A keyless verification walk reports every gap it never tried to cross as a broken chain, and blames the keyring
## Problem
A walk at `VerificationDepth::None` reports every live-purged range as a broken chain link, on a host holding every signing secret. The two stricter depths report the same chain verified.
Measured on a chain of five rows with rows 1-3 archived and live-purged:
```
with every secret: strict ok=true tolerant ok=true none ok=FALSE
none: Chain "webdav" broken at id 4: previous_hash does not link to the
predecessor (expected <head>, found 8d6f6002bb50c162…).
holding no secret: tolerant ok=true none ok=FALSE
tolerant: Chain "webdav" verified: 5 entries intact. Bridged purged ranges
via 1 WORM archive covering rows 1-3. 1 segment(s) account for rows
whose forensic context was purged, and the secrets that signed them
could not be resolved here: #6.
```
So the ladder inverts. `None` is documented as the cheapest and most permissive of the three, and it is the strictest of them about a purged gap.
## Why it is wrong
`AuditTrailVerifier::walkChain()` gates the whole gap walk on the depth:
```php
if ($depth->readsSignatures() && $id > $last_seen_id + 1) {
$gap = $this->walkGap($chain_id, $last_seen_id, $id, $expected_previous);
```
so `None` never reaches `bridgeGapStep()` at all. The comment above it gives the reason:
> `findArchiveBridging()` validates the segment's identity and archive-content HMACs before it will chain across a purged gap, so a keyless reader cannot use it.
That was true when it was written. Since #3620386 `SegmentAttestations::findArchiveBridging()` has a second route that needs no secret: when a signature will not resolve it falls through to the chain's segment spine, marks the bridge `spine_attested`, names the segment in `unauthenticated_attestations` and leaves `authenticated_ok` NULL. That is exactly the reader `None` exists for, and the gate keeps it from ever being asked.
The consequence is a false BROKEN verdict, which is the one verdict this module exists to make mean something:
- `docs/verification.md` and `drush audit_trail:verify --full --depth=none` tell an operator this is the walk to run before handing a copy of a chain to an auditor. On any install whose retention has run, it comes back BROKEN.
- `VerificationDepth::None` documents itself as "exactly the walk a reader without the keys gets". It is not: that reader gets `Tolerant`, and `Tolerant` crosses the gap.
- The enum's own docblock says collapsing "the secret cannot be resolved" and "the signature does not re-derive" into one boolean "is what made a walk without the operator's secrets report every healthy purged chain as broken". That is the state `None` is still in.
## The change
The gap walk is asked at every depth, and `bridgeGapStep()` decide on the evidence it has, which is what it already does for `Tolerant`: a bridge the spine carried is named in `unauthenticated_attestations` and sets `authenticationIncomplete`, so `None` keeps saying that it authenticated nothing and gains no claim it has not earned. A gap nothing accounts for still falls through to the per-row check as a bad link.
`AuditTrailVerifierTest::testTheNoneDepthNeverBridgesOnSegmentMetadata()` pinned the old behavior and moved with it. Its docblock reasoned from a premise the spine had since falsified: "The only ways to do it are to hand it the secrets, which removes the reason it exists, or to bridge on anchors nothing validated." The spine is a third way, and it is checked rather than taken on trust.
The same gate held the other route across a gap. An acknowledgment is trusted against the chain's own `acknowledgment_recorded` event, which needs no secret, so `docs/security.md` says the depths agree about acknowledged ranges. They do over a row still present, and did not over rows an operator acknowledged and then removed: that crossing is `walkGap()` too.
Two operator surfaces answer for the crossing and both said too little once `None` makes it. The verdict sentence naming a segment accepted unchecked gave one reason for it, that the secrets would not resolve here, which is the reason at `Tolerant` and not at `None`, where they may all be present and simply unread; it now takes the depth. And the `--depth` note promised the spine crossing without its condition: the crossing needs a surviving chain event still corroborating the spine at that segment's height, so a range whose lifecycle events have aged out, or that compaction has folded into a consolidated row, still reports a broken link at `none` and crosses at `strict`. The note says so.
Where the spine cannot carry the crossing it is still refused, and `testTheNoneDepthRefusesTheRangeCompactionFolded()` is what holds that: replacing `isSpineAttested()` with a constant left both suites green before it existed.
## User interface changes
None.
---
AI-Generated: Yes (Claude Code was used to help draft this issue summary and to write the fix and its tests. I reviewed both, and the new tests were confirmed to fail against the unpatched module and to pass with the change.)
issue
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