Beer, Betting and the Nxt Blockchain

Suppose you have a circle of friends who you meet with in a sports bar to drink beer, watch sports and make bets. You can make bets on the Nxt Blockchain and have a basic Python app automate it for you.

In my first article I introduced basics of a blockchain. As we learned, the most important value proposition of the blockchain is immutability of data submitted in transactions recorded into blocks, from which a blockchain is built.

Today I want to show how to build a simple but fully functional betting app in python. For the betting app that I present, immutability of data may be required, but in the case a bet should need to be canceled, the app can be modified to take into account this additional functionality.

Pre-requisites:
NRS (Nxt Reference software) 1.10.2: https://bitbucket.org/JeanLucPicard/nxt/downloads/Python 3 (or Python 2 with slightest modification). A beer or two (optional).

Application logic

A user submits a bet (transfers any amount of NXT coins) to the betting account and adds a public unencrypted attachment. In the attachment he should write the id of the bet, a colon and a number indicating his choice in the corresponding bet. Let’s give an example: the sports bar runs a bet which football team wins. They assign an ID to this game, let’s say “30”.

The choices are:
“1” – home team wins. “2” – other team wins. “3” – tie.

To bet on the victory of the home team, the player must submit a transaction of any amount of NXT, and attach a public unencrypted (for auditability) message with the content: 30:1

When the time comes, the betting host runs the app to find winners and process payments. The host can take a small fee for the service.

Implementation

The Nxt Server listens for queries on port 7876. If you run it on the same machine as your python code by default you must submit queries to http://127.0.0.1:7876/nxt

For our app we will need to import a few modules and write the function to submit queries which will be used a few times through the app.

import sys, json, re

def sendQuery(Query):
  from urllib import parse, request
  params = parse.urlencode(Query)
  headers = {"Content-type": "application/x-www-form-urlencoded", 
"Accept": "text/plain"}
  req = request.Request("http://127.0.0.1:7876/nxt", 
params.encode('utf-8'))
  response = request.urlopen(req)
  return response.read()
	
myAccount = "NXT-AAAA-BBBB-CCCC-DDDDD"
totalAmount = 0
serviceFee = 0.05
players = []
secretPhrase = "bla bla bla"

The function returns a JSON string to be decoded, with transactions and attachments to be parsed. Also a few variables are initialized.

Next we must query the betting account to fetch all incoming transactions.
This is the request you can send to the NXT server to execute this:

Query = {'requestType': 'getBlockchainTransactions', 
'account': myAccount, 'type': '0', 'subtype': '0', 'executedOnly': 'true'}

decode JSON and load it to a variable for further processing.

output = sendQuery(Query)
data = json.loads(output.decode('utf-8'))

As we have all the data now, we loop through the list and discard some of the transactions we don’t need. For instance, we don’t need to process outgoing transactions, so we continue the loop from the next item.

for i in data['transactions']:
  txAmount = i['amountNQT']
  txSender = i['senderRS']
  if txSender == myAccount:
      continue
  txRecipient = i['recipientRS']
  if txRecipient != myAccount:
      continue

Next we pull out the attachment, check if it is not encrypted and strip whitespace.

  line = i['attachment']['message']
  lineIsText = i['attachment']['messageIsText']
  if line.strip() != "" and lineIsText == True:
    line = line.strip()

Next step is to match the submitted message against the regular expression. It must satisfy the format “Number:Number”, the first number can be up to 3 digits.

    matchLine = re.match(r'(\d{1,3}:\d{1})(.*)', line, re.M|re.I)

If the attached message is of correct format we process it further…

The betting app accepts up to 3 command-line arguments: id of the game, the choice number that has won, and third optional parameter in double quotes to name the winner choice for verbosity in the payout message.

For instance, the betting host must run it like this:
python3 bet.py 30 1 “Home team wins”

The app will process transactions with game id 30 and rewards players who chose ‘1’.
The third argument can be omitted: python3 bet.py 30 1

Next we must calculate the sum of money submitted for our game, make a list of dictionaries of players who guessed the correct number, and calculate the total sum for distribution after the betting host gets his optional service cut.

    if matchLine:
      if matchLine.group(1).split(':')[0] == sys.argv[1]:
        totalAmount += int(txAmount)
        if matchLine.group(1).split(':')[1] == sys.argv[2]:
          d = {txSender: matchLine.group(1).split(':')[1], 
'amount': txAmount}
          players.append(dict(d))
afterFee = float(totalAmount - totalAmount * serviceFee)

