feat: add GPG signing for release PDFs

hooks/pre-commit

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+#!/usr/bin/env bash
+# Optional convenience for the GhostNet signer: when a PDF is committed, re-sign
+# it so its .asc never goes stale, and stage the signature in the same commit.
+#
+# Install (signer only):  git config core.hooksPath hooks
+#
+# ponytail: convenience only, not enforcement. Git does not distribute hooks to
+# people who clone, and this is bypassable with `git commit --no-verify`. The
+# real mechanism is signing/sign-ghostnet.sh; this just runs it automatically.
+set -euo pipefail
+
+cd "$(git rev-parse --show-toplevel)"
+mapfile -t pdfs < <(git diff --cached --name-only --diff-filter=ACM -- '*.pdf')
+[ "${#pdfs[@]}" -gt 0 ] || exit 0
+
+signing/sign-ghostnet.sh "${pdfs[@]}"
+git add "${pdfs[@]/%/.asc}"

signing/README.md

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+GhostNet PDF Signing
+====================
+
+**Status: Proposed.** Opened in response to
+[issue #13](https://github.com/s2underground/GhostNet/issues/13). Signing is not
+active until S2 Underground publishes a signing key (see "For the signer").
+
+Why
+---
+The concern in issue #13 is forged copies of the GhostNet manual circulating
+with altered information, especially once a PDF leaves GitHub and travels by
+radio, sneakernet, or reposts. A detached GPG signature lets anyone confirm a
+PDF is the genuine S2 Underground release, wherever they got it.
+
+The signature (`<file>.pdf.asc`) is committed next to each PDF and travels with
+the file. This is deliberately not git commit/tag signing: that would only help
+people who obtained the file through GitHub, who are the ones least at risk.
+
+The trust model (read this)
+---------------------------
+A valid signature only proves the PDF matches **the key in this repository**.
+Anyone can fork the repo, swap in their own key, and re-sign a forged PDF. The
+signature is therefore only meaningful if you verify the key's **fingerprint
+against a copy published out of band**, somewhere an attacker cannot quietly
+change:
+
+- read aloud / sent during a live net,
+- printed inside the manual PDF itself,
+- posted on QRZ and on more than one independent platform,
+- anywhere the real S2 Underground identity is already established.
+
+If the fingerprint the verify script prints does not match a source you trust,
+the signature means nothing. Do not skip this step.
+
+For anyone: verify a PDF
+------------------------
+Requires `gpg`.
+
+```sh
+signing/verify-ghostnet.sh                 # verify all signed PDFs
+signing/verify-ghostnet.sh GhostNet_Version_1.5.pdf
+```
+
+It imports the bundled key into a throwaway keyring (your own keyring is never
+touched), prints the fingerprint to cross-check, and reports `OK` or `FAIL` per
+file. A non-zero exit means at least one file failed.
+
+For the signer (S2 Underground)
+-------------------------------
+1. Publish your public key and its fingerprint out of band (see trust model).
+2. Add the public key to this repo as `signing/GHOSTNET-signing-key.asc`:
+   ```sh
+   gpg --armor --export <your-key-id> > signing/GHOSTNET-signing-key.asc
+   ```
+3. Sign the release PDFs and commit the `.asc` files:
+   ```sh
+   signing/sign-ghostnet.sh                 # signs all tracked PDFs
+   git add *.pdf.asc signing/GHOSTNET-signing-key.asc
+   ```
+   Pick a specific key with `-u <key-id>` or `GHOSTNET_SIGNING_KEY=<key-id>`.
+
+Optional: sign automatically on commit:
+```sh
+git config core.hooksPath hooks
+```
+The `hooks/pre-commit` hook then re-signs any staged PDF and stages its
+signature. This is a convenience for the signer only; git does not distribute
+hooks to people who clone.
+
+Checking the scripts
+--------------------
+`signing/selftest.sh` generates a throwaway key, signs a sample, and asserts a
+good signature verifies while a tampered file is rejected. No network, no
+changes to your keyring.

signing/selftest.sh

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+#!/usr/bin/env bash
+# Self-test for sign-ghostnet.sh / verify-ghostnet.sh.
+# Generates a throwaway key in a temp keyring (no network, your real keyring is
+# untouched), signs a dummy file, and asserts: good signature verifies, a
+# tampered file does NOT. Exercises both scripts end to end.
+#
+# Run:  signing/selftest.sh
+set -euo pipefail
+
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+work="$(mktemp -d)"
+export GNUPGHOME="$work/gnupg"
+mkdir -p "$GNUPGHOME"; chmod 700 "$GNUPGHOME"
+trap 'rm -rf "$work"' EXIT
+
+echo "1. generating throwaway signing key"
+gpg --homedir "$GNUPGHOME" --batch --quiet \
+    --quick-generate-key "GhostNet Selftest <selftest@example.invalid>" \
+    default default none
+fpr="$(gpg --homedir "$GNUPGHOME" --batch --with-colons --fingerprint \
+        | awk -F: '/^fpr:/{print $10; exit}')"
+gpg --homedir "$GNUPGHOME" --batch --armor --export "$fpr" > "$work/pub.asc"
+
+cd "$work"
+printf 'GhostNet selftest payload\n' > sample.pdf
+
+echo "2. signing with sign-ghostnet.sh"
+GHOSTNET_SIGNING_KEY="$fpr" "$here/sign-ghostnet.sh" sample.pdf
+[ -f sample.pdf.asc ] || { echo "FAIL: no signature produced"; exit 1; }
+
+echo "3. verifying a good signature (expect OK)"
+GHOSTNET_KEY_FILE="$work/pub.asc" "$here/verify-ghostnet.sh" sample.pdf \
+  || { echo "FAIL: good signature did not verify"; exit 1; }
+
+echo "4. tampering the file (expect FAIL)"
+printf 'forged content\n' > sample.pdf
+if GHOSTNET_KEY_FILE="$work/pub.asc" "$here/verify-ghostnet.sh" sample.pdf; then
+  echo "FAIL: tampered file verified as good"; exit 1
+fi
+
+echo "PASS: good signature verifies, tampered file rejected"

