Snapshot reproducibility contract

When you paste a Subspace Trace URL into an IC memo, you need the URL to still resolve to the same facts 6 months later. This page documents the contract that guarantees it.

What's frozen

• The JSON payload returned by the snapshot URL is byte-identical across reads, forever.
• The SHA-256 hash printed on the PDF watermark matches the JSON.
• The PDF visually-identical on regeneration (content + layout + fonts pinned).

"Byte-identical PDF" is intentionally NOT a guarantee — PDF metadata (creation timestamp embedded in the file) varies across regeneration runs. We hash the JSON, not the PDF bytes. Diligence verification proceeds against the JSON.

Verification flow

Given any Subspace snapshot URL — say one you found in an IC memo:

https://thesubspace.io/trace/cloudflare.com?snapshot=<HASH>&sig=<SIG>

You can verify it hasn't been tampered with in three steps:

1. Fetch the JSON payload:

curl https://thesubspace.io/api/v1/snapshots/<HASH>?sig=<SIG>

2. Canonicalize the response per RFC 8785 (JSON Canonicalization Scheme) and compute its SHA-256:

# Using the npm package canonicalize:
node -e "const c=require('canonicalize');const crypto=require('crypto');
const json=require('fs').readFileSync('/dev/stdin','utf8');
process.stdout.write(crypto.createHash('sha256').update(c(JSON.parse(json))).digest('hex'));"

3. Compare the computed hash to the <HASH> in the URL — and to the hash printed in the PDF footer.

• Match: the JSON has not been modified since snapshot creation. The PDF you're looking at is reproducible from this URL.
• Mismatch: someone has corrupted Subspace's snapshot store, or your PDF was edited after generation. Contact us — this should never happen.

Canonicalization (RFC 8785)

We canonicalize the JSON payload before hashing so that key reordering, whitespace, or number-format drift across JSON serializers does not change the hash. The canonical form:

• Object keys sorted by UTF-16 code unit at every depth.
• No insignificant whitespace.
• Numbers serialized via ECMAScript 2019 Number.prototype.toString(); no trailing zeros, no scientific notation for typical magnitudes.
• String escaping per RFC 8259.

Tampering defense (the sig parameter)

The sig query parameter is an HMAC-SHA256 over {hash}|{owner_org}|{tier_at_creation} keyed by a server-side secret. It defends against a forged URL that claims a higher tier-at-creation (which would otherwise let a free-tier snapshot bypass the paid-tier-resolvability gate).

If the sig is invalid or missing, the /api/v1/snapshots/<HASH> endpoint returns 401. If the underlying hash is real but the sig was tampered, the same 401 fires — you can't bypass tier gating by rewriting query params.

Correction policy

If a fact in our pipeline turns out to be wrong, we issue a correction in the live /trace/<domain> view but we do not modify historical snapshots. The snapshot you cited remains exactly as it was at creation — including the now-known-wrong fact. This is the IC-memo-perpetuity promise; tampering with snapshot contents to "fix" facts would invalidate every prior citation.

If a snapshot must be redacted (e.g. legal takedown), we delete the row entirely and the URL begins returning 404. We log every such deletion in our internal audit log.