Implement Necessary LibSodium Primitives in Golang

[Moopli]

For interoperability with related projects (eg, ACA-Py), we need to use or reimplement certain libsodium primitives (for example, crypto_box_seal), some of which are not available in golang.org/x/crypto (for example, golang.org/x/crypto/nacl/box.Seal is equivalent to NaCl crypto_box_easy, despite the name)

[Moopli]

The current primitives we know we need are equivalent to NaCl crypto_box_seal and crypto_box_seal_open. I've implemented and tested them in my fork of the golang libsodium wrapper: https://github.com/Moopli/libsodium-go/blob/master/cryptobox/crypto_box_seal_test.go

How we should regression-test these is an open question - do we introduce a testing-only dependency to GoKillers/libsodium-go? Do we accept that these functions are tested on a fork of another repo, and that they won't be broken someday once we begin using them?

[Moopli]

@troyronda assign me

[Moopli]

Looks like we'll need an equivalent to:

• crypto_sign_ed25519_pk_to_curve25519
• crypto_sign_ed25519_sk_to_curve25519

For python interop.

The secret key conversion is done, but the public key conversion requires elliptic curve arithmetic, which is implemented in /x/crypto but not exposed.

[Moopli]

Found a way to avoid reimplementing ed25519 -> curve25519 key conversion - testing against ACA-Py is the only reason we're working with key conversions, and testcases and keys can be generated in Python, so no key conversion needs to be done in Go.

[Moopli]

Update: key conversion mentioned above was indeed necessary for ACA-Py interop - keys need to be transmitted as Ed255519 keys, and encryption (eg, of CEK) is done using their Curve25519 counterparts.

[Moopli]

Key conversion was implemented in #292

Closing for now, can reopen if we find more primitives are needed.