Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 29 Feb 2024 13:45:35 -0500 | Subject | Re: [PATCH v3 00/10] Add support for NIST P521 to ecdsa | From | Stefan Berger <> |
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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
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