Restructure the repository

- Make OpenPGP only one of many protocols
  + KEYS.md only holds the current keys
  + OPENPGP.md describes all OpenPGP details
- Make the current public key available at the root
- Add scripts/ to automate the various steps:
  + bootstrap.sh
  + publish.sh
  + sign.sh
  + snapshot.sh
  + verify.sh

scripts/snapshot.sh

@@ -0,0 +1,72 @@
+#!/bin/sh
+
+# Export the current OpenPGP public key as a yearly snapshot
+#
+# Usage:  ./scripts/snapshot.sh [YEAR]
+#
+# YEAR defaults to the current UTC year
+#
+# Writes snapshots/<KEYID>-public-<YEAR>.asc and refreshes public-key.asc
+
+set -eu
+
+
+# Internal [S] subkey; only used to look up the key
+SIGNING_KEY='D6D1989DF1F6B23FD776293DB46EDB4DFC26805D'
+
+
+REPO=$(cd "$(dirname "$0")/.." && pwd)
+YEAR=${1:-$(date -u +%Y)}
+
+# Notes:
+#  - the first "fpr" line is the primary key's fingerprint
+#  - a subkey resolves to it
+FPR=$(gpg --list-keys --with-colons "$SIGNING_KEY" \
+    | awk -F: '$1 == "fpr" { print $10; exit }')
+if [ -z "$FPR" ]; then
+    echo "Error: Key $SIGNING_KEY not found" >&2
+    exit 1
+fi
+# The last 16 hex chars, as used in OPENPGP.md
+KEYID=${FPR#"${FPR%????????????????}"}
+
+OUT="$REPO/snapshots/$KEYID-public-$YEAR.asc"
+
+
+mkdir -p "$REPO/snapshots"
+
+
+# Merge the repo copy in first, so a lagging keyring cannot shrink the export
+gpg --quiet \
+    --import "$REPO/public-key.asc"
+
+# The retired keys must be present,
+# or "export-clean" below drops their certifications as unverifiable
+gpg --quiet \
+    --import "$REPO"/history/*.asc
+
+
+gpg --armor \
+    --export-options export-clean \
+    --export "$FPR" \
+    > "$OUT" \
+    || {
+        rm -f "$OUT"
+        exit 1
+    }
+
+
+if [ ! -s "$OUT" ]; then
+    rm -f "$OUT"
+    echo "Error: Nothing exported for $FPR" >&2
+    exit 1
+fi
+
+
+cp "$OUT" "$REPO/public-key.asc"
+
+
+gpg --show-keys "$OUT" >&2
+
+
+echo "Wrote snapshots/$KEYID-public-$YEAR.asc and public-key.asc"