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<div class="section" id="key-agreement">
<span id="id1"></span><h1>10.9. Key agreement</h1>
<div class="section" id="key-agreement-algorithms">
<span id="id2"></span><h2>10.9.1. Key agreement algorithms</h2>
<div class="section" id="PSA_ALG_KEY_AGREEMENT">
<span id="c.PSA_ALG_KEY_AGREEMENT"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT</span></code> (macro)</h3>
<p>Macro to build a combined algorithm that chains a key agreement with a key derivation.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT">PSA_ALG_KEY_AGREEMENT</a>(ka_alg, kdf_alg) \
<em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">ka_alg</span></code></dt><dd><p>A key agreement algorithm (<code class="docutils literal notranslate"><span class="pre">PSA_ALG_XXX</span></code> value such that <a class="reference internal" href="algorithms.html#c.PSA_ALG_IS_KEY_AGREEMENT" title="PSA_ALG_IS_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">ka_alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true).</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">kdf_alg</span></code></dt><dd><p>A key derivation algorithm (<code class="docutils literal notranslate"><span class="pre">PSA_ALG_XXX</span></code> value such that <a class="reference internal" href="algorithms.html#c.PSA_ALG_IS_KEY_DERIVATION" title="PSA_ALG_IS_KEY_DERIVATION"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_KEY_DERIVATION</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">kdf_alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p>The corresponding key agreement and derivation algorithm.</p>
<p>Unspecified if <code class="docutils literal notranslate"><span class="pre">ka_alg</span></code> is not a supported key agreement algorithm or <code class="docutils literal notranslate"><span class="pre">kdf_alg</span></code> is not a supported key derivation algorithm.</p>
<p class="rubric">Description</p>
<p>A combined key agreement algorithm is used with a multi-part key derivation operation, using a call to <a class="reference internal" href="#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a>.</p>
<p>The component parts of a key agreement algorithm can be extracted using <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_BASE" title="PSA_ALG_KEY_AGREEMENT_GET_BASE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_BASE()</span></code></a> and <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_KDF" title="PSA_ALG_KEY_AGREEMENT_GET_KDF"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_KDF()</span></code></a>.</p>
</div>
<div class="section" id="PSA_ALG_FFDH">
<span id="c.PSA_ALG_FFDH"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_FFDH</span></code> (macro)</h3>
<p>The finite-field Diffie-Hellman (DH) key agreement algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_FFDH" title="PSA_ALG_FFDH">PSA_ALG_FFDH</a> ((<a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t">psa_algorithm_t</a>)0x09010000)</pre>
<p>This algorithm can be used directly in a call to <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a>, or combined with a key derivation operation using <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT()</span></code></a> for use with <a class="reference internal" href="#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a>.</p>
<p>When used as part of a multi-part key derivation operation, this implements a Diffie-Hellman key agreement scheme using a single Diffie-Hellman key-pair for each participant. This includes the <em>dhEphem</em>, <em>dhOneFlow</em>, and <em>dhStatic</em> schemes. The input step <a class="reference internal" href="kdf.html#c.PSA_KEY_DERIVATION_INPUT_SECRET" title="PSA_KEY_DERIVATION_INPUT_SECRET"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_INPUT_SECRET</span></code></a> is used when providing the secret and peer keys to the operation.</p>
<p>The shared secret produced by this key agreement algorithm is <code class="docutils literal notranslate"><span class="pre">g^{ab}</span></code> in big-endian format. It is <code class="docutils literal notranslate"><span class="pre">ceiling(m</span> <span class="pre">/</span> <span class="pre">8)</span></code> bytes long where <code class="docutils literal notranslate"><span class="pre">m</span></code> is the size of the prime <code class="docutils literal notranslate"><span class="pre">p</span></code> in bits.</p>
<p>This key agreement scheme is defined by <span><em>NIST Special Publication 800-56A: Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography</em> <a class="reference internal" href="../../about.html#citation-sp800-56a"><span class="cite">[SP800-56A]</span></a></span> §5.7.1.1 under the name FFC DH.</p>
</div>
<div class="section" id="PSA_ALG_ECDH">
<span id="c.PSA_ALG_ECDH"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_ECDH</span></code> (macro)</h3>