We have winners in the ‘players’ list of dictionaries, we have money to distribute in ‘afterFee’, let us now process payouts.

sumCorrect = 0
for i in players:
  for key in i:
    if i[key] == sys.argv[2]:
      sumCorrect += float(i['amount'])
for i in players:
  for key in i:
    if i[key] == sys.argv[2]:
      t = float(i['amount']) * (afterFee / sumCorrect)
      if t > afterFee:
        strSendAmount = str(int(afterFee))
      else:
        strSendAmount = str(int(t))
      if len(sys.argv) > 3:
        reply = sys.argv[3]
      else:
        reply = sys.argv[2]
      message = "Payout for correct bet in poll " 
+ sys.argv[1] + "; your reply: " + reply
      Query = {'requestType': 'sendMoney', 'recipient': key, 
      'amountNQT': strSendAmount, 'secretPhrase': secretPhrase, 
      'feeNQT': '100000000', 'deadline': '1440', 'broadcast': 'true', 
'message': message, 'messageIsText': 'true', 'messageIsPrunable': 'true'}
      sendQuery(Query)

Needless to say the app can be improved to process errors, cancel bets or optionally allow other parameters in attached messages. Attached messages can be submitted with the Nxt Client (NRS) or from a web wallet that allows sending attachments.

To make transactions on the Nxt Blockchain you need NXT coins, which you can purchase directly, by exchanging Bitcoin for NXT or by attending the “Lucky Node” project by running a public Nxt node (Nxt server).

To learn how to participate in the Lucky node project, please visit https://nxtforum.org.

There are over a hundred API calls you can make to the Nxt server, a full list with mandatory and optional parameters and examples is available at https://nxtwiki.org/wiki/The_Nxt_API.

Cheers!

blockchain-betting

Lottery on the Nxt Blockchain in Golang

In my first article I introduced blockchain theory, what it can do for your software project and the basics of interacting with the Nxt blockchain in PHP.

Today I am going to present a small Lottery program written in Go.

Pre-requisites:
Golang (tested with Go 1.6.2)
NRS 1.10.1 (https://bitbucket.org/JeanLucPicard/nxt/downloads/)

The program is fully functional and runs a lottery every Sunday [1].
It was originally written in PHP, both sources are available for download from the Nxt Data Cloud [2].

Application logic

Data feed for this app is ‘tickets’ submitted by users in attachments.

A lottery player sends 10 NXT to the lottery account and attaches a public unencrypted message with a string of 5 numbers, ranging from 0 to 30 separated by commas. The attachment must be unencrypted for rewards to be publicly auditable using the blockchain.

The app makes a request to the NXT server to fetch all transactions from the lottery account, sorts through them, selects only valid transactions and creates a slice of maps (multidimensional array in PHP) of players’ accounts and their strings of numbers. It also calculates the total sum of NXT payable to the players from the sums of all valid tickets.

Upon receiving all the valid data the app runs three rounds of the lottery. Each round receives a portion of the total payable sum, split between concurrent winners. In the round of 5, the app finds user(s) who have correctly guessed 5 numbers, and sends out rewards. In the round of 4, the app does the same for users who have guessed 4 numbers, the slice of participating tickets is now short of winner(s) of round of 5. Rinse and repeat for the round of 3.

This is the gist of it.

A little bit on internals

For each of the three rounds the lottery generates sequences of 5 numbers and compares them to the strings of numbers in tickets until one or more winners are found. It can be said that the lottery “forces” the winning sequence onto ticket(s).

With a limited number of users this seems to be the only sensible way of running a lottery and not having to collect and keep a large jackpot for months and years.

Let’s take a look at the function that generates sequences of 5 numbers and returns an array of them to caller function. On average, this function is called hundreds of thousands of times to find the sequence of 5 matching one of the tickets, when we have a very limited number of participants. It takes fractions of a second. In PHP it takes a tiny bit longer (a second or two), although PHP 7 performance is really good.

func genFive(seed string) [5]int {
   var r [5]int
   seedInt, _ := strconv.Atoi(seed)
   d := false
   for a := offset; a < offset+5; a++ { 
      rand.Seed(int64(seedInt + offset)) 
      var dup [31]int 
      d = false 
      r[0] = rand.Intn(31) 
      r[1] = rand.Intn(31) 
      r[2] = rand.Intn(31) 
      r[3] = rand.Intn(31) 
      r[4] = rand.Intn(31) 
      for _, v := range r { 
         dup[v]++ 
      } 
      for k, _ := range dup { 
         if dup[k] > 1 {
            d = true
         }
      }
      offset = offset + 5
      if d == false {
         return r
      }
   }
   return r
}

An important characteristic of a blockchain lottery app is: it must be completely trust-free.
Everybody must be able to validate that results of the lottery haven’t been gamed. A logical and simple solution to this is to generate sequences of numbers with a deterministic seed.

