Ensure we have correct padding everywhere

[adam-fowler]

No description provided.

[sebsto]

I just tested xcodeinstall against this branch, end-to-end, and it works for me.
Both the padding and the calculateSharedSecret with binary password allows to authenticate on Apple's SRP.

Thank you Adam

[adam-fowler]

@sebsto There are some more changes in this PR. Can you verify again.
You might need to change a couple of parameter names

[sebsto]

@adam-fowler thank you for the heads up I just test this branch with latest commit (9468111) and it still works for me.

[abiligiri]

@adam-fowler thanks for these changes. Can you keep the public constructor for keypair? It will enable unit tests like these https://github.com/XcodesOrg/XcodesApp/blob/main/XcodesTests/AppStateUpdateTests.swift#L274 (specifically https://github.com/XcodesOrg/XcodesApp/blob/main/XcodesTests/AppStateUpdateTests.swift#L296)

[adam-fowler]

@adam-fowler thanks for these changes. Can you keep the public constructor for keypair? It will enable unit tests like these https://github.com/XcodesOrg/XcodesApp/blob/main/XcodesTests/AppStateUpdateTests.swift#L274 (specifically https://github.com/XcodesOrg/XcodesApp/blob/main/XcodesTests/AppStateUpdateTests.swift#L296)

The API has been re-designed to ensure we don't get bad values. If you still feel the need to re-test the library I guess I could use an @_spi(Testing) to make the symbol available for testing.