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Record that pdp_digest does not authenticate the decision

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flex-auth reported that its response channel is unauthenticated: pins serve
plain HTTP and the envelope carries no signature, so a responder knowing the
published package id and version can return a well-formed allow that passes
every check a consumer makes. Their framing is the useful one -- fail-closed
protects against a PDP that is absent, not against one that lies.

That lands directly on this engine's correspondence chain. Matching pdp_digest
proves two artifacts describe one request; it proves nothing about whether the
decision is genuine, and an approval whose pdp_digest matches a forged decision
matches perfectly. Every input to the comparison is either sent by the caller or
published by the PDP.

Our doc already said what pdp_digest cannot cover but did not say this, which is
the limit most easily mistaken for coverage -- the digest chain looks like it
closes authenticity and does not. Stated plainly, with the fix named as the
PDP's to make (FLEX-WP-0024, signing the envelope) rather than something a
consumer can recover.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PM5HnEAhokxdfcPqBNpT7D

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 715850@bnt-lap001
Assistant-Session: eb557e93-7cb1-45d0-9e57-7d15b3edc60e
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tegwick 2026-09-07 00:20:59 +02:00
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docs/approval-claim.md
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@ -148,6 +148,27 @@ produces a confident wrong answer, and comparing two digests derived under
different rules fails open in the direction of accepting a claim bound to a
different request.
**`pdp_digest` does not authenticate the decision.** This is the limit most
easily mistaken for coverage, so it is stated plainly: matching
`claim.binding.pdp_digest` against the PDP's published digest proves the two
artifacts describe *one request*. It proves nothing about whether the decision
is genuine. **An approval whose `pdp_digest` matches a forged decision matches
perfectly** — every input to the comparison is either sent by the caller or
published by the PDP, so a responder that knows a package id and version can
return a well-formed allow that passes every check a consumer makes.
Fail-closed protects against a PDP that is *absent*, not against one that
*lies*. The distinction matters because the digest chain looks like it closes
this and does not: a consumer that verified correspondence has verified
correspondence, not authenticity.
Closing it is the PDP's to do, by signing the decision envelope —
`FLEX-WP-0024`, reported by flex-auth against its own interest after finding its
response channel served plain HTTP with no envelope signature. Until a signature
exists and a consumer verifies it, treat a matching `pdp_digest` as evidence
that the right request was approved, never as evidence that the decision came
from the PDP.
### The comparison target at execute time
The issue-time description above is unchanged: this engine records the