feat(lab8): cosign sign + SBOM/provenance attestations + blob signing

[ratteperk]

Goal

Sign the Juice Shop image with Cosign in a local registry, attach the CycloneDX SBOM from Lab 4 as an attestation, and (bonus) sign a tarball using cosign sign-blob to mitigate the Codecov 2021 attack class.

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Changes

• labs/lab8/keys/cosign.pub — public key for verifying the signatures/attestations (private key stays local, gitignored/gitleaks-blocked)
• submissions/lab8.md — full write-up:
  • Task 1: local registry:3 instance, image pushed and signed by digest, tamper demo (re-tagged alpine under the same name — verify correctly fails), sanity recheck on the original digest
  • Task 2: CycloneDX SBOM attestation (matches Lab 4's SBOM exactly, diff is empty) + SLSA provenance attestation, both verified
  • Bonus: cosign sign-blob on a tarball, verify-blob passes on the original and fails on a tampered copy

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Testing

# Task 1
docker run -d --name lab8-registry -p 127.0.0.1:5000:5000 registry:3
docker push localhost:5000/juice-shop:v20.0.0
cosign sign --key labs/lab8/keys/cosign.key --yes "$DIGEST"
cosign verify --key labs/lab8/keys/cosign.pub --insecure-ignore-tlog "$DIGEST"
# → Verified, exit 0; tampered re-tag → "no signatures found", exit 10; original still verifies after

# Task 2
cosign attest --key labs/lab8/keys/cosign.key --type cyclonedx --predicate labs/lab4/juice-shop.cdx.json "$DIGEST"
cosign attest --key labs/lab8/keys/cosign.key --type slsaprovenance --predicate /tmp/predicate-only.json "$DIGEST"
cosign verify-attestation --type cyclonedx "$DIGEST"    # component count matches Lab 4 (3069), diff empty
cosign verify-attestation --type slsaprovenance "$DIGEST"

# Bonus
cosign sign-blob --key labs/lab8/keys/cosign.key --bundle my-tool.tar.gz.bundle my-tool.tar.gz
cosign verify-blob --key cosign.pub --bundle my-tool.tar.gz.bundle my-tool.tar.gz   # → Verified OK
# tampered copy → "invalid signature", exit 1

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Artifacts & Screenshots

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• Task 1 — Image signed + tamper demo (both shown)
• Task 2 — SBOM + provenance attestations attached and verified
• Bonus — Blob signed + verify-blob success + tamper failure