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39 lines
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<title>Crypto.verify_hmac — Ludic</title>
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<meta name="description" content="Constant-time check that a MAC matches.">
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<header class="nav"><div class="wrap nav-in"><a class="brand" href="index.html"><span class="logo">L</span> Ludic</a><button class="nav-toggle" aria-label="Toggle menu" aria-expanded="false">☰</button><nav class="nav-links"><a href="index.html">Home</a><a href="api.html">API Reference</a><a class="nav-cta" href="https://git.workshopsoft.io/workshopsoft/ludic">Source ↗</a></nav></div></header>
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<div class="crumbs"><a href="api.html">API Reference</a> <span>›</span> <a href="ns-crypto.html">Crypto</a> <span>›</span> <span class="here">Crypto.verify_hmac</span></div>
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<div class="item-head">
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<span class="kind-badge kind-method">method</span>
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<h1 id="top">Crypto.verify_hmac</h1>
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</div>
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<code class="sig">Crypto.verify_hmac(key, msg, mac) -> bool</code>
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<div class="desc"><p>Recomputes HMAC-SHA256(<code>key</code>, <code>msg</code>) and compares it to the supplied <code>mac</code> hex string, returning <code>true</code> only if they match. The comparison is <em>constant-time</em>: it never stops early on the first differing character, so it does not leak — through how long the check took — how many leading bytes of a forged tag happened to be right. That leak is exactly what lets an attacker guess a MAC one byte at a time, which is why you should always verify with this and never with <code>==</code>. A mismatched length returns <code>false</code> immediately (the length of a MAC is not a secret).</p></div>
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<div class="params"><h2>Parameters</h2><div class="param" id="param-key"><code class="pname">key</code><span class="pdesc">the shared secret used to sign</span></div><div class="param" id="param-msg"><code class="pname">msg</code><span class="pdesc">the payload as received</span></div><div class="param" id="param-mac"><code class="pname">mac</code><span class="pdesc">the hex tag to check, e.g. from <a href="crypto-hmac_sha256"><code>Crypto.hmac_sha256</code></a></span></div></div>
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<div class="examples"><h2>Example</h2><pre data-lang="ludic">program CheckSave {
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entry {
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let key = "s3cret"
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let payload = "score=9001;level=12"
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let mac = Crypto.hmac_sha256(key, payload)
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if Crypto.verify_hmac(key, payload, mac) {
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print(1) # untampered
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} else {
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print(0) # altered or forged
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}
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}
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}</pre></div>
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