar viewshed safety gates,
• Validating 57 synthetic Q5 test vectors with 100% pass rates.