[rpc]blindrawtransaction accepts more commitments

[AkioNak]

In #510 / #550 a requirement that the blinding commitments were equal in amount to the inputs was introduced. This requirement is correct for the final case, but not for partial transactions (e.g. where party 1 blinds their inputs before even handing the transaction over to party 2 to add theirs). In order to hide party 1's input amounts from party 2, blindrawtransaction must allow >= the inputs, since the commitments are shared beforehand (if not, the blinding fails; this may need further investigating).

This PR restores the functionality to allow blinding commitments more than or equal to the amount of inputs, which makes the confidential assets demo https://github.com/ElementsProject/confidential-assets-demo functional again.

[instagibbs]

If we re-give this ability it should be documented in the RPCs that use it. It was hidden functionality IIRC.

[kallewoof]

@instagibbs Without this, people have to provide placeholder inputs before they can start blinding stuff, and people have to know which entries to replace. How is that better?

[instagibbs]

Sorry I must have been unclear: please change the help to make it clear this functionality exists.

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On Tue, Jul 9, 2019, 2:19 AM kallewoof ***@***.***> wrote: @instagibbs <https://github.com/instagibbs> Without this, people have to provide placeholder inputs before they can start blinding stuff, and people have to know which entries to replace. How is that better? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#674?email_source=notifications&email_token=ABMAFU4VJWAHBXTC43SXQ5TP6QUXDA5CNFSM4H6XCFG2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODZPG6LY#issuecomment-509505327>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABMAFU4PQFGSMDNO352OL7DP6QUXDANCNFSM4H6XCFGQ> .

[instagibbs]

e.g., rawblindrawtransaction

[kallewoof]

Oh, I misunderstood. Sorry about that. You are saying the rawblindtransaction RPC docs need to be updated?

[instagibbs]

Actually this is used in blindrawtransaction, currently text says

"An array of input asset generators. If provided, this list must be empty, or match the final input commitment list, including ordering, to make a valid surjection proof. This list does not include generators for issuances, as these assets are inherently unblinded."

I suppose this is technically correct but might want to make it even more explicit that this "final list" can include inputs not given in hexstring.

[instagibbs]

I'll touch up the help myself, ACK d584b65

[AkioNak]

@instagibbs Oh, I'm sorry to bother you.

[instagibbs]

No worries I just don't want to hold up this improvement as is.

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On Tue, Jul 23, 2019, 8:58 PM Akio Nakamura ***@***.***> wrote: @instagibbs <https://github.com/instagibbs> Oh, I'm sorry to bother you. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#674?email_source=notifications&email_token=ABMAFU2HCXNICUGCYDNE3N3QA6SMVA5CNFSM4H6XCFG2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD2U275Q#issuecomment-514437110>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABMAFUZOR5EJYZLJSHGYI4TQA6SMVANCNFSM4H6XCFGQ> .