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# Provably fair

One of the main advantages of PokerDAO compared to traditional online poker solutions is that players do not need to trust a centralized authority to apply the rules to each game fairly. Our mental poker scheme enables each player to partake in shuffling the deck with an assigned seed number — effectively a digital form of ‘cutting the deck.’

Before the seed numbers are applied, the initial shuffling is handled algorithmically as a client-side process. Cards are encrypted and shuffled between all participating players so that every player encrypts every card. The openness of this process ensures any attempt to disrupt or manipulate the game can be observed by other players and challenged using the Optimistic Rollup protocol. Players who attempt to disrupt the game will be subject to a penalty in the form of fees.

The poker protocol used by PokerDAO relies on elliptic curves and their unique properties.

Below is an overview of what makes elliptic curves helpful for this purpose.

First, given a point on the elliptic curve P and a scalar k, we can multiply P by k to yield kP:

Finding k given the starting point P and the result of the multiplication KP is called an ‘elliptic curve discrete logarithm problem.’ This operation is computationally complicated to solve.

Second, the nature of elliptic curves is such that the starting point P can be multiplied by multiple different scalars a, b, and c in any order and still yield the same result. As in normal math, the order of multiplication does not matter: a * b = b * a.

**b. Shuffling Algorithm**

To best understand the PokerDAO algorithm, we will follow an example poker game. Let's say that we have three players on the table: Alice, Bob, and Charlie.

**Stage 1: Shuffle**

We start with a set of 52 points on the elliptic curve. These are points generated by our servers. These points represent the 52 cards we will play poker with. Each card is mapped to a single point on the curve.

Let the starting points be

*P1, P2, P3, ..., P52.*We send these points to all players on the table.

Alice is the one to start. She generates a scalar on her computer (the browser) that only she knows. She takes the set of points, shuffles it on her computer (the browser), and multiplies each point by her scalar

*a*.The result is the set of points:

aP1, aP2, aP3, ..., aP52.

She sends this set to all players on the table.

The next player to act is Bob. On his computer, he generates a scalar b. He shuffles Alice's set and multiplies each point by his scalar

*b*.The result is the set of points:

*abP1, abP2, abP3, ..., abP52.*

He sends this to all players on the table.

It is Charlie's turn. He does the same as Bob and Alice:

Now that we have a shuffled and encrypted deck of cards, we proceed to the next stage of the algorithm.

**Stage 2: Locking**

It is Alice's turn again. She takes Charlie's set of points and multiplies each point of the set by the multiplicative inverse of her scalar a - the one she used during the shuffle stage. This removes their shuffle key from the set. The set looks like this:

*bcP1, bcP2, bcP3, ..., bcP52.*

Alice then generates 52 new unique scalars:

*a1, a2, a3, ..., a52.*She multiplies each one of the 52 points sequentially using these new scalars. This time each point gets multiplied by a different scalar. The result is:

*a1bcP1, a2bcP2, a3bcP3, ..., a52bcP5.*

Note, now each card is encrypted by unique 52 scalars by Alice and by a single scalar from all other players.

Alice sends this set of cards to all players on the table.

It is Bob's turn. He does the same as Alice. He multiplies the set of cards by the multiplicative inverse of his shuffle key b. This removes it from the set. He then generates 52 unique keys and encrypts each card with a unique key. The result is:

*a1b1cP1, a2b2cP2, a3b3cP3, ..., a52b52cP5.*

It is Charlie's turn. He repeats what Alice and Bob did:

The deck circled in red is the final deck of cards. It has been shuffled and encrypted by each player. No player knows the order of the cards, and no player can decrypt any card without the cooperation of all other players. The poker game is played with this deck.

**c. Opening Cards**

To open a card at index

*i,*all players have to broadcast their respective locking keys at that index.Once the cards have been dealt, the necessary de-encryption keys are supplied via a smart contract to each player according to the cards they hold, and the game commences.

Last modified 3mo ago