The problem with deterministic seeds: if they are known in advance, sequences of numbers can be predicted and the lottery can be gamed. To address this problem we turn to the Nxt blockchain again, to find a source of seed with a getSeed() function.

func getSeed() (string, string) {
 type BlockchainStatus struct {
    NumberOfBlocks int `json:"numberOfBlocks"`
 }
 var status BlockchainStatus
 if seedBlockOutput, b := 
sendQuery("requestType=getBlockchainStatus", 
true); 
b != false { if err := 
json.Unmarshal([]byte(seedBlockOutput), &status); 
err != nil {
     fmt.Println(err)
   }
  }
 seedBlockHeight := 
strconv.Itoa(status.NumberOfBlocks - 11)

 type BlockId struct {
  Block string `json:"block"`
 }
 var block BlockId
 if seedBlockId, b := 
sendQuery("requestType=getBlockId&height="
+seedBlockHeight, true); b != false {
if err := json.Unmarshal([]byte(seedBlockId), 
&block); err != nil {
         fmt.Println(err)
    }
  }
 seed := block.Block[len(block.Block)-5:]
 return seed, seedBlockHeight
}

The app runs at 18:00 UTC on Sunday.

The first thing it does in the getSeed() function is to fetch the block id of the block which was generated 10 blocks before the app started (as seen in a local copy of the blockchain on the lottery node) and take the 5 last digits of the block id as a seed. Due to network latency and occasional blockchain reorganizations of 1-3 blocks, the lottery node may not see the same block as other nodes. The number 10 for getting the block for seed was chosen to be reasonably sure that block would not be reorganized.

It can be argued that there is a theoretical possibility that the block id is predictable. The odds of this are tiny in my opinion but I’ll leave it to the readers to debate and decide.

Now that the app has its seed, it can do its job in a way so users don’t need to trust the lottery host.

The Go source code doesn’t include the routine to verify past results.
The PHP source code does have it, it is fully functional and can be used to independently verify all past results with the deterministic seeds from the blockchain.

For Go I use this function to send queries to the Nxt Server and return results.

func sendQuery(Query string, Active bool) 
(output string, b bool) {
   output = ""
   b = false
   if Active == false {
      output = "Function disabled"
      return
   }
   body := strings.NewReader(Query)
   req, err := http.NewRequest("POST", 
"http://127.0.0.1:7876/nxt", body)
   if err != nil {
      output = fmt.Sprintf("%s", err)
      return
   }
   req.Header.Set("Content-Type", 
"application/x-www-form-urlencoded")

   resp, err := http.DefaultClient.Do(req)
   if err != nil {
      output = fmt.Sprintf("%s", err)
      return
   }
   bo, err := ioutil.ReadAll(resp.Body)
   defer resp.Body.Close()
   output = fmt.Sprintf("%s", bo)
   match, _ := 
regexp.MatchString(".*errorDescription.*", 
output)
   if match == true {
      fileHandle, _ := 
os.OpenFile("./error.log", os.O_APPEND, 0666)
      writer := bufio.NewWriter(fileHandle)
      defer fileHandle.Close()
      fmt.Fprintln(writer, output)
      writer.Flush()
      return
   }
   b = true
   return
}

The results are returned as a JSON string and need to be unmarshaled into proper structs.

validPlayers := make([]map[string]string, 0)

lotteryAccount := "NXT-YXC4-RB92-F6MQ-2ZRA6"

type Attachment struct {
   Message       string `json:"message"`
   MessageIsText bool   `json:"messageIsText"`
}

type Transaction struct {
   Timestamp   int        `json:"timestamp"`
   AmountNQT   string     `json:"amountNQT"`
   ID          string     `json:"transaction"`
   SenderRS    string     `json:"senderRS"`
   RecipientRS string     `json:"recipientRS"`
   Attached    Attachment `json:"attachment"`
}

type Response struct {
   Transactions []Transaction 
`json:"transactions"`
}
Query := 
"requestType=getBlockchainTransactions&account=" +
lotteryAccount + 
"&type=0&subtype=0&executedOnly=true"

if v, a := sendQuery(Query, true); a == true {
   var transactions Response
   if err := json.Unmarshal([]byte(v), 
&transactions); err != nil {
      fmt.Println(err)
   }

 p := 0
 for k, _ := range transactions.Transactions {
    // code to check tickets for validity.
    // if transaction satisfies all criteria 
    // add it to the slice of valid tickets.
		
