ECDSA signature format

When 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 are fairly strict about the formatting of the ECDSA signature (specifically, mbed TLS). This currently means that the ECDSA SECP224R1 (EC) signature checking code will fail to boot about 1 out of every 256 images, because the signature itself will end in a 0x00 byte, and the code will remove too much data, invalidating the signature.

There are a couple of ways to fix this:

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

  2. Remove the padding entirely. Depending on which tool, this will require 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 all written in higher-level languages and this change should not be difficult.

    However, this will also break compatibility with older versions, specifically in that 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.

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

This will require a change to any scripts that are relying on a default, but not specifying 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 sim 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 being encoded as:

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

where 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 of both of these numbers has the high bit set.

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

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