Download JoinSplit Groth16 verification parameters and load them from disk

[dconnolly]

Motivation

In order to verify Sprout JoinSplit-on-Groth16 proofs, we need to load the public verification parameters, which are several hundred MB in size.

Specifications

Designs

• If not cached, download parameter files in parallel with sync
  • https://github.com/zcash/zcash/blob/49ffee3f20b972dc3aa75e422c67523251cf088b/zcutil/fetch-params.sh
• Read from disk location
• Check the hash to match
• Turn into JoinSplit on Groth16 VerificationKey type
• Instantiate static instance of groth16::Verifier with these parameters

Design Questions

Where do we store the parameters?

• Zebra already has a cache directory for its database

How do we avoid loading them in tests that don't need them?

How do we test this code without downloading 500 MB?

Related Work

#3038

[dconnolly]

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[str4d]

Where do we store the parameters?

If you look for / use the same folder as zcashd then you may be able to avoid downloading anything at all: ~/.zcash-params. Likewise, if you download to there, then zcashd will also Just Work without any downloading.

I had grand plans to also enable zcashd to find the parameters at other predictable locations (so e.g. providing the parameters as a separate Debian package for Debian installs, that could be shared between projects), but never got around to it; we could pick this back up.

[str4d]

The issue I was thinking of is zcash/zcash#3147

[teor2345]

Where do we store the parameters?

If you look for / use the same folder as zcashd then you may be able to avoid downloading anything at all: ~/.zcash-params. Likewise, if you download to there, then zcashd will also Just Work without any downloading.

This is the solution I settled on in PR #3057.

I had grand plans to also enable zcashd to find the parameters at other predictable locations (so e.g. providing the parameters as a separate Debian package for Debian installs, that could be shared between projects), but never got around to it; we could pick this back up.

This is out of scope for our current sprints - we're focused on validating Sprout before NU5 mainnet activation.