param_bld: add a padded BN call.

[paulidale]

To aviod leaking size information when passing private value using the
OSSL_PARAM builder, a padded BN call is required.

• documentation is added or updated
• tests are added or updated

[levitte]

The manual might need a teeny bit more

[slontis]

make update required..

[paulidale]

Feedback addressed.

[paulidale]

Ping reviewer.

[richsalz]

I wish there were a resolution to #10840 (comment) :(

[paulidale]

I think we disagree.
We'll see what any reviewers say -- it's an = sign change to the code.

[levitte]

I wish there were a resolution to #10840 (comment) :(

There's nothing to resolve. That usage example is just that, but does not affect this PR per se.
(and I agree that it wasn't an example of passing a BIGNUM with padding, it's an example of passing a BIGNUM without)

[levitte]

I expected to see this rewrite of ossl_param_bld_push_BN(OSSL_PARAM_BLD:

int ossl_param_bld_push_BN(OSSL_PARAM_BLD *bld, const char *key,
                           const BIGNUM *bn)
{
    return ossl_param_bld_push_BN_pad(bld, key, bn, bn == NULL ? 0 : BN_num_bytes(bn));
}

UPDATE to handle NULL bn

[paulidale]

That rewrite isn't quite right. A NULL bn argument is permitted by that function.

[levitte]

Er, what? Looking at the code, they seem to treat a NULL bn exactly the same way...

[levitte]

Sorry, now I see. I've update my proposed rewrite

[paulidale]

I've made the change.

[levitte]

Please check the CIs before merging

[paulidale]

Merged to master. Thanks for the input.