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+# ADR-0006: Sigstore Integration
+
+**Status**: Proposed
+**Date**: 2026-04-29
+**Authors**: Igor Brandao
+**Reviewers**:
+
+## Context
+
+From the roadmap: "As AI skills become a standard way to inject context into agents, they also become a vector for prompt injection attacks and supply chain compromises." Skills can contain executable scripts, and without verification there is no way to validate their integrity or origin.
+
+Issue #84 proposes an MVP for signature verification at `lola mod add` time using [Sigstore](https://www.sigstore.dev/) — the same standard used by PyPI, npm, and Maven Central. Issue #62 introduces the nono trust system as a reference implementation.
+
+The Lola + Skill Image collaboration establishes that skillctl handles signing (via cosign at pack time) and Lola handles verification (at add/install time). In Go, Lola can use [sigstore-go](https://github.com/sigstore/sigstore-go) — a stable, production-ready Go library for sigstore verification with minimal dependencies.
+
+The module package format (ADR-0005) defines `lola.mod` + `lola.sum` for dependency management and integrity checking. This ADR addresses cryptographic verification of module origin and tamper resistance on top of that foundation.
+
+## Decision
+
+Integrate sigstore bundle verification into the fetch pipeline for all add operations (`lola mod add`, `lola skill add`, `lola plugin add`, `lola ext add`). Use `sigstore-go` as a compiled-in library dependency.
+
+**Verification flow**: fetch → verify → cache. Verification happens after fetching to a temp directory and before promoting to the local cache. If verification fails, the fetched content is discarded (strict mode) or cached with a warning (warn mode).
+
+**Identity derivation**: For GitHub-hosted modules, the expected OIDC issuer and subject are derived automatically from the repository URL:
+```
+https://github.com/org/repo
+  → issuer:  https://token.actions.githubusercontent.com
+  → subject: repo:org/repo:*
+```
+No additional configuration needed for GitHub sources.
+
+**Enforcement levels**: Opt-in via the repo/marketplace YAML `sign:` field:
+
+```yaml
+modules:
+  - name: "openssf-skill"
+    repository: "https://github.com/ryanwaite/openssf-skill.git"
+    sign:
+      enforcement: warn    # warn | strict
+```
+
+| `sign:` present | Bundle present | `enforcement` | Result |
+|---|---|---|---|
+| no | any | — | skip verification, cache |
+| yes | no bundle | `warn` | warn, cache |
+| yes | no bundle | `strict` | error, discard |
+| yes | invalid bundle | `warn` | warn, cache |
+| yes | invalid bundle | `strict` | error, discard |
+| yes | valid bundle | any | verified, cache |
+
+**OCI sources**: For OCI-distributed modules, cosign signature verification is handled by skillimage's `pkg/oci` which has built-in cosign support. No separate verification step needed.
+
+**Go library**: `sigstore-go` (`github.com/sigstore/sigstore-go`) v1.0+:
+- `pkg/verify` — core verification APIs
+- `pkg/bundle` — sigstore bundle handling
+- `pkg/root` — TUF-based trusted root management
+
+## Rationale
+
+- Sigstore is the industry standard for software signing — adopted by PyPI, npm, Maven Central, and the Linux kernel
+- `sigstore-go` is stable (v1.0+), production-ready, and has minimal dependencies
+- Verification at add time (not install time) ensures unsigned or tampered content never enters the cache
+- Enforcement is opt-in — existing workflows are not broken; signing adoption is gradual
+- Identity derivation from repository URL eliminates configuration overhead for the common case (GitHub Actions)
+- For OCI sources, skillimage already handles cosign verification — no duplication
+
+## Consequences
+
+### Positive Consequences
+
+- Skills from trusted sources are verifiable — publisher identity and tamper resistance
+- Enforcement is progressive: start with `warn`, move to `strict` as signing adoption grows
+- No configuration needed for GitHub-hosted modules — identity derived automatically
+- OCI verification reuses skillimage's existing cosign support
+- Verification is a compiled-in library call, not a subprocess — aligns with single-binary goal
+
+### Negative Consequences
+
+- `sigstore-go` adds to the transitive dependency tree (sigstore crypto libs)
+- Skills without signatures are not blocked by default — requires marketplace authors to opt in
+- GitLab, Bitbucket, and self-hosted sources need custom issuer/subject configuration (not covered in MVP)
+
+## Alternatives Considered
+
+### Alternative 1: No verification
+- Description: Skip signature verification entirely
+- Pros: No additional complexity or dependencies
+- Cons: No protection against tampered skills or impersonated publishers
+- Reason for rejection: The roadmap explicitly identifies this as a security priority
+
+### Alternative 2: Shell out to cosign CLI
+- Description: Execute `cosign verify` as a subprocess
+- Pros: Simpler integration, no library dependency
+- Cons: Requires cosign binary to be installed; defeats single-binary distribution goal
+- Reason for rejection: Go library is available and stable; subprocess adds a runtime dependency
+
+### Alternative 3: Custom signing with GPG
+- Description: Implement a custom GPG-based signing scheme
+- Pros: Well-understood technology, widely available
+- Cons: Key management overhead, no keyless mode, no transparency log
+- Reason for rejection: Sigstore solves key management with keyless signing (Fulcio + OIDC)
+
+## Implementation Notes
+
+See paired design document: `docs/dev-guide/design/sigstore-integration.md`
+
+**Out of scope for MVP** (future work):
+- Per-module `lola.yml` signing override
+- GitLab / self-hosted OIDC issuers
+- Key-pair (offline) signing mode
+- SLSA provenance attestation verification
+- nono integration (issue #62)
+
+## References
+
+- [GitHub Issue #84](https://github.com/LobsterTrap/lola/issues/84) — sigstore MVP proposal
+- [GitHub Issue #62](https://github.com/LobsterTrap/lola/issues/62) — skill attestation / nono
+- [CY26 Roadmap](../concepts/roadmap.md) — trusted catalogs vision
+- [ADR-0005: Module Package Format](0005-module-package-format.md) — what gets signed
+- [sigstore-go](https://github.com/sigstore/sigstore-go) — Go verification library
+- [Sigstore](https://www.sigstore.dev/) — signing standard
+- [skillimage.dev](https://skillimage.dev/) — cosign integration for OCI skills
diff --git a/docs/dev-guide/design/sigstore-integration.md b/docs/dev-guide/design/sigstore-integration.md
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+# Sigstore Integration — Implementation Design
+
+Paired with [ADR-0006: Sigstore Integration](../../adr/0006-sigstore-integration.md).