signing/sign-ghostnet.sh

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+#!/usr/bin/env bash
+# Sign GhostNet release PDFs with a detached, armored GPG signature.
+# Intended for the S2 Underground signer only. Produces "<file>.asc" next to
+# each PDF, which travels with the file even when it leaves GitHub.
+#
+# Usage:
+#   signing/sign-ghostnet.sh [file ...]
+#     With no arguments, signs every *.pdf tracked in the repository root.
+#
+# Key selection (first match wins):
+#   -u / --local-user KEYID      explicit key
+#   $GHOSTNET_SIGNING_KEY         environment override
+#   gpg default key               otherwise
+set -euo pipefail
+
+command -v gpg >/dev/null || { echo "error: gpg not found" >&2; exit 127; }
+
+key="${GHOSTNET_SIGNING_KEY:-}"
+case "${1:-}" in
+  -u|--local-user) key="$2"; shift 2 ;;
+esac
+
+# Default target list: tracked PDFs in the repo root.
+if [ "$#" -gt 0 ]; then
+  pdfs=("$@")
+else
+  mapfile -t pdfs < <(git ls-files -- '*.pdf' 2>/dev/null || true)
+fi
+[ "${#pdfs[@]}" -gt 0 ] || { echo "no PDFs to sign" >&2; exit 0; }
+
+for pdf in "${pdfs[@]}"; do
+  [ -f "$pdf" ] || { echo "skip (missing): $pdf" >&2; continue; }
+  gpg --batch --yes --armor --detach-sign \
+      ${key:+--local-user "$key"} \
+      --output "$pdf.asc" "$pdf"
+  echo "signed: $pdf -> $pdf.asc"
+done

signing/verify-ghostnet.sh

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+#!/usr/bin/env bash
+# Verify GhostNet release PDFs against their detached GPG signatures.
+# Anyone can run this. It imports the bundled GhostNet signing key into a
+# throwaway keyring (your own keyring is never touched) and checks each PDF.
+#
+# IMPORTANT: a good signature only proves the file matches the key in this
+# repo. Confirm the printed fingerprint against a copy published OUT OF BAND
+# (read during a net, printed inside the PDF, posted on QRZ / multiple sites)
+# before you trust it. See signing/README.md.
+#
+# Usage:
+#   signing/verify-ghostnet.sh [file ...]
+#     With no arguments, verifies every *.pdf that has a matching *.asc.
+#
+# Key file: signing/GHOSTNET-signing-key.asc (override with $GHOSTNET_KEY_FILE).
+set -euo pipefail
+
+command -v gpg >/dev/null || { echo "error: gpg not found" >&2; exit 127; }
+
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+keyfile="${GHOSTNET_KEY_FILE:-$here/GHOSTNET-signing-key.asc}"
+if [ ! -f "$keyfile" ]; then
+  echo "error: signing key not found: $keyfile" >&2
+  echo "The S2 Underground public key has not been added yet. Once published," >&2
+  echo "save it as signing/GHOSTNET-signing-key.asc." >&2
+  exit 2
+fi
+
+# Throwaway keyring so we never touch the user's real GnuPG home.
+tmp="$(mktemp -d)"
+trap 'rm -rf "$tmp"' EXIT
+chmod 700 "$tmp"
+gpg --homedir "$tmp" --batch --quiet --import "$keyfile"
+echo "Imported signing key. Cross-check this fingerprint out of band:"
+gpg --homedir "$tmp" --batch --fingerprint --with-colons \
+  | awk -F: '/^fpr:/{print "  " $10; exit}'
+echo
+
+# Target list: explicit args, or every PDF that has a signature.
+if [ "$#" -gt 0 ]; then
+  pdfs=("$@")
+else
+  pdfs=()
+  for sig in *.asc; do
+    [ -e "$sig" ] || continue
+    pdfs+=("${sig%.asc}")
+  done
+fi
+[ "${#pdfs[@]}" -gt 0 ] || { echo "no signed PDFs found" >&2; exit 0; }
+
+failed=0
+for pdf in "${pdfs[@]}"; do
+  if gpg --homedir "$tmp" --batch --verify "$pdf.asc" "$pdf" 2>/dev/null; then
+    echo "OK:   $pdf"
+  else
+    echo "FAIL: $pdf"
+    failed=1
+  fi
+done
+exit "$failed"