elements: derive confidential addresses using SLIP-0077

[romanz]

Following #66 and #398, it would allow receiving confidential
transactions to addresses generated on the devices.

Blinding, unblinding and signing will be added in separate PRs.

[romanz]

Please let me know if I should "wrap" the Elements-related parts with if not utils.BITCOIN_ONLY.

[prusnak]

Yes, please

[romanz]

Tested on bitcoin-only firmware with:

$ BITCOIN_ONLY=1 make -C core clean build_unix
$ py.test tests/device_tests/ -k 'getaddress and not altcoin'

and on altcoin-enabled firmware:

$ BITCOIN_ONLY=0 make -C core clean build_unix
$ py.test tests/device_tests/ -k 'getaddress'

[romanz]

Rebased (to resolve a conflict) and force-pushed 1e9563b.
Please let me know if there is anything else that I can help with :)

[matejcik]

perhaps this should be deferred until #617 ? OTOH this seems to only affect GetAddress, in which case maybe that's not necessary?

[romanz]

OTOH this seems to only affect GetAddress, in which case maybe that's not necessary?

Indeed, currently it affects only GetAddress (for deriving the blinding public key).

In future PR (after #617), we will add support for unblinding confidential UTXOs (during transaction signing) and it will use SLIP-0077 to derive the corresponding blinding private key.

[romanz]

Rebased over the latest master.

[tsusanka]

Hi there, thank you for your PR. Same as with #473 we are interested in adding this to our firmware but we just don't have enough capacity to review it right now. We would be more than happy to take a closer look at it later, when our capacity frees up a little, likely during the course of 2021. Thank you for your understanding.