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+<!DOCTYPE html>
+ <meta charset="utf-8"/>
+ <style>
+ table.head, table.foot { width: 100%; }
+ td.head-rtitle, td.foot-os { text-align: right; }
+ td.head-vol { text-align: center; }
+ div.Pp { margin: 1ex 0ex; }
+ </style>
+ <link rel="stylesheet" href="style.css" type="text/css" media="all"/>
+ <title>CRYPTO_POLY1305(3MONOCYPHER)</title>
+<table class="head">
+ <tr>
+ <td class="head-ltitle">CRYPTO_POLY1305(3MONOCYPHER)</td>
+ <td class="head-vol">3MONOCYPHER</td>
+ <td class="head-rtitle">CRYPTO_POLY1305(3MONOCYPHER)</td>
+ </tr>
+<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_poly1305</b>,
+ <b class="Nm" title="Nm">crypto_poly1305_init</b>,
+ <b class="Nm" title="Nm">crypto_poly1305_update</b>,
+ <b class="Nm" title="Nm">crypto_poly1305_final</b> &#x2014;
+ <span class="Nd" title="Nd">Poly1305 one-time message authentication
+ codes</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>
+<b class="Fn" title="Fn">crypto_poly1305</b>(<var class="Fa" title="Fa">uint8_t
+ mac[16]</var>, <var class="Fa" title="Fa">const uint8_t *message</var>,
+ <var class="Fa" title="Fa">size_t message_size</var>,
+ <var class="Fa" title="Fa">const uint8_t key[32]</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">void</var>
+<b class="Fn" title="Fn">crypto_poly1305_init</b>(<var class="Fa" title="Fa">crypto_poly1305_ctx
+ *ctx</var>, <var class="Fa" title="Fa">const uint8_t key[32]</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">void</var>
+<b class="Fn" title="Fn">crypto_poly1305_update</b>(<var class="Fa" title="Fa">crypto_poly1305_ctx
+ *ctx</var>, <var class="Fa" title="Fa">const uint8_t *message</var>,
+ <var class="Fa" title="Fa">size_t message_size</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">void</var>
+<b class="Fn" title="Fn">crypto_poly1305_final</b>(<var class="Fa" title="Fa">crypto_poly1305_ctx
+ *ctx</var>, <var class="Fa" title="Fa">uint8_t mac[16]</var>);
+<h1 class="Sh" title="Sh" id="DESCRIPTION"><a class="selflink" href="#DESCRIPTION">DESCRIPTION</a></h1>
+Poly1305 is a one-time message authentication code. &#x201C;One-time&#x201D;
+ means the authentication key can be used only once.
+ <b class="Sy" title="Sy">This makes Poly1305 easy to misuse</b>. On the other
+ hand, Poly1305 is fast, and provably secure if used correctly.
+<div class="Pp"></div>
+Poly1305 is a low-level primitive. Consider using authenticated encryption,
+ implemented by
+ <a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>.
+<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">mac</var></dt>
+ <dd class="It-tag">The authentication code.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">key</var></dt>
+ <dd class="It-tag">The secret authentication key. Use only once per message.
+ Do not use the session key to authenticate messages. It should be wiped
+ with
+ <a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>
+ after use.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">message</var></dt>
+ <dd class="It-tag">The message to authenticate. May overlap with the
+ <var class="Fa" title="Fa">mac</var> argument.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">message_size</var></dt>
+ <dd class="It-tag">Length of <var class="Fa" title="Fa">message</var>, in
+ bytes.</dd>
+<h2 class="Ss" title="Ss" id="Direct_interface"><a class="selflink" href="#Direct_interface">Direct
+ interface</a></h2>
+<b class="Fn" title="Fn">crypto_poly1305</b>() produces a message authentication
+ code for the given message and authentication key. To verify the integrity of
+ a message, use
+ <a class="Xr" title="Xr" href="crypto_verify16.html">crypto_verify16(3monocypher)</a>
+ to compare the received MAC to the output
+ <var class="Fa" title="Fa">mac</var>.
+<h2 class="Ss" title="Ss" id="Incremental_interface"><a class="selflink" href="#Incremental_interface">Incremental
+ interface</a></h2>
+<b class="Fn" title="Fn">crypto_poly1305_init</b>() initialises a context.
+ <var class="Fa" title="Fa">key</var> should be wiped once the context is
+ initialised. Then, <b class="Fn" title="Fn">crypto_poly1305_update</b>()
+ authenticates the message chunk by chunk. Once the message is entirely
+ processed, <b class="Fn" title="Fn">crypto_poly1305_final</b>() yields the
+ message authentication code.
