Define a Bech32 encoding for Sapling diversifiable full/incoming viewing keys about zips HOT 3 CLOSED

I suggest the following encodings [edit: suggestion withdrawn, see below].

• For a diversifiable FVK: Bech32 of $\mathsf{EncodeExtFVKParts}(\mathsf{ak}, \mathsf{nk}, \mathsf{ovk}, \mathsf{dk})$ with HRP "zdviews" on mainnet or "zdviewtestsapling" on testnet.
• For a diversifiable IVK: Bech32 of $\mathsf{dk}\ ||\ \mathsf{I2LEOSP}_ {256}(\mathsf{ivk})$ with HRP "zdivks" on mainnet or "zdivktestsapling" on testnet.

Rationale:

• The diversifiable FVK encodes $\mathsf{dk}$ last because that matches the already-defined $\mathsf{EncodeExtFVKParts}$ function.
• The diversifiable IVK encodes $\mathsf{dk}$ first because that matches the corresponding encoding in Orchard.
• Bech32 is used instead of Bech32m, for consistency with existing Sapling encodings.
• The HRPs are the same as the corresponding HRPs for non-diversifiable encodings, but starting with "zd" in place of "z".

Please don't put software that uses this "in the wild" yet. We need to discuss it at a ZIP Editors' meeting.

from zips.

Actually there is already a way to express a diversifiable Sapling FVK or IVK, as Items of a Unified Viewing Key encoded as described here.

Sorry, I should have remembered that. We should close this issue, but file another one to document that the Bech32 encodings aren't diversifiable and that you might want to use a UVK instead.

from zips.

Closing in favour of #779.

from zips.