<p>The elliptic curve Diffie-Hellman (ECDH) key agreement algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_ECDH" title="PSA_ALG_ECDH">PSA_ALG_ECDH</a> ((<a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t">psa_algorithm_t</a>)0x09020000)</pre>
<p>This algorithm can be used directly in a call to <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a>, or combined with a key derivation operation using <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT()</span></code></a> for use with <a class="reference internal" href="#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a>.</p>
<p>When used as part of a multi-part key derivation operation, this implements a Diffie-Hellman key agreement scheme using a single elliptic curve key-pair for each participant. This includes the <em>Ephemeral unified model</em>, the <em>Static unified model</em>, and the <em>One-pass Diffie-Hellman</em> schemes. The input step <a class="reference internal" href="kdf.html#c.PSA_KEY_DERIVATION_INPUT_SECRET" title="PSA_KEY_DERIVATION_INPUT_SECRET"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_INPUT_SECRET</span></code></a> is used when providing the secret and peer keys to the operation.</p>
<p>The shared secret produced by key agreement is the x-coordinate of the shared secret point. It is always <code class="docutils literal notranslate"><span class="pre">ceiling(m</span> <span class="pre">/</span> <span class="pre">8)</span></code> bytes long where <code class="docutils literal notranslate"><span class="pre">m</span></code> is the bit size associated with the curve, i.e. the bit size of the order of the curve’s coordinate field. When <code class="docutils literal notranslate"><span class="pre">m</span></code> is not a multiple of 8, the byte containing the most significant bit of the shared secret is padded with zero bits. The byte order is either little-endian or big-endian depending on the curve type.</p>
<ul>
<li><p>For Montgomery curves (curve family <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_MONTGOMERY" title="PSA_ECC_FAMILY_MONTGOMERY"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_MONTGOMERY</span></code></a>), the shared secret is the x-coordinate of <code class="docutils literal notranslate"><span class="pre">Z</span> <span class="pre">=</span> <span class="pre">d_A</span> <span class="pre">Q_B</span> <span class="pre">=</span> <span class="pre">d_B</span> <span class="pre">Q_A</span></code> in little-endian byte order.</p>
<ul class="simple">
<li><p>For Curve25519, this is the X25519 function defined in <span><em>Curve25519: new Diffie-Hellman speed records</em> <a class="reference internal" href="../../about.html#citation-curve25519"><span class="cite">[Curve25519]</span></a></span>. The bit size <code class="docutils literal notranslate"><span class="pre">m</span></code> is 255.</p></li>
<li><p>For Curve448, this is the X448 function defined in <span><em>Ed448-Goldilocks, a new elliptic curve</em> <a class="reference internal" href="../../about.html#citation-curve448"><span class="cite">[Curve448]</span></a></span>. The bit size <code class="docutils literal notranslate"><span class="pre">m</span></code> is 448.</p></li>
</ul>
</li>
<li><p>For Weierstrass curves (curve families <code class="docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_SECP_XX</span></code>, <code class="docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_SECT_XX</span></code>, <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_BRAINPOOL_P_R1" title="PSA_ECC_FAMILY_BRAINPOOL_P_R1"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_BRAINPOOL_P_R1</span></code></a> and <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_FRP" title="PSA_ECC_FAMILY_FRP"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_FRP</span></code></a>) the shared secret is the x-coordinate of <code class="docutils literal notranslate"><span class="pre">Z</span> <span class="pre">=</span> <span class="pre">h</span> <span class="pre">d_A</span> <span class="pre">Q_B</span> <span class="pre">=</span> <span class="pre">h</span> <span class="pre">d_B</span> <span class="pre">Q_A</span></code> in big-endian byte order. This is the Elliptic Curve Cryptography Cofactor Diffie-Hellman primitive defined by <span><em>SEC 1: Elliptic Curve Cryptography</em> <a class="reference internal" href="../../about.html#citation-sec1"><span class="cite">[SEC1]</span></a></span> §3.3.2 as, and also as ECC CDH by <span><em>NIST Special Publication 800-56A: Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography</em> <a class="reference internal" href="../../about.html#citation-sp800-56a"><span class="cite">[SP800-56A]</span></a></span> §5.7.1.2.</p>
<ul class="simple">
