ECDSA signature format

When the ECDSA SECP256R1 (EC256) signature support was added to MCUboot, a shortcut was taken, and these signatures were padded to make them always a fixed length. Unfortunately, this padding was done in a way that is not easily reversible. Some crypto libraries (specifically, Mbed TLS) are fairly strict about the formatting of the ECDSA signature.

There are two ways to fix this:

• Use a reversible padding scheme. This solution requires at least one pad byte to always be added (to set the length). This padding would be somewhat incompatible across versions (older EC256 would work, while newer MCUboot code would reject old signatures. The EC code would work reliably only in the new combination).

• Remove the padding entirely. Depending on the tool used, this solution requires some rethinking of how TLV generation is implemented so that the length does not need to be known until the signature is generated. These tools are usually written in higher-level languages, so this change should not be difficult.

  However, this will also break compatibility with older versions, because images generated with newer tools will not work with older versions of MCUboot.

This document proposes a multi-stage approach to give a transition period:

1. Add a --no-pad-sig argument to the sign command in imgtool.py.

   Without this argument, the images are padded with the existing scheme. With this argument, the ECDSA is encoded without any padding. The --pad-sig argument is also accepted, but it is already the default.

2. MCUboot will be modified to allow unpadded signatures right away. The existing EC256 implementations will still work (with or without padding), and the existing EC implementation will be able to accept padded and unpadded signatures.

3. An Mbed TLS implementation of EC256 can be added, but it will require the --no-pad-sig signature to be able to boot all generated images. Without the argument, 3 out of 4 images generated will have padding and will be considered invalid.

After one or more MCUboot release cycles and announcements in the relevant channels, the arguments to imgtool.py will change:

• --no-pad-sig will still be accepted but will have no effect.

• --pad-sig will now bring back the old padding behavior.

This will require an update to any scripts that will rely on the default behavior, but will not specify a specific version of imgtool.

The signature generation in the simulator can be changed at the same time the boot code begins to accept unpadded signatures. The simulator is always run out of the same tree as the MCUboot code, so there should not be any compatibility issues.

Background

ECDSA signatures are encoded as ASN.1, notably with the signature itself encoded as follows:

    ECDSA-Sig-Value ::= SEQUENCE {
      r  INTEGER,
      s  INTEGER
    }

Both r and s are 256-bit numbers. Because these are unsigned numbers that are being encoded in ASN.1 as signed values, if the high bit of the number is set, the DER-encoded representation will require 33 bytes instead of 32. This means that the length of the signature will vary by a couple of bytes, depending on whether one or both of these numbers have the high bit set.

Originally, MCUboot added padding to the entire signature and just removed any trailing 0 bytes from the data block. This turned out to be fine 255 out of 256 times, each time the last byte of the signature was non-zero, but if the signature ended in a zero, MCUboot would remove too many bytes and render the signature invalid.

The correct approach here is to accept that ECDSA signatures are of variable length, and to make sure that we can handle them as such.