Dockerイメージを署名して検証する方法｜cosignとDocker Content Trustでサプライチェーン攻撃を防ぐ実践手順

この記事のポイント

・cosign sign --keyでイメージに署名し、改ざん・なりすましを検知できるようにする
・cosign verify --keyで署名を検証し、未署名や改ざんイメージのデプロイを防ぐ
・GitHub Actions上ではKeyless署名（OIDC連携）で秘密鍵の管理自体を不要にできる
・レガシーなDocker Content Trust（Notary）との違いを理解し、現在はcosignを標準とする

# 最新版のバイナリを取得してインストールする
$ curl -O -L "https://github.com/sigstore/cosign/releases/latest/download/cosign-linux-amd64"
$ sudo mv cosign-linux-amd64 /usr/local/bin/cosign
$ sudo chmod +x /usr/local/bin/cosign

# バージョンを確認する
$ cosign version
GitVersion:    v2.4.1
GitCommit:     a1b2c3d
GitTreeState:  clean
BuildDate:     2026-06-12T10:14:22Z
GoVersion:     go1.22.4
Compiler:      gc
Platform:      linux/amd64

$ cosign generate-key-pair
Enter password for private key:
Enter password for private key again:
Private key written to cosign.key
Public key written to cosign.pub

$ ls -l cosign.*
-rw-------  1 admin admin  384  8月 25 10:02 cosign.key
-rw-r--r--  1 admin admin  178  8月 25 10:02 cosign.pub

$ docker build -t registry.example.jp/myapp:1.4.0 .
$ docker push registry.example.jp/myapp:1.4.0
The push refers to repository [registry.example.jp/myapp]
5f70bf18a086: Pushed
3c9c1e388db3: Pushed
1.4.0: digest: sha256:7f2c1e9dcb1a4f6e0b8d2a9c3e5f1b7a6d4c8e0f2a1b3c5d7e9f0a1b2c3d4e5f size: 3049

# 署名を付与する
$ cosign sign --key cosign.key registry.example.jp/myapp:1.4.0
Enter password for private key:
WARNING: Image reference registry.example.jp/myapp:1.4.0 uses a tag, not a digest, to identify the image to sign.
This can lead you to sign a different image than the intended one. Please use a digest (example.com/ubuntu@sha256:abc123...) rather than tag as a best practice.
Pushing signature to: registry.example.jp/myapp

$ cosign verify --key cosign.pub registry.example.jp/myapp:1.4.0

Verification for registry.example.jp/myapp:1.4.0 --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - The signatures were verified against the specified public key

[{"critical":{"identity":{"docker-reference":"registry.example.jp/myapp"},"image":{"docker-manifest-digest":"sha256:7f2c1e9dcb1a4f6e0b8d2a9c3e5f1b7a6d4c8e0f2a1b3c5d7e9f0a1b2c3d4e5f"},"type":"cosign container image signature"},"optional":null}]

$ cosign verify --key cosign.pub registry.example.jp/myapp:latest

Error: no matching signatures:
no signatures found for image
main.go:69: error during command execution: no matching signatures:
no signatures found for image

# GitHub ActionsのワークフローステップでKeyless署名する（--keyを付けない）
- name: Sign the image with cosign
  run: cosign sign --yes registry.example.jp/myapp@${{ steps.build.outputs.digest }}
  env:
    COSIGN_EXPERIMENTAL: "false"

$ cosign verify \
    --certificate-identity "https://github.com/example-org/myapp/.github/workflows/build.yml@refs/heads/main" \
    --certificate-oidc-issuer "https://token.actions.githubusercontent.com" \
    registry.example.jp/myapp:1.4.0

# syftなどで生成したSBOM（SPDX形式）をイメージにアテステーションとして添付する
$ cosign attest --key cosign.key --predicate sbom.spdx.json --type spdxjson registry.example.jp/myapp:1.4.0

# DCTを有効にした状態でpushすると、署名鍵の作成を求められる
$ export DOCKER_CONTENT_TRUST=1
$ docker push registry.example.jp/myapp:1.4.0
Signing and pushing trust metadata
You are about to create a new root signing key passphrase.

$ cosign verify --certificate-identity "wrong-identity" --certificate-oidc-issuer "https://token.actions.githubusercontent.com" registry.example.jp/myapp:1.4.0
Error: none of the expected identities matched what was in the certificate

# 鍵をローテーションする（既存のcosign.pubは無効化し、配布物を差し替える）
$ mv cosign.key cosign.key.revoked-20260825
$ cosign generate-key-pair