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<li class="toctree-l2 current"><a class="current reference internal" href="#">Signatures: Ed25519</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#key-generation">Key Generation</a></li>
<li class="toctree-l3"><a class="reference internal" href="#eddsa-signing">EdDSA Signing</a></li>
<li class="toctree-l3"><a class="reference internal" href="#eddsa-verify">EdDSA Verify</a></li>
<li class="toctree-l3"><a class="reference internal" href="#eddsa-sign-expanded">EdDSA Sign Expanded</a></li>
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<div class="section" id="signatures-ed25519">
<span id="hacl-ed25519"></span><h1>Signatures: Ed25519<a class="headerlink" href="#signatures-ed25519" title="Permalink to this headline">¶</a></h1>
<p>HACL* implements the Edwards-Curve Digital Signature Algorithm (EdDSA)
construction for the Ed25519 elliptic curve as specified in <a class="reference external" href="https://tools.ietf.org/html/rfc8032">IETF RFC 8032</a>.
The API for this signature algorithm is in <code class="docutils literal notranslate"><span class="pre">Hacl_Ed25519.h</span></code>.</p>
<div class="section" id="key-generation">
<h2>Key Generation<a class="headerlink" href="#key-generation" title="Permalink to this headline">¶</a></h2>
<p>Any 32 byte array can be used as an Ed25519 private key. In
practice, private keys should be generated using a cryptographically
strong pseudo-random number generator (CSPRNG). In some cases, the
private key may be derived as the result of a key derivation function
such as HKDF.</p>
<p>Given a private key, the corresponding public key can be computed
using the <code class="docutils literal notranslate"><span class="pre">secret_to_public</span></code> function:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">Hacl_Ed25519_secret_to_public</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">pub</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">priv</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>The first argument is a pointer to the output public key <code class="docutils literal notranslate"><span class="pre">pub</span></code> (64 bytes);
the second argument is a pointer to the input private key <code class="docutils literal notranslate"><span class="pre">priv</span></code> (32 bytes).</p>
</div>
<div class="section" id="eddsa-signing">
<h2>EdDSA Signing<a class="headerlink" href="#eddsa-signing" title="Permalink to this headline">¶</a></h2>
<p>The signature operation is implemented by the following function:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">Hacl_Ed25519_sign</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">signature</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">priv</span><span class="p">,</span><span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">len</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">msg</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>The first argument is a pointer to the output signature <code class="docutils literal notranslate"><span class="pre">signature</span></code>;
the second argument is the private key of the signer <code class="docutils literal notranslate"><span class="pre">priv</span></code>;
the third argument is the length <code class="docutils literal notranslate"><span class="pre">len</span></code> of the message to be signed <code class="docutils literal notranslate"><span class="pre">msg</span></code>.
The size of <code class="docutils literal notranslate"><span class="pre">signature</span></code> must be (at least) 64 bytes; the size of the private
key is 32 bytes.</p>
</div>
<div class="section" id="eddsa-verify">
<h2>EdDSA Verify<a class="headerlink" href="#eddsa-verify" title="Permalink to this headline">¶</a></h2>
<p>To verify an Ed25519 signature, one may call the following function:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">bool</span><span class="w"> </span><span class="nf">Hacl_Ed25519_verify</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">pub</span><span class="p">,</span><span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">len</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">msg</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">signature</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>The first argument is a pointer to the public key <code class="docutils literal notranslate"><span class="pre">pub</span></code> (64 bytes);
the second argument is the length <code class="docutils literal notranslate"><span class="pre">len</span></code> of the message to be signed <code class="docutils literal notranslate"><span class="pre">msg</span></code>;
the last argument is the input signature <code class="docutils literal notranslate"><span class="pre">signature</span></code>.
If the signature verification succeeds the function returns the boolean <code class="docutils literal notranslate"><span class="pre">true</span></code>,
otherwise it returns <code class="docutils literal notranslate"><span class="pre">false</span></code>.</p>
</div>
<div class="section" id="eddsa-sign-expanded">
<h2>EdDSA Sign Expanded<a class="headerlink" href="#eddsa-sign-expanded" title="Permalink to this headline">¶</a></h2>
<p>In situations where a signer needs to sign many times with the same
signature key, a part of the signature computation can be shared
between these invocations for efficiency. The caller first calls
<code class="docutils literal notranslate"><span class="pre">Hacl_Ed25519_expand_keys</span></code> to compute an expanded signing key <code class="docutils literal notranslate"><span class="pre">ks</span></code>,
and then can use <code class="docutils literal notranslate"><span class="pre">ks</span></code> to call <code class="docutils literal notranslate"><span class="pre">Hacl_Ed25519_sign_expanded</span></code> multiple
times with different arguments.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">Hacl_Ed25519_expand_keys</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">ks</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">priv</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">Hacl_Ed25519_sign_expanded</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">signature</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">ks</span><span class="p">,</span><span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">len</span><span class="p">,</span><span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">msg</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
</div>
<div class="section" id="other-signature-algorithms-ecdsa-with-p-256">
<h2>Other Signature Algorithms: ECDSA with P-256<a class="headerlink" href="#other-signature-algorithms-ecdsa-with-p-256" title="Permalink to this headline">¶</a></h2>
<p>A development branch includes a verified implementation of ECDSA
signatures with the P-256 elliptic curve, which has not yet been
merged to master. Contact the HACL* maintainers if you wish to use
this code.</p>
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