artifacts/incoming

Ephemeral Provenance Capsule (HD + Commit/Reveal)

artifacts/incoming/ephemeral_provenance_capsule_hd_commit_reveal.md

Rendered from markdown source. Open raw source on GitHub.

Ephemeral Provenance Capsule (HD + Commit/Reveal)

Intent

Create a portable provenance capsule (“data cube”) whose contents are freely copyable, but whose authenticity standing is only provable by consent.

Default state: anyone can copy pixels/messages; verification yields only “internally consistent, signed by some ephemeral pseudonym.”

Reveal state: the author can later prove anti-backdating and (optionally) authorship/control for a specific session without linking other sessions or devices.

This implements unlinkable-by-default, linkable-by-consent 🝁 and forms a self-contained loop 🝳.

---

Threat model + guarantees (plain language)

Guarantees

Integrity: the media/message bytes are detectable as altered.
Session authenticity: the session signing key existed by time T (anti-backdating) when revealed.
Unlinkability: separate sessions (and separate devices) are not linkable by observers by default.
Selective opening: the author can open one session without revealing anything about other sessions.

Non-guarantees (honest limits)

Does not prove the depicted scene is true (staging, screen re-filming, sensor spoofing remain possible).
Hardware can be compromised in principle; this design raises cost and adds audit signals.

---

Core primitives

Hardware primitive

A secure element (IronKey/TPM-class) that:
Generates secrets internally
Enforces non-exportable private keys
Optionally wipes/locks on tamper

Cryptographic primitives

Hash: SHA-256
Signature: Ed25519 (or similar)
Commitment: Commit(x, r)
Minimal: SHA-256( "EPC|v1|" || x || r ) (where r is 32 random bytes)
Stronger: a formal commitment scheme (Pedersen) if desired
Merkle tree over chunk hashes for large media

Anchoring primitive

A time witness / transparency anchor for commitments:
Witness circle (N-of-M signatures) OR
An append-only public log (CT-style) OR
Both

---

Keying model (HD-style, unlinkable sessions)

Device-level secret (never exported)

Secure element generates S_master (random seed). Not externally readable.

Session key derivation options

Two viable models:

Model A (stateless ephemeral keys — simplest + strongest privacy):

For each session, device generates a fresh sk_session internally.
Device exports only pk_session.
Device may discard sk_session after sealing the capsule (or keep it for later live proofs).

Model B (HD derivation — convenience + reproducibility):

Device derives per-session key using hardened KDF:
sk_session = KDF_hardened(S_master, session_entropy)
session_entropy should be random per session and can be stored in the capsule or withheld.

Default recommendation: Model A unless you truly need deterministic regeneration.

---

Lifecycle

0) Provisioning (optional; per session is fine)

The ephemeral root must be per session, not per device.

For each session:

Device creates sk_session internally, exports pk_session.
Controller app generates random r_session (32 bytes).
Compute commitment:

C_session = Commit(pk_session, r_session)

Anchor C_session using one of:

witness circle receipts
transparency log receipt

Store r_session offline (this is the opening key).

Destroying r_session = session becomes permanently unclaimable.

---

1) Capture / package capsule

Capture media/message.
Chunk if needed (e.g., 1–4MB chunks).
Compute chunk hashes h_i = SHA-256(chunk_i).
Compute Merkle root root = MerkleRoot(h_1..h_n).
Build manifest (see format below).
Device signs manifest hash:

mhash = SHA-256(canonical_manifest_without_signature)
sig_session = Sign(sk_session, mhash)

Package capsule:

media chunks
manifest
sig_session

---

2) Default verification (no reveal)

Verifier can check:

chunk hashes match Merkle root in manifest
signature validates under pk_session
anchor receipts exist (but may be opaque without opening)

Conclusion available to verifier:

“This artifact is internally consistent and signed by an ephemeral pseudonym.”

No link to other sessions. No link to a real identity.