    validPlayers = append(validPlayers, 
make(map[string]string))
    validPlayers[p][txSender] = lotteryNumbers
    p++
			
   }
}

Now that ‘validPlayers’ has all the good tickets we can start the game.

process() receives an integer (5, 4, or 3) and other parameters, including validPlayers and runs three rounds of the lottery. It makes a call to getWinners() function, that one calls genFive() to generate sequences of numbers until at least one winner is found. getWinners() returns results to process() which sends out reward, removes the winner ticket from eligible tickets and returns remaining tickets into main() for subsequent rounds. There is a helper function preparePlayers() which recreates validPlayers without the empty spots freed by the removed tickets.

I encourage every programmer to try coding on the Nxt blockchain. It is very easy with its rich API hooking into all the functionality of the core engine. https://nxtwiki.org/wiki/The_Nxt_API

My next app will probably be a poll app, with immutable records of votes saved into the blockchain. Do you think an app like that can find usage in the modern world? By the way, Nxt has its own built-in Voting. It’s too easy to forget what Nxt has, because it has so many features and all those features are accessible through API, kindly programmed by the core developers for userland. You can ‘mine’ your first NXT coins to send transactions in the Lucky node project running a public node, come to nxtforum.org and you will find out how.

Please leave your comments and suggestions.


1.
Lottery at nxtforum.org.

2.
To access Nxt Data Cloud, download and install the NRS (Nxt Reference Software 1.10.1) and search by ‘lottery’ keyword. Source code can also be downloaded from any of the public Open API Nxt servers, for instance:

Go: http://23.94.134.161:7876/nxt?requestType=downloadTaggedData&transaction=7872865106538381099&retrieve=true

PHP: http://23.94.134.161:7876/nxt?requestType=downloadTaggedData&transaction=13031806327722095646&retrieve=true
BACK TO POST.

Programming the Nxt Blockchain for fun and profit

Blockchains can be useful in various applications, facing global audience and supporting company internal infrastructure. A blockchain is a distributed database, a copy of the blockchain is stored on each node in a peer-to-peer network. This extreme redundancy can be duely considered inefficient, but please stay with me for a couple of minutes of blockchain theory.

As each node validates all transactions recorded into the blockchain and as past transactions cannot be rolled back or tampered with like in traditional RDBMS, this redundancy makes the blockchain *immutable*, and that is a very important value proposition of blockchains. Immutability of data is something traditional databases cannot provide. You may or may not need immutability of data and trust-free confirmation that data hasn’t been altered.

In this tutorial I assume you do need this.

One of the most versatile and flexible blockchains is the Nxt Blockchain (https://nxt.org). It has over a hundred API calls https://nxtwiki.org/wiki/The_Nxt_API.

Today you will learn basics of programming the Nxt blockchain. I will use only two API calls in this tutorial. With over a hundred API calls opportunities for programmers are limitless.

Application logic

A customer of a company or an employee in an organization uploads a file through a web form.
The file is renamed to a unique name and is saved in some location on the server.

A year later the customer / employee needs to verify, using the Nxt Blockchain, that the file hadn’t been altered. For instance, this may be required for legal purposes. It doesn’t have to be files. Internal company memos can be hashed and stored in the database, to be verified in a future audit.

Nxt allows us to send and record arbitrary messages (AM) into its blockchain.

Every transaction on the blockchain comes at a fee. If the transaction size is large, it can be expensive; fortunately Nxt has a subtype of AM called prunable messages. These are pruned after 90 days which make them cheap, they are available for retrieval from archival nodes after 90 days.

The maximum size of an arbitrary message in the Nxt blockchain is approx. 42 KB, the size of one block. A prunable message of 1 KB costs 1 NXT ($0.03). 1 KB is enough to store a hash of a file and this is our final cost to permanently record one hash in the immutable distributed Nxt blockchain.

As the customer uploads a file I create a SHA256 hash of the file and store the hash in the database in the organization’s server. For simplicity I’ve chosen SQlite, but you can use Mysql, Postgresql, Oracle. I will use PDO to access SQlite database in PHP.

