The Principia Machina/TPS/2.5 Digests and the digest gate

Instruction artifact: TPS 0.1.0, page 2.5 Digests and the digest gate. Part of TPS 0.1.0 and the TPM hub. Human-only edit (TPS 5.13). Version 0.1.0, proposed; declaration pending. Governing section: this page, TPS 2.5. The text below is this page's unit of the TPS 0.1.0 artifact, verbatim; the whole artifact and its file digest are on The Principia Machina/TPS 0.1.0.

TPS 2.5  DIGESTS AND THE DIGEST GATE
purpose: D(TPK), D(TrM), D(TMS), D(TFK); TPX the sole implementation;
         the combination rule owed; the gate before every write and at
         every CAC; RESTORE or FORK by human declaration
governs: 2.5 | version: 0.1.0 (proposed) | class: instruction artifact
audience: operator, agent

2.5.1  Digests D(TPK), D(TrM), D(TMS) and D(TFK) are computed at CAC by
       TPX, the sole implementation of every digest, written into the
       CAC certificate that assigns the UUID, and appended as a BUILD
       CHAIN line (see 5.6). [I-062]
2.5.2  A hand-transcribed digest is not a digest (D9). [I-062]
2.5.3  D(TFK) is computed over the three zone digests in fixed order.
       The container binds to D(TFK). [I-033]
       OWED: the combination rule for D(TFK) over the three zone digests,
       fixed order, fixed algorithm, stated once, so independent
       verifiers agree; kind: convention; C-02.
       OWED: cross-implementation hash tests, since serialization drift
       is the known failure of hash chains; kind: computation; C-03.
2.5.4  Chain link hash SHA-256; page hash may be MediaWiki's SHA-1 (see
       5.6). [I-063]
2.5.5  A content digest is verified before every write and at every CAC.
       A digest-gate failure halts writes. [I-064]
2.5.6  The gate reports which of the three zones mismatched. [B-05]
2.5.7  Recovery is a human declaration between RESTORE (return the TFK
       pages to the revision whose content hash matches the chain; writes
       resume only after the digest matches) and FORK (copy-forward
       under a newly declared TFK). Never an agent's choice. [I-064]
2.5.8  Before every write, TPX recomputes D(TFK) and matches it against
       the certificate; on mismatch it halts the write and names the
       zone (see 4.2, row 1). [B-18]