docs: clarify certFingerprint does not govern matrix federation tls

docs/swarm.md

@@ -53,6 +53,46 @@ optional:
   for peers whose self-signed TLS cert doesn't chain to a CA your
   host trusts.
 
+`certFingerprint` scopes **only** to hive-c0re's own peer HTTPS checks
+(the P33RS dashboard links and agent peer discovery below). It is
+**not** consulted by matrix federation — tuwunel validates a peer's
+federation certificate against the system CA bundle independently (see
+*Matrix federation* below), so pinning a fingerprint here does nothing
+for a self-signed matrix gateway cert.
+
+### Fingerprint format
+
+The value is the string `sha256:` followed by exactly 64 hexadecimal
+digits — the SHA-256 digest of the peer's DER-encoded TLS leaf
+certificate. The hex is case-insensitive (upper or lower both parse),
+carries no colon separators between bytes, and any value not matching
+this shape is ignored with a warning rather than weakening trust.
+
+```
+sha256:b1946ac92492d2347c6235b4d2611184a3f5b6cae6c19d6e3c2f0a8e7d4c9f12
+```
+
+Generate it from the peer's certificate with openssl. The
+`-fingerprint -sha256` output is uppercase and colon-separated, so
+strip the colons, lowercase, and prepend the `sha256:` prefix:
+
+```sh
+# from a PEM/CRT file
+openssl x509 -in peer.crt -noout -fingerprint -sha256 \
+  | sed 's/^.*=//; s/://g' | tr 'A-Z' 'a-z' | sed 's/^/sha256:/'
+
+# straight from the live endpoint (port 443)
+echo | openssl s_client -connect peer.example.com:443 -servername peer.example.com 2>/dev/null \
+  | openssl x509 -noout -fingerprint -sha256 \
+  | sed 's/^.*=//; s/://g' | tr 'A-Z' 'a-z' | sed 's/^/sha256:/'
+```
+
+Pin the leaf certificate, not an intermediate or the CA — the
+digest must match the exact cert the peer serves on its HTTPS
+endpoint. When the peer rotates its cert, update the pin to the new
+fingerprint (or switch the peer to a CA-trusted cert and drop the
+field).
+
 The nix module serialises the attrset to a `HYPERHIVE_PEERS` JSON
 array (`[{ domain, cert_fingerprint }]`) injected into the c0re
 environment and forwarded to agent containers.
@@ -72,10 +112,15 @@ environment and forwarded to agent containers.
    qualified names (`agent@domain`).
 
 3. **Matrix federation** — when `matrix.enable` is on, tuwunel
-   federates with the peer's matrix server at
-   `matrix.{peer-domain}:8448`. No extra config needed; federation
-   works as soon as the domains are reachable and TLS validates.
-   See `docs/matrix.md` for federation firewall requirements.
+   federates with the peer's matrix server (discovered via the peer's
+   `.well-known/matrix/server` delegation, which the gateway serves).
+   Federation validates the peer's TLS certificate against the
+   **system CA bundle** — independently of `certFingerprint`, which it
+   never consults. A self-signed gateway certificate therefore won't
+   federate even with a fingerprint pinned above: the peers need
+   CA-issued certs (ACME) or a shared private CA trusted on both
+   gateway hosts. See `docs/matrix.md` for federation firewall + TLS
+   requirements.
 
 ## Bilateral setup
 

nix/modules/hive-c0re.nix

@@ -120,11 +120,19 @@ in
           certFingerprint = lib.mkOption {
             type = lib.types.nullOr lib.types.str;
             default = null;
-            example = "sha256:abc123...";
+            example = "sha256:b1946ac92492d2347c6235b4d2611184a3f5b6cae6c19d6e3c2f0a8e7d4c9f12";
             description = ''
               Expected TLS certificate fingerprint for this peer's HTTPS
               endpoint. Null = trust the system CA bundle (for Let's
               Encrypt peers). Set to pin a self-signed cert.
+
+              Format: the literal `sha256:` followed by exactly 64
+              hex digits (case-insensitive, no colon separators) — the
+              SHA-256 digest of the peer's DER-encoded leaf certificate.
+              Generate with `openssl x509 -noout -fingerprint -sha256`,
+              then strip the colons and prepend `sha256:`. A malformed
+              value is ignored with a warning rather than weakening
+              trust. See docs/swarm.md for the full recipe.
             '';
           };
 
@@ -173,7 +181,7 @@ in
     default = { };
     example = {
       "lab.example.com" = {
-        certFingerprint = "sha256:abc123";
+        certFingerprint = "sha256:b1946ac92492d2347c6235b4d2611184a3f5b6cae6c19d6e3c2f0a8e7d4c9f12";
       };
       "edge.corp" = { };
     };