---

3) Reveal / opening (by consent)

To prove anti-backdating and session authorship/control:

Reveal package includes:

r_session
(optionally) additional proof of custody/intent (witness statements, live device challenge)

Verifier checks:

Commit(pk_session, r_session) == C_session
C_session was anchored at/before time T via witness receipts/log receipt
media signatures and Merkle integrity

Optional stronger proof (live):

device signs a verifier-provided challenge using sk_session (requires device retained/regenerated key)

---

4) Discard

Destroy r_session to remove future ability to prove pre-commitment.
Destroy device/session key to remove ability to sign as that session again.

---

Manifest format suggestion (EPC-MANIFEST v1)

Encoding: canonical JSON (JCS) or CBOR.

{
  "schema": "epc.manifest.v1",
  "capsule_id": "b32:...",
  "created_utc": "2026-02-22T14:03:12Z",

  "media": {
    "type": "video/mp4",
    "chunk_bytes": 2097152,
    "chunk_count": 184,
    "hash_alg": "sha256",
    "merkle_root": "hex:...",
    "total_bytes": 385882112,
    "codec_hint": "h264+aac"
  },

  "session": {
    "sig_alg": "ed25519",
    "pk_session": "b64:...",

    "commitment": {
      "commit_alg": "sha256-commit-v1",
      "C_session": "hex:...",

      "anchor": {
        "type": "witness_circle",
        "threshold": 3,
        "receipts": [
          { "witness_id": "w1", "receipt": "b64:..." },
          { "witness_id": "w2", "receipt": "b64:..." },
          { "witness_id": "w3", "receipt": "b64:..." }
        ]
      }
    }
  },

  "privacy": {
    "metadata_policy": "minimal",
    "notes": "No location/sensor metadata included"
  },

  "lineage": {
    "prev_capsule_manifest_hash": null
  }
}

Notes:

capsule_id can be SHA-256(pk_session || merkle_root || created_utc).
created_utc is informational unless also timestamped/anchored; treat as advisory.
prev_capsule_manifest_hash enables a private or revealable chain (optional).

---

Receipt format suggestion (EPC-WITNESS-RECEIPT v1)

Witnesses sign only the commitment, not the media.

Receipt payload (what is signed):

{
  "schema": "epc.receipt.v1",
  "witness_id": "w1",
  "issued_utc": "2026-02-22T13:58:00Z",
  "commit_alg": "sha256-commit-v1",
  "C_session": "hex:...",

  "context": {
    "purpose": "commitment_witness",
    "channel": "email|https|inperson",
    "nonce": "b64:..."
  }
}

Receipt object included in manifest:

{
  "payload": { /* as above */ },
  "sig_alg": "ed25519",
  "witness_pk": "b64:...",
  "signature": "b64:..."
}

Verifier checks:

witness signature validates on payload
witness public key is one of the accepted witness set (local policy)
issued_utc is credible because the verifier trusts the witness (or multiple witnesses)

---

Anchor strategies (practical)

Witness circle (governance-light)

Choose 3–7 witnesses (people or services).
Require 2–4 receipts for high confidence.
Diversity matters more than count.

Transparency log (CT-style)

Submit C_session to an append-only Merkle log.
Include log inclusion proof in manifest.

Hybrid

Log inclusion proof + 1–2 human witnesses.

---

Linkability control knobs

Default (max privacy)

One session key per capsule.
Do not reuse pk_session.
No cross-capsule references.

Optional (thread continuity)

Add prev_capsule_manifest_hash for a chain.
Still keep session keys rotating; chain link can be revealed later.

---

Verification summary (what a verifier does)

Hash chunks → Merkle root; compare to manifest.
Canonicalize manifest (exclude signature fields) → hash.
Verify sig_session under pk_session.
If opened: verify Commit(pk_session, r_session) == C_session.
Verify anchor receipts/log proofs for C_session.

---

Open questions to decide later

Do we require a manufacturer hardware attestation blob at all? (Pros: anti-cloning. Cons: adds centralized root.)
Should r_session be split via Shamir secret sharing for safer custody?
Should we support redaction/derivative proofs (e.g., cropped video) via Merkle segment proofs?

---

One-line mantra

Copyable pixels; non-copyable standing.