Check jubjub key correctness independent of redjubjub / jubjub

[dconnolly]

Motivation

In the spec there are a bunch of small-order, identity or prime-order checks. Some of those were being validated in redjubjub crate but that was not appropriate since it's a Zcash-specific rule. See #2549

Additionally, some checks were missing.

Specifications

Designs

Solution

• Sapling Spend and Output cv (value commitments): add new type to specify a not-small-order value commitment and use those in the Spend and Output structs
• Sapling rk validating key: create type for it, enforce not small order
• Sapling epk (EphemeralPublicKey): enforce not small order
• Sapling pkd (TransmisisonKey): enforce prime-order
• Sapling ak: enforce prime order (not a consensus rule, used by wallets)
• Orchard Action epk: enforce not identity

Orchard Action cv and rk can be the identity, so no change was needed.

Closes #2549

Review

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

[conradoplg]

I think this is done, so I'm moving out of draft, but I want to review it myself to make sure I didn't forget anything.

[conradoplg]

I fixed the issue which was caused by some ValueCommitments being the identity point in the middle of computations (e.g. binding_verification_key()) which caused a panic. The non-small order rule only applies to the ValueCommitments that are inside Spends and Outputs. So I created a NotSmallOrderValueCommiment for those. (Feel free to suggest a better name 😛 )

@dconnolly could you please take a look on these recent changes? b8760ce and 0531b5f

[dconnolly]

Looks good! I can't approve a PR I opened, so @conradoplg I think can approve this 😁

[codecov]

Codecov Report

Merging #3154 (df59366) into main (3611250) will increase coverage by 0.68%.
The diff coverage is 93.75%.

❗ Current head df59366 differs from pull request most recent head fbad308. Consider uploading reports for the commit fbad308 to get more accurate results

@@            Coverage Diff             @@
##             main    #3154      +/-   ##
==========================================
+ Coverage   77.03%   77.71%   +0.68%     
==========================================
  Files         265      264       -1     
  Lines       31267    31060     -207     
==========================================
+ Hits        24086    24139      +53     
+ Misses       7181     6921     -260     

[conradoplg]

Approving as requested