EVP_KEY_cmp and -_parameters issues

[Thulasi Goriparthi]

RSA keys wouldn't have parameters that are separated from key
components. So, EVP_PKEY_cmp() is applicable, but not
EVP_PKEY_cmp_parameters().

DH keys, which are generally used for key exchange, are short lived,
though the group parameters can be comparatively valid for longer
duration (let's say for a whole session) and can be used to generate
multiple DH keys. So, EVP_PKEY_cmp_parameters() is useful to validate
peer's public key parameters during key exchange to confirm that both
peers are working in the same group.

Though EVP_PKEY_cmp() function can be extended to compare both
parameters and key components for DH keys,  it wouldn't be of much use
as DH keys are ephemeral and we never need to compare two of them for
their key components.

Thanks,
Thulasi.

On Wed, 7 Aug 2019 at 12:27, <Johannes.Heinz at telekom.de> wrote:
>
> I have a question to following situation with RSA and DH structures:
>
> I’m testing these in separated unit tests.
>
> Both test cases (each one for RSA and DH) are doing the same:
>
>
>
> I’m creating a new DH or RSA structure, filling it with my params (pqg …) and convert it to an EVP_PKEY (for example: EVP_PKEY_assign_DH => pkey1)
>
> Next step, I’m writing and reading this structure with these functions:
>
> PEM_write_bio_PrivateKey() (not PEM_write_bio_PrivateKey_traditional() ) (with password)
>
> EVP_PKEY* pkey2 = PEM_read_bio_PrivateKey() (with same password)
>
> (or even without a password)
>
>
>
> Now I want to compare these two EVP_PKEY* variables (pkey1 and pkey2) and for that I can use these two functions:
>
> EVP_PKEY_cmp(pkey1, pkey2) (compares components and params)
>
> EVP_PKEY_cmp_parameters(pkey1, pkey2) (compares params)
>
>
>
> Now the Problem:
>
> Even the tests work the same way, the one with RSA only accept the compare-function “EVP_PKEY_cmp” and not the other one.
>
> The one with DH is only with the “EVP_PKEY_cmp_parameters” successful.
>
>
>
> Question:
>
> Why can the first compare function find the components and params of the RSA structure (and even after the PEM_write_bio…) and not of the DH?
>
> Also, why it’s with the second compare function (only params) the other way around (keys match in DH unit test and not in RSA unit test)?
>
>
>
> Thanks