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<title>CRYPTO_KEY_EXCHANGE(3MONOCYPHER)</title>
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<table class="head">
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<td class="head-ltitle">CRYPTO_KEY_EXCHANGE(3MONOCYPHER)</td>
<td class="head-vol">3MONOCYPHER</td>
<td class="head-rtitle">CRYPTO_KEY_EXCHANGE(3MONOCYPHER)</td>
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<div class="manual-text">
<h1 class="Sh" title="Sh" id="NAME"><a class="selflink" href="#NAME">NAME</a></h1>
<b class="Nm" title="Nm">crypto_key_exchange</b>,
<b class="Nm" title="Nm">crypto_key_exchange_public_key</b> &#x2014;
<span class="Nd" title="Nd">Elliptic Curve Diffie-Hellman key exchange</span>
<h1 class="Sh" title="Sh" id="SYNOPSIS"><a class="selflink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<b class="In" title="In">#include
&lt;<a class="In" title="In">monocypher.h</a>&gt;</b>
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_key_exchange</b>(<var class="Fa" title="Fa">uint8_t
shared_key[32]</var>, <var class="Fa" title="Fa">const uint8_t
your_secret_key[32]</var>, <var class="Fa" title="Fa">const uint8_t
their_public_key[32]</var>);
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>(<var class="Fa" title="Fa">uint8_t
your_public_key[32]</var>, <var class="Fa" title="Fa">const uint8_t
your_secret_key[32]</var>);
<h1 class="Sh" title="Sh" id="DESCRIPTION"><a class="selflink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<b class="Fn" title="Fn">crypto_key_exchange</b>() computes a shared key with
your secret key and their public key.
<div class="Pp"></div>
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>() deterministically
computes the public key from a random secret key.
<div class="Pp"></div>
The arguments are:
<dl class="Bl-tag">
<dt class="It-tag">&#x00A0;</dt>
<dd class="It-tag">&#x00A0;</dd>
<dt class="It-tag"><var class="Fa" title="Fa">shared_key</var></dt>
<dd class="It-tag">The shared secret, known only to those who know a relevant
secret key (yours or theirs). It is cryptographically random, and suitable
for use with the
<a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>
family of functions.</dd>
<dt class="It-tag">&#x00A0;</dt>
<dd class="It-tag">&#x00A0;</dd>
<dt class="It-tag"><var class="Fa" title="Fa">your_secret_key</var></dt>
<dd class="It-tag">A 32-byte random number, known only to you. See
<a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a> for
advice about generating random bytes (use the operating system's random
number generator).</dd>
<dt class="It-tag">&#x00A0;</dt>
<dd class="It-tag">&#x00A0;</dd>
<dt class="It-tag"><var class="Fa" title="Fa">their_public_key</var></dt>
<dd class="It-tag">The public key of the other party.</dd>
<dt class="It-tag">&#x00A0;</dt>
<dd class="It-tag">&#x00A0;</dd>
<dt class="It-tag"><var class="Fa" title="Fa">your_public_key</var></dt>
<dd class="It-tag">Your public key, generated from
<var class="Fa" title="Fa">your_secret_key</var> with
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>().</dd>
</dl>
<div class="Pp"></div>
<var class="Fa" title="Fa">shared_key</var> and
<var class="Fa" title="Fa">your_secret_key</var> may overlap if the secret is
no longer required.
<div class="Pp"></div>
Some poorly designed protocols require to test for &#x201C;contributory&#x201D;
behaviour, which ensures that no untrusted party forces the shared secret to a
known constant. Protocols should instead be designed in such a way that no
such check is necessary, namely by authenticating the other party or
exchanging keys over a trusted channel.
<div class="Pp"></div>
Do not use the same secret key for both key exchanges and signatures. The public
keys are different, and revealing both may leak information. If there really
is no room to store or derive two different secret keys, consider generating a
key pair for signatures and then converting it with
<a class="Xr" title="Xr" href="crypto_from_eddsa_private.html">crypto_from_eddsa_private(3monocypher)</a>
and
<a class="Xr" title="Xr" href="crypto_from_eddsa_public.html">crypto_from_eddsa_public(3monocypher)</a>.
<h1 class="Sh" title="Sh" id="RETURN_VALUES"><a class="selflink" href="#RETURN_VALUES">RETURN
VALUES</a></h1>
<b class="Fn" title="Fn">crypto_key_exchange</b>() and
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>() return nothing.
<h1 class="Sh" title="Sh" id="EXAMPLES"><a class="selflink" href="#EXAMPLES">EXAMPLES</a></h1>
The following examples assume the existence of
<b class="Fn" title="Fn">arc4random_buf</b>(), which fills the given buffer
with cryptographically secure random bytes. If
<b class="Fn" title="Fn">arc4random_buf</b>() does not exist on your system,
see <a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a> for
advice about how to generate cryptographically secure random bytes.
<div class="Pp"></div>
Generate a public key from a randomly generated secret key:
<div class="Pp"></div>
<div class="Bd" style="margin-left: 5.00ex;">
<pre class="Li">
uint8_t sk[32]; /* Random secret key */
uint8_t pk[32]; /* Public key */
arc4random_buf(sk, 32);
crypto_key_exchange_public_key(pk, sk);
/* Wipe secrets if they are no longer needed */
crypto_wipe(sk, 32);
</pre>
</div>
<div class="Pp"></div>
Generate a shared, symmetric key with your secret key and their public key. (The
other party will generate the same shared key with your public key and their
secret key.)
<div class="Pp"></div>
<div class="Bd" style="margin-left: 5.00ex;">
<pre class="Li">
const uint8_t their_pk [32]; /* Their public key */
uint8_t your_sk [32]; /* Your secret key */
uint8_t shared_key[32]; /* Shared session key */
crypto_key_exchange(shared_key, your_sk, their_pk);
/* Wipe secrets if they are no longer needed */
crypto_wipe(your_sk, 32);
</pre>
</div>
<h1 class="Sh" title="Sh" id="SEE_ALSO"><a class="selflink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>,
<a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a>
<h1 class="Sh" title="Sh" id="STANDARDS"><a class="selflink" href="#STANDARDS">STANDARDS</a></h1>
These functions implement X25519, described in RFC 7748.
<b class="Fn" title="Fn">crypto_key_exchange</b>() uses HChacha20 as well.
<h1 class="Sh" title="Sh" id="HISTORY"><a class="selflink" href="#HISTORY">HISTORY</a></h1>
The <b class="Fn" title="Fn">crypto_key_exchange</b>() function first appeared
in Monocypher 0.2. The
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>() macro alias
first appeared in Monocypher 1.1.0.
<h1 class="Sh" title="Sh" id="SECURITY_CONSIDERATIONS"><a class="selflink" href="#SECURITY_CONSIDERATIONS">SECURITY
CONSIDERATIONS</a></h1>
If either of the long term secret keys leaks, it may compromise
<i class="Em" title="Em">all past messages</i>. This can be avoided by using
protocols that provide forward secrecy, such as the X3DH key agreement
protocol.
<h1 class="Sh" title="Sh" id="IMPLEMENTATION_DETAILS"><a class="selflink" href="#IMPLEMENTATION_DETAILS">IMPLEMENTATION
DETAILS</a></h1>
<b class="Fn" title="Fn">crypto_key_exchange_public_key</b>() is an alias to
<a class="Xr" title="Xr" href="crypto_x25519_public_key.html">crypto_x25519_public_key(3monocypher)</a>.</div>
<table class="foot">
<tr>
<td class="foot-date">March 31, 2020</td>
<td class="foot-os">Linux 4.15.0-106-generic</td>
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