<li><p>Over prime fields (curve families <code class="docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_SECP_XX</span></code>, <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_BRAINPOOL_P_R1" title="PSA_ECC_FAMILY_BRAINPOOL_P_R1"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_BRAINPOOL_P_R1</span></code></a> and <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_FRP" title="PSA_ECC_FAMILY_FRP"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_FRP</span></code></a>), the bit size is <code class="docutils literal notranslate"><span class="pre">m</span> <span class="pre">=</span> <span class="pre">ceiling(log_2(p))</span></code> for the field <code class="docutils literal notranslate"><span class="pre">F_p</span></code>.</p></li>
<li><p>Over binary fields (curve families <code class="docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_SECT_XX</span></code>), the bit size is <code class="docutils literal notranslate"><span class="pre">m</span></code> for the field <code class="docutils literal notranslate"><span class="pre">F_{2^m}</span></code>.</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The cofactor Diffie-Hellman primitive is equivalent to the standard elliptic curve Diffie-Hellman calculation <code class="docutils literal notranslate"><span class="pre">Z</span> <span class="pre">=</span> <span class="pre">d_A</span> <span class="pre">Q_B</span> <span class="pre">=</span> <span class="pre">d_B</span> <span class="pre">Q_A</span></code> (<a class="reference internal" href="../../about.html#citation-sec1"><span class="cite psa_c psa_c-cite">[SEC1]</span></a> §3.3.1) for curves where the cofactor <code class="docutils literal notranslate"><span class="pre">h</span></code> is <code class="docutils literal notranslate"><span class="pre">1</span></code>. This is true for all curves in the <code class="docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_SECP_XX</span></code>, <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_BRAINPOOL_P_R1" title="PSA_ECC_FAMILY_BRAINPOOL_P_R1"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_BRAINPOOL_P_R1</span></code></a>, and <a class="reference internal" href="../keys/types.html#c.PSA_ECC_FAMILY_FRP" title="PSA_ECC_FAMILY_FRP"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ECC_FAMILY_FRP</span></code></a> families.</p>
</div>
</li>
</ul>
</div>
</div>
<div class="section" id="standalone-key-agreement">
<h2>10.9.2. Standalone key agreement</h2>
<div class="section" id="psa_raw_key_agreement">
<span id="c.psa_raw_key_agreement"></span><h3><code class="docutils literal notranslate"><span class="pre">psa_raw_key_agreement</span></code> (function)</h3>
<p>Perform a key agreement and return the raw shared secret.</p>
<pre class="literal-block"><a class="reference internal" href="../library/status.html#c.psa_status_t" title="psa_status_t">psa_status_t</a> <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement">psa_raw_key_agreement</a>(<a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t">psa_algorithm_t</a> alg,
<a class="reference internal" href="../keys/ids.html#c.psa_key_id_t" title="psa_key_id_t">psa_key_id_t</a> private_key,
const uint8_t * peer_key,
size_t peer_key_length,
uint8_t * output,
size_t output_size,
size_t * output_length);</pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>The key agreement algorithm to compute (<code class="docutils literal notranslate"><span class="pre">PSA_ALG_XXX</span></code> value such that <a class="reference internal" href="#c.PSA_ALG_IS_RAW_KEY_AGREEMENT" title="PSA_ALG_IS_RAW_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_RAW_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true).</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">private_key</span></code></dt><dd><p>Identifier of the private key to use.
It must allow the usage <a class="reference internal" href="../keys/policy.html#c.PSA_KEY_USAGE_DERIVE" title="PSA_KEY_USAGE_DERIVE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_USAGE_DERIVE</span></code></a>.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">peer_key</span></code></dt><dd><p>Public key of the peer. It must be in the same format that <a class="reference internal" href="../keys/management.html#c.psa_import_key" title="psa_import_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_import_key()</span></code></a> accepts. The standard formats for public keys are documented in the documentation of <a class="reference internal" href="../keys/management.html#c.psa_export_public_key" title="psa_export_public_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_export_public_key()</span></code></a>.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">peer_key_length</span></code></dt><dd><p>Size of <code class="docutils literal notranslate"><span class="pre">peer_key</span></code> in bytes.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">output</span></code></dt><dd><p>Buffer where the raw shared secret is to be written.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">output_size</span></code></dt><dd><p>Size of the <code class="docutils literal notranslate"><span class="pre">output</span></code> buffer in bytes.
