diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index 5826396..c947373 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -7,8 +7,10 @@ This repository follows the Bugs5382 standard workflow.
 1. Open an issue from a template (free-form issues are disabled). For multi-step work,
    use a parent issue with ordered sub-issues, and put it on the active milestone.
 2. Branch from `main` as `<type>/<issue#>-<slug>` (for example `feat/12-add-listener`).
-3. Commit using Conventional Commits (`type(scope): description`). No attribution
-   trailers, no emoji in source or commit messages (emoji are fine in Markdown).
+3. Commit using Conventional Commits (`type(scope): description`) and sign off every
+   commit with `git commit -s` (see [Developer Certificate of Origin](#developer-certificate-of-origin)).
+   The sign-off is the only trailer: no attribution trailers, and no emoji in source or
+   commit messages (emoji are fine in Markdown).
 4. Open a PR with a Conventional Commit title. The autolabeler sets the category label
    from the title; fill the PR template, reference the issue (`Closes #N`), and add a
    closing summary before merge.
@@ -19,6 +21,26 @@ Keep one concern per PR, even small ones. When editing GitHub Actions workflows,
 plain identifier (a letter or `_`, then alphanumerics/`-`/`_`); put emoji and display text in the
 job's `name:`. The Actionlint check enforces this.
 
+## Developer Certificate of Origin
+
+Contributions are accepted under the [Developer Certificate of Origin](https://developercertificate.org/)
+(DCO) 1.1. Signing off a commit certifies that you wrote the change, or otherwise have the
+right to submit it under this repository's licence (Apache License 2.0), and that the
+contribution and your sign-off are public and kept permanently. The organization's
+[contributing guide](https://github.com/CryptOS-PKI/.github/blob/main/CONTRIBUTING.md#developer-certificate-of-origin)
+quotes the full certificate.
+
+Sign off with `git commit -s`. Git builds the trailer from your `user.name` and
+`user.email`, and it has to match the commit's author:
+
+```text
+Signed-off-by: Jane Doe <jane@example.com>
+```
+
+The DCO check fails a pull request when any commit, other than a merge commit, has no
+matching sign-off. To sign off commits you have already pushed, run
+`git rebase --signoff origin/main` and force-push the branch.
+
 ## Local setup
 
 Install the governance hooks once per clone: `bash .claude/hooks/install.sh`. They
diff --git a/.github/workflows/job-dco.yaml b/.github/workflows/job-dco.yaml
new file mode 100644
index 0000000..66d9bf4
--- /dev/null
+++ b/.github/workflows/job-dco.yaml
@@ -0,0 +1,67 @@
+name: DCO
+# Developer Certificate of Origin: every commit in a pull request must carry a
+# `Signed-off-by:` trailer (`git commit -s`) naming the commit's author, with
+# the same name and email (compared case-insensitively). Merge commits are
+# skipped, since they add no authored change of their own.
+#
+# The commit list comes from the pull request API, so the job needs no
+# checkout, no Node action and no third-party DCO app: `gh` and `jq` ship on
+# the hosted runner. The API returns at most 250 commits for a pull request;
+# a longer one fails with a message rather than passing unchecked.
+#
+# Pull requests only, and not while the PR is a draft. Title and body edits
+# don't change the commits, so they skip; a base change (retarget) reruns.
+on:
+  pull_request:
+    branches: [main]
+    types: [opened, synchronize, reopened, ready_for_review, edited]
+# One run per PR per workflow: a new push cancels the older PR run. A title or
+# body edit gets its own group, so its skipped run never cancels a real one.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}${{ github.event.action == 'edited' && github.event.changes.base == null && format('-edit-{0}', github.run_id) || '' }}
+  cancel-in-progress: true
+permissions:
+  contents: read
+  pull-requests: read
+jobs:
+  dco:
+    name: ✍️ DCO sign-off${{ github.event.action == 'edited' && github.event.changes.base == null && ' (edit, not run)' || '' }}
+    if: github.event.pull_request.draft == false && (github.event.action != 'edited' || github.event.changes.base != null)
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: ✍️ Check every commit is signed off by its author
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          PR: ${{ github.event.pull_request.number }}
+          EXPECTED: ${{ github.event.pull_request.commits }}
+        run: |
+          set -euo pipefail
+          commits="$(gh api --paginate "repos/$REPO/pulls/$PR/commits?per_page=100" --jq '.[]' | jq -s '.')"
+          total="$(jq 'length' <<<"$commits")"
+          if [ "$total" -lt "$EXPECTED" ]; then
+            echo "::error::The API listed $total of $EXPECTED commits (it stops at 250). Split the pull request so every commit can be checked."
+            exit 1
+          fi
+          missing="$(jq -r '
+            .[]
+            | select((.parents | length) < 2)
+            | . as $k
+            | ($k.commit.author.name | ascii_downcase) as $name
+            | ($k.commit.author.email | ascii_downcase) as $email
+            | select([$k.commit.message
+                | scan("(?m)^Signed-off-by:[ \t]*([^<\n]*?)[ \t]*<([^>\n]*)>[ \t]*$")
+                | select((.[0] | ascii_downcase) == $name and (.[1] | ascii_downcase) == $email)
+              ] | length == 0)
+            | "\($k.sha[0:12]) \($k.commit.author.name) <\($k.commit.author.email)>"
+          ' <<<"$commits")"
+          if [ -n "$missing" ]; then
+            while IFS= read -r line; do
+              echo "::error::Commit $line has no Signed-off-by line matching its author."
+            done <<<"$missing"
+            echo "Sign off each commit with 'git commit -s'. To fix commits already pushed,"
+            echo "run 'git rebase --signoff origin/main' and force-push the branch."
+            exit 1
+          fi
+          echo "All $total commits carry a sign-off matching their author."