diff --git a/libtock-sync/Makefile b/libtock-sync/Makefile
index 13d62e794..cdd1b301f 100644
--- a/libtock-sync/Makefile
+++ b/libtock-sync/Makefile
@@ -10,6 +10,7 @@ $(LIBNAME)_SRC_ROOT := $(TOCK_USERLAND_BASE_DIR)
 # List all C and Assembly files
 $(LIBNAME)_SRCS  := $(wildcard $($(LIBNAME)_DIR)/*.c)
 $(LIBNAME)_SRCS  += $(wildcard $($(LIBNAME)_DIR)/crypto/*.c)
+$(LIBNAME)_SRCS  += $(wildcard $($(LIBNAME)_DIR)/crypto/syscalls/*.c)
 $(LIBNAME)_SRCS  += $(wildcard $($(LIBNAME)_DIR)/display/*.c)
 $(LIBNAME)_SRCS  += $(wildcard $($(LIBNAME)_DIR)/interface/*.c)
 $(LIBNAME)_SRCS  += $(wildcard $($(LIBNAME)_DIR)/kernel/*.c)
diff --git a/libtock-sync/crypto/hmac.c b/libtock-sync/crypto/hmac.c
index e38f188c9..ffe520b6a 100644
--- a/libtock-sync/crypto/hmac.c
+++ b/libtock-sync/crypto/hmac.c
@@ -1,16 +1,6 @@
-#include "hmac.h"
-
-struct hmac_data {
-  bool fired;
-  returncode_t ret;
-};
+#include <libtock/defer.h>
 
-static struct hmac_data result = {.fired = false};
-
-static void hmac_cb_hmac(returncode_t ret) {
-  result.fired = true;
-  result.ret   = ret;
-}
+#include "hmac.h"
 
 returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                      uint8_t* key_buffer, uint32_t key_length,
@@ -18,24 +8,29 @@ returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                      uint8_t* hmac_buffer, uint32_t hmac_length) {
   returncode_t ret;
 
-  result.fired = false;
-
-  ret = libtock_hmac_simple(hmac_type, key_buffer, key_length, input_buffer, input_length, hmac_buffer, hmac_length,
-                            hmac_cb_hmac);
+  ret = libtock_hmac_command_set_algorithm((uint32_t) hmac_type);
   if (ret != RETURNCODE_SUCCESS) return ret;
 
-  // Wait for the callback.
-  yield_for(&result.fired);
-  if (result.ret != RETURNCODE_SUCCESS) return result.ret;
+  ret = libtock_hmac_set_readonly_allow_key_buffer(key_buffer, key_length);
+  if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readonly_allow_key_buffer(NULL, 0);
+  };
 
-  ret = libtock_hmac_set_readonly_allow_key_buffer(NULL, 0);
+  ret = libtock_hmac_set_readonly_allow_data_buffer(input_buffer, input_length);
   if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readonly_allow_data_buffer(NULL, 0);
+  };
 
-  ret = libtock_hmac_set_readonly_allow_data_buffer(NULL, 0);
+  ret = libtock_hmac_set_readwrite_allow_destination_buffer(hmac_buffer, hmac_length);
   if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readwrite_allow_destination_buffer(NULL, 0);
+  };
 
-  ret = libtock_hmac_set_readwrite_allow_destination_buffer(NULL, 0);
+  ret = libtock_hmac_command_run();
   if (ret != RETURNCODE_SUCCESS) return ret;
 
-  return RETURNCODE_SUCCESS;
+  // Wait for the operation.
+  ret = libtocksync_hmac_yield_wait_for();
+
+  return ret;
 }
diff --git a/libtock-sync/crypto/hmac.h b/libtock-sync/crypto/hmac.h
index 5655b0a62..7139896ec 100644
--- a/libtock-sync/crypto/hmac.h
+++ b/libtock-sync/crypto/hmac.h
@@ -1,8 +1,10 @@
 #pragma once
 
-#include <libtock/crypto/hmac.h>
+#include <libtock/crypto/hmac_types.h>
 #include <libtock/tock.h>
 