This must be appropriate for the keys:</p>
<ul class="simple">
<li><p>The required output size is <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE"><code class="docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">type</span></code><code class="docutils literal notranslate"><span class="pre">,</span> </code><code class="docutils literal notranslate"><span class="pre">bits</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> where <code class="docutils literal notranslate"><span class="pre">type</span></code> is the type of <code class="docutils literal notranslate"><span class="pre">private_key</span></code> and <code class="docutils literal notranslate"><span class="pre">bits</span></code> is the bit-size of either <code class="docutils literal notranslate"><span class="pre">private_key</span></code> or the <code class="docutils literal notranslate"><span class="pre">peer_key</span></code>.</p></li>
<li><p><a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE</span></code></a> evaluates to the maximum output size of any supported raw key agreement algorithm.</p></li>
</ul>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">output_length</span></code></dt><dd><p>On success, the number of bytes that make up the returned output.</p>
</dd>
</dl>
<p class="rubric">Returns: <a class="reference internal" href="../library/status.html#c.psa_status_t" title="psa_status_t"><code class="docutils literal notranslate"><span class="pre">psa_status_t</span></code></a></p>
<dl class="simple">
<dt> <a class="reference internal" href="../library/status.html#c.PSA_SUCCESS" title="PSA_SUCCESS"><code class="docutils literal notranslate"><span class="pre">PSA_SUCCESS</span></code></a></dt><dd><p>Success.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_HANDLE" title="PSA_ERROR_INVALID_HANDLE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_HANDLE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_NOT_PERMITTED" title="PSA_ERROR_NOT_PERMITTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_NOT_PERMITTED</span></code></a></dt><dd><p>The key does not have the <a class="reference internal" href="../keys/policy.html#c.PSA_KEY_USAGE_DERIVE" title="PSA_KEY_USAGE_DERIVE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_USAGE_DERIVE</span></code></a> flag, or it does not permit the requested algorithm.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_ARGUMENT" title="PSA_ERROR_INVALID_ARGUMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_ARGUMENT</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a key agreement algorithm</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_ARGUMENT" title="PSA_ERROR_INVALID_ARGUMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_ARGUMENT</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">private_key</span></code> is not compatible with <code class="docutils literal notranslate"><span class="pre">alg</span></code>, or <code class="docutils literal notranslate"><span class="pre">peer_key</span></code> is not valid for <code class="docutils literal notranslate"><span class="pre">alg</span></code> or not compatible with <code class="docutils literal notranslate"><span class="pre">private_key</span></code>.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_BUFFER_TOO_SMALL" title="PSA_ERROR_BUFFER_TOO_SMALL"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_BUFFER_TOO_SMALL</span></code></a></dt><dd><p>The size of the <code class="docutils literal notranslate"><span class="pre">output</span></code> buffer is too small.
<a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE()</span></code></a> or <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE</span></code></a> can be used to determine the required buffer size.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_NOT_SUPPORTED" title="PSA_ERROR_NOT_SUPPORTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_NOT_SUPPORTED</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a supported key agreement algorithm.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INSUFFICIENT_MEMORY" title="PSA_ERROR_INSUFFICIENT_MEMORY"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INSUFFICIENT_MEMORY</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_COMMUNICATION_FAILURE" title="PSA_ERROR_COMMUNICATION_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_COMMUNICATION_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_HARDWARE_FAILURE" title="PSA_ERROR_HARDWARE_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_HARDWARE_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_CORRUPTION_DETECTED" title="PSA_ERROR_CORRUPTION_DETECTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_CORRUPTION_DETECTED</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_STORAGE_FAILURE" title="PSA_ERROR_STORAGE_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_STORAGE_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_DATA_CORRUPT" title="PSA_ERROR_DATA_CORRUPT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_DATA_CORRUPT</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_DATA_INVALID" title="PSA_ERROR_DATA_INVALID"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_DATA_INVALID</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_BAD_STATE" title="PSA_ERROR_BAD_STATE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_BAD_STATE</span></code></a></dt><dd><p>The library has not been previously initialized by <a class="reference internal" href="../library/library.html#c.psa_crypto_init" title="psa_crypto_init"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_crypto_init()</span></code></a>. It is implementation-dependent whether a failure to initialize results in this error code.</p>
</dd>
</dl>
<p class="rubric">Description</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The raw result of a key agreement algorithm such as finite-field Diffie-Hellman or elliptic curve Diffie-Hellman has biases, and is not suitable for use as key material. Instead it is recommended that the result is used as input to a key derivation algorithm. To chain a key agreement with a key derivation, use <a class="reference internal" href="#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a> and other functions from the key derivation interface.</p>
