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<div class="section" id="hashing-evercrypt-hash-h">
<h1>Hashing (<code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash.h</span></code>)<a class="headerlink" href="#hashing-evercrypt-hash-h" title="Permalink to this headline">¶</a></h1>
<p>This API is:</p>
<ul class="simple">
<li><strong>agile</strong></li>
<li><strong>multiplexing</strong>: portable C (all); SHAEXT (SHA2-224, SHA2-256)</li>
<li><strong>stateful</strong></li>
</ul>
<p>Possible values for the agility argument (<code class="docutils literal notranslate"><span class="pre">Hacl_Spec.h</span></code>) :</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="cp">#define Spec_Hash_Definitions_SHA2_224 0</span>
<span class="cp">#define Spec_Hash_Definitions_SHA2_256 1</span>
<span class="cp">#define Spec_Hash_Definitions_SHA2_384 2</span>
<span class="cp">#define Spec_Hash_Definitions_SHA2_512 3</span>
<span class="cp">#define Spec_Hash_Definitions_SHA1 4</span>
<span class="cp">#define Spec_Hash_Definitions_MD5 5</span>
<span class="cp">#define Spec_Hash_Definitions_Blake2S 6</span>
<span class="cp">#define Spec_Hash_Definitions_Blake2B 7</span>
</pre></div>
</div>
<p>Supported values for the agility argument: all</p>
<div class="section" id="block-based-api">
<h2>Block-based API<a class="headerlink" href="#block-based-api" title="Permalink to this headline">¶</a></h2>
<p>The block-based functions require the client to follow this exact state
machine:</p>
<ol class="arabic simple">
<li>one call to <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_create_in</span></code></li>
<li>one call to <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_init</span></code></li>
<li>any number of calls to: <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_update</span></code> (passing data <strong>exactly</strong> of
the right block size for the chosen algorithm) or
<code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_update_multi</span></code> (passing a multiple of the block size)</li>
<li>one call to <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_update_last</span></code> (data strictly smaller than the
block size)</li>
<li>one call to <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_finish</span></code></li>
<li>one call to <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_free</span></code></li>
</ol>
<p>Clients may jump to state 2 at any point before state 6.</p>
<p>As evidenced by the state machine, this API requires clients to buffer data and
chunk it along the block size (“block-based”). Furthermore, this API does
not allow feeding more data into the hash function after <code class="docutils literal notranslate"><span class="pre">update_last</span></code> has
been called: the client must either proceed with <code class="docutils literal notranslate"><span class="pre">finish</span></code> or reset the state
with <code class="docutils literal notranslate"><span class="pre">init</span></code>.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">This API is error-prone and is not recommended for unverified clients. We
advise clients use the streaming API described below.</p>
</div>
</div>
<div class="section" id="streaming-api">
<h2>Streaming API<a class="headerlink" href="#streaming-api" title="Permalink to this headline">¶</a></h2>
<p>The streaming hash API wraps the block-based API with an internal buffer
(at the expense of an extra indirection) and relieves the client from having to
perform modulo computations and block-size management. Furthermore, the
block-based API allows extracting intermediary hashes without invalidating the
state, meaning clients can compute a hash, then later feed more data into the
hash function (at the expense of a state copy).</p>
<p>Clients can allocate state via <code class="docutils literal notranslate"><span class="pre">create_in</span></code>. This is a non-faillible function
and as such does not return an error code. This is possible because i) we always
have a fallback portable C implementation available for all algorithms and ii)
we do not yet model allocation failures.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="n">Hacl_Streaming_Functor_state_s___EverCrypt_Hash_state_s____</span><span class="w"></span>
<span class="o">*</span><span class="nf">EverCrypt_Hash_Incremental_create_in</span><span class="p">(</span><span class="n">Spec_Hash_Definitions_hash_alg</span><span class="w"> </span><span class="n">a</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>Clients can feed an arbitrary amount of data via <code class="docutils literal notranslate"><span class="pre">update</span></code>:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"></span>
<span class="nf">EverCrypt_Hash_Incremental_update</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">Hacl_Streaming_Functor_state_s___EverCrypt_Hash_state_s____</span><span class="w"> </span><span class="o">*</span><span class="n">p</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">data</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">len</span><span class="w"></span>
<span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>A hash can be extracted at any time by the client without invalidating the
state:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"></span>
<span class="nf">EverCrypt_Hash_Incremental_finish_md5</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">Hacl_Streaming_Functor_state_s___EverCrypt_Hash_state_s____</span><span class="w"> </span><span class="o">*</span><span class="n">p</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">dst</span><span class="w"></span>
<span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>Once done, clients should use <code class="docutils literal notranslate"><span class="pre">free</span></code> which frees all internal buffers and
underlying block-based state:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"></span>
<span class="nf">EverCrypt_Hash_Incremental_free</span><span class="p">(</span><span class="n">Hacl_Streaming_Functor_state_s___EverCrypt_Hash_state_s____</span><span class="w"> </span><span class="o">*</span><span class="n">s</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">There is no streaming HACL* API for hashes, i.e. clients must go through
agility and multiplexing to enjoy the streaming hash API.</p>
</div>
</div>
<div class="section" id="one-shot-api">
<h2>One-shot API<a class="headerlink" href="#one-shot-api" title="Permalink to this headline">¶</a></h2>
<p>If all data is available at once, clients can use the (slightly more efficient),
agile, multiplexing <code class="docutils literal notranslate"><span class="pre">EverCrypt_Hash_hash</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">EverCrypt_Hash_hash</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">Spec_Hash_Definitions_hash_alg</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">dst</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">input</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">len</span><span class="w"></span>
<span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>This function merely dispatches onto one of the numerous non-agile specialized
variants. As such, the cost of agility is one test.</p>
<p>For SHA2-256 and SHA2-224, the EverCrypt API provides non-agile, multiplexing
variants:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">EverCrypt_Hash_hash_256</span><span class="p">(</span><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">input</span><span class="p">,</span><span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">input_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">dst</span><span class="p">);</span><span class="w"></span>
</pre></div>
</div>
<p>For other hash algorithms, for which only one implementation (portable C) is
currently available, clients can use the non-agile, non-multiplexing HACL Hash
API.</p>
</div>
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