diff --git a/test/mocks/HKDF.h b/test/mocks/HKDF.h
new file mode 100644
index 000000000..afd1ab136
--- /dev/null
+++ b/test/mocks/HKDF.h
@@ -0,0 +1,16 @@
+#pragma once
+
+#include <stddef.h>
+
+// Mock HKDF for native testing — deterministic but not cryptographic.
+template <typename T> void hkdf
+    (void *out, size_t outLen, const void *key, size_t keyLen,
+     const void *salt, size_t saltLen, const void *info, size_t infoLen)
+{
+    int j = 0, k = 0;
+    for (int i = 0; i < outLen; i++) {
+        ((uint8_t *)out)[i] = ((const uint8_t *)key)[k] ^ ((const uint8_t *)info)[j];
+        k = (k + 1) % keyLen;
+        j = (j + 1) % infoLen;
+    }
+}