implementing-sigstore-for-software-signing

Implements Sigstore-based software signing and verification using Cosign keyless signing, Rekor transparency log verification, and Fulcio certificate authority integration to establish cryptographic provenance for container images, binaries, and software artifacts. The practitioner configures OIDC-based identity binding, verifies signing events against the Rekor transparency log, and integrates signing workflows into CI/CD pipelines. Activates for requests involving software supply chain signing, keyless container signing, Sigstore deployment, or artifact provenance verification.

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name	implementing-sigstore-for-software-signing
description	Implements Sigstore-based software signing and verification using Cosign keyless signing, Rekor transparency log verification, and Fulcio certificate authority integration to establish cryptographic provenance for container images, binaries, and software artifacts. The practitioner configures OIDC-based identity binding, verifies signing events against the Rekor transparency log, and integrates signing workflows into CI/CD pipelines. Activates for requests involving software supply chain signing, keyless container signing, Sigstore deployment, or artifact provenance verification.
domain	cybersecurity
subdomain	supply-chain-security
tags	["sigstore","cosign","rekor","fulcio","software-signing","supply-chain","keyless-signing","OIDC","transparency-log"]
version	1.0.0
author	mukul975
license	Apache-2.0
nist_csf	["GV.SC-01","GV.SC-03","GV.SC-06","GV.SC-07"]

Implementing Sigstore for Software Signing

When to Use

Signing container images and software artifacts without managing long-lived cryptographic keys
Establishing verifiable provenance for build outputs in CI/CD pipelines using OIDC identity binding
Querying the Rekor transparency log to audit when and by whom an artifact was signed
Verifying that container images pulled from registries were signed by authorized identities and issuers
Integrating Sigstore verification into Kubernetes admission controllers to enforce signed-image policies

Do not use for signing artifacts that require air-gapped or offline signing workflows where OIDC authentication is unavailable, for environments that cannot reach the public Sigstore infrastructure (Fulcio, Rekor) and have no private instance deployed, or as a replacement for traditional PGP/GPG signing where regulatory compliance mandates specific key management procedures.

Prerequisites

Cosign CLI v2.4+ installed (go install github.com/sigstore/cosign/v2/cmd/cosign@latest or binary release)
Access to an OIDC identity provider supported by Fulcio (Google, GitHub, Microsoft, or a custom OIDC issuer)
Container registry credentials (for signing container images) with push access to store signature objects
Python 3.9+ with sigstore, requests, and cryptography packages for the automation agent
Network access to fulcio.sigstore.dev, rekor.sigstore.dev, and tuf-repo-cdn.sigstore.dev (or private Sigstore instance URLs)

Workflow

Step 1: Install and Configure Cosign

Install Cosign and verify it can reach the Sigstore infrastructure:

Install from binary release: Download the appropriate binary from the Cosign GitHub releases page and verify its checksum. On Linux: curl -LO https://github.com/sigstore/cosign/releases/latest/download/cosign-linux-amd64 && chmod +x cosign-linux-amd64 && sudo mv cosign-linux-amd64 /usr/local/bin/cosign
Verify installation: Run cosign version to confirm the version and check connectivity to Sigstore services with cosign initialize which fetches the TUF root of trust
Configure custom infrastructure (optional): If running a private Sigstore stack, set --fulcio-url, --rekor-url, and --oidc-issuer flags or use environment variables and

COSIGN_REKOR_URL

COSIGN_FULCIO_URL

Sign a container image: Run cosign sign <IMAGE_DIGEST> which triggers an OIDC authentication flow. Cosign generates an ephemeral key pair, obtains a short-lived certificate from Fulcio binding the public key to the OIDC identity, signs the image digest, and records the signing event in Rekor. The private key is destroyed immediately after signing.
Sign a blob (file): Run cosign sign-blob <file> --bundle artifact.sigstore.json to sign arbitrary files. The bundle contains the signature, certificate, timestamp, and Rekor inclusion proof.

Non-interactive signing in CI: Set SIGSTORE_ID_TOKEN environment variable with a valid OIDC token (e.g., from GitHub Actions OIDC or GCP workload identity) to skip the browser-based authentication flow:

export SIGSTORE_ID_TOKEN=$(curl -sH "Authorization: bearer $ACTIONS_ID_TOKEN_REQUEST_TOKEN" \
  "$ACTIONS_ID_TOKEN_REQUEST_URL&audience=sigstore" | jq -r '.value')
cosign sign $IMAGE_DIGEST

Supported OIDC providers: Google (https://accounts.google.com), GitHub (https://github.com/login/oauth), Microsoft (https://login.microsoftonline.com), GitLab (https://gitlab.com), and custom providers registered with a private Fulcio instance

Verify a container image: Run cosign verify <IMAGE_URI> --certificate-identity=name@example.com --certificate-oidc-issuer=https://accounts.google.com to confirm the image was signed by the specified identity. Cosign validates the certificate chain, checks the Rekor inclusion proof, and verifies the signature matches the current image digest.
Verify a signed blob: Run cosign verify-blob <file> --bundle artifact.sigstore.json --certificate-identity=name@example.com --certificate-oidc-issuer=https://accounts.google.com

Regex matching for CI identities: Use --certificate-identity-regexp to match CI workflow identities:

cosign verify $IMAGE --certificate-identity-regexp="https://github.com/myorg/myrepo/.*" \
  --certificate-oidc-issuer=https://token.actions.githubusercontent.com

Verification failure modes: Cosign returns a non-zero exit code on failure. Common failures include certificate identity mismatch, expired certificates without a valid Rekor timestamp, missing Rekor entry, and image digest mismatch (image was modified after signing).

Term	Definition
Keyless Signing	Identity-based signing that uses short-lived certificates from Fulcio bound to OIDC identities instead of long-lived cryptographic keys, eliminating key management overhead
Fulcio	Sigstore's certificate authority that issues short-lived X.509 certificates after verifying OIDC tokens, binding an ephemeral public key to a verified identity
Rekor	Sigstore's immutable, append-only transparency log that records signing events with timestamps, enabling auditors to verify when and by whom an artifact was signed
Cosign	The primary CLI tool for signing and verifying container images and blobs using the Sigstore infrastructure (Fulcio + Rekor)
TUF Root of Trust	The Update Framework distribution mechanism for Sigstore's root CA certificate and Rekor public key, ensuring clients trust the correct Sigstore infrastructure
OIDC Identity Binding	The process where Fulcio verifies a user's identity through an OpenID Connect token and binds it to a short-lived signing certificate
Inclusion Proof	A cryptographic proof from Rekor demonstrating that a signing event entry exists within the transparency log's Merkle tree

## Sigstore Signing Verification Report

**Artifact**: ghcr.io/myorg/myapp@sha256:a1b2c3d4...
**Verification Status**: PASSED

**Certificate Details**:
  Subject: https://github.com/myorg/myapp/.github/workflows/build.yml@refs/heads/main
  Issuer: https://token.actions.githubusercontent.com
  Valid From: 2026-03-19T10:00:00Z
  Valid To: 2026-03-19T10:10:00Z

**Rekor Entry**:
  UUID: 24296fb24b8ad77a8d52...
  Log Index: 89234567
  Integrated Time: 2026-03-19T10:00:05Z
  Inclusion Proof: VERIFIED (tree size: 92000000, root hash: e4f5a6...)

**Policy Check**: Image signed by authorized CI workflow identity