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
