Pin every GitHub Action to a commit SHA, and verify the nak download

Not one action reference in this repository was pinned. Every uses: line named
a mutable tag, and one named a branch. That includes the jobs holding the AUR
deploy key, the jobs with release write scope, and the packaging jobs that run
with a signing key in the environment, so whoever controls one of those action
repositories could repoint a tag into a job holding our credentials.

Sixty-two of the sixty-six references are now full commit SHAs with the
original tag kept as a trailing comment, so a reader can still tell which
release a pin is. Each SHA was resolved from the upstream peeled tag. Four
references are left unpinned and justified in one place rather than silently:
two actions select the tool they install from the ref name itself, so a bare
SHA hands them a hex string where a toolchain name belongs and the step fails.
Pinning those means moving the selection into with:, which changes what
resolves, and that is a separate decision from pinning.

A guard enforces the form on every sweep, wired into the parity job and the
local runner beside the existing checkers. It accepts only owner/repo@40-hex
with a mandatory trailing comment, treats an unreadable tree as exit 2 rather
than as a pass, and its header names what it does not cover: the actions that
pinned actions themselves invoke, the pip and cargo installs that are version
pinned at best, and anything fetched at run time.

The sharper hole was not the tags. The OpenWrt packaging workflow fetched a
helper binary straight from a release URL with no verification, in two jobs
that hold a signing key, which is code execution from a third-party host into a
credentialed job and needs nobody to retag anything. That download now goes
through a shared script with per-architecture pinned SHA-256 constants,
modelled on the zig block already in that workflow. Upstream publishes no
checksum document, so the provenance comment records the asset URL and the date
the hashes were taken by downloading rather than pretending they were verified
against a published sum.

.github/scripts/install-nak.sh

@@ -0,0 +1,105 @@
+#!/bin/bash
+# ── Install nak, checksum-verified ──────────────────────────────────────────
+# nak signs the release announcement events, and the jobs that call this script
+# hand it the publishing nsec on argv. An unverified download therefore runs
+# with the signing key in reach, so the binary is staged, checked against a
+# pinned SHA-256, and only then installed.
+#
+# Called from .github/workflows/package-openwrt.yml by both the .ipk (`build`)
+# and .apk (`build-apk`) jobs, which is why it lives here rather than under
+# packaging/openwrt-ipk/ — that directory is the .ipk payload tree.
+#
+# Exit 0 = installed and verified. Any non-zero exit means nothing was
+# installed.
+# ─────────────────────────────────────────────────────────────────────────────
+set -euo pipefail
+
+NAK_VERSION="0.16.2"
+INSTALL_PATH="/usr/local/bin/nak"
+
+ARCH=$(uname -m)
+# Each arch carries the expected SHA-256 of its upstream release asset.
+#
+# Unlike the zig hashes in the same workflow, which come from ziglang.org's own
+# https://ziglang.org/download/index.json, these are NOT upstream-attested:
+# fiatjaf/nak publishes no checksum document, sidecar or SHA256SUMS alongside
+# its release assets, so the only way to obtain a hash is to download the asset
+# and compute it. These were derived that way on 2026-08-11 from
+#   https://github.com/fiatjaf/nak/releases/download/v0.16.2/nak-v0.16.2-linux-<arch>
+# What the pin buys is therefore continuity, not authenticity: it detects the
+# asset changing under a fixed tag, a corrupted or truncated transfer, and a
+# substituted download, but it cannot attest that the bytes captured on that
+# date were the bytes upstream intended. Bumping NAK_VERSION means re-deriving
+# every hash below, and adding an arch means adding its hash here too.
+case "$ARCH" in
+  x86_64|amd64)
+    NAK_ARCH="amd64"
+    NAK_SHA256="495243c070c4533ce96e98b6f34b7e97fd4be2da3353488b400233ed7ed0d4da"
+    ;;
+  aarch64|arm64)
+    NAK_ARCH="arm64"
+    NAK_SHA256="1fb8868c60ebf77dd86f90d6374ebf8557412baa37026d2844af932776085b88"
+    ;;
+  *)
+    echo "Unsupported architecture: $ARCH"
+    exit 1
+    ;;
+esac
+
+if [ -z "${NAK_SHA256:-}" ]; then
+  echo "No SHA-256 pinned for nak ${NAK_VERSION} on ${NAK_ARCH}."
+  echo "Add one to the case above, derived by downloading the asset:"
+  echo "  curl -fsSL <asset-url> | sha256sum"
+  exit 1
+fi
+
+NAME="nak-v${NAK_VERSION}-linux-${NAK_ARCH}"
+URL="https://github.com/fiatjaf/nak/releases/download/v${NAK_VERSION}/${NAME}"
+# Stage outside the checkout so a failed attempt cannot leave a stray binary in
+# the working tree, and so nothing lands at $INSTALL_PATH before it verifies.
+NAK_TMP="$(mktemp -d)"
+trap 'rm -rf "$NAK_TMP"' EXIT
+TMP="${NAK_TMP}/${NAME}"
+
+# Download to a file and check it before installing. curl's own --retry does
+# not cover a short read (exit 18), and a checksum mismatch needs a fresh
+# download anyway, so the retry is an explicit bounded loop — the same failure
+# mode that forced one on the zig step in this workflow.
+verified=""
+previous=""
+for attempt in 1 2 3; do
+  rm -f "$TMP"
+  if curl -fsSL -o "$TMP" "$URL" && [ -s "$TMP" ]; then
+    actual="$(sha256sum < "$TMP" | cut -d' ' -f1)"
+    if [ "$actual" = "$NAK_SHA256" ]; then
+      echo "nak binary matches its pinned SHA-256 (${actual})"
+      verified=yes
+      break
+    fi
+    echo "nak binary failed its checksum on attempt ${attempt}:"
+    echo "  expected ${NAK_SHA256}"
+    echo "  actual   ${actual}"
+    echo "  size     $(wc -c < "$TMP") bytes"
+    if [ "$actual" = "$previous" ]; then
+      echo "Two attempts fetched byte-identical content, so retrying is not"
+      echo "going to help: the pin is stale, upstream re-published, or the"
+      echo "source is serving the same bad file every time."
+      break
+    fi
+    previous="$actual"
+  else
+    echo "nak binary download failed on attempt ${attempt}"
+  fi
+  if [ "$attempt" -lt 3 ]; then
+    sleep $((attempt * 10))
+  fi
+done
+
+if [ -z "$verified" ]; then
+  echo "nak ${NAK_VERSION} (${NAK_ARCH}) did not download with its pinned"
+  echo "SHA-256 ${NAK_SHA256} in 3 attempts. Nothing installed at ${INSTALL_PATH}."
+  exit 1
+fi
+
+install -m 0755 "$TMP" "$INSTALL_PATH"
+echo "Installed nak ${NAK_VERSION} (${NAK_ARCH}) at ${INSTALL_PATH}"