doc: fixed ecdsa.md
Rephrased a few sentences.
Fixed a few typos.
Fixed lists.
Added additional small fixes.
Signed-off-by: Francesco Domenico Servidio <francesco.servidio@nordicsemi.no>
diff --git a/docs/ecdsa.md b/docs/ecdsa.md
index bb2e376..50ca961 100644
--- a/docs/ecdsa.md
+++ b/docs/ecdsa.md
@@ -1,90 +1,94 @@
# ECDSA signature format
-When ECDSA SECP256R1 (EC256) signature support was added to MCUboot, a
+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 are fairly
-strict about the formatting of the ECDSA signature (specifically, Mbed
-TLS). This currently means that the ECDSA SECP224R1 (EC) signature
+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.
+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:
+There are two 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).
+ - 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).
- 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.
+ - 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,
- specifically in that images generated with newer tools will not
- work with older versions of MCUboot.
+ 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.
+This document proposes a multi-stage approach to give a transition
+period:
- - First, add a `--no-pad-sig` argument to the sign command in
- `imgtool.py`. Without this, the images will be padded with the
- existing scheme, and with the argument, the ecdsa will be encoded
- without any padding. The `--pad-sig` argument will also be
- accepted, but this will initially be the default.
+ 1. Add a `--no-pad-sig` argument to the sign command in
+ `imgtool.py`.
- - 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 begin
- accepting padded and unpadded signatures.
+ 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.
- - An Mbed TLS implementation of EC256 can be added, but will require
- the `--no-pad-sig` signature to be able to boot all generated
- images (without the argument 3 of out 4 images generated will have
- padding, and be considered invalid).
+ 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.
-After one or more MCUboot release cycles, and announcements over
+ 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 have no effect.
+ - `--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 a change to any scripts that are relying on a
-default, but not specifying a specific version of imgtool.
+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 sim is
+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 being encoded as:
+itself encoded as follows:
+```
ECDSA-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
+```
-where both `r` and `s` are 256-bit numbers. Because these are
+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.
+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 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.
+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
-variable length, and make sure that we can handle them as such.
+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.