+
+## Verification Flow
+
+Based on [issue #84](https://github.com/LobsterTrap/lola/issues/84):
+
+```mermaid
+flowchart TD
+    A([lola mod add source]) --> B{Source type?}
+    B -->|OCI| C[Pull OCI artifact via skillimage pkg/oci]
+    C --> D{Cosign signature present?}
+    D -->|yes| E[Verify via pkg/oci cosign support]
+    D -->|no| F{Enforcement?}
+
+    B -->|git / tar / zip| G[Fetch to temp/]
+    G --> H{.bundle file present?}
+    H -->|yes| I[Verify bundle via sigstore-go]
+    H -->|no| F
+
+    F -->|strict| J[ERROR: unsigned module]
+    F -->|warn| K[WARN: unsigned, cache anyway]
+    F -->|absent| L[Skip verification, cache]
+
+    E --> M{Valid?}
+    M -->|yes| N[Cache module]
+    M -->|no + strict| O[ERROR: invalid signature]
+    M -->|no + warn| K
+
+    I --> P{Valid?}
+    P -->|yes| N
+    P -->|no + strict| O
+    P -->|no + warn| K
+```
+
+## Enforcement Matrix
+
+| `sign:` in repo | Bundle present | `enforcement` | Result |
+|---|---|---|---|
+| no | any | — | skip verification, cache |
+| yes | no bundle | `warn` | warn, cache anyway |
+| yes | no bundle | `strict` | error, discard |
+| yes | invalid bundle | `warn` | warn, cache anyway |
+| yes | invalid bundle | `strict` | error, discard |
+| yes | valid bundle | any | verified, cache |
+
+## Repo YAML Sign Field
+
+```yaml
+modules:
+  - name: "openssf-skill"
+    description: "OpenSSF Security Instructions"
+    version: "v0.1.0"
+    repository: "https://github.com/ryanwaite/openssf-skill.git"
+    tags: ["openssf", "security"]
+    sign:
+      enforcement: warn    # warn | strict
+```
+
+## Identity Derivation
+
+For GitHub-hosted modules, identity is derived from the repository URL:
+
+```
+Repository: https://github.com/org/repo
+  → OIDC Issuer:  https://token.actions.githubusercontent.com
+  → Subject:      repo:org/repo:*
+```
+
+This means skill authors sign using GitHub Actions OIDC (keyless) and Lola verifies against the expected identity without any additional configuration.
+
+## Sigstore Components
+
+```mermaid
+graph LR
+    Author["Skill Author"] -->|signs with| Cosign["cosign / GH Actions"]
+    Cosign -->|gets cert from| Fulcio["Fulcio<br/>OIDC CA"]
+    Cosign -->|logs to| Rekor["Rekor<br/>Transparency Log"]
+    Cosign -->|pushes| Registry["OCI Registry<br/>or .bundle file"]
+
+    Lola["Lola (verifier)"] -->|reads| Registry
+    Lola -->|verifies cert via| Fulcio
+    Lola -->|checks log via| Rekor
+```
+
+- **Fulcio**: Free OIDC-based certificate authority, issues short-lived (10 min) certificates
+- **Rekor**: Append-only transparency log for non-repudiation
+- **cosign**: CLI/library for signing and verifying OCI artifacts and bundles
+
+## GitHub Actions Signing Workflow
+
+Reference workflow for skill authors to sign their modules:
+
+```yaml
+# .github/workflows/sign.yml
+name: Sign Skills
+on:
+  push:
+    tags: ['v*']
+
+permissions:
+  id-token: write    # Required for keyless signing
+  contents: read
+
+jobs:
+  sign:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: sigstore/cosign-installer@v3
+      - run: |
+          for f in skills/*/SKILL.md commands/*.md agents/*.md; do
+            [ -f "$f" ] && cosign sign-blob "$f" --bundle "$f.bundle"
+          done
+```
+
+## Scope Boundaries
+
+**In scope (MVP):**
+- Verification at add time for all add operations
+- GitHub Actions OIDC identity derivation
+- `sigstore-go/pkg/verify` for bundle verification
+- OCI verification via skillimage `pkg/oci`
+- Enforcement via repo YAML `sign:` field
+
+**Out of scope (future):**
+- GitLab, Bitbucket, self-hosted OIDC issuers
+- Key-pair (offline) signing verification
+- SLSA provenance attestation verification
+- nono trust system integration (issue #62)
+- Per-module `lola.yml` signing override