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This document might still be updated until the beginning of the CS Games 2017. We recommend that interested parties watch the repository to be notified as soon as a new change is introduced. For any issue, bug or needed clarification, please open an issue in the GitHub project.

This repository contains the CSCoin specification and the source code for the Central Authority server. More samples should be added in the near future to manage a CSCoin wallet and a (slow) reference implementation for a miner.


CSCoins Documentation


The Pre-CS Games challenge is a new concept for the CS Games 2017 proposing a competition that teams can work on before the CS Games. This competition should still be doable in the span of the week-end for late teams and a minimal PoC will be available. During the week-end, points will be awarded to the best miners as part of the Puzzle Hero challenge.

The concept is to create a nano-economy for the CS Games event through the implementation of a cryptocurrency called "CSCoins". Therefore, teams will have to develop a miner to mine and get CSCoins for the week-end. The team will then be able to spend the coins in exchange of goods, hints and extra points! The CSCoins is a currency exclusively for the CS Games 2017 week-end and only blocks mined during the event can be used for trading. Teams are still encouraged to start mining beforehand to properly test their miner!

CSCoins are a currency gained by solving a challenge. Unlike distributed peer-to-peer currencies such as Bitcoin, a server authority hosted by the organization committee will handle the challenge distribution and synchronization between all peers. The server authority will be proposing a challenge to be solved and awarding CSCoins for the first miner's wallet to solve it, after which a new challenge will be proposed.

Miners environment

For more information about the environment visit docker-client/


Pseudo Random Number Generator

All challenges require the miners to generate random data from a given seed. In order to generate the same random number on each computer for a given seed, the same pseudorandom number generator has to be used by all the teams and the Central Authority. The used PRNG is the Mersenne Twister (MT19937-64). Implementations are freely available on the internet and in most standard libraries.

The Challenge

The challenge is about finding a SHA256 hash that matches the target prefix given by the Central Authority. All previous solutions are also included into the current solution.

When a miner fetches the current challenge from the Central Authority, a challenge name will specify which problem needs to be solved. For example, the server will tell the miner to solve the "sorted_list" challenge type.

Along with the challenge name, the Central Authority server might also specify additional parameters for the challenge. For example, the sorted list challenge will have the "nb_elements" parameter. This parameter will tell the miner how many numbers need to be generated for the challenge.

Once the miner has the challenge parameters, it can start generating the challenge dataset. It does so by generating nb_elements 64 bits unsigned integer with the Pseudo Random Number Generator. The seed state used by the PRNG is the first 8 bytes of the SHA256 Hash of the concatenation of the last solution hash (in stringified hex) and a given nonce (SHA256(HEX(LastSolutionHash) | Nonce)[:8]). The nonce is the decimal representation of a chosen unsigned 64-bit integer.

Once the challenge dataset is generated, the miner has to solve the challenge. In the sorted list challenge, it must sort the elements to generate a solution string. To build that string, the miner has to put all values of the list in sorted order, convert them to their string representation (in base 10) and concatenate them together. Example: Given a list of 5 numbers, [9, 4, 5, 9, 2]. The sorted list solution string is: "24599".

Finally, the solution string is hashed with SHA256 and the result compared to the prefix provided by the Central Authority server. If it does match, the miner can submit the nonce used to generate the challenge and get awarded a CSCoin!

Otherwise, the miner needs to try again with a new random nonce, generate a new challenge dataset, sort it, and keep trying until a solution hash matches the wanted prefix.


The miner starts by requesting the current challenge properties from the Central Authority. It gives it the first challenge:

    "challenge_id": 0,
    "challenge_name": "sorted_list",
    "last_solution_hash": "0000000000000000000000000000000000000000000000000000000000000000",
    "hash_prefix": "94f9",
    "parameters": {
        "nb_elements": 20

This is the first challenge of the blockchain, so the last solution hash is "0..0000". We can try our first mining attempts with a nonce set to "0".

So we hash "00000000000000000000000000000000000000000000000000000000000000000" ('0' x 65). Which give us the hex digest: e531ef0f962409170917abf9de3287afec23dd1c42c9e1fea66c5feab99e8f7c.

We use the first 8 bytes to generate an integer, this integer will be our seed number. "\xe5\x31\xef\x0f\96\x24\x09\x17" or 0x170924960fef31e5 as an unsigned 64 bits integer.

