Face down cards in exile being shown/revealed improperly

[Cguy7777]

Tested on commit 7696839.

Face-down cards in exile are being shown to players who shouldn't have access to that information:

1. They are being shown to the player who exiled them, even with cards that don't grant permission to see them. (406.3: ... Cards "exiled face down" can't be examined by any player except when instructions allow it.) This didn't happen yesterday.
2. They are being revealed to your opponent at the end of the game. AFAIK, no rule requires this: 708.9 requires face-down permanents, components of merged permanents, and spells to be revealed at the end of each game, but not exiled face down cards. I didn't test this until today.

This is likely related to #11873.

Known affected cards:

• Bomat Courier
• Duplicity
• Knowledge Vault
• Kyren Archive
• Moonring Mirror
• Necropotence

[JayDi85]

It's interesting. There are another rule -- you can't select exiled face down card:

• 406.4. Face-down cards in exile should be kept in separate piles based on when they were exiled and how they were exiled. If a player is instructed to choose an exiled card, the player may choose a specific face-down card only if the player is allowed to look at that card. Otherwise, they may choose a pile of face-down exiled cards, and then a card is chosen at random from within that pile. If choosing such a card is part of casting a spell or activating an ability, the chosen card isn't revealed until after that cost is fully paid. (See rule 601.2i.)

[JayDi85]

I don't like 406.4 with face down piles, etc. Xmage uses objects with id all around -- it's impossible to fully anonymize cards/objects. GUI uses ids inside and you can dig it from inner data (e.g. cheating).

So here is my conclusion:

• Rules support 708.9:
  • ok: show faced-down permanents and spells to all players at the game end -- already implemented;
  • ok: keep hiding faced-down cards in other zones at the game end -- will be implemented;
• Rules support 406.3:
  • ???: keep hiding moved faced-down cards (from hidden zones: library, opponent's hand) -- need research, can be implemented in theory;
  • ignore: hiding moved faced-down cards (from visible zones: own hand, graveyard, exile, etc) -- no needs, see object ids problem above;
• Rules support 406.4:
  • ???: restrict to target or select a hidden faced-down card -- need research, must be implemented already, but not fully support (looks like NamePredicate do not check face down status, other filters can ignore something too);
  • ignore: allow to random choice from a pile of hidden faced-down cards -- paper specific, impossible to implement;

[JayDi85]

Another one thing for 406.3 -- owner of the card must be visible to all, so a player has another legal "cheat" to identify manifested card:

Jeskai Infiltrator
A card’s owner is public information at all times. If the two cards you exile are owned by different players (perhaps because you gained control of a Jeskai Infiltrator owned by your opponent), which card is which is no longer hidden from your opponent. That player will know which face-down creature they own. (2014-11-24)

[JayDi85]

One more for 406.3 -- face down cards must be identifiable:

Jeskai Infiltrator
You must ensure that your face-down spells and permanents can easily be differentiated from each other. You’re not allowed to mix up the cards that represent them on the battlefield to confuse other players. The order they entered the battlefield should remain clear. Common methods for indicating this include using markers or dice, or simply placing them in order on the battlefield. You must also track how each became face down (manifested, cast face down using a morph ability, and so on). (2018-07-13)

[Cguy7777]

Something I forgot about: with Foretell ability, face-down foretold cards in exile must be revealed when a player leaves or the game ends.
702.143f. If a player leaves the game, all face-down foretold cards that player owns must be revealed to all players. At the end of each game, all face-down foretold cards must be revealed to all players.