tree c7306086614184bfed07cdd5b5b01a3ee80da92e
parent 9d306d20dc3f5840cc2a8c8cf281c53bbef020cc
author Fabio Utzig <utzig@apache.org> 1537270174 -0300
committer Fabio Utzig <utzig@utzig.org> 1539362173 -0300

Add bootutil support for encrypted images

This allows storing encrypted images in slot1, that are automatically
decrypted when copying to slot0 and re-encrypted when copying from slot0
to slot1.

The encryption works by applying AES-CTR-128 on the image blocks
(excluding the header and TLVs) using a random key. This random key
is itself encrypted using either RSA-OAEP-2048 or AES-KW-128 (AES keywrap
as defined by RFC3394), and appended to the image as newly defined TLVs.

AES-CTR-128 was chosen primarily for having stream cipher proporties,
which basically means that any block being encrypted/decrypted does not
depend on any other previous blocks results.

The TLV adds about 256 bytes to the image in RSA-OAEP-2048 mode, and 24
bytes in AES-KW-128 mode. Resulting sizes for a Mynewt generated mcuboot
(frdm-k64f):

- swap mode and no signing: 12KB
- adding encryption with RSA-OAEP-2048: 28KB
- adding encryption with AES-KW-128: 20KB

Some extra comments:

- AES-KW-128 requires a fairly new mbedtls with nist_kw support.
- An alternative methods which could be added later are ECIES.
- Key-wrapping seems easy enough to implement using just standard
  AES-ECB mode that it should be straight-forward to also add support to
  tinycrypt.

Signed-off-by: Fabio Utzig <utzig@apache.org>