+<h1 class="Sh" title="Sh" id="RETURN_VALUES"><a class="selflink" href="#RETURN_VALUES">RETURN
+ VALUES</a></h1>
+These functions 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>
+To authenticate a message:
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+const uint8_t msg[ 5] = &quot;Lorem&quot;; /* Message to authenticate */
+uint8_t key[32]; /* Random secret key (use only once) */
+uint8_t mac[16]; /* Message authentication code (MAC) */
+arc4random_buf(key, 32);
+crypto_poly1305(mac, msg, 5, key);
+/* Wipe the key */
+crypto_wipe(key, 32);
+<div class="Pp"></div>
+To verify the above message:
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+const uint8_t msg [ 5] = &quot;Lorem&quot;; /* Message to verify */
+uint8_t key [32]; /* The above key */
+const uint8_t mac [16]; /* The above MAC */
+uint8_t real_mac[16]; /* The actual MAC */
+crypto_poly1305(real_mac, msg, 5, key);
+/* Wipe the key */
+crypto_wipe(key, 32);
+if (crypto_verify16(mac, real_mac)) {
+ /* Corrupted message, abort processing */
+} else {
+ /* Genuine message */
+/* The real mac is secret. Wipe it */
+crypto_wipe(real_mac, 16);
+<div class="Pp"></div>
+Incremental authentication:
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+const uint8_t msg[500]= {1}; /* Message to authenticate */
+uint8_t key[ 32]; /* Random secret key (use only once) */
+uint8_t mac[ 16]; /* Message authentication code (MAC) */
+crypto_poly1305_ctx ctx;
+arc4random_buf(key, 32);
+crypto_poly1305_init(&amp;ctx, key);
+/* Wipe the key */
+crypto_wipe(key, 32);
+for (int i = 0; i &lt; 500; i += 100) {
+ crypto_poly1305_update(&amp;ctx, msg, 100);
+crypto_poly1305_final(&amp;ctx, mac);
+<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_blake2b.html">crypto_blake2b(3monocypher)</a>,
+ <a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>,
+ <a class="Xr" title="Xr" href="crypto_verify16.html">crypto_verify16(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 Poly1305, described in RFC 8439.
+<h1 class="Sh" title="Sh" id="HISTORY"><a class="selflink" href="#HISTORY">HISTORY</a></h1>
+The <b class="Fn" title="Fn">crypto_poly1305_init</b>(),
+ <b class="Fn" title="Fn">crypto_poly1305_update</b>(), and
+ <b class="Fn" title="Fn">crypto_poly1305_final</b>() functions first appeared
+ in Monocypher 0.1. <b class="Fn" title="Fn">crypto_poly1305</b>() first
+ appeared in Monocypher 1.1.0.
+<h1 class="Sh" title="Sh" id="SECURITY_CONSIDERATIONS"><a class="selflink" href="#SECURITY_CONSIDERATIONS">SECURITY
+Poly1305 is difficult to use correctly. Do not use it unless you are absolutely
+ sure what you are doing. Use authenticated encryption instead; see
+ <a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>.
+ If you are certain you do not want encryption, refer to
+ <a class="Xr" title="Xr" href="crypto_blake2b.html">crypto_blake2b(3monocypher)</a>
+ on how to use Blake2b to generate message authentication codes.
+<h2 class="Ss" title="Ss" id="Authentication_key_requirements"><a class="selflink" href="#Authentication_key_requirements">Authentication
+ key requirements</a></h2>
+Poly1305 is a <i class="Em" title="Em">one-time</i> authenticator. This puts
+ rather stringent constraints on the authentication key:
+<ul class="Bl-bullet">
+ <li class="It-bullet">Any given key must be used only once. Using the same key
+ for two different messages reveals it to the attacker. Do not use the
+ session key, or it will void all security.</li>
+ <li class="It-bullet">Authentication keys must be random, and independent from
+ each other. Do not use non-random nonces. Do not use related keys. Use
+ fresh, unpredictable, uniformly distributed random numbers.</li>
+ <li class="It-bullet">The key must be transmitted to the recipient without
+ revealing it to the attacker. Somehow.</li>
+<div class="Pp"></div>
+The only practical source for the authentication key is a chunk of the
+ encryption stream used to encrypt the message. That chunk must be
+ <i class="Em" title="Em">dedicated</i> to the authentication key: if it is
+ reused to encrypt the message itself, the attacker may recover that chunk by
+ guessing the message, then forge arbitrary messages.
+<div class="Pp"></div>
+To get this right, you need a session key, a <i class="Em" title="Em">unique</i>
+ nonce, and a stream cipher. Generate a stream with the session key and nonce.
+ Take the first 32 bytes of that stream as your authentication key, then use
+ the <i class="Em" title="Em">rest</i> of the stream to encrypt your message.
+ This is the approach used by
+ <a class="Xr" title="Xr" href="crypto_lock_aead.html">crypto_lock_aead(3monocypher)</a>.
+<h2 class="Ss" title="Ss" id="Protection_against_side_channels"><a class="selflink" href="#Protection_against_side_channels">Protection
+ against side channels</a></h2>
+ <a class="Xr" title="Xr" href="crypto_verify16.html">crypto_verify16(3monocypher)</a>
+ to compare message authentication codes. Avoid standard buffer comparison
+ functions. They may not run in constant time, enabling an attacker to exploit
+ timing attacks to recover the MAC.
+<div class="Pp"></div>
+The authentication key should be wiped with
+ <a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>
+ after use.
+<div class="Pp"></div>
+The incremental interface automatically wipes its context when finished so users
+ do not need to do it themselves.</div>
+<table class="foot">
+ <tr>
+ <td class="foot-date">March 31, 2020</td>
+ <td class="foot-os">Linux 4.15.0-106-generic</td>
+ </tr>