With the seeded Mersenne Twister PRNG, generate 20 unsigned 64 bits random integer:

[187434852114612846, 10837196899528407950, 13231065775368684808, 4942314897761499926, 17028306872668076666, 6479076410078001012, 1361812409744256450, 3291217005668754618, 4966164133636131008, 7642938895213376385, 2143391747588726128, 10354190044461362030, 7773684818964863771, 10794580392141786114, 16492712020482564264, 9905729874405870466, 5103580236542799915, 15262620284228700467, 4872192301904809974, 2466619936935862033]

We order them:

[187434852114612846, 1361812409744256450, 2143391747588726128, 2466619936935862033, 3291217005668754618, 4872192301904809974, 4942314897761499926, 4966164133636131008, 5103580236542799915, 6479076410078001012, 7642938895213376385, 7773684818964863771, 9905729874405870466, 10354190044461362030, 10794580392141786114, 10837196899528407950, 13231065775368684808, 15262620284228700467, 16492712020482564264, 17028306872668076666]

We build the solution string by concatenating those numbers together:


We finally hash this solution string to know if it matches the challenge hash prefix: e9528071d01ab08cc4c9e74fee45df95af0256a469f17aa88918dc14a4008eb7

e952 != 94f9, So we need to try again. We generate a new random nonce and reseed the random number generator.

Our new random nonce is "96436717027".

So we hash "000000000000000000000000000000000000000000000000000000000000000096436717027". Which give us: b998963f3add4b84a5bb2b28bff28e2c556cb860e73d110679b5f9c2ac65bad4

\xb9\x98\x96\x3f\x3a\xdd\x4b\x84 or 0x844bdd3a3f9698b9 as 64 bits integer.

Our new numbers are:

13916525834144929239, 14267992374683960415, 17447482481089031970, 17757524129420908411, 3262888185675309556, 4651484254165491713, 14892328834357397553, 642435218906306919, 12375157785219615227, 17110841558370943827, 17014653408604181717, 16941248359459129302, 448482909903223685, 13547565333399307760, 7472403995188135213, 7093177824549920619, 4562417252290276756, 17541524771507166271, 10898898378669729391, 9285048143772460569

The numbers are sorted and the solution string becomes:


Hashing the solution string yields the digest: 90c7e5020a21158beeaabe40d1c736aae12006cf7e42e54ade214aa0d117f76f. Still not matching 94f9...

Let's keep trying.

Nonce: "81784174387" Solution string:


Solution Hash: 94f983bf4d2e06610db4f15bfc6237f5d9e5f79a2e9502e52cd0ddfca905b163. The first bytes match 94f9!

So we can submit the nonce to the Central Authority Server and win more CSCoins!

A new challenge is then generated. The miner fetches the updated information from the server:

    "challenge_id": 0,
    "challenge_name": "sorted_list",
    "last_solution_hash": "94f983bf4d2e06610db4f15bfc6237f5d9e5f79a2e9502e52cd0ddfca905b163",
    "hash_prefix": "9098",
    "parameters": {
        "nb_elements": 20

This is the second challenge, so the miner now has the previous hash to seed the PRNG. We hash the last solution and a random nonce to seed the PRNG: SHA256("94f983bf4d2e06610db4f15bfc6237f5d9e5f79a2e9502e52cd0ddfca905b163" + "248565"). This gives us the digest: 83d51291b621b0a96c6a871256c911ca431b1ff24102f069951659457cb0d8fb.

We use the first 8 bytes to generate a 64 bits unsigned integer, this integer will be our seed number: \x83\xd5\x12\x91\xb6\x21\xb0\xa9 or 0xa9b021b69112d583.

We use the seeded PRNG to generate 20 random 64 bits unsigned integers:

7554992472722769382, 8495646299498670407, 16283171823286969215, 14362607901913361150, 6692862674045639924, 9015192917032127462, 327830662700023863, 9371480197312631464, 16474393573432639799, 11349000119560803050, 3722572335198690747, 11617180555990659053, 14400604751394908311, 16917897951851071986, 4035298408703112487, 5375153024761395986, 1544660713920769741, 16852903019784931493, 16173382640596610454, 17545493636135106882

We order them:

327830662700023863, 1544660713920769741, 3722572335198690747, 4035298408703112487, 5375153024761395986, 6692862674045639924, 7554992472722769382, 8495646299498670407, 9015192917032127462, 9371480197312631464, 11349000119560803050, 11617180555990659053, 14362607901913361150, 14400604751394908311, 16173382640596610454, 16283171823286969215, 16474393573432639799, 16852903019784931493, 16917897951851071986, 17545493636135106882

We build the solution string by concatenating those numbers together:


We hash the solution: 7d5fb906c70df5d43275e30bb044f64fbd5b4c2a2c8a92597700c7d1709a202b.

