Pin key fingerprints so a corrupted key cannot deploy

README.md

@@ -42,9 +42,13 @@ images where `/bin/sh` is dash.
 Edit `keys.txt` and deploy. That is the entire workflow — nothing else references
 the list.
 
-    vi keys.txt            # add: ssh-ed25519 AAAA... aswin@NewLaptop
+    vi keys.txt                     # add: ssh-ed25519 AAAA... aswin@NewLaptop
+    sh scripts/fingerprints.sh      # regenerate fingerprints.txt
     npm run deploy
-    npm run check          # verifies production
+    npm run check                   # verifies production
+
+Commit `keys.txt` and `fingerprints.txt` together — `validate.sh` refuses to build
+while they disagree.
 
 Or edit `keys.txt` on GitHub and let Workers Builds deploy it — useful precisely
 because this endpoint exists to set up *other* machines, and you may not be at the
@@ -75,6 +79,24 @@ the SSH wire format inside it holds just the algorithm name and the raw public
 key, with no field for a comment. Renaming one changes nothing about who can log
 in; the fingerprint is identical.
 
+### Pinned fingerprints
+
+`fingerprints.txt` lists the SHA256 of every key in `keys.txt`, and `validate.sh`
+refuses to build if the two disagree.
+
+This exists because `ssh-keygen -l` validates *structure*, not authenticity. Flip a
+single bit in a key's material and the line still parses, still reports
+`256 ... (ED25519)`, and still passes every other check — only the fingerprint
+moves. A key corrupted that way would deploy, land in a new machine's
+`authorized_keys`, and silently not work, because no private key matches it. You
+would believe you had four ways into that box and have three.
+
+The pin also makes key changes reviewable. A reviewer skims past a 68-character
+base64 blob; a changed `SHA256:` line is legible. Any change to key material has to
+appear in `fingerprints.txt` in the same commit or the build fails.
+
+    sh scripts/fingerprints.sh      # after any change to keys.txt
+
 ### A bad key cannot be deployed
 
 `scripts/validate.sh` runs as wrangler's `build.command`, so it fires on
@@ -151,7 +173,8 @@ offline — no secrets, no Cloudflare token — so there is nothing to gate it o
 
 **Checks**
 
-- `validate.sh` — the same script the deploy runs, but now *before* merge
+- `validate.sh` — the same script the deploy runs, but now *before* merge,
+  including the pinned-fingerprint comparison
 - `check-readme.sh` — the README table must match `keys.txt`
 - `tsc --noEmit`
 - `wrangler deploy --dry-run` — config and bundling, without deploying

fingerprints.txt

@@ -0,0 +1,15 @@
+# Fingerprints of every key in keys.txt. Generated -- do not hand-edit.
+# Regenerate with: sh scripts/fingerprints.sh
+#
+# These exist so a corrupted key cannot deploy. ssh-keygen -l validates
+# structure, not authenticity: flip one bit in the key material and it still
+# parses, still reports 256-bit ED25519, and still passes -- only the
+# fingerprint changes. Pinning them turns 'these look like keys' into 'these
+# are exactly the keys we intend'.
+
+SHA256:dxiyBcPf+pkSLO/WNrEYnGEKkXpu67jKaS47Ky8ECFs  aswin@AswinPC
+SHA256:SvP/8B22PR2Q4rCGcQmvF8yUsSGWBcIKAuCD+P6oYDg  aswin@Aswin-Laptop
+SHA256:cCz8aDx6zovq4vWWcEUEmRzkdf7LLgNbzM3BKUqpeJ4  aswin@Aswins-MacBook-Air.local
+SHA256:J0Xs4eAxSVzI/XhTmESrauM48xEMtFmN78GxEc0CkV4  aswin@Aswin-Macbook-Pro
+SHA256:ubg9S7gxhwKqTvmgRxS/SRIvfEo1ZayOa+FqfVl+qJ8  mail@ubuntu
+SHA256:gWAoI59eLhaNK6FGvQDobXNmxPZUCkf+HUNh329gZAg  aswin@truenas-host

scripts/fingerprints.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+# Regenerates fingerprints.txt from keys.txt.
+#
+#     sh scripts/fingerprints.sh
+#
+# Run this after changing keys.txt and commit both files together. validate.sh
+# refuses to build while the two disagree, so the fingerprint of any key that
+# changed lands in the same diff as the change -- which is the point. A reviewer
+# skims past a 68-character base64 blob; a changed SHA256 line is legible.
+set -eu
+cd "$(dirname "$0")/.."
+
+{
+  echo "# Fingerprints of every key in keys.txt. Generated -- do not hand-edit."
+  echo "# Regenerate with: sh scripts/fingerprints.sh"
+  echo "#"
+  echo "# These exist so a corrupted key cannot deploy. ssh-keygen -l validates"
+  echo "# structure, not authenticity: flip one bit in the key material and it still"
+  echo "# parses, still reports 256-bit ED25519, and still passes -- only the"
+  echo "# fingerprint changes. Pinning them turns 'these look like keys' into 'these"
+  echo "# are exactly the keys we intend'."
+  echo
+  ssh-keygen -lf keys.txt | awk '{print $2"  "$3}'
+} > fingerprints.txt
+
+echo "fingerprints.sh: wrote $(grep -v '^#' fingerprints.txt | grep -c .) fingerprints"
+grep -v '^#' fingerprints.txt | grep . | sed 's/^/    /'

scripts/validate.sh

@@ -73,6 +73,35 @@ if [ -n "$dupes" ]; then
   fail=1
 fi
 
+# Pinned fingerprints. ssh-keygen -l above proved every line is STRUCTURALLY a
+# key; it cannot prove it is the RIGHT key. Flip one bit in the key material and
+# the line still parses, still reports 256-bit ED25519 and still passes — only the
+# fingerprint moves. A key corrupted that way would deploy, reach a new machine's
+# authorized_keys, and silently not work, because no private key matches it.
+#
+# Comparing against a checked-in list closes that, and also puts any change to key
+# material into the diff in a form a reviewer can actually read.
+FP=fingerprints.txt
+if [ ! -f "$FP" ]; then
+  echo "validate: $FP is missing — run: sh scripts/fingerprints.sh"
+  fail=1
+else
+  want=$(grep -v '^#' "$FP" | grep . | awk '{print $1"  "$2}' | sort)
+  got=$(ssh-keygen -lf "$F" 2>/dev/null | awk '{print $2"  "$3}' | sort)
+  if [ "$want" != "$got" ]; then
+    echo "validate: keys.txt and $FP disagree."
+    echo
+    echo "  pinned but not in keys.txt:"
+    printf '%s\n' "$want" | grep -vxF "$(printf '%s' "$got")" 2>/dev/null | grep . | sed 's/^/    /' || echo "    (none)"
+    echo "  in keys.txt but not pinned:"
+    printf '%s\n' "$got" | grep -vxF "$(printf '%s' "$want")" 2>/dev/null | grep . | sed 's/^/    /' || echo "    (none)"
+    echo
+    echo "  If the key change was intended:  sh scripts/fingerprints.sh"
+    echo "  and commit fingerprints.txt alongside keys.txt."
+    fail=1
+  fi
+fi
+
 if [ "$fail" != 0 ]; then
   echo
   echo "validate: FAILED — not deploying."