Re: [PATCH v3 00/10] Add support for NIST P521 to ecdsa

[Stefan Berger]

On 2/29/24 04:34, Lukas Wunner wrote:
> On Fri, Feb 23, 2024 at 03:41:39PM -0500, Stefan Berger wrote:
> > This series adds support for the NIST P521 curve to the ecdsa module.
> >
> > An issue with the current code in ecdsa is that it assumes that input
> > arrays providing key coordinates for example, are arrays of digits
> > (a 'digit' is a 'u64'). This works well for all currently supported
> > curves, such as NIST P192/256/384, but does not work for NIST P521 where
> > coordinates are 8 digits + 2 bytes long. So some of the changes deal with
> > converting byte arrays to digits and adjusting tests on input byte
> > array lengths to tolerate arrays not providing multiples of 8 bytes.
>
> Don't you also need to amend software_key_query()?  In the "issig" case,
> it calculates len = crypto_sig_maxsize(sig), which is 72 bytes for P521,
> then further below calculates "info->max_sig_size = 2 * (len + 3) + 2;"
>
> I believe the ASN.1 encoded integers are just 66 bytes instead of 72,
> so info->max_sig_size is 6 bytes too large.  Am I missing something?

Right! Good catch. While the 'keyctl pkey_verify' interface was already 
working the space was too generous with 72 bytes. So I adjusted 
ecdsa_max_size now to base the size calculations on nbits rather than 
ndigits and we now get 66 bytes.

For so-far supported curves the max_sig_size is:

2 bytes for sequence (0x30) + following length as single byte
Each coordinate may have a 0 prepended to make a possibly negative 
number positive:

 => 2 + 2 * (2 + 1 + len)

In case of NIST P521 the max signature length is calculated as follows:

3 bytes for sequence (0x30) + following length as 2 bytes
The coordinates won't have a preprended 0 byte since only 1 bit is used 
in the highest bit, so only 2 bytes for

 => 3 + 2 * (2 + len)

We would have to adjust the math there as well. The max. signature size 
for NIST P521 is 139 rather than 140 with the first formula.

   Stefan



> Thanks,
>
> Lukas