</div>
</div>
</div>
<div class="section" id="combining-key-agreement-and-key-derivation">
<h2>10.9.3. Combining key agreement and key derivation</h2>
<div class="section" id="psa_key_derivation_key_agreement">
<span id="c.psa_key_derivation_key_agreement"></span><h3><code class="docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement</span></code> (function)</h3>
<p>Perform a key agreement and use the shared secret as input to a key derivation.</p>
<pre class="literal-block"><a class="reference internal" href="../library/status.html#c.psa_status_t" title="psa_status_t">psa_status_t</a> <a class="reference internal" href="#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement">psa_key_derivation_key_agreement</a>(<a class="reference internal" href="kdf.html#c.psa_key_derivation_operation_t" title="psa_key_derivation_operation_t">psa_key_derivation_operation_t</a> * operation,
<a class="reference internal" href="kdf.html#c.psa_key_derivation_step_t" title="psa_key_derivation_step_t">psa_key_derivation_step_t</a> step,
<a class="reference internal" href="../keys/ids.html#c.psa_key_id_t" title="psa_key_id_t">psa_key_id_t</a> private_key,
const uint8_t * peer_key,
size_t peer_key_length);</pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">operation</span></code></dt><dd><p>The key derivation operation object to use. It must have been set up with <a class="reference internal" href="kdf.html#c.psa_key_derivation_setup" title="psa_key_derivation_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_setup()</span></code></a> with a key agreement and derivation algorithm <code class="docutils literal notranslate"><span class="pre">alg</span></code> (<code class="docutils literal notranslate"><span class="pre">PSA_ALG_XXX</span></code> value such that <a class="reference internal" href="algorithms.html#c.PSA_ALG_IS_KEY_AGREEMENT" title="PSA_ALG_IS_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true and <a class="reference internal" href="#c.PSA_ALG_IS_RAW_KEY_AGREEMENT" title="PSA_ALG_IS_RAW_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_RAW_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is false). The operation must be ready for an input of the type given by <code class="docutils literal notranslate"><span class="pre">step</span></code>.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">step</span></code></dt><dd><p>Which step the input data is for.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">private_key</span></code></dt><dd><p>Identifier of the private key to use.
It must allow the usage <a class="reference internal" href="../keys/policy.html#c.PSA_KEY_USAGE_DERIVE" title="PSA_KEY_USAGE_DERIVE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_USAGE_DERIVE</span></code></a>.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">peer_key</span></code></dt><dd><p>Public key of the peer. The peer key must be in the same format that <a class="reference internal" href="../keys/management.html#c.psa_import_key" title="psa_import_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_import_key()</span></code></a> accepts for the public key type corresponding to the type of private_key. That is, this function performs the equivalent of <a class="reference internal" href="../keys/management.html#c.psa_import_key" title="psa_import_key"><code class="docutils literal notranslate"><span class="pre">psa_import_key</span></code></a><code class="docutils literal notranslate"><span class="pre">(...,</span> </code><code class="docutils literal notranslate"><span class="pre">peer_key</span></code><code class="docutils literal notranslate"><span class="pre">,</span> </code><code class="docutils literal notranslate"><span class="pre">peer_key_length</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> where with key attributes indicating the public key type corresponding to the type of <code class="docutils literal notranslate"><span class="pre">private_key</span></code>. For example, for EC keys, this means that peer_key is interpreted as a point on the curve that the private key is on. The standard formats for public keys are documented in the documentation of <a class="reference internal" href="../keys/management.html#c.psa_export_public_key" title="psa_export_public_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_export_public_key()</span></code></a>.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">peer_key_length</span></code></dt><dd><p>Size of <code class="docutils literal notranslate"><span class="pre">peer_key</span></code> in bytes.</p>
</dd>
</dl>
<p class="rubric">Returns: <a class="reference internal" href="../library/status.html#c.psa_status_t" title="psa_status_t"><code class="docutils literal notranslate"><span class="pre">psa_status_t</span></code></a></p>
<dl class="simple">
<dt> <a class="reference internal" href="../library/status.html#c.PSA_SUCCESS" title="PSA_SUCCESS"><code class="docutils literal notranslate"><span class="pre">PSA_SUCCESS</span></code></a></dt><dd><p>Success.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_BAD_STATE" title="PSA_ERROR_BAD_STATE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_BAD_STATE</span></code></a></dt><dd><p>The operation state is not valid for this key agreement <code class="docutils literal notranslate"><span class="pre">step</span></code>.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_HANDLE" title="PSA_ERROR_INVALID_HANDLE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_HANDLE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_NOT_PERMITTED" title="PSA_ERROR_NOT_PERMITTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_NOT_PERMITTED</span></code></a></dt><dd><p>The key does not have the <a class="reference internal" href="../keys/policy.html#c.PSA_KEY_USAGE_DERIVE" title="PSA_KEY_USAGE_DERIVE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_USAGE_DERIVE</span></code></a> flag, or it does not permit the requested algorithm.