diff --git a/captcha/nonce.go b/captcha/nonce.go
new file mode 100644
index 0000000..1605c8a
--- /dev/null
+++ b/captcha/nonce.go
@@ -0,0 +1,39 @@
+package captcha
+
+import (
+	"sync"
+	"time"
+)
+
+// NonceStore tracks used challenge nonces for single-use enforcement. The
+// default MemoryNonceStore is per-process (no DB) — sufficient for the 5a
+// model. A durable cross-process store (5b) can implement this interface later.
+type NonceStore interface {
+	// MarkUsed records nonce; returns true on first use, false if already seen.
+	MarkUsed(nonce string, ttl time.Duration) bool
+}
+
+type MemoryNonceStore struct {
+	mu   sync.Mutex
+	used map[string]time.Time
+}
+
+func NewMemoryNonceStore() *MemoryNonceStore {
+	return &MemoryNonceStore{used: make(map[string]time.Time)}
+}
+
+func (m *MemoryNonceStore) MarkUsed(nonce string, ttl time.Duration) bool {
+	now := time.Now()
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for k, exp := range m.used {
+		if !exp.After(now) {
+			delete(m.used, k)
+		}
+	}
+	if _, seen := m.used[nonce]; seen {
+		return false
+	}
+	m.used[nonce] = now.Add(ttl)
+	return true
+}
diff --git a/captcha/nonce_test.go b/captcha/nonce_test.go
new file mode 100644
index 0000000..d7ec502
--- /dev/null
+++ b/captcha/nonce_test.go
@@ -0,0 +1,27 @@
+package captcha
+
+import (
+	"testing"
+	"time"
+)
+
+func TestMemoryNonceStoreSingleUse(t *testing.T) {
+	st := NewMemoryNonceStore()
+	if !st.MarkUsed("n1", time.Minute) {
+		t.Fatal("first use should return true")
+	}
+	if st.MarkUsed("n1", time.Minute) {
+		t.Fatal("second use should return false")
+	}
+	if !st.MarkUsed("n2", time.Minute) {
+		t.Fatal("distinct nonce should return true")
+	}
+}
+
+func TestMemoryNonceStoreExpiry(t *testing.T) {
+	st := NewMemoryNonceStore()
+	st.MarkUsed("n1", -time.Second) // already expired
+	if !st.MarkUsed("n1", time.Minute) {
+		t.Fatal("expired nonce should be reusable")
+	}
+}