imgtool: Add clear image generation with encryption capability

Create an option to generate a clear image with encryption capability
that can be installed on a primary slot. Since image has encryption
capability image can be swapped encrypted in secondary slot

Signed-off-by: Michel Jaouen <michel.jaouen@st.com>

scripts/imgtool/image.py

@@ -305,7 +305,7 @@ class Image():
         return cipherkey, ciphermac, pubk
 
     def create(self, key, public_key_format, enckey, dependencies=None,
-               sw_type=None, custom_tlvs=None, encrypt_keylen=128):
+               sw_type=None, custom_tlvs=None, encrypt_keylen=128, clear=False):
         self.enckey = enckey
 
         # Calculate the hash of the public key
@@ -472,13 +472,14 @@ class Image():
                 else:
                     tlv.add('ENCX25519', enctlv)
 
-            nonce = bytes([0] * 16)
-            cipher = Cipher(algorithms.AES(plainkey), modes.CTR(nonce),
-                            backend=default_backend())
-            encryptor = cipher.encryptor()
-            img = bytes(self.payload[self.header_size:])
-            self.payload[self.header_size:] = \
-                encryptor.update(img) + encryptor.finalize()
+            if not clear:
+                nonce = bytes([0] * 16)
+                cipher = Cipher(algorithms.AES(plainkey), modes.CTR(nonce),
+                                backend=default_backend())
+                encryptor = cipher.encryptor()
+                img = bytes(self.payload[self.header_size:])
+                self.payload[self.header_size:] = \
+                    encryptor.update(img) + encryptor.finalize()
 
         self.payload += prot_tlv.get()
         self.payload += tlv.get()

scripts/imgtool/main.py

@@ -254,6 +254,10 @@ class BasedIntParamType(click.ParamType):
               type=click.Choice(['128','256']),
               help='When encrypting the image using AES, select a 128 bit or '
                    '256 bit key len.')
+@click.option('-c', '--clear', required=False, is_flag=True, default=False,
+              help='Output a non-encrypted image with encryption capabilities,'
+                   'so it can be installed in the primary slot, and encrypted '
+                   'when swapped to the secondary.')
 @click.option('-e', '--endian', type=click.Choice(['little', 'big']),
               default='little', help="Select little or big endian")
 @click.option('--overwrite-only', default=False, is_flag=True,
@@ -303,7 +307,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
          pad_header, slot_size, pad, confirm, max_sectors, overwrite_only,
          endian, encrypt_keylen, encrypt, infile, outfile, dependencies,
          load_addr, hex_addr, erased_val, save_enctlv, security_counter,
-         boot_record, custom_tlv, rom_fixed, max_align):
+         boot_record, custom_tlv, rom_fixed, max_align, clear):
 
     if confirm:
         # Confirmed but non-padded images don't make much sense, because
@@ -350,7 +354,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
             custom_tlvs[tag] = value.encode('utf-8')
 
     img.create(key, public_key_format, enckey, dependencies, boot_record,
-               custom_tlvs, int(encrypt_keylen))
+               custom_tlvs, int(encrypt_keylen), clear)
     img.save(outfile, hex_addr)