Support importing RSA parameters manually for encryption with Pkcs1PublicEncryptingKey

[Vadmeme]

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Problem:

At the moment it seems there is no way to get a Pkcs1PublicEncryptingKey with a custom n and e.
There already exists PublicKeyComponents::verify but this only handles signature verification, so having something for encryption does not seem out of scope.

Solution:

I think the simplest and ideal solution is to allow getting a PublicEncryptingKey from a PublicKeyComponents. This would best be done by adding a build_encrypting_key () helper. This should be trivial as all that would be required is to call build_rsa() and pass the resulting EVP_PKEY to PublicEncryptingKey::new.

• Does this change any public APIs? Yes, would ideally allow converting a PublicKeyComponents to a PublicEncryptingKey
• Which algorithm(s) will this impact? RSA

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: N/A
• Related Issues: N/A
• Will the Usage Guide or other documentation need to be updated? Yes
• Testing: I am not sure about the testing requirements, from my eyes it seems trivial to implement.

Out of scope:

Enforcing the secure usage of these apis would likeley be out of scope, if the user wants to manually supply RSA parameters its likely for some more low-level operation.

[Vadmeme]

Did a preliminary implementation in #582
Please let me know if any changes need to be made.

[justsmth]

Thanks for the PR!

Your request and proposed change make sense to me. Before a PR for this can be merged, more testing for it will need to be added. We will also need to consider how the API change might fit in with some longer-term plans that we have.

[Vadmeme]

Thanks for your consideration, I went ahead an implemented a test near the test for RsaPublicKeyComponents (just checks if building the key succeeds + encrypting a test message). Also fixed import order.