Add security.txt

- Add scripts/security-txt.sh
  to generate a security.txt file
- Add the current security.txt
- Extend verify.sh to verify security.txt by default
  and cross-check that it expires together with the signing key
- Describe security.txt in OPENPGP.md and README.md

scripts/security-txt.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+
+# Generate security.txt (RFC 9116), clearsigned with the [S] subkey
+#
+# Usage:  ./scripts/security-txt.sh
+#
+# Expires is read from the key, so the January ceremony only re-runs this
+# after the key's expiry was extended
+
+set -eu
+
+
+# Internal [S] subkey; pinned with "!" below so `gpg` never picks another subkey
+SIGNING_KEY='D6D1989DF1F6B23FD776293DB46EDB4DFC26805D'
+
+
+REPO=$(cd "$(dirname "$0")/.." && pwd)
+OUT="$REPO/security.txt"
+
+
+# Notes:
+#  - the 7th field of the pub line is the primary key's expiry in epoch seconds
+#  - a subkey resolves to the primary key
+EPOCH=$(gpg --list-keys --with-colons "$SIGNING_KEY" \
+    | awk -F: '$1 == "pub" { print $7; exit }')
+if [ -z "$EPOCH" ]; then
+    echo "Error: No expiry found for key $SIGNING_KEY" >&2
+    exit 1
+fi
+EXPIRES=$(date -u -d "@$EPOCH" +%Y-%m-%dT%H:%M:%SZ)
+
+
+UNSIGNED=$(mktemp)
+trap 'rm -f "$UNSIGNED"' EXIT
+
+
+cat > "$UNSIGNED" <<EOT
+Canonical: https://www.alexander-hess.com/.well-known/security.txt
+Canonical: https://www.webartifex.de/.well-known/security.txt
+Contact: mailto:ah@alexander-hess.com
+Contact: mailto:alexander@webartifex.de
+Encryption: https://www.alexander-hess.com/public-key.asc
+Encryption: https://www.webartifex.de/public-key.asc
+Expires: $EXPIRES
+Preferred-Languages: en, de
+EOT
+
+
+gpg \
+    --clearsign \
+    --digest-algo SHA512 \
+    --local-user "${SIGNING_KEY}!" \
+    --yes \
+    --output "$OUT" \
+    "$UNSIGNED"
+
+gpg --verify "$OUT"
+
+
+echo "Wrote security.txt (Expires: $EXPIRES)"

scripts/verify.sh

@@ -4,17 +4,18 @@
 #
 # Usage:  ./scripts/verify.sh [FILE...]
 #
-# FILE defaults to KEYS.md OPENPGP.md ceremonies/*.md AGE.md
+# FILE defaults to KEYS.md OPENPGP.md ceremonies/*.md AGE.md security.txt
 #
 # A FILE with a FILE.asc next to it is checked as a detached signature,
 # any other FILE as a clearsigned document
 #
 # Also checks the key's expiry, and,
 # if KEYS.md or AGE.md is among the FILEs,
-# that age-recipient.txt matches the recipient in them
+# that age-recipient.txt matches the recipient in them, and,
+# if security.txt is among the FILEs, that it expires with the key
 #
 # Exits non-zero if any file lacks a valid signature from the pinned key,
-# or if the key has expired, or if a cross-check fails
+# or if the key or security.txt has expired, or if a cross-check fails
 
 set -eu
 
@@ -36,6 +37,7 @@ if [ $# -eq 0 ]; then
         "$REPO/OPENPGP.md" \
         "$REPO"/ceremonies/*.md \
         "$REPO/AGE.md" \
+        "$REPO/security.txt" \
         # Keep this list aligned with the documentation at the top
 fi
 
@@ -104,10 +106,13 @@ fi
 
 
 # age-recipient.txt is the only unsigned artifact,
-# so it must match the recipient in the signed documents
+# so it must match the recipient in the signed documents;
+# security.txt must expire together with the signing key,
+# but only cross-check it when it was among the verified files
 for FILE; do
 
     case "$FILE" in
+
         */KEYS.md | KEYS.md | */AGE.md | AGE.md)
             # shellcheck disable=SC2016  # The backticks are literal Markdown
             EXPECTED=$(sed -n 's/^> `\(age1[0-9a-z]*\)`$/\1/p' "$FILE")
@@ -120,6 +125,16 @@ for FILE; do
                 FAILED=1
             fi
             ;;
+
+        */security.txt | security.txt)
+            EXPECTED=$(date -u -d "@$EXPIRY" +%Y-%m-%dT%H:%M:%SZ)
+            ACTUAL=$(sed -n 's/^Expires: //p' "$FILE")
+            if [ "$ACTUAL" != "$EXPECTED" ]; then
+                echo "FAIL  security.txt expires $ACTUAL, key expires $EXPECTED" >&2
+                FAILED=1
+            fi
+            ;;
+
     esac
 
 done