* Use GetRealOut... instead of Capped

Signed-off-by: pasta <[email protected]>

* Add "ps_salt" value to walletdb

This value is used to deterministically pick a random number of rounds to mix, between N and N + GetRandomRounds. A salt is needed in addition to the inputs hash to ensure that an attacker learns nothing from looking at the blockchain.

Signed-off-by: pasta <[email protected]>

* Implement Random Round Mixing

This implements "Random Round Mixing." Previously, attempted attacks on PrivateSend assumed that all inputs had been mixed for the same number of rounds. Noramlly assuming 2,4,8 or 16.

While none of these attacks have been successful, they still teach what we can do to make our system more robust, and one of those ways is to implement "Random Round Mixing".

Under the previous system, inputs were mixed up until N rounds (configured by user). At this point, the input was considered mixed, and could be private-sent. Under this new system, an input will be mixed to N rounds like prior. However, at this point, Sha256d(input, salt) will be calculated (note: this likely could be a more efficient hash function than double sha256, but that can be done in another PR / version if needed). If (hash % 2 == 0), then the input will be mixed again.
This results in an exponential decay where if you mix a set of inputs, half of those inputs will be mixed for N rounds, 1/4 will be mixed N+1, 1/8 will be mixed N+2, etc. This current implementation caps it at N+2. This results in mixing an average of N+0.875 rounds. If you removed the cap, you would mix on average N+1 rounds.

Signed-off-by: pasta <[email protected]>

* Make PS salt a private member of CWallet, tweak the way it's initialized

* Introduce `CWallet::IsFullyMixed` and use it everywhere instead of comparing rounds directly to ensure consistency between coin selection logic, balance calculations and gui

* Tweak `GetRealOutpointPrivateSendRounds` to respect random rounds

* Tweak IsFullyMixed to make decision on a per-outpoint basis instead of a per-tx one

* make a comment doxygen

Signed-off-by: pasta <[email protected]>

* Rename GetPrivateSendSalt InitPrivateSendSalt, since it is not a getter

Signed-off-by: pasta <[email protected]>

* move the comment below GetRounds call

Signed-off-by: pasta <[email protected]>

* don't use GetCappedOutpointPrivateSendRounds when printing to RPC

Signed-off-by: pasta <[email protected]>

* Simplify hashing in IsFullyMixed

Uses just 1 sha256 instead of 3 (1 in SerializeHash + 2 in Hash)

* undo comment change

Signed-off-by: pasta <[email protected]>

Co-authored-by: UdjinM6 <[email protected]>

Signed-off-by: pasta <[email protected]>