NUT-06: Deprecate amount in POST /split

[callebtc]

The amount field in POST /split was an unnecessary contraint on the split operation. Now, wallets can choose the amount distribitions of the inputs (proofs) and outputs (and thus returned promises) as they wish, as long as they sum up to the same value.

Closes #32

Changes:

Wallets:

Wallets simply don't send the amount field with the POST /split request anymore. They receive a new object with the field promises from which they then need to separate the promises to keep (first elements) and send (last elements).

Previously, they took these two sets of elements from the fields fst and snd from the response. This is no longer the case! Note that wallets already should know how many elements of the promises response they need to slice to get the same as the fst and snd responses because they have previously generated an array of outputs that has the same distribution.

Mints:

Mints now do no receive an amount field anymore. They can simply proceed as usual but remove any security checks that might have previously asserted that the distribution of amounts of outputs must adhere to a certain pattern (for example perfectly decompose in powers of two). This is not necessary anymore. They MUST still check whether the sum of proofs is equal to the sum of outputs and they SHOULD also check whether it equals the sum of generated promises (as a sanity check).

PostSplitRequest

Before:

{
  "proofs": Proofs,
  "outputs": BlindedMessages,
  "amount": int //deprecate
}

Now:

{
  "proofs": Proofs,
  "outputs": BlindedMessages,
}

PostSplitResponse

Before:

{
  "fst": BlindSignatures,
  "snd": BlindSignatures
}

Now:

{
  "promises": BlindSignatures
}

Testing:

For wallet development, use this Nutshell testnut mint that runs on the branch nut06/no_amount_in_split

https://nut06.testnut.cashu.space

Tracking progress of NUT-06 update (please report):

Mints

• Nutshell (Status: ✅ PR Backwards compatible!)
• LNbits (Status: ✅ PR Backwards compatible!)
• Feni (Status: 🛠️ PR)
• ngutech21/moksha (Status: ✅ PR Backwards compatible!)
• cashu-crab (Status: ✅ commit Backwards compatible!)
• clarkmoody/cashu-rs

Wallets

• Cashu.me (Status: 🛠️ WIP)
• Cashu-TS (Status: 🛠️ WIP)
• Nutstash
• eNuts
• cashu-ts (Status: 🛠️ WIP)
• cashu-crab (Status: ✅ commit)
• cashu-kotlin(Status: ✅)
• Minibits

Ping: @BilligsterUser @gandlafbtc @clarkmoody @ngutech21 @gohumble @thesimplekid @KKA11010 @thunderbiscuit

[thesimplekid]

Cashu-crab updated in cashubtc/cdk@e8303fc, mint handles both with and without amount and responds accordingly, wallet chooses which to follow. Will deprecate amount fully after transition.

[gandlafbtc]

If the amount is not sent anymore, mints that haven't upgraded yet will error, right? Shouldn't we keep the amount in the request, at least till most mints have upgraded?

[callebtc]

If the amount is not sent anymore, mints that haven't upgraded yet will error, right? Shouldn't we keep the amount in the request, at least till most mints have upgraded?

Same as @thesimplekid, I've upgraded Nutshell to be backwards compatible (has two different responses depending on whether amount is sent or not). I will add the same to LNbits and we should be relatively good. Only issue will be clients that have upgraded but want to talk to old mints but there will always be pain!

[thunderbiscuit]

I'm not there yet so all good on my end! (not sure if the cashu-kotlin item above was intended for me? if so you can check it off).

[ngutech21]

Moksha mint and wallet upgraded ngutech21/moksha@d0d39f5

All changes are backwards compatible. I will remove the deprecated code, after all the other implementations have upgraded as well.