diff --git a/internal/crypto/session.go b/internal/crypto/session.go
new file mode 100644
index 0000000..2c938d0
--- /dev/null
+++ b/internal/crypto/session.go
@@ -0,0 +1,44 @@
+package crypto
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// token = base64(user) "." expiryUnix "." base64(hmac)
+func SignSession(secret []byte, user string, expiry time.Time) string {
+	payload := base64.RawURLEncoding.EncodeToString([]byte(user)) + "." +
+		strconv.FormatInt(expiry.Unix(), 10)
+	return payload + "." + sign(secret, payload)
+}
+
+func VerifySession(secret []byte, token string, now time.Time) (string, bool) {
+	parts := strings.Split(token, ".")
+	if len(parts) != 3 {
+		return "", false
+	}
+	payload := parts[0] + "." + parts[1]
+	if !hmac.Equal([]byte(parts[2]), []byte(sign(secret, payload))) {
+		return "", false
+	}
+	exp, err := strconv.ParseInt(parts[1], 10, 64)
+	if err != nil || now.Unix() > exp {
+		return "", false
+	}
+	user, err := base64.RawURLEncoding.DecodeString(parts[0])
+	if err != nil {
+		return "", false
+	}
+	return string(user), true
+}
+
+func sign(secret []byte, payload string) string {
+	m := hmac.New(sha256.New, secret)
+	fmt.Fprint(m, payload)
+	return base64.RawURLEncoding.EncodeToString(m.Sum(nil))
+}
diff --git a/internal/crypto/session_test.go b/internal/crypto/session_test.go
new file mode 100644
index 0000000..8d844f9
--- /dev/null
+++ b/internal/crypto/session_test.go
@@ -0,0 +1,34 @@
+package crypto
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSessionRoundTrip(t *testing.T) {
+	secret := []byte("s3cr3t")
+	now := time.Unix(1_700_000_000, 0)
+	tok := SignSession(secret, "admin", now.Add(time.Hour))
+	user, ok := VerifySession(secret, tok, now)
+	if !ok || user != "admin" {
+		t.Fatalf("verify = %q,%v want admin,true", user, ok)
+	}
+}
+
+func TestSessionRejectsExpired(t *testing.T) {
+	secret := []byte("s3cr3t")
+	now := time.Unix(1_700_000_000, 0)
+	tok := SignSession(secret, "admin", now.Add(-time.Second))
+	if _, ok := VerifySession(secret, tok, now); ok {
+		t.Fatal("expired token must be rejected")
+	}
+}
+
+func TestSessionRejectsTampered(t *testing.T) {
+	secret := []byte("s3cr3t")
+	now := time.Unix(1_700_000_000, 0)
+	tok := SignSession(secret, "admin", now.Add(time.Hour))
+	if _, ok := VerifySession([]byte("other"), tok, now); ok {
+		t.Fatal("wrong secret must be rejected")
+	}
+}