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Encrypted images

Rationale

To provide confidentiality of image data while in transport to the device or while residing on an external flash, MCUBoot has support for encrypting/decrypting images on-the-fly while upgrading.

The image header needs to flag this image as ENCRYPTED (0x04) and a TLV with the key must be present in the image. When upgrading the image from the secondary slot to the primary slot it is automatically decrypted (after validation). If swap upgrades are enabled, the image located in the primary slot, also having the ENCRYPTED flag set and the TLV present, is re-encrypted while swapping to the secondary slot.

Threat model

The encrypted image support is supposed to allow for confidentiality if the image is not residing on the device or is written to external storage, eg a SPI flash being used for the secondary slot.

It does not protect against the possibility of attaching a JTAG and reading the internal flash memory, or using some attack vector that enables dumping the internal flash in any way.

Since decrypting requires a private key (or secret if using symmetric crypto) to reside inside the device, it is the responsibility of the device manufacturer to guarantee that this key is already in the device and not possible to extract.

Design

When encrypting an image, only the payload (FW) is encrypted. The header, TLVs are still sent as plain data.

Hashing and signing also remain functionally the same way as before, applied over the un-encrypted data. Validation on encrypted images, checks that the encrypted flag is set and TLV data is OK, then it decrypts each image block before sending the data to the hash routines.

The image is encrypted using AES-CTR-128, with a counter that starts from zero (over the payload blocks) and increments by 1 for each 16-byte block. AES-CTR-128 was chosen for speed/simplicity and allowing for any block to be encrypted/decrypted without requiring knowledge of any other block (allowing for simple resume operations on swap interruptions).

The key used is a randomized when creating a new image, by imgtool or newt. This key should never be reused and no checks are done for this, but randomizing a 16-byte block with a TRNG should make it highly improbable that duplicates ever happen.

To distribute this AES-CTR-128 key, new TLVs were defined. The key can be encrypted using either RSA-OAEP, AES-KW-128, ECIES-P256 or ECIES-X25519.

For RSA-OAEP a new TLV with value 0x30 is added to the image, for AES-KW-128 a new TLV with value 0x31 is added to the image, for ECIES-P256 a new TLV with value 0x32 is added, and for ECIES-X25519 a newt TLV with value 0x33 is added. The contents of those TLVs are the results of applying the given operations over the AES-CTR-128 key.

ECIES encryption

ECIES follows a well defined protocol to generate an encryption key. There are multiple standards which differ only on which building blocks are used; for MCUBoot we settled on some primitives that are easily found on our crypto libraries. The whole key encryption can be summarized as:

  • Generate a new private key and derive the public key; when using ECIES-P256 this is a secp256r1 keypair, when using ECIES-X25519 this will be a x25519 keypair. Those keys will be our ephemeral keys.
  • Generate a new secret (DH) using the ephemeral private key and the public key that corresponds to the private key embedded in the HW.
  • Derive the new keys from the secret using HKDF (built on HMAC-SHA256). We are not using a salt and using an info of MCUBoot_ECIES_v1, generating 48 bytes of key material.
  • A new random encryption key of 16 bytes is generated (for AES-128). This is the AES key used to encrypt the images.
  • The key is encrypted with AES-128-CTR and a nonce of 0 using the first 16 bytes of key material generated previously by the HKDF.
  • The encrypted key now goes through a HMAC-SHA256 using the remaining 32 bytes of key material from the HKDF.

The final TLV is built from the 65 bytes for ECIES-P256 or 32 bytes for ECIES-X25519, which correspond to the ephemeral public key, followed by the 32 bytes of MAC tag and the 16 bytes of the encrypted key, resulting in a TLV of 113 bytes for ECIES-P256 or 80 bytes for ECIES-X25519.

The implemenation of ECIES-P256 is named ENC_EC256 in the source code and artifacts while ECIES-X25519 is named ENC_X25519.

Upgrade process

When starting a new upgrade process, MCUBoot checks that the image in the secondary slot has the ENCRYPTED flag set and has the required TLV with the encrypted key. It then uses its internal private/secret key to decrypt the TLV containing the key. Given that no errors are found, it will then start the validation process, decrypting the blocks before check. A good image being determined, the upgrade consists in reading the blocks from the secondary slot, decrypting and writing to the primary slot.

If swap is used for the upgrade process, the encryption happens when copying the sectors of the secondary slot to the scratch area.

The scratch area is not encrypted, so it must reside in the internal flash of the MCU to avoid attacks that could interrupt the upgrade and dump the data.

Also when swap is used, the image in the primary slot is checked for presence of the ENCRYPTED flag and the key TLV. If those are present the sectors are re-encrypted when copying from the primary slot to the secondary slot.

PS: Each encrypted image must have its own key TLV that should be unique and used only for this particular image.

Also when swap method is employed, the sizes of both images are saved to the status area just before starting the upgrade process, because it would be very hard to determine this information when an interruption occurs and the information is spread across multiple areas.

Creating your keys with imgtool

imgtool can generate keys by using imgtool keygen -k <output.pem> -t <type>, where type can be one of rsa-2048, rsa-3072, ecdsa-p256, ecdsa-p224 or ed25519. This will generate a keypair or private key.

To extract the public key in source file form, use imgtool getpub -k <input.pem> -l <lang>, where lang can be one of c or rust (defaults to c).

If using AES-KW-128, follow the steps in the next section to generate the required keys.

Creating your keys with Unix tooling

  • If using RSA-OAEP, generate a keypair following steps similar to those described in signed_images to create RSA keys.
  • If using ECIES-P256, generate a keypair following steps similar to those described in signed_images to create ECDSA256 keys.
  • If using ECIES-X25519, generate a private key passing the option -t x25519 to imgtool keygen command. To generate public key PEM file the following command can be used: openssl pkey -in <generated-private-key.pem> -pubout
  • If using AES-KW-128 (newt only), the kek can be generated with a command like dd if=/dev/urandom bs=1 count=16 | base64 > my_kek.b64