boot: Add MCUBOOT_HW_KEY support for image encryption

Currently encryption supports only private key embed in mcuboot itself. To support MCUBOOT_HW_KEY for image encryption boot_retrieve_private_key() hook is added. This hook helps retrieving private key from trusted sources like OTP, TPM. Signed-off-by: Dinesh Kumar K <[email protected]>
mcu-tools · Jun 15, 2023 · b557325 · b557325
1 parent a5db515
commit b557325
Show file tree

Hide file tree

Showing 5 changed files with 48 additions and 4 deletions.
diff --git a/boot/bootutil/include/bootutil/enc_key.h b/boot/bootutil/include/bootutil/enc_key.h
@@ -32,6 +32,7 @@
 #include <flash_map_backend/flash_map_backend.h>
 #include "bootutil/crypto/aes_ctr.h"
 #include "bootutil/image.h"
+#include "bootutil/sign_key.h"
 #include "bootutil/enc_key_public.h"
 
 #ifdef __cplusplus
@@ -45,7 +46,17 @@ struct enc_key_data {
     bootutil_aes_ctr_context aes_ctr;
 };
 
-extern const struct bootutil_key bootutil_enc_key;
+/**
+ * Retrieve the private key for image encryption.
+ *
+ * @param[out]  private_key  structure to store the private key and
+ *                           its length.
+ *
+ * @return                   0 on success; nonzero on failure.
+ *
+ */
+int boot_retrieve_private_key(const struct bootutil_key *private_key);
+
 struct boot_status;
 
 int boot_enc_init(struct enc_key_data *enc_state, uint8_t slot);

diff --git a/boot/bootutil/src/encrypted.c b/boot/bootutil/src/encrypted.c
@@ -67,7 +67,7 @@ static int bootutil_constant_time_compare(const uint8_t *a, const uint8_t *b, si
 
 #if defined(MCUBOOT_ENCRYPT_KW)
 static int
-key_unwrap(const uint8_t *wrapped, uint8_t *enckey)
+key_unwrap(const uint8_t *wrapped, uint8_t *enckey, const struct bootutil_enc_key)
 {
     bootutil_aes_kw_context aes_kw;
     int rc;
@@ -438,8 +438,14 @@ boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey)
     uint8_t counter[BOOTUTIL_CRYPTO_AES_CTR_BLOCK_SIZE];
     uint16_t len;
 #endif
+    const struct bootutil_key bootutil_enc_key = {0};
     int rc = -1;
 
+    rc = boot_retrieve_private_key(&bootutil_enc_key);
+    if (rc) {
+        return rc;
+    }
+
 #if defined(MCUBOOT_ENCRYPT_RSA)
 
     bootutil_rsa_init(&rsa);
@@ -464,7 +470,7 @@ boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey)
 #if defined(MCUBOOT_ENCRYPT_KW)
 
     assert(*bootutil_enc_key.len == BOOT_ENC_KEY_SIZE);
-    rc = key_unwrap(buf, enckey);
+    rc = key_unwrap(buf, enckey, bootutil_enc_key);
 
 #endif /* defined(MCUBOOT_ENCRYPT_KW) */
 

diff --git a/boot/cypress/MCUBootApp/keys.c b/boot/cypress/MCUBootApp/keys.c
@@ -167,3 +167,12 @@ const struct bootutil_key bootutil_enc_key = {
     .key = enc_priv_key,
     .len = &enc_priv_key_len,
 };
+
+#if !defined(MCUBOOT_HW_KEY)
+int boot_retrieve_private_key(const struct bootutil_key *private_key)
+{
+        private_key = &bootutil_enc_key;
+
+        return 0;
+}
+#endif /* !MCUBOOT_HW_KEY */
diff --git a/boot/mbed/app_enc_keys.c b/boot/mbed/app_enc_keys.c
@@ -69,3 +69,12 @@ const struct bootutil_key bootutil_enc_key = {
 #endif
 
 #endif
+
+#if !defined(MCUBOOT_HW_KEY)
+int boot_retrieve_private_key(const struct bootutil_key *private_key)
+{
+        private_key = &bootutil_enc_key;
+
+        return 0;
+}
+#endif /* !MCUBOOT_HW_KEY */
diff --git a/boot/zephyr/keys.c b/boot/zephyr/keys.c
@@ -186,7 +186,7 @@ const struct bootutil_key bootutil_enc_key = {
     .len = &enc_priv_key_len,
 };
 #elif defined(MCUBOOT_ENCRYPT_EC256)
-extern const unsigned char enc_priv_key[];
+extern unsigned char enc_priv_key[];
 extern unsigned int enc_priv_key_len;
 const struct bootutil_key bootutil_enc_key = {
     .key = enc_priv_key,
@@ -207,3 +207,12 @@ const struct bootutil_key bootutil_enc_key = {
 #elif defined(MCUBOOT_ENCRYPT_KW)
 #error "Encrypted images with AES-KW is not implemented yet."
 #endif
+
+#if !defined(MCUBOOT_HW_KEY)
+int boot_retrieve_private_key(const struct bootutil_key *private_key)
+{
+        private_key = &bootutil_enc_key;
+
+        return 0;
+}
+#endif /* !MCUBOOT_HW_KEY */