+#include "syscalls/hmac_syscalls.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/libtock-sync/crypto/syscalls/hmac_syscalls.c b/libtock-sync/crypto/syscalls/hmac_syscalls.c
new file mode 100644
index 000000000..7bdf3a468
--- /dev/null
+++ b/libtock-sync/crypto/syscalls/hmac_syscalls.c
@@ -0,0 +1,8 @@
+#include "hmac_syscalls.h"
+
+returncode_t libtocksync_hmac_yield_wait_for(void) {
+  yield_waitfor_return_t ret;
+  ret = yield_wait_for(DRIVER_NUM_HMAC, 0);
+
+  return (returncode_t) ret.data0;
+}
diff --git a/libtock-sync/crypto/syscalls/hmac_syscalls.h b/libtock-sync/crypto/syscalls/hmac_syscalls.h
new file mode 100644
index 000000000..b8a47274d
--- /dev/null
+++ b/libtock-sync/crypto/syscalls/hmac_syscalls.h
@@ -0,0 +1,15 @@
+#pragma once
+
+#include <libtock/crypto/syscalls/hmac_syscalls.h>
+#include <libtock/tock.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Wait for an HMAC operation to finish.
+returncode_t libtocksync_hmac_yield_wait_for(void);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/libtock/crypto/hmac.c b/libtock/crypto/hmac.c
index 0984b1e81..0f4790cf4 100644
--- a/libtock/crypto/hmac.c
+++ b/libtock/crypto/hmac.c
@@ -11,7 +11,7 @@ static void hmac_upcall(int ret,
 returncode_t libtock_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                  uint8_t* key_buffer, uint32_t key_length,
                                  uint8_t* input_buffer, uint32_t input_length,
-                                 uint8_t* hash_buffer, uint32_t hash_length,
+                                 uint8_t* hmac_buffer, uint32_t hmac_length,
                                  libtock_hmac_callback_hmac cb) {
 
   returncode_t ret;
@@ -25,7 +25,7 @@ returncode_t libtock_hmac_simple(libtock_hmac_algorithm_t hmac_type,
   ret = libtock_hmac_set_readonly_allow_data_buffer(input_buffer, input_length);
   if (ret != RETURNCODE_SUCCESS) return ret;
 
-  ret = libtock_hmac_set_readwrite_allow_destination_buffer(hash_buffer, hash_length);
+  ret = libtock_hmac_set_readwrite_allow_destination_buffer(hmac_buffer, hmac_length);
   if (ret != RETURNCODE_SUCCESS) return ret;
 
   ret = libtock_hmac_set_upcall(hmac_upcall, cb);
diff --git a/libtock/crypto/hmac.h b/libtock/crypto/hmac.h
index b9e496b25..4a7c9881f 100644
--- a/libtock/crypto/hmac.h
+++ b/libtock/crypto/hmac.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "../tock.h"
+#include "hmac_types.h"
 #include "syscalls/hmac_syscalls.h"
 
 #ifdef __cplusplus
@@ -12,13 +13,6 @@ extern "C" {
 // - `arg1` (`returncode_t`): Status from computing the HMAC.
 typedef void (*libtock_hmac_callback_hmac)(returncode_t);
 
-typedef enum {
-  LIBTOCK_HMAC_SHA256 = 0,
-  LIBTOCK_HMAC_SHA384 = 1,
-  LIBTOCK_HMAC_SHA512 = 2,
-} libtock_hmac_algorithm_t;
-
-
 
 // Compute an HMAC using `keyb_buffer` over `input_buffer` and store the result
 // in `hash_buffer`.
@@ -27,7 +21,7 @@ typedef enum {
 returncode_t libtock_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                  uint8_t* key_buffer, uint32_t key_length,
                                  uint8_t* input_buffer, uint32_t input_length,
-                                 uint8_t* hash_buffer, uint32_t hash_length,
+                                 uint8_t* hmac_buffer, uint32_t hmac_length,
                                  libtock_hmac_callback_hmac cb);
 
 #ifdef __cplusplus
diff --git a/libtock/crypto/hmac_types.h b/libtock/crypto/hmac_types.h
new file mode 100644
index 000000000..b31a7e765
--- /dev/null
+++ b/libtock/crypto/hmac_types.h
@@ -0,0 +1,17 @@
+#pragma once
+
+#include "../tock.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef enum {
+  LIBTOCK_HMAC_SHA256 = 0,
+  LIBTOCK_HMAC_SHA384 = 1,
+  LIBTOCK_HMAC_SHA512 = 2,
+} libtock_hmac_algorithm_t;
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/libtock/defer.h b/libtock/defer.h
new file mode 100644
index 000000000..49b1c977b
--- /dev/null
+++ b/libtock/defer.h
@@ -0,0 +1,27 @@
+#pragma once
+
+// Implementation of the C `defer {}` feature.
+//
+// As of July 2025, upcoming versions of C will support `defer` as a way to run
+// code when a value goes out of scope. See:
+// https://thephd.dev/_vendor/future_cxx/technical%20specification/C%20-%20defer/C%20-%20defer%20Technical%20Specification.pdf
+//
+// This implements the same feature with a macro. This implementation used from
+// https://gustedt.wordpress.com/2025/01/06/simple-defer-ready-to-use/.
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define __DEFER__(F, V)                \
+        auto void F(int* V##SENTINEL); \
+        [[gnu::cleanup(F)]] int V;     \
+        auto void F(__attribute__ ((unused)) int* V##SENTINEL)
+
+#define defer __DEFER(__COUNTER__)
+#define __DEFER(N) __DEFER_(N)
+#define __DEFER_(N) __DEFER__(__DEFER_FUNCTION_##N, __DEFER_VARIABLE_##N)
+
+#ifdef __cplusplus
+}
+#endif