Not matching the 9098 prefix sent by the Central Authority server...

We keep trying with new random nonce.

Nonce: 814267888 Seed: \x1e\xcd\xad\x5d\xa4\x6f\x9c\xd1 Solution String:


We hash the solution: 90985e24a9e0b8ece598cf643e303424ae5d1c3f5a9903c08f1dcd3b8e1a3454. Matching the prefix sent by the Central Authority server.

How lucky we are, another CSCoin awarded to our wallet. That's pretty much what mining is about. Generate a challenge dataset, solve it and check if the solution hash is valid. Rinse and repeat!

Challenge Type

Sorted List

This challenge is simple and yet offers many ways to the miners to optimize their processing. The challenge begins by generating 64 bits unsigned integers from the PRNG. Afterward the miner has to sort all the numbers to hash the solution.

Challenge Name: sorted_list Parameters:

  • nb_elements: Number of integer to generate and sort.

Solution String Formatting: Convert all integers into their decimal string representation and join all together without any whitespaces.

Example: Given the numbers [400, 2, 4, 3, 5], the solution string would be "2345400".

Reverse Sorted List

Just like the Sorted List challenge, this challenge begins by generating 64 bits unsigned integers from the PRNG. Afterward the miner has to sort all the numbers in descending order to hash the solution.

Challenge Name: reverse_sorted_list Parameters:

  • nb_elements: Number of integer to generate and sort.

Solution String Formatting: Convert all integers into their decimal string representation and join all together without any whitespaces.

Example: Given the numbers [400, 2, 4, 3, 5], the solution string would be "4005432".

Shortest Path

You need to find the optimal path in a Grid. You can only go up, down, right and left.

Challenge Name: shortest_path Parameters:

  • grid_size: Grid size, the grid is square.
  • nb_blockers: Numbers of blocker tile in the grid.

Solution String Formatting: Join all coordinates of the path, a coordinate is the row and the column in decimal string representation. Including starting and ending coordinate.

Grid generation

  • First, Place some walls at each border tiles. (First row, last row, first column and last column).
  • We generate an starting coordinate and a ending coordinate. (PRNG.extract_number % grid_size, PRNG.extract_number % grid_size)
    • While starting coordinate is a wall/blocker, generate another starting coordinate.
    • Do the same with the ending coordinate, but also if the generated ending coordinate is the same as the starting one, generating another ending coordinate.
  • Generate nb_blockers walls with the PRNG (PRNG.extract_number % grid_size, PRNG.extract_number % grid_size).
    • If the coordinate is the same as the starting coord or the ending coord, skip this blockers.
    • If the coordinate is already blocker, skip this blockers.
  • Find the shortest path. If there's multiple possibility, use the first one, and we check the nearest neighbors in that order : down, up, right, left.

Example: We got a grid of size 10 with 5 blockers, "x" is a blocker, "s" is the starting point, "e" is the ending point and "p" is the path. You can also look at to know how the server solve the grid.

x        x
xsx      x
xpppx    x
x  p x   x
x  ex    x
x    x   x
x        x
x        x

The solution path is [(2, 1), (3, 1), (3, 2), (3, 3), (4, 3), (5, 3)]. The solution string would be "213132435354".


A wallet is a RSA key pair of 1024 bits. To send or receive coins, we use the Wallet Id. The Wallet Id is a SHA256 Hash of a client public key in DER (Distinguished Encoding Rules) format.

A client needs its private key to sign the submission message and create a transaction message to prove that it is the owner of the wallet.

A client will need to register its public key on the Central Authority Server to be able to do submission and create a transaction.

To calculate a client's Wallet balance, a client needs to retrieve all the transactions and compute the transaction outcomes.