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_ARGUMENT" title="PSA_ERROR_INVALID_ARGUMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_ARGUMENT</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">private_key</span></code> is not compatible with <code class="docutils literal notranslate"><span class="pre">alg</span></code>, or <code class="docutils literal notranslate"><span class="pre">peer_key</span></code> is not valid for <code class="docutils literal notranslate"><span class="pre">alg</span></code> or not compatible with <code class="docutils literal notranslate"><span class="pre">private_key</span></code>.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_NOT_SUPPORTED" title="PSA_ERROR_NOT_SUPPORTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_NOT_SUPPORTED</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">alg</span></code> is not supported or is not a key derivation algorithm.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INVALID_ARGUMENT" title="PSA_ERROR_INVALID_ARGUMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INVALID_ARGUMENT</span></code></a></dt><dd><p><code class="docutils literal notranslate"><span class="pre">step</span></code> does not allow an input resulting from a key agreement.</p>
</dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_INSUFFICIENT_MEMORY" title="PSA_ERROR_INSUFFICIENT_MEMORY"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_INSUFFICIENT_MEMORY</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_COMMUNICATION_FAILURE" title="PSA_ERROR_COMMUNICATION_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_COMMUNICATION_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_HARDWARE_FAILURE" title="PSA_ERROR_HARDWARE_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_HARDWARE_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_CORRUPTION_DETECTED" title="PSA_ERROR_CORRUPTION_DETECTED"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_CORRUPTION_DETECTED</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_STORAGE_FAILURE" title="PSA_ERROR_STORAGE_FAILURE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_STORAGE_FAILURE</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_DATA_CORRUPT" title="PSA_ERROR_DATA_CORRUPT"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_DATA_CORRUPT</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_DATA_INVALID" title="PSA_ERROR_DATA_INVALID"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_DATA_INVALID</span></code></a></dt><dd></dd>
<dt> <a class="reference internal" href="../library/status.html#c.PSA_ERROR_BAD_STATE" title="PSA_ERROR_BAD_STATE"><code class="docutils literal notranslate"><span class="pre">PSA_ERROR_BAD_STATE</span></code></a></dt><dd><p>The library has not been previously initialized by <a class="reference internal" href="../library/library.html#c.psa_crypto_init" title="psa_crypto_init"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_crypto_init()</span></code></a>. It is implementation-dependent whether a failure to initialize results in this error code.</p>
</dd>
</dl>
<p class="rubric">Description</p>
<p>A key agreement algorithm takes two inputs: a private key <code class="docutils literal notranslate"><span class="pre">private_key</span></code> a public key <code class="docutils literal notranslate"><span class="pre">peer_key</span></code>. The result of this function is passed as input to a key derivation. The output of this key derivation can be extracted by reading from the resulting operation to produce keys and other cryptographic material.</p>
<p>If this function returns an error status, the operation enters an error state and must be aborted by calling <a class="reference internal" href="kdf.html#c.psa_key_derivation_abort" title="psa_key_derivation_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_abort()</span></code></a>.</p>
</div>
</div>
<div class="section" id="support-macros">
<h2>10.9.4. Support macros</h2>
<div class="section" id="PSA_ALG_KEY_AGREEMENT_GET_BASE">
<span id="c.PSA_ALG_KEY_AGREEMENT_GET_BASE"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_BASE</span></code> (macro)</h3>
<p>Get the raw key agreement algorithm from a full key agreement algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_BASE" title="PSA_ALG_KEY_AGREEMENT_GET_BASE">PSA_ALG_KEY_AGREEMENT_GET_BASE</a>(alg) <em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>A key agreement algorithm identifier (value of type <a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a> such that <a class="reference internal" href="algorithms.html#c.PSA_ALG_IS_KEY_AGREEMENT" title="PSA_ALG_IS_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p>The underlying raw key agreement algorithm if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is a key agreement algorithm.</p>
<p>Unspecified if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a key agreement algorithm or if it is not supported by the implementation.</p>
<p class="rubric">Description</p>
<p>See also <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT()</span></code></a> and <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_KDF" title="PSA_ALG_KEY_AGREEMENT_GET_KDF"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_KDF()</span></code></a>.</p>
</div>
<div class="section" id="PSA_ALG_KEY_AGREEMENT_GET_KDF">
<span id="c.PSA_ALG_KEY_AGREEMENT_GET_KDF"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_KDF</span></code> (macro)</h3>
