add(chain): Adds ViewingKey type in zebra-chain

[arya2]

Motivation

This PR adds a ViewingKey type to zebra-chain behind the shielded-scan feature for parsing viewing keys into.

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Complex Code or Requirements

Safely hashing viewing keys may need special attention.

Solution

Adds:

• shielded-scan feature to zebra-chain to depend on zcash_client_backend
• ViewingKey, SaplingViewingKey, and OrchardViewingKey types
• parse() method to ViewingKey for decoding keys

Testing

• Tests that zecpages sapling efvk parses successfully

Review

Anyone can review.

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[upbqdn]

@arya2 could you please describe how we are going to use the key hashes?

[arya2]

please describe how we are going to use the key hashes?

I added to its docs that it's for authorizing access via the gRPC interface, it's so that the client doesn't have to keep re-sending their viewing keys once they're registered in the scanner.

[upbqdn]

I still struggle to see the advantages of hashing the keys instead of using them directly. What would be worse if we didn't hash the keys?

[arya2]

I still struggle to see the advantages of hashing the keys instead of using them directly. What would be worse if we didn't hash the keys?

It could be useful if the private key for the TLS encryption were compromised.

[upbqdn]

Looks great.

We'll need tests that check the "to bytes" and "from bytes" round-trips, but we can do them later.