toychain/docs/pow.md

---
layout: simple
---
# Proof of Work

We use a [hashcash][1] type of proof of work (see also [Adam Back][ab] annonce)
where the difficulities is set to <span id=dif><i>:dif</i></span>.


hash algo: <input name="algo" value="SHA256" size="7">
<br>data: <input name="data" value="some data" size=24>
<!-- perl -Ilib -Mmisc -e 'print $misc::inonce'
5: 12131054599640000000
6: 12131054600130000000
8: 12131054604670000000
9: 12131054702320000000
-->
<br>nonce: <input name="nonce" value="12131054702320000000" size=20>
<br>difficulty: <input name="dif" value="7" size="2">
<br>hashcash: <span name="hashcash"><i>?</i></span>
<br>proof: <span id="pow"><i>:pow</i></span>
<button>compute</button>

--- 

## note:

 - perl hashcash : [/lib/Chain.pm#hashcash](/lib/Chain.pm#hashcash)
 - original hashcash's [gi(s)t]: &lt;[gist:8748be59087e244f916618ac2d66ae3b][hologit]> (SHA1 based)

### code
```perl
# -----------------------------------------------------
sub hashcash { # ex: hashcash('SHA256',$data,$nonce,7);
  my $alg = shift;
  my $difficulty = pop; # difficulty < 8
  my $nonce = pop; # 64 bits of nonce
  my $n = $nonce;
  printf "hashcash.nonce: %s\n",$nonce;
  printf "hashcash.difficulty: %s\n",$difficulty;
  #printf "data: %s\n",join'',@_;
  my $l = 0;
  my $match = '8'.substr('0'x$difficulty,0,$l); # set the MSB to 1 to avoid padding problems
  my $iv;
  use Crypt::Digest qw();
  my $msg = Crypt::Digest->new($alg) or die $!;
     $msg->add(join'',@_);
  #printf "iv: %s\n",$msg->hexdigest();
  my $h16;
  my $pn = pack'Q',$nonce; # Quad
  while (1) { # length($pn) < $difficulty) {
     $iv = $msg->clone;
     $iv->add($pn);
     $h16 = $iv->hexdigest();
     if (substr($h16,0,$l+1) eq $match) {
       my $elapse = time - $^T + 1;
       if ($0 =~ m/\.t$/) { $elapse = 3; }
       my $rate = ($n - $nonce) / $elapse / 1000;
       #printf "%d: %s %s %s %.0fkH/s %.1fmin\n",$l, $n,unpack('H*',$pn),$h16,$rate,$elapse/60;
       # count any zeros : ($h16 =~ m/0/g) !
       my $zc = ($h16 =~ /80*/) ? length($&) : 0;
       #print "zc: $zc\n";
       $l = $zc;
       last if $zc >= $difficulty;
       $match = '8'.substr('0'x$l,0,$l);
       #} else {
       # printf "%d: %s %s %s %s\r",$l, $n,unpack('H*',$pn),$h16,$match;
     }
     $pn = pack('Q',$n++);
  }
  return pack('H*',$h16),$n-1;
}
# -----------------------------------------------------


```

[1]: http://www.hashcash.org/papers/hashcash.pdf
[ab]: http://www.hashcash.org/papers/announce.txt
[2]: https://www.cs.jhu.edu/~rdas/finalreport.pdf
[3]: https://www.arijuels.com/wp-content/uploads/2013/09/JB99.pdf
[4]: http://pubs.sciepub.com/jcsa/5/2/2/index.html
[4d]: http://www.sciepub.com/portal/downloads?doi=10.12691/jcsa-5-2-2&filename=jcsa-5-2-2.pdf
[hologit]: https://hologit-ml.ipns.cf-ipfs.com/cbuquo/hashcash.git
[gist]: git@gist.github.com:8748be59087e244f916618ac2d66ae3b.git



<script src="pow.js"></script>