bootutil: Refactor boot_read_enc_key

Move code around to reduce ifdes and make it more clear,
and allow to reuse TLV read check loop for key read verification.

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

Author: de-nordic

boot/bootutil/src/bootutil_misc.c

@@ -244,28 +244,47 @@ int
 boot_read_enc_key(const struct flash_area *fap, uint8_t slot, struct boot_status *bs)
 {
     uint32_t off;
-#if MCUBOOT_SWAP_SAVE_ENCTLV
     uint32_t i;
-#endif
     int rc;
+    uint8_t *read_dst;
+    uint32_t read_size;
 
-    off = boot_enc_key_off(fap, slot);
 #if MCUBOOT_SWAP_SAVE_ENCTLV
-    rc = flash_area_read(fap, off, bs->enctlv[slot], BOOT_ENC_TLV_ALIGN_SIZE);
+    /* In this case we have stored entire encryted TLV in swap-state and bs->enckey
+     * will be decrypted from the TLV.
+     */
+    BOOT_LOG_DBG("boot_read_enc_key: TLV");
+    read_dst = bs->enctlv[slot];
+    read_size = BOOT_ENC_TLV_ALIGN_SIZE;
+#else
+    BOOT_LOG_DBG("boot_read_enc_key: RAW key");
+    read_dst = bs->enckey[slot];
+    read_size = BOOT_ENC_KEY_ALIGN_SIZE;
+#endif
+
+    off = boot_enc_key_off(fap, slot);
+
+    rc = flash_area_read(fap, off, read_dst, read_size);
     if (rc == 0) {
-        for (i = 0; i < BOOT_ENC_TLV_ALIGN_SIZE; i++) {
-            if (bs->enctlv[slot][i] != 0xff) {
+        for (i = 0; i < read_size; i++) {
+            if (read_dst[i] != 0xff) {
                 break;
             }
         }
-        /* Only try to decrypt non-erased TLV metadata */
-        if (i != BOOT_ENC_TLV_ALIGN_SIZE) {
+
+        if (i == read_size) {
+            BOOT_LOG_ERR("boot_read_enc_key: No key, read all 0xFF");
+            rc = 1;
+        }
+#if MCUBOOT_SWAP_SAVE_ENCTLV
+        else {
+            /* read_dst is the same as bs->enctlv[slot], and serves as a source
+             * of the encrypted key.
+             */
             rc = boot_decrypt_key(bs->enctlv[slot], bs->enckey[slot]);
         }
-    }
-#else
-    rc = flash_area_read(fap, off, bs->enckey[slot], BOOT_ENC_KEY_ALIGN_SIZE);
 #endif
+    }
 
     return rc;
 }