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Is it possible for devices on an online group chat to toss a coin without trusting each other and the server? (discuss.tchncs.de)

submitted 6 months ago by [email protected] to c/[email protected]

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[–] [email protected] 4 points 6 months ago* (last edited 6 months ago) (3 children)

Everyone tosses three coins, and posts it in the chat
- If a player tosses three of the same, they have to toss again.
Everyone chooses the mode coin from their neighbour, and adds it to their stack
Each player, with 3+N coins, picks the mode coin in their own collection.
- Ideally: the player's own bias, is outweighed by the other player's biases.
The final coin is the mode of all players coins.

spoiler

from numpy import median
from pprint import pprint

players = {"p1" : [1,0,1],  ## playing fair
           "p2" : [0,0,1],  ## cheating
           "p3" : [1,1,0],  ## cheating
           "p4" : [1,1,0],  ## cheating
           "p5" : [0,0,1]}   ## playing fair
print("Initial rolls:")
pprint(players)

get_mode_coin = lambda x: int(median(x))
get_all_mode_coins = lambda x: [get_mode_coin(y) for y in x]

for play in players: ## Players add the mode coin from their neigbours
    players[play] = players[play] + get_all_mode_coins(players.values())
print("First picks:")
pprint(players)

for play in players: ## Players collapse their collections to mode
    players[play] = [get_mode_coin(players[play])]
print("Last modes:", players)

print("Final choice:", get_mode_coin([x for x in players.values()]))

Which as you can see, is no better than simply picking the median coin from the initial rolls. I thank you for wasting your time.

[–] [email protected] 5 points 6 months ago

The last player (or server) still can choose a result, because it knows other tosses before making it's own.

[–] [email protected] 2 points 6 months ago

First person gets a box showing heads tails. Once that is picked player 2 is shown a flip coin button. This isn't fucking hard except the sync between apps which you do via db on the back end.

[–] [email protected] 2 points 6 months ago

Second attempt that factors in cheating.

spoiler

from numpy import median
from random import choice
from pprint import pprint

# Functions
get_mode_coin = lambda x: int(median(x))
def pick(player, wants):
    for neighbor in players:
        if player != neighbor:
            neighbor_purse = players[neighbor]["purse"]
            if wants:
                if wants in neighbor_purse: # Cheat
                    players[play]["purse"] = players[play]["purse"] + [wants]
                    continue
            players[play]["purse"] = players[play]["purse"] + [choice(neighbor_purse)]


# Main
players = {"p1" : {"purse": [1,0,1], "wants": False}, ## playing fair
           "p2" : {"purse": [0,0,1], "wants": 0}, ## cheating
           "p3" : {"purse": [1,1,0], "wants": 1}, ## cheating
           "p4" : {"purse": [1,1,0], "wants": 0}, ## cheating
           "p5" : {"purse": [0,0,1], "wants": False}}   ## playing fair

for play in players: ## Players pick a desired coin from each of their neighbours
    pick(play, players[play]["wants"])
print("First picks:")
pprint(players)

for play in players: ## Players collapse their collections to mode
    players[play] = [get_mode_coin(players[play]["purse"])]
print("Last modes:", players)

print("Final choice:", get_mode_coin([x for x in players.values()]))

So, my method doesn't work