bootutil: Allow bypassing ASN.1 encoding for ED25519 key import

The commit adds MCUBOOT_KEY_IMPORT_BYPASS_ASN configuration option
that allows bypassing ASN.1 decoding of ED25519 public key, compiled
into MCUboot.
When the option is enabled the key will be accessed directly
and ASN.1 processing is not compiled in, resulting in smaller
footprint of MCUboot, at a cost of reduced detection of invalid
key, i.e. public key designated for different method than
compiled in.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

boot/bootutil/src/image_ed25519.c

@@ -26,16 +26,18 @@
 
 #define EDDSA_SIGNATURE_LENGTH 64
 
-static const uint8_t ed25519_pubkey_oid[] = MBEDTLS_OID_ISO_IDENTIFIED_ORG "\x65\x70";
 #define NUM_ED25519_BYTES 32
 
 extern int ED25519_verify(const uint8_t *message, size_t message_len,
                           const uint8_t signature[EDDSA_SIGNATURE_LENGTH],
                           const uint8_t public_key[NUM_ED25519_BYTES]);
 
+#if !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN)
 /*
  * Parse the public key used for signing.
  */
+static const uint8_t ed25519_pubkey_oid[] = MBEDTLS_OID_ISO_IDENTIFIED_ORG "\x65\x70";
+
 static int
 bootutil_import_key(uint8_t **cp, uint8_t *end)
 {
@@ -71,6 +73,7 @@ bootutil_import_key(uint8_t **cp, uint8_t *end)
 
     return 0;
 }
+#endif /* !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN) */
 
 fih_ret
 bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
@@ -89,11 +92,25 @@ bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
     pubkey = (uint8_t *)bootutil_keys[key_id].key;
     end = pubkey + *bootutil_keys[key_id].len;
 
+#if !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN)
     rc = bootutil_import_key(&pubkey, end);
     if (rc) {
         FIH_SET(fih_rc, FIH_FAILURE);
         goto out;
     }
+#else
+    /* Directly use the key contents from the ASN stream,
+     * these are the last NUM_ED25519_BYTES.
+     * There is no check whether this is the correct key,
+     * here, by the algorithm selected.
+     */
+    if (*bootutil_keys[key_id].len < NUM_ED25519_BYTES) {
+        FIH_SET(fih_rc, FIH_FAILURE);
+        goto out;
+    }
+
+    pubkey = end - NUM_ED25519_BYTES;
+#endif
 
     rc = ED25519_verify(hash, IMAGE_HASH_SIZE, sig, pubkey);
 

boot/zephyr/include/mcuboot_config/mcuboot_config.h

@@ -34,6 +34,10 @@
 #     error "One crypto library implementation allowed at a time."
 #endif
 
+#if defined(CONFIG_BOOT_KEY_IMPORT_BYPASS_ASN)
+#define MCUBOOT_KEY_IMPORT_BYPASS_ASN
+#endif
+
 #ifdef CONFIG_BOOT_USE_MBEDTLS
 #define MCUBOOT_USE_MBED_TLS
 #elif defined(CONFIG_BOOT_USE_TINYCRYPT)