diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index 257c892..684c6b3 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -1,6 +1,6 @@
 # Copyright 2018 Nordic Semiconductor ASA
 # Copyright 2017-2020 Linaro Limited
-# Copyright 2019-2020 Arm Limited
+# Copyright 2019-2021 Arm Limited
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -51,7 +51,8 @@
 # Image header flags.
 IMAGE_F = {
         'PIC':                   0x0000001,
-        'ENCRYPTED':             0x0000004,
+        'ENCRYPTED_AES128':      0x0000004,
+        'ENCRYPTED_AES256':      0x0000008,
         'NON_BOOTABLE':          0x0000010,
         'RAM_LOAD':              0x0000020,
         'ROM_FIXED':             0x0000100,
@@ -67,7 +68,7 @@
         'RSA3072': 0x23,
         'ED25519': 0x24,
         'ENCRSA2048': 0x30,
-        'ENCKW128': 0x31,
+        'ENCKW': 0x31,
         'ENCEC256': 0x32,
         'ENCX25519': 0x33,
         'DEPENDENCY': 0x40,
@@ -288,7 +289,7 @@
         return cipherkey, ciphermac, pubk
 
     def create(self, key, public_key_format, enckey, dependencies=None,
-               sw_type=None, custom_tlvs=None):
+               sw_type=None, custom_tlvs=None, encrypt_keylen=128):
         self.enckey = enckey
 
         # Calculate the hash of the public key
@@ -361,7 +362,10 @@
                     self.payload.extend(pad)
 
         # This adds the header to the payload as well
-        self.add_header(enckey, protected_tlv_size)
+        if encrypt_keylen == 256:
+            self.add_header(enckey, protected_tlv_size, 256)
+        else:
+            self.add_header(enckey, protected_tlv_size)
 
         prot_tlv = TLV(self.endian, TLV_PROT_INFO_MAGIC)
 
@@ -429,7 +433,10 @@
             self.payload = self.payload[:protected_tlv_off]
 
         if enckey is not None:
-            plainkey = os.urandom(16)
+            if encrypt_keylen == 256:
+                plainkey = os.urandom(32)
+            else:
+                plainkey = os.urandom(16)
 
             if isinstance(enckey, rsa.RSAPublic):
                 cipherkey = enckey._get_public().encrypt(
@@ -462,12 +469,15 @@
 
         self.check_trailer()
 
-    def add_header(self, enckey, protected_tlv_size):
+    def add_header(self, enckey, protected_tlv_size, aes_length=128):
         """Install the image header."""
 
         flags = 0
         if enckey is not None:
-            flags |= IMAGE_F['ENCRYPTED']
+            if aes_length == 128:
+                flags |= IMAGE_F['ENCRYPTED_AES128']
+            else:
+                flags |= IMAGE_F['ENCRYPTED_AES256']
         if self.load_addr != 0:
             # Indicates that this image should be loaded into RAM
             # instead of run directly from flash.
diff --git a/scripts/imgtool/main.py b/scripts/imgtool/main.py
index 6188e78..dd6c044 100755
--- a/scripts/imgtool/main.py
+++ b/scripts/imgtool/main.py
@@ -1,7 +1,7 @@
 #! /usr/bin/env python3
 #
 # Copyright 2017-2020 Linaro Limited
-# Copyright 2019-2020 Arm Limited
+# Copyright 2019-2021 Arm Limited
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -250,6 +250,10 @@
 @click.option('-E', '--encrypt', metavar='filename',
               help='Encrypt image using the provided public key. '
                    '(Not supported in direct-xip or ram-load mode.)')
+@click.option('--encrypt-keylen', default='128',
+              type=click.Choice(['128','256']),
+              help='When encrypting the image using AES, select a 128 bit or '
+                   '256 bit key len.')
 @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,
@@ -295,9 +299,9 @@
                .hex extension, otherwise binary format is used''')
 def sign(key, public_key_format, align, version, pad_sig, header_size,
          pad_header, slot_size, pad, confirm, max_sectors, overwrite_only,
-         endian, encrypt, infile, outfile, dependencies, load_addr, hex_addr,
-         erased_val, save_enctlv, security_counter, boot_record, custom_tlv,
-         rom_fixed):
+         endian, encrypt_keylen, encrypt, infile, outfile, dependencies,
+         load_addr, hex_addr, erased_val, save_enctlv, security_counter,
+         boot_record, custom_tlv, rom_fixed):
 
     if confirm:
         # Confirmed but non-padded images don't make much sense, because
@@ -344,7 +348,7 @@
             custom_tlvs[tag] = value.encode('utf-8')
 
     img.create(key, public_key_format, enckey, dependencies, boot_record,
-               custom_tlvs)
+               custom_tlvs, int(encrypt_keylen))
     img.save(outfile, hex_addr)