mix inputs with highest number of rounds first

[UdjinM6]

Currently we are mixing like this (imagine columns are number of rounds and we mix simple input, for simplicity sake, and inputs are selected in some order (which is not the case, we randomize them and even drop some to break possible patterns)):

1st session: 100000000
2nd session: 110000000
3rd session: 111000000
...
nth sesssion: 111111111
nth+1 session: 211111111

and so on until finally we get to nPrivateSendRounds round (N, M=N-1):
NMMMMMMMM

This is the first time user will finally see smth in his Private balance (input with N rounds).

What I'm proposing is changing this scheme to:

1st session: 100000000
2nd session: 200000000
...
Nth session: N00000000
Nth+1 session: N10000000
Nth+2 session: N20000000

and so on.
This way user will see (and will be able to use) mixed funds much earlier.

One thing to note is that together with multi-session option this scheme actually morphs into a mixed one because we don't use non-confirmed inputs for mixing. Smth like this:

1st session: 100000000
2nd session: 110000000
_block accepted_
3nd session: 210000000
4th session: 220000000
5th session: 221000000
...

and so on. This way user will be able to get even more private funds faster.

I was thinking about it for a while and wasn't able to find any reasonable arguments why this could not work or why this would defeat/leak some privacy. Suggestions and possible attack vector ideas are highly welcome.

[SCDeveloper]

Couldn't this mean that inputs with 0 rounds might never be mixed and that potentially inputs with a high number of rounds will get continuously mixed over and over?

[UdjinM6]

No, this doesn't change the way inputs are selected, this changes the order.

EDIT: not sure if I expressed it right so I'll try again - max number of rounds for PrepareDenominate call is still nPrivateSendRounds - 1 and min number of rounds is still 0 so mixing will happen for all compatible inputs like before, we just swap which one is selected first.

[SCDeveloper]

utACK. Makes complete sense, highest rounds first.

[SCDeveloper]

Why is this being held back until version v0.12.2.x ?

[schinzelh]

Because we are 2 weeks before 12.1 release and 12.1 is feature freezed since 24th Dec.

[tgflynn]

utACK