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<title>Proof-of-work challenge</title>
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<main>
<header>
<h1>Proof-of-work challenge</h1>
</header>
<section>
<p>This site is protected by <a href="{{ .Global.SourceURL }}">Powxy</a>{{ if .Global.Version }} {{ .Global.Version }}{{ end }}.</p>
<p>You must complete this proof-of-work challenge before you can access this site.</p>
</section>
<section>
<p>Select a nonce no longer than 32 bytes, such that when it is appended to the decoded form of the challenge identifier, and the entire result is hashed with SHA-256, the first {{ .Global.NeedBits }} bits of the SHA-256 hash are all zeros. Within one octet, higher bits are considered to come before lower bits.</p>
<p>In other words, find an nonce <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>N</mi><annotation encoding="application/x-tex">N</annotation></semantics></math> with <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false" form="prefix">|</mo><mi>N</mi><mo stretchy="false" form="postfix">|</mo><mo>≤</mo><mn>32</mn><mspace width="0.167em"></mspace><mi mathvariant="normal">B</mi></mrow><annotation encoding="application/x-tex">\lvert N\rvert \le 32\,\mathrm{B}</annotation></semantics></math>, such that <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mi mathvariant="normal">S</mi><mi mathvariant="normal">H</mi><mi mathvariant="normal">A</mi><mn mathvariant="normal">256</mn></mrow><mrow><mo stretchy="true" form="prefix">(</mo><mi>I</mi><mspace width="0.167em"></mspace><mo stretchy="false" form="postfix">∥</mo><mspace width="0.167em"></mspace><mi>N</mi><mo stretchy="true" form="postfix">)</mo></mrow><mo><</mo><msup><mn>2</mn><mn>{{ .Global.NeedBitsReverse }}</mn></msup></mrow><annotation encoding="application/x-tex">\mathrm{SHA256}(I \,\|\, N) < 2^{{{ .Global.NeedBitsReverse }}}</annotation></semantics></math> in big endian, where <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>I</mi><annotation encoding="application/x-tex">I</annotation></semantics></math> is the identifier provided below.</p>
<label for="identifier">Challenge identifier (read-only)</label>
<pre id="identifier">{{ .Identifier }}</pre>
</section>
<section>
<form method="POST">
<p>Encode your selected nonce in base64 and submit it below.</p>
<p>Please note that if your submission is successful, you will be given a cookie that will allow you to access this site for a period of time without having to complete the challenge again. By pressing the submit button, you agree to be given cookies for this purpose.</p>
<label for="nonce">Nonce</label>
<input id="nonce" name="powxy" type="text" />
<input type="submit" value="Submit" />
</form>
</section>
{{- if .Message }}
<section>
<p><strong>{{ .Message }}</strong></p>
</section>
{{- end }}
<section>
<p class="somewhat-transparent" id="solver_status">JavaScript seems to be disabled. You must solve the challenge externally.</p>
</section>
<section>
<p class="somewhat-transparent"><a href="/.powxy/static/solver.c">A C implementation of the challenge solver</a> is available.</p>
</section>
</main>
<script>
document.addEventListener("DOMContentLoaded", async function() {
let challenge_b64 = "{{ .Identifier }}";
let difficulty = {{ .Global.NeedBits }};
let form = document.querySelector("form");
let field = form.querySelector("input[name='powxy']");
let status_el = document.getElementById("solver_status");
let identifier_bytes = Uint8Array.from(
atob(challenge_b64),
ch => ch.charCodeAt(0)
);
status_el.textContent = "Starting WebAssembly solver as a worker...";
let worker = new Worker("/.powxy/static/solver.js");
worker.onmessage = function(e) {
let { nonce, nonce_bytes } = e.data;
let nonce_str = String.fromCharCode(...nonce_bytes);
field.value = btoa(nonce_str);
status_el.textContent = "Challenge solved automatically in " + nonce + " iterations";
};
worker.postMessage({ identifier_bytes, difficulty });
});
</script>
</body>
</html>