Message Signature

We are using RSA digital signature protocol according to PKCS#1 v1.5. Some messages required a signature to validate that the client is the owner of the wallet. Usually, the signed contents are the arguments of the command, joined together by a comma (,). After, the contents in hashed using SHA256, and you sign the digest generated. A signature is always represented in a stringified hexadecimal format. The Central Authority server will validate the signature against the registered public key by the client.

Communication with the Central Authority

The Central Authority Server use the WebSocket protocol to communicate. The server URI is wss:// Once a client is connected, the server will be waiting for any incoming commands. All data sent or received are serialized in JSON.

All responses sent by the Central Authority Server contains a type attribute. This attribute will tell you the type of the response object.

Available commands

Command object

Field Name Type Description
command String The name of the command. Example: get_challenge_solution
args Dictionary<String, Object> Parameters for the command, see each command's documentation

Get Current Challenge

Fetch the current problem set from the Central Authority.

After the first call of this command, you will automatically receive the new challenge when it's available, until the connection is closed.

Command Name: get_current_challenge


There's no argument.

Field Name Type Description
type String current_challenge
time_left Integer Time left in seconds to solve the problem set.
challenge_id Integer Current challenge id
challenge_name String Current challenge type name. Example: sorted_list
hash_prefix String Solution hash prefix used to validate a solution hash. The prefix is represented in a hexadecimal string format. Example: a4f1
last_solution_hash String Last valid solution hash. Must be used as part of the seed for the current challenge. Example: 30fe523cb3c9007b50242736a8a099215e6ea533f3c8012c9dac34756257d6dc
parameters Dictionary<String, Object> Parameters of the challenge, they are specific to the challenge type. See the Challenge Type section for more details.

Get previous solutions

Fetch the solution of a challenge

Command Name: get_challenge_solution

Field Name Type Description
challenge_id Integer Challenge id for which a solution is requested.

The response has the same content as the get_current_challenge command, with the addition of those two fields:

Field Name Type Description
type String challenge_solution
nonce String Nonce used to seed the PRNG.
solution_hash String Solution hash that validated with the challenge prefix.

Close connection

Close the connection with the Central Authority Server

Command Name: close


There's no argument.


There's no response.

Register a new Wallet

Register your Wallet's public key with the Central Authority.

Command Name: register_wallet

Field Name Type Description
name String Your Wallet Name, you should use your Team Name here.
key String Your wallet's public key, in PEM format.
signature String Message signature in hexadecimal format. The message used for the signature is the public key in DER format.
Field Name Type Description
type String register_wallet
error String Error message if something went wrong

Get Transactions

Get transactions history from the Central Authority.

Command Name: get_transactions

Field Name Type Description
start Integer Starting index of the requested transactions.
count Integer Number of transaction requested.
Field Name Type Description
type String transactions
error String Error message if something went wrong
transactions Array<Transaction> List of transaction(s)
Transaction Object
Field Name Type Description
id Integer Transaction id
source String Source Wallet id
recipient String Recipient Wallet id
amount Decimal Transaction amount

Create a new Transaction (Send coins)

Create a new Transaction, sending coins to another wallet

Command Name: create_transaction

Field Name Type Description
source String Source Wallet id
recipient String Recipient Wallet id
amount Decimal Amount to send, Minimum 0.00001.
signature String Message Signature in hexadecimal format. The signed message is source,recipient,amount. The amount is a decimal number formatted with 5 digit of precision. Example: amount = 2, would be written as 2.00000 in the message. The signature must be validated by the public key related to the source Wallet id.
Field Name Type Description
type String create_transaction
error String Error message if something went wrong.
id Integer New transaction id.

Submit a problem solution

Submit a solution for the current challenge, awarding CSCoins to the miner if the solution is valid.

Command Name: submission

Field Name Type Description
wallet_id String Miner Wallet id.
nonce String Nonce used to find a valid solution hash.
Field Name Type Description
type String submission
error String Error message if something went wrong.

Get Central Authority Server Information

Fetch the current information of the Central Authority server

Command Name: ca_server_info


There's no argument.

Field Name Type Description
type String ca_server_info
minutes_per_challenge Integer Maximum time of a challenge in minutes.
coins_per_challenge Integer Coins awarded for a valid submission.
min_transaction_amount Decimal Minimum transaction amount.
ca_public_key String Central Authority PublicKey in (PEM) format.


CSCoin Specification and tools




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