When we do not use the immutable database (blockchain), the file can be modified, the new hash of the modified file saved in the database, making it hard to prove the file was like this from the beginning.

Blockchain comes to the rescue

Every prunable message can be retrieved from archival nodes. Every record in the blockchain is immutable. You can be sure the hash of the file you uploaded a year ago, when it is retrieved from the blockchain, is the same hash. All you need now is compare it with the hash in the organization’s internal RDBMS.

Pre-requisites:
PHP with curl, json and some db extension (I use sqlite3). A web server is optional, you can use php-cli. Java 8 (Oracle or OpenJDK to run Nxt). Nxt reference software: https://nxtforum.org/nrs-releases/nrs-v1-10-1/.

Install the NRS (Nxt Reference Software (depending on the context interchangeably called Nxt Client or Nxt Server)) and create an account. Fund it with a few coins. You can exchange Bitcoin to NXT at an exchange service like https://shapeshift.io or exchange with someone at https://nxtforum.org. It is also possible to “mine” some free NXT as reward for running a node; http://test.nxter.org/the-forging-bounty-lottery-campaign-will-pay-5-million-in-rewards-to-forgers-and-nxt-nodes/.

First we create a simple database table for our application, nothing fancy, you can add more column types if you must store more information. I like to use DB Browser for SQLite of http://sqlitebrowser.org.

Let’s make an empty database ‘files.db’ and save it in /home/lurker10/phptutorial/files.db

Using DB Browser for SQLite create the following table.

CREATE TABLE "files" (
`id` INTEGER PRIMARY KEY AUTOINCREMENT,
`txid` TEXT,
`hash` TEXT,
`uploadtime` TEXT,
`filename` TEXT
)

‘txid’ is the field for storing the id of a transaction we receive from Nxt when the transaction is accepted. It is unique. ‘hash’ is the sha256 hash of the file.

In this tutorial I skip the file upload part of code to keep it concise.

Let’s assume the file is already uploaded and stored on the web server. We define the file location variable in the code.

$uploadDir = "/home/lurker10/phptutorial/tmp/";
$fileName = "copy12345.tar";

By default the Nxt Server listens for API requests on port 7876. If you run it on the same machine as your php code, your code must send queries to http://127.0.0.1:7876/nxt

The other important variables are the passphrase of the Nxt account you’ve created and funded and the recipient’s account.

You can send the message to yourself, recipient can be your own account.

$host = "http://127.0.0.1:7876/nxt";
$secretPhrase = "your passphrase";
$recipientID = "NXT-XXXX-XXXX-XXXX-XXXXX";

The next part of code is the function that submits query using curl in a POST request.

To make a query we must define $payload and $payload_string variables and feed them to sendRequest(). It is possible to run the Nxt Server over HTTPS and use curl to verify the SSL certificate, but for this simple app we have disabled SSL verification in the curl connection.

Another point of interest is the $errorDescription, json-decoded from server response.

If there is a problem with the query (“Not enough funds” in your account when your balance is zero),
you must add error handling routine. I omit this too. For this app I assume the server responded properly, and return the response into the app for further processing.

function sendRequest($host, $payload, $payload_string) {
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $host);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT_MS, 10000);
curl_setopt($ch, CURLOPT_TIMEOUT_MS, 10000);
curl_setopt($ch, CURLOPT_POST, count($payload));
curl_setopt($ch, CURLOPT_POSTFIELDS, $payload_string);
$output = curl_exec($ch);
$curl_error = curl_error($ch);
curl_close($ch);
$errorDescription = trim(@json_decode($output)->errorDescription);
if ($errorDescription != "") { // perform error handling; return false; }
return $output;
}

As the file is uploaded already, I create a sha256 hash of the file and timestamp.

$fileHash = hash_file("sha256", $uploadDir.$fileName);
$txTime = time();

I will use PDO to work with the database.

Open the db and insert a new record.

We don’t know the txid until we talk to the Nxt Server that can give it to us when transaction is accepted on the NXT network, so for now I’ll insert null for txid.

$pdo = new PDO('sqlite:/home/lurker10/phptutorial/files.db');
$sql = "INSERT INTO files (txid, hash, uploadtime, filename)
VALUES (null, '$fileHash', '$txTime', '$fileName')";
$result = $pdo->exec($sql);

Next we create a query to send to the NXT server.

