dream: signed session cookies (tamper-evident sid) + 11 tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/dream/session.sx

@@ -72,6 +72,48 @@
   dream-drop-cookie
   (fn (resp name) (dream-set-cookie resp name "" {:max-age 0})))
 
+;; ── signed cookie values (tamper-evident) ──────────────────────────
+;; NOTE: pure-SX keyed hash — not cryptographic; production should inject a host
+;; HMAC. Value carries no "." so the first "." splits value from signature.
+(define
+  dr/sess-hash
+  (fn (s) (dr/sess-hash-loop s 0 (string-length s) 7)))
+(define
+  dr/sess-hash-loop
+  (fn
+    (s i n h)
+    (if
+      (>= i n)
+      h
+      (dr/sess-hash-loop
+        s
+        (+ i 1)
+        n
+        (mod (+ (* h 131) (char-code (char-at s i))) 2147483647)))))
+(define
+  dr/sess-sig
+  (fn (secret val) (str (dr/sess-hash (str secret "|" val)))))
+
+(define
+  dream-cookie-sign
+  (fn (secret val) (str val "." (dr/sess-sig secret val))))
+(define
+  dream-cookie-unsign
+  (fn
+    (secret signed)
+    (if
+      (or (nil? signed) (= signed ""))
+      nil
+      (let
+        ((dot (index-of signed ".")))
+        (if
+          (< dot 0)
+          nil
+          (let
+            ((val (substr signed 0 dot))
+              (sig (substr signed (+ dot 1))))
+            (if (= sig (dr/sess-sig secret val)) val nil)))))))
+
 ;; ── in-memory session store (tests + demos) ────────────────────────
 ;; A backend is (fn (op) result) where op is a dict {:op ... :sid ... :key ...}.
 (define
@@ -143,6 +185,32 @@
                     sid
                     {:path "/" :http-only true :same-site "Lax"}))))))))))
 
+;; signed variant: the cookie value is signed so a guessed/forged sid is rejected
+(define
+  dream-sessions-signed
+  (fn
+    (backend secret)
+    (fn
+      (next)
+      (fn
+        (req)
+        (let
+          ((sid0 (dream-cookie-unsign secret (dream-cookie req dream-session-cookie-name))))
+          (let
+            ((have (and sid0 (backend {:op "session/exists" :sid sid0}))))
+            (let
+              ((sid (if have sid0 (backend {:op "session/create"}))))
+              (let
+                ((resp (next (assoc req :dream-session {:io backend :sid sid}))))
+                (if
+                  have
+                  resp
+                  (dream-set-cookie
+                    resp
+                    dream-session-cookie-name
+                    (dream-cookie-sign secret sid)
+                    {:path "/" :http-only true :same-site "Lax"}))))))))))
+
 ;; ── handler-facing session API ─────────────────────────────────────
 (define dr/session-of (fn (req) (get req :dream-session)))
 (define dream-session-id (fn (req) (get (dr/session-of req) :sid)))