diff --git a/muxplex/auth.py b/muxplex/auth.py
index 05d907f..d340eca 100644
--- a/muxplex/auth.py
+++ b/muxplex/auth.py
@@ -5,6 +5,8 @@ muxplex authentication — password and signing secret file management.
 import secrets
 from pathlib import Path
 
+from itsdangerous import BadSignature, SignatureExpired, TimestampSigner
+
 
 # ---------------------------------------------------------------------------
 # Config directory
@@ -66,3 +68,28 @@ def load_or_create_secret() -> str:
     path.write_text(secret + "\n")
     path.chmod(0o600)
     return secret
+
+
+# ---------------------------------------------------------------------------
+# Session cookie signing / verification
+# ---------------------------------------------------------------------------
+
+
+def create_session_cookie(secret: str, ttl_seconds: int) -> str:
+    """Create a signed, timestamped session cookie value."""
+    signer = TimestampSigner(secret)
+    return signer.sign("muxplex-session").decode()
+
+
+def verify_session_cookie(secret: str, cookie: str, ttl_seconds: int) -> bool:
+    """Verify a session cookie's signature and expiry. Returns True/False.
+
+    ttl_seconds=0 means session cookie — no server-side expiry check.
+    """
+    signer = TimestampSigner(secret)
+    try:
+        max_age = ttl_seconds if ttl_seconds > 0 else None
+        signer.unsign(cookie, max_age=max_age)
+        return True
+    except (BadSignature, SignatureExpired):
+        return False
diff --git a/muxplex/tests/test_auth.py b/muxplex/tests/test_auth.py
index 7f690b3..70990c8 100644
--- a/muxplex/tests/test_auth.py
+++ b/muxplex/tests/test_auth.py
@@ -119,3 +119,54 @@ def test_load_or_create_secret_returns_same_value_on_second_call(monkeypatch, tm
     first = load_or_create_secret()
     second = load_or_create_secret()
     assert first == second
+
+
+# ---------------------------------------------------------------------------
+# Session cookie signing / verification
+# ---------------------------------------------------------------------------
+
+
+def test_create_session_cookie_returns_string():
+    """create_session_cookie() returns a non-empty string."""
+    from muxplex.auth import create_session_cookie
+
+    cookie = create_session_cookie("test-secret", ttl_seconds=3600)
+    assert isinstance(cookie, str)
+    assert len(cookie) > 0
+
+
+def test_verify_session_cookie_valid_roundtrip():
+    """A cookie created by create_session_cookie verifies successfully."""
+    from muxplex.auth import create_session_cookie, verify_session_cookie
+
+    cookie = create_session_cookie("test-secret", ttl_seconds=3600)
+    assert verify_session_cookie("test-secret", cookie, ttl_seconds=3600) is True
+
+
+def test_verify_session_cookie_tampered():
+    """A tampered cookie fails verification."""
+    from muxplex.auth import create_session_cookie, verify_session_cookie
+
+    cookie = create_session_cookie("test-secret", ttl_seconds=3600)
+    tampered = cookie + "X"
+    assert verify_session_cookie("test-secret", tampered, ttl_seconds=3600) is False
+
+
+def test_verify_session_cookie_wrong_secret():
+    """A cookie signed with a different secret fails verification."""
+    from muxplex.auth import create_session_cookie, verify_session_cookie
+
+    cookie = create_session_cookie("secret-A", ttl_seconds=3600)
+    assert verify_session_cookie("secret-B", cookie, ttl_seconds=3600) is False
+
+
+def test_verify_session_cookie_expired():
+    """A cookie verified with a very short TTL fails (simulates expiry)."""
+    import time
+    from muxplex.auth import create_session_cookie, verify_session_cookie
+
+    cookie = create_session_cookie("test-secret", ttl_seconds=1)
+    # itsdangerous uses integer-second timestamps; sleep 2s to guarantee
+    # age = 2 > max_age = 1, ensuring reliable expiry detection
+    time.sleep(2)
+    assert verify_session_cookie("test-secret", cookie, ttl_seconds=1) is False