Hardened: encrypt stored records with AES-256-GCM

[DavidCozens]

What this tag adds

SolidSyslogMbedTlsAesGcmPolicy replaces the HMAC policy on the store. Records at rest
gain confidentiality on top of tamper-evidence — the body is encrypted, the record
header is authenticated as associated data, and nonce and tag go in the trailer.
Completes hardened, and completes the sequence.

  wire ... [logPipeline@32473 transport="mtls" atRest="aes-256-gcm"] device started

  Flash        14,652 B     (+148)
  Static RAM    5,060 B       (+8)
  Heap         17,784 B       (+8)
  Log stack       712 B         (0)
  Service stack 3,800 B         (0)

148 bytes, because AES-GCM is already linked: a device that negotiates a GCM
ciphersuite for TLS carries the same primitive the store now uses. No stack movement —
the policy encrypts in place, into the buffer the store already owns.

Two things worth noting

The store key is unchanged. Its name says what it protects, not which algorithm
protects it, so escalating the policy needs no second key provisioned. That is the
payoff of the named-key design from the HMAC step.

GCM needs a fresh nonce per record, and mbedTLS has no context-free RNG, so the
policy takes the device's DRBG alongside the key. That is the only wiring difference
from the HMAC policy.

Also in this commit

atRest in the pipeline element was hard-coded, the same defect caught on transport
in PR #24 but not yet exercised. Both values now derive from what was actually
configured, and both default to the weakest honest answer — "tls" and "none" — so a
missed Init understates protection rather than overstating it.

Checklist

• Application-only.
• measurements/<State>.csv committed, and a row added to measurements/tags.tsv.
  Tag boundaries deferred to the release pass.
• README cost table regenerated — nothing to regenerate until a state's CSV is frozen.
• ./run.sh green; the record round-tripped through an encrypted store and
  arrived intact, so Seal and Open are both proven end to end.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Syslog pipelines now support AES-256-GCM protection when a device random-number source is available.
  • Syslog metadata reports the active transport and at-rest protection settings.

• Bug Fixes

  • Improved security configuration and nonce generation for protected syslog records.

• Tests

  • Updated resource usage measurements and syslog output expectations to reflect the new security settings.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ef0ed9a7-de5f-4970-99c6-d68cc92a2902

📥 Commits

Reviewing files that changed from the base of the PR and between bcf4e19 and c4569b9.

📒 Files selected for processing (4)

• app/syslog/Syslog.c
• app/syslog/SyslogPipelineSd.c
• app/syslog/SyslogPipelineSd.h
• run-report.txt

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📝 Walkthrough

Walkthrough

Syslog security changes from HMAC-SHA256 to RNG-backed AES-256-GCM, while structured data now reports configurable transport and at-rest protection values. Runtime reports update measurements and collector output accordingly.

Changes

Syslog security and pipeline configuration

Layer / File(s)	Summary
RNG-backed AES-GCM policy wiring app/syslog/Syslog.c	Device RNG is cached and supplied to TLS stream and AES-256-GCM policy configuration; transport and cipher selection depend on RNG availability.
Active protection values in structured data app/syslog/SyslogPipelineSd.c, app/syslog/SyslogPipelineSd.h, run-report.txt	SyslogPipelineSd_Init accepts transport and at-rest strings, emits the configured at-rest value, and reports AES-GCM collector output with updated measurements.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• cososo-ltd/solid-syslog-example#21: Both update the syslog block-store at-rest security policy wiring.
• cososo-ltd/solid-syslog-example#23: Both modify the syslog pipeline structured-data implementation.
• cososo-ltd/solid-syslog-example#24: Both change transport initialization for the syslog pipeline structured data.

Sequence Diagram(s)

sequenceDiagram
  participant Syslog as Syslog initialization
  participant DeviceCertStore_Rng
  participant TLSStream as SolidSyslogMbedTlsStream
  participant AESGCM as SolidSyslogMbedTlsAesGcmPolicy
  participant PipelineSD as SyslogPipelineSd
  Syslog->>DeviceCertStore_Rng: obtain RNG handle
  Syslog->>TLSStream: configure TLS stream with RNG
  Syslog->>AESGCM: configure key callback and RNG
  Syslog->>PipelineSD: initialize transport and atRest values
  PipelineSD-->>Syslog: emit configured logPipeline structured data

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately captures the main change: switching stored records to AES-256-GCM encryption.
Description check	✅ Passed	The description covers the required tag-add explanation and includes the checklist, though some checklist items remain unchecked.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch hardened-store-aesgcm

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