This particular query is “sendMessage”, you can find a whole lot more queries to interact with the blockchain and their mandatory and optional parameters at https://nxtwiki.org/wiki/The_Nxt_API.

As I said earlier the fee to transact is 1 NXT. 1 NXT = 100,000,000 NQT (nanoquants).
1 NQT is the smallest unit of denomination in NXT same as 1 satoshi in Bitcoin.
The Nxt Server accepts the fee in NQT, so we pay exactly 100 million NQT ($0.03)

The “broadcast” parameter can be changed to false, in this case you will receive ‘transactionBytes’ in the response, which can be broadcast to the network later using the ‘broadcastTransaction’ request. But today I have set it to ‘true’ to broadcast the transaction instantly.

Remember to urlencode() the message. I insert the filename separated from the hash with a colon into the message.

$payload = array(
"requestType" => "sendMessage",
"recipient" => $recipientID,
"secretPhrase" => urlencode($secretPhrase),
"feeNQT" => 100000000,
"deadline" => 1440,
"broadcast" => "true",
"message" => urlencode($fileName . ":" . $fileHash),
"messageIsPrunable" => "true"
);
$payload_string = "";
foreach ($payload as $key => $value) {
$payload_string .= $key . "=" . $value . "&";
}
rtrim($payload_string, "&");

Send the query to NXT server using sendRequest() function:

$output = sendRequest($host, $payload, $payload_string);

and decode the JSON response from the server to get the transaction id:

if ($output != false) {
$txId = json_decode($output)->transaction;
}

Now that there is a positive response on the accepted transaction and its ID is known, let’s update the record in the in-house db.

$lastId = $pdo->lastInsertId();
$sql = "UPDATE files SET txid = '$txId' where id = '$lastId'";
$result = $pdo->exec($sql);

We can optionally provide these links to the customer for future reference and to prove that the hash was uploaded:

echo "NXT Transaction ID: " . $txId . ",
JSON response";
echo "

Use these links to verify Sha256 hash of your file saved in our internal database against the permanent record in the NXT blockchain:

" . $fileHash;

Optionally email the customer the $txId that they can later use to verify hash or somehow else give them basic information on retrieval of the hash from in-house db and comparing it to blockchain stored hash in the future, by timestamp or other criteria.

This app doesn’t include user authentication. Normally the customer or user of an intranet service would be able to see their files after having authenticated to the site.

This app also assumes that the verification app is out of reach of the in-house database maintainer to prevent faking verification outcome.

Now the record is saved in the company’s database. Show the db record to confirm it’s there.

$sth = $pdo->prepare("SELECT id, txid, hash, uploadtime, filename FROM files ORDER BY id DESC");
$sth->execute();
$result = $sth->fetch(PDO::FETCH_OBJ);
if ($result != false) {
var_dump($result);
}

The verification app

To use the hash verification app the customer must have the transaction ID in the NXT blockchain
passed to them when the transaction was submitted to the Nxt Blockchain.

Suppose the customer does have it, saved in the mail archive or retrieved by other means.
Here it is.

$txId = "111111111111111111";

Let us see what our in-house database has for the hash of the file. Fetch and save it in $hashInDb.

$pdo = new PDO('sqlite:/home/lurker10/phptutorial/files.db');
$sth = $pdo->prepare("SELECT hash FROM files where txid = '$txId'");
$sth->execute();
$result = $sth->fetch(PDO::FETCH_OBJ);
if ($result != false) {
$hashInDb = $result->hash;
}

Send a request to the NXT server and fetch all information stored in the NXT blockchain for transaction with the given ID.

$payload = array (
"requestType" => "getTransaction",
"transaction" => $txId
);
$payload_string = "";
foreach ($payload as $key => $value) {
$payload_string .= $key . "=" . $value . "&";
}
rtrim($payload_string, "&");

$output = sendRequest($host, $payload, $payload_string);

Decode the JSON response and extract the attachment field where the hash is stored.

In the first part of the app we recorded the filename separated from the hash with a colon.
Now we extract just the hash portion of the arbitrary message.

$attachmentPlainData = json_decode($output)->attachment->message;
$hashInBlockchain = explode(":", $attachmentPlainData)[1];

And compare what we have in the company’s database with what was recorded 1 year ago into the Nxt Blockchain.

if ($hashInDb == $hashInBlockchain)
echo "Hashes are identical";
else
echo "Hashes are not identical";

Next part of this series: Lottery on the Nxt Blockchain in Golang (and PHP)

NXT-crypto-developer