<p>Get the key derivation algorithm used in a full key agreement algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_KDF" title="PSA_ALG_KEY_AGREEMENT_GET_KDF">PSA_ALG_KEY_AGREEMENT_GET_KDF</a>(alg) <em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>A key agreement algorithm identifier (value of type <a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a> such that <a class="reference internal" href="algorithms.html#c.PSA_ALG_IS_KEY_AGREEMENT" title="PSA_ALG_IS_KEY_AGREEMENT"><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_KEY_AGREEMENT</span></code></a><code class="docutils literal notranslate"><span class="pre">(</span></code><code class="docutils literal notranslate"><span class="pre">alg</span></code><code class="docutils literal notranslate"><span class="pre">)</span></code> is true).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p>The underlying key derivation algorithm if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is a key agreement algorithm.</p>
<p>Unspecified if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a key agreement algorithm or if it is not supported by the implementation.</p>
<p class="rubric">Description</p>
<p>See also <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT()</span></code></a> and <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_BASE" title="PSA_ALG_KEY_AGREEMENT_GET_BASE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_BASE()</span></code></a>.</p>
</div>
<div class="section" id="PSA_ALG_IS_RAW_KEY_AGREEMENT">
<span id="c.PSA_ALG_IS_RAW_KEY_AGREEMENT"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_RAW_KEY_AGREEMENT</span></code> (macro)</h3>
<p>Whether the specified algorithm is a raw key agreement algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_IS_RAW_KEY_AGREEMENT" title="PSA_ALG_IS_RAW_KEY_AGREEMENT">PSA_ALG_IS_RAW_KEY_AGREEMENT</a>(alg) <em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>An algorithm identifier (value of type <a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a>).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p><code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is a raw key agreement algorithm, <code class="docutils literal notranslate"><span class="pre">0</span></code> otherwise. This macro can return either <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a supported algorithm identifier.</p>
<p class="rubric">Description</p>
<p>A raw key agreement algorithm is one that does not specify a key derivation function. Usually, raw key agreement algorithms are constructed directly with a <code class="docutils literal notranslate"><span class="pre">PSA_ALG_xxx</span></code> macro while non-raw key agreement algorithms are constructed with <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT" title="PSA_ALG_KEY_AGREEMENT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT()</span></code></a>.</p>
<p>The raw key agreement algorithm can be extracted from a full key agreement algorithm identifier using <a class="reference internal" href="#c.PSA_ALG_KEY_AGREEMENT_GET_BASE" title="PSA_ALG_KEY_AGREEMENT_GET_BASE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_KEY_AGREEMENT_GET_BASE()</span></code></a>.</p>
</div>
<div class="section" id="PSA_ALG_IS_FFDH">
<span id="c.PSA_ALG_IS_FFDH"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_FFDH</span></code> (macro)</h3>
<p>Whether the specified algorithm is a finite field Diffie-Hellman algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_IS_FFDH" title="PSA_ALG_IS_FFDH">PSA_ALG_IS_FFDH</a>(alg) <em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>An algorithm identifier (value of type <a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a>).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p><code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is a finite field Diffie-Hellman algorithm, <code class="docutils literal notranslate"><span class="pre">0</span></code> otherwise. This macro can return either <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a supported key agreement algorithm identifier.</p>
<p class="rubric">Description</p>
<p>This includes the raw finite field Diffie-Hellman algorithm as well as finite-field Diffie-Hellman followed by any supporter key derivation algorithm.</p>
</div>
<div class="section" id="PSA_ALG_IS_ECDH">
<span id="c.PSA_ALG_IS_ECDH"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_ALG_IS_ECDH</span></code> (macro)</h3>
<p>Whether the specified algorithm is an elliptic curve Diffie-Hellman algorithm.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_ALG_IS_ECDH" title="PSA_ALG_IS_ECDH">PSA_ALG_IS_ECDH</a>(alg) <em><a class="reference internal" href="../../overview/implementation.html#specification-defined-value"><span class="std std-ref">/* specification-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">alg</span></code></dt><dd><p>An algorithm identifier (value of type <a class="reference internal" href="algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a>).</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p><code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is an elliptic curve Diffie-Hellman algorithm, <code class="docutils literal notranslate"><span class="pre">0</span></code> otherwise. This macro can return either <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">1</span></code> if <code class="docutils literal notranslate"><span class="pre">alg</span></code> is not a supported key agreement algorithm identifier.</p>
<p class="rubric">Description</p>
<p>This includes the raw elliptic curve Diffie-Hellman algorithm as well as elliptic curve Diffie-Hellman followed by any supporter key derivation algorithm.</p>
</div>
<div class="section" id="PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE">
<span id="c.PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE</span></code> (macro)</h3>
<p>Sufficient output buffer size for <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a>.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE">PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE</a>(key_type, key_bits) \
<em><a class="reference internal" href="../../overview/implementation.html#implementation-defined-value"><span class="std std-ref">/* implementation-defined value */</span></a></em></pre>
<p class="rubric">Parameters</p>
<dl class="simple">
