EVP_PKEY_get_int_param is not getting degree from EC key

[Matt Caswell]

On 05/01/2022 10:11, Wolf wrote:
> On 2022-01-05 09:45:14 +0100, Tomas Mraz wrote:
>>> How does one know (without going through
>>> EVP_PKEY_gettable_params(EVP_PKEY *pkey) and
>>> EVP_PKEY_get_params(const EVP_PKEY *pkey, OSSL_PARAM params[])) what
>>> method to use to retrieve what parameter?
>>
>> So you're basically asking to put something like - "The parameter most
>> probably won't fit into unsigned int." - to every such parameter
>> documented for PKEYs?
> 
> I think changing the description of EVP_PKEY_get_int_param to something
> like:
> 
>    EVP_PKEY_get_int_param() retrieves a key pkey integer value *out
>    associated with a name of key_name. This function might fail if the
>    number is too large too fit, so for some parameters you should use
>    (or retry with) EVP_PKEY_get_bn_param.
> 
> Might suffice? To give some additional context (since I'm the original
> poster): I definitely did read the man pages. I've really tried to
> understand. But the fact the `unsigned integer` does not mean `unsigned
> int` was totally lost on me.
> 
> Even now, when I know what to look for, I have hard time finding the
> relevant content in the man page. Sure, I guess this could have been the
> correct hint:
> 
>    The parameter data is an integer (signed or unsigned) of arbitrary
>    length [..]
> 
> But I so did not connect the dots there. I think adding into this
> paragraph mention then it can be access by both _int_param and _bn_param
> (depending on the size) would work as well.
> 
> Also it's bit weird that responder *may* choose to return error if
> data_size is not suitable. What else it might do? Is it valid (from
> responder's point of view) to just truncate the value to that it would
> fit into unsigned int (that would obviously be useless behavior, I'm
> just curious if it would be compliant based on the man page).
> 
> On separate note, my code is now passing the tests when running on
> openssl 3.0.1, so thank you very much for the guidance you've provided.

I've tried to clarify the EVP_PKEY_get_int_param() documentation in this 
PR to make it clearer:

https://github.com/openssl/openssl/pull/17445


On 05/01/2022 13:26, Jakob Bohm via openssl-users wrote:
 > What many API users probably want is a quick way to get the nominal bit
 > length
 > of a public key or group, as a proxy for the cryptographic strength and
 > as a rough
 > guide to allocating data buffers.  This API should not give access to or
 > reveal the
 > exact group parameters or public key, that would be different (but still
 > needed)
 > APIs/parameters.  For example, it would return 4096 for RSA4096, 384 for
 > the
 > NIST P-384 curve etc.

You can use EVP_PKEY_get_size(), EVP_PKEY_get_bits() or 
EVP_PKEY_get_security_bits() functions for this purpose:

https://www.openssl.org/docs/manmaster/man3/EVP_PKEY_size.html

Matt