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# Encrypted images

## [Rationale](#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](#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](#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 or AES-CTR-256, with a counter
that starts from zero (over the payload blocks) and increments by 1 for each
16-byte block. AES-CTR 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 key, new TLVs were defined. The key can be
encrypted using either RSA-OAEP, AES-KW (128 or 256 bits depending on the
AES-CTR key length), ECIES-P256 or ECIES-X25519.

For RSA-OAEP a new TLV with value `0x30` is added to the image, for
AES-KW 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 key.

## [ECIES encryption](#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 is generated (for AES). This is
  the AES key used to encrypt the images.
* The key is encrypted with AES-128-CTR or AES-256-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 or 32 bytes of the encrypted key, resulting in
a TLV of 113 or 129 bytes for ECIES-P256 and 80 or 96 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](#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 using scratch is used for the upgrade process, the decryption happens
when copying the content of the scratch area to the `primary slot`, which means
the scratch area does not contain the image unencrypted. However, unless
`MCUBOOT_SWAP_SAVE_ENCTLV` is enabled, the decryption keys are stored in
plaintext in the scratch area. Therefore, `MCUBOOT_SWAP_SAVE_ENCTLV` must be
enabled if the scratch area does not reside in the internal flash memory of the
MCU, to avoid attacks that could interrupt the upgrade and read the plaintext
decryption keys from external flash memory.

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`.

---
***Note***

*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](#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`
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> -e <encoding>`, where `encoding` can be one of
`lang-c` or `lang-rust` (defaults to `lang-c`). To extract a public key in PEM
format, use `imgtool getpub -k <input.pem> -e pem`.

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

## [Creating your keys with Unix tooling](#creating-your-keys-with-unix-tooling)

* If using RSA-OAEP, generate a keypair following steps similar to those
  described in [signed_images](signed_images.md) to create RSA keys.
* If using ECIES-P256, generate a keypair following steps similar to those
  described in [signed_images](signed_images.md) 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 (`newt` only), the `kek` can be generated with a
  command like (change count to 32 for a 256 bit key)
  `dd if=/dev/urandom bs=1 count=16 | base64 > my_kek.b64`
