Challenges MSC

proposals/3182-challenges.md

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+# MSC3182: Challenges
+
+In matrix we already have vibrant discussions going on in rooms, but we often miss a fair method to resolve a dispute between several parties.
+
+This proposal improve matrix by adding several methods to fairly choose between several options by:
+
+ - Defining the standard shape for extensible challenges messages, such as 
+     - Rock/Paper/scissors
+     - Coin Flip
+     - Drawing Straws
+ - Defining extensible commitment schemes allowing to define the level for binding vs concealing property of the challenge
+
+
+
+
+## Proposal
+
+
+This proposal does not require new server APIs, and only define new message types as well as new event relations ( as per MSC2674: Event Relationships)
+
+### General framework
+
+#### Creating a challenge
+
+To start a challenge, Alice will send an `m.challenge` event with the following properties in its contents:
+
+- `body`: a fallback message to alert users that their client does not support challenges
+- `challenge_type`: Type of challenge  like `m.challenge.rock_paper_scissors` or `m.challenge.coin`
+- `commitment_scheme`: the commitment scheme to use, e.g `random_sha_256`
+- `to`: a list of user ids, Users can only respond to this challenge if
+  they are named in this field. `m.challenge.*` events that have relation to this challenge but which was not sent by one the these users should be ignored
+
+````
+{
+    "type": "m.challenge",
+    "content": {
+        "body": "Alice is challenging Bob to Rock Paper Scissors",
+        "m.challenge_type": "m.challenge.rock_paper_scissors",
+        "commitment_scheme" : "random_sha_256",
+        "to": ["@bob:exemple.com"]
+    }
+}
+````
+
+
+#### Commitment phase
+
+
+Now each participant must send a commitment message regarding the created challenge. For example in the case of rock paper scissors, the participant choose one of ✌️, 🤚, or ✊ and then apply the commitment scheme that will hide the choice to the other participant, while binding to it (the reveal phase will proove that the user did not cheat)
+
+````
+{
+    "type": "m.challenge.commit",
+    "content": {
+        "commitment": "kvGuecVHbPcjqSecX3Yz+DrGsNWEezdrfHyk6y1T4iE",
+        "m.relates_to": {
+            "rel_type": "m.reference",
+            "event_id": "$challenge_event_id"
+        }
+    }
+}
+````
+
+#### Reveal phase
+
+When each participant in the challenge did send their commitment, now can start the reveal phase.
+
+Each participant will send a reveal message, the reveal message will contain their answer to the challenge as well as the info to verify the commitment
+
+````
+{
+    "type": "m.challenge.reveal",
+    "content": {
+        "secret": "✌️",
+        "random": "dnNwdmR2bGYsZ21sa2Rudm1xZGZvbmRxbW5mZHFtbHbDpycow6DDpychKA=="
+        "m.relates_to": {
+            "rel_type": "m.reference",
+            "event_id": "$challenge_event_id"
+        }
+    }
+}
+````
+
+
+#### Verification phase
+
+Now each participant can verify that the reveal match the commitments and determinate the challenge result
+
+Using the provided info and commitment scheme, all participants can check if the reveal is valid and matches the initial commitment
+
+`Commit(secret, random) = commitment`
+
+For `m.challenge.rock_paper_scissors` it's just determining if it's win/loose/draw based on the revealed secret ✌️🤚✊
+
+
+## Commitment schemes
+
+A first commitment scheme `random_sha_256` is defined in this MSC, but more could be added later.
+
+Let `alice_secret` be the answer to the challenge done by alice, and `random` a random 256bit number base64 encoded.
+
+`Commit(alice_secret) = sha256(alice_secret || random)`
+
+
+In case of a Rock-Paper_Scissors, alice computes a random `rA` number, concatenates ✌️ and rA then compute the hash   that will be sent as `commitment` of the `m.challenge.commit`message
+
+````
+{
+    "type": "m.challenge.commit",
+    "content": {
+        "commitment": <sha256(✌️ || rA)>
+        [...]
+````
+
+In the reveal phase, alice then sends
+
+````
+{
+    "type": "m.challenge.reveal",
+    "content": {
+        "secret": "✌️",
+        "random": "rA"
+````
+
+Bob can then check that `sha256(secret, random) == commitment`
+
+
+## Challenge types
+
+This MSC defines 3 challenges:
+
+| Challenge        | Id         |
+| ------------- |:-------------:| 
+| Rock Paper Scissors    | `m.challenge.rock_paper_scissors`|
+| Coin Flip  | `m.challenge.coin` |
+| Drawing Straws | `m.challenge.straws` |
+
+
+### Rock Paper Scissors
+
+**Use Case:** 
+Settle a dispute or make an unbiased group decision. 
+
+**Participant number:** 2
+
+Possible `secret` values are ✌️🤚✊
+Each participant commit on a secret value, and results is defined as follow:
+- ✌️ beats 🤚
+- 🤚 beats ✊
+- ✊ beats ✌️
+
+### Coin Flip
+
+**Use Case:**
+Randomly choose between two alternatives
+
+**Participant number:** 2
+
+Possible `secret` values are `head` or `tail`
+
+The challenge creator (alice) commits on the outcome of the coin flip (`head`|`tail`)
+The other partipant (Bob) choose one of `head`|`tail`
+
+After the reveal phase, Bob wins if he corretly guess the coin flip outcome commited by Alice
+
+
+### Drawing Straws
+
+**Use case:**
+Choose one member of the group to perform a task after none has volunteered for it
+
+**Participant number:** 2 or more
+
+The challenge creator (alice), commits on the ordering of n straws, e.g:
+`===`
+`=`
+`==`
+`====`
+
+
+Each participant will choose one of the straws, by commiting to a number [1,n].
+In order to resolve disputes, each participant must commit the straws by order of preference, e.g (1,3,2,4). If straw 1 was already taken, then try to peek 3, and so on.
+
+Alice as the creator must take the last remaining straw.
+
+The participant with the shortest straw wins the challenge.
+
+
+
+## Potential issues
+
+
+## Alternatives
+
+
+## Security considerations
+
+## Unstable prefix
+
+The following mapping will be used for identifiers in this MSC during development:
+
+
+Proposed final identifier       | Purpose | Development identifier
+------------------------------- | ------- | ----
+`m.challenge` | event type | `org.billcarsonfr.challenge`
+`m.challenge.commit` | event type | `org.billcarsonfr.challenge.commit`
+`m.challenge.reveal` | event type | `org.billcarsonfr.challenge.reveal`
+`commitment_scheme` | event content field | `org.billcarsonfr.commitment_scheme` 
+`m.challenge_type` | event content field | `org.billcarsonfr.challenge_type`
+`commitment` | event content field | `org.billcarsonfr.commitment`
+`secret` | event content field | `org.billcarsonfr.secret`
+`random` | event content field | `org.billcarsonfr.random`