<dt> <code class="docutils literal notranslate"><span class="pre">key_type</span></code></dt><dd><p>A supported key type.</p>
</dd>
<dt> <code class="docutils literal notranslate"><span class="pre">key_bits</span></code></dt><dd><p>The size of the key in bits.</p>
</dd>
</dl>
<p class="rubric">Returns</p>
<p>If the parameters are valid and supported, return a buffer size in bytes that guarantees that <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a> will not fail with <a class="reference internal" href="../library/status.html#c.PSA_ERROR_BUFFER_TOO_SMALL" title="PSA_ERROR_BUFFER_TOO_SMALL"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_BUFFER_TOO_SMALL</span></code></a>. If the parameters are a valid combination that is not supported by the implementation, this macro must return either a sensible size or <code class="docutils literal notranslate"><span class="pre">0</span></code>. If the parameters are not valid, the return value is unspecified.</p>
<p class="rubric">Description</p>
<p>This macro returns a compile-time constant if its arguments are compile-time constants.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>This function might evaluate its arguments multiple times or zero times. Providing arguments that have side effects will result in implementation-specific behavior, and is non-portable.</p>
</div>
<p>See also <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE</span></code></a>.</p>
</div>
<div class="section" id="PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE">
<span id="c.PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE"></span><h3><code class="docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE</span></code> (macro)</h3>
<p>Maximum size of the output from <a class="reference internal" href="#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a>.</p>
<pre class="literal-block">#define <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE">PSA_RAW_KEY_AGREEMENT_OUTPUT_MAX_SIZE</a> \
<em><a class="reference internal" href="../../overview/implementation.html#implementation-defined-value"><span class="std std-ref">/* implementation-defined value */</span></a></em></pre>
<p>This macro must expand to a compile-time constant integer.
It is recommended that this value is the maximum size of the output any raw key agreement algorithm supported by the implementation, in bytes. The value must not be smaller than this maximum.</p>
<p>See also <a class="reference internal" href="#c.PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE" title="PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE()</span></code></a>.</p>
</div>
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<div class="sphinxsidebarwrapper"><h3><a href="../../index.html"><b>PSA Crypto API</b></a></h3>
IHI 0086<br/>
Non-confidential<br/>
Version 1.0.1
<span style="color: red; font-weight: bold;"></span>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../about.html">About this document</a></li>
</ul>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="../../overview/intro.html">1. Introduction</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/goals.html">2. Design goals</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/functionality.html">3. Functionality overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/sample-arch.html">4. Sample architectures</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/conventions.html">5. Library conventions</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/implementation.html">6. Implementation considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../overview/usage.html">7. Usage considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../library/index.html">8. Library management reference</a></li>
<li class="toctree-l1"><a class="reference internal" href="../keys/index.html">9. Key management reference</a></li>
<li class="toctree-l1 current"><a class="reference internal" href="index.html">10. Cryptographic operation reference</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="algorithms.html">10.1. Algorithms</a></li>
<li class="toctree-l2"><a class="reference internal" href="hashes.html">10.2. Message digests</a></li>
<li class="toctree-l2"><a class="reference internal" href="macs.html">10.3. Message authentication codes (MAC)</a></li>
<li class="toctree-l2"><a class="reference internal" href="ciphers.html">10.4. Unauthenticated ciphers</a></li>
<li class="toctree-l2"><a class="reference internal" href="aead.html">10.5. Authenticated encryption with associated data (AEAD)</a></li>
<li class="toctree-l2"><a class="reference internal" href="kdf.html">10.6. Key derivation</a></li>
<li class="toctree-l2"><a class="reference internal" href="sign.html">10.7. Asymmetric signature</a></li>
<li class="toctree-l2"><a class="reference internal" href="pke.html">10.8. Asymmetric encryption</a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">10.9. Key agreement</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#key-agreement-algorithms">10.9.1. Key agreement algorithms</a></li>
<li class="toctree-l3"><a class="reference internal" href="#standalone-key-agreement">10.9.2. Standalone key agreement</a></li>
<li class="toctree-l3"><a class="reference internal" href="#combining-key-agreement-and-key-derivation">10.9.3. Combining key agreement and key derivation</a></li>
<li class="toctree-l3"><a class="reference internal" href="#support-macros">10.9.4. Support macros</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="rng.html">10.10. Other cryptographic services</a></li>
</ul>
</li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../appendix/example_header.html">Example header file</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../appendix/specdef_values.html">Example macro implementations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../appendix/history.html">Changes to the API</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../psa_c-identifiers.html">Index of API elements</a></li>
</ul>
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