Evaluate cryptography in use and decide what we will support

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Currently we make wide use of ed25519 keys (via ed25519-zebra), except on the web where we use RSASSA-PKCS1-v1_5 (via the Web Crypto API).

ed25519 has various virtues (small public key size, speedy verification time among them), but also comes with some baggage:

• It's not in broad use in Web Crypto API, so we have to use Web Crypto's RSA there (implementation status)
• There are issues when verifying signatures across implementations

Both ed25519 keys and Web Crypto RSA keys are supported "out of the box" in the rs-ucan project, which is the basis for using these as the defaults in Noosphere.

For CID hashing, we currently use blake2b-256 everywhere. Blake2b's main virtue is its speed compared to most plausible alternatives.

Recently, there was some thought provoking discussion of the default schemes used in Bluesky: https://staging.bsky.app/profile/gtank.bsky.social/post/3jvsidg6mgh2f
And less recently, some discussion on Bluesky about usage of the BLAKE3 hash by n0 computer: https://staging.bsky.app/profile/b5.bsky.social/post/3jv2zaqze5s2x (see also this screencast).

This issue is intended to kick off a discussion weighing the pros and cons of our default cryptography in use before we get too comfy with our priors. Some provocations to kick off the discussion:

• There is compelling evidence that we should switch to BLAKE3 for hashing; what is the counter-arguement?
• Compatibility across ed25519 implementations seems like it could be a real problem; is it worth blessing a (possibly less popular / less available) alternative as our favored default?

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Another thread on the ed25519 validation topic:

• https://staging.bsky.app/profile/hdevalence.bsky.social/post/3jx2gp723kl2x
• https://docs.rs/ed25519-consensus/latest/ed25519_consensus/