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authorpukkamustard <pukkamustard@posteo.net>2021-02-05 21:02:47 +0100
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+<!DOCTYPE html>
+<html>
+<head>
+ <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_CHACHA20(3MONOCYPHER)</title>
+</head>
+<body>
+<table class="head">
+ <tr>
+ <td class="head-ltitle">CRYPTO_CHACHA20(3MONOCYPHER)</td>
+ <td class="head-vol">3MONOCYPHER</td>
+ <td class="head-rtitle">CRYPTO_CHACHA20(3MONOCYPHER)</td>
+ </tr>
+</table>
+<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_chacha20</b>,
+ <b class="Nm" title="Nm">crypto_chacha20_ctr</b>,
+ <b class="Nm" title="Nm">crypto_xchacha20</b>,
+ <b class="Nm" title="Nm">crypto_xchacha20_ctr</b> &#x2014;
+ <span class="Nd" title="Nd">Chacha20 and XChacha20 encryption functions</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_chacha20</b>(<var class="Fa" title="Fa">uint8_t
+ *cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
+ *plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
+ <var class="Fa" title="Fa">const uint8_t key[32]</var>,
+ <var class="Fa" title="Fa">const uint8_t nonce[8]</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">void</var>
+<br/>
+<b class="Fn" title="Fn">crypto_xchacha20</b>(<var class="Fa" title="Fa">uint8_t
+ *cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
+ *plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
+ <var class="Fa" title="Fa">const uint8_t key[32]</var>,
+ <var class="Fa" title="Fa">const uint8_t nonce[24]</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">uint64_t</var>
+<br/>
+<b class="Fn" title="Fn">crypto_chacha20_ctr</b>(<var class="Fa" title="Fa">uint8_t
+ *cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
+ *plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
+ <var class="Fa" title="Fa">const uint8_t key[32]</var>,
+ <var class="Fa" title="Fa">const uint8_t nonce[8]</var>,
+ <var class="Fa" title="Fa">uint64_t ctr</var>);
+<div class="Pp"></div>
+<var class="Ft" title="Ft">uint64_t</var>
+<br/>
+<b class="Fn" title="Fn">crypto_xchacha20_ctr</b>(<var class="Fa" title="Fa">uint8_t
+ *cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
+ *plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
+ <var class="Fa" title="Fa">const uint8_t key[32]</var>,
+ <var class="Fa" title="Fa">const uint8_t nonce[24]</var>,
+ <var class="Fa" title="Fa">uint64_t ctr</var>);
+<h1 class="Sh" title="Sh" id="DESCRIPTION"><a class="selflink" href="#DESCRIPTION">DESCRIPTION</a></h1>
+These functions provide an interface for the Chacha20 encryption primitive.
+<div class="Pp"></div>
+Chacha20 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">key</var></dt>
+ <dd class="It-tag">A 32-byte secret key.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">nonce</var></dt>
+ <dd class="It-tag">An 8-byte or 24-byte number, used only once with any given
+ key. It does not need to be secret or random, but it does have to be
+ unique. Repeating a nonce with the same key reveals the XOR of two
+ different messages, which allows decryption. 24-byte nonces can be
+ selected at random. 8-byte nonces <i class="Em" title="Em">cannot</i>.
+ They are too small, and the same nonce may be selected twice by accident.
+ See <a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a> for
+ advice about generating random numbers (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">plain_text</var></dt>
+ <dd class="It-tag">The message to encrypt. It is allowed to be
+ <code class="Dv" title="Dv">NULL</code>, in which case it will be
+ interpreted as an all zero input.
+ <var class="Fa" title="Fa">cipher_text</var> will then contain the raw
+ Chacha20 stream.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">cipher_text</var></dt>
+ <dd class="It-tag">The encrypted message.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">text_size</var></dt>
+ <dd class="It-tag">Length of both <var class="Fa" title="Fa">plain_text</var>
+ and <var class="Fa" title="Fa">cipher_text</var>, in bytes.</dd>
+ <dt class="It-tag">&#x00A0;</dt>
+ <dd class="It-tag">&#x00A0;</dd>
+ <dt class="It-tag"><var class="Fa" title="Fa">ctr</var></dt>
+ <dd class="It-tag">The number of 64-byte blocks since the beginning of the
+ stream.</dd>
+</dl>
+<div class="Pp"></div>
+The <var class="Fa" title="Fa">key</var> and
+ <var class="Fa" title="Fa">nonce</var> buffers may overlap.
+ <var class="Fa" title="Fa">plain_text</var> and
+ <var class="Fa" title="Fa">cipher_text</var> must either be the same buffer
+ (for in-place encryption), or non-overlapping.
+<div class="Pp"></div>
+<b class="Fn" title="Fn">crypto_chacha20</b>() performs a Chacha20 operation. It
+ uses an 8-byte nonce, which is too small to be selected at random. Use a
+ message counter as a nonce instead.
+<div class="Pp"></div>
+<b class="Fn" title="Fn">crypto_xchacha20</b>() performs an XChacha20 operation.
+ It uses a 24-byte nonce, which is large enough to be selected at random.
+<div class="Pp"></div>
+<b class="Fn" title="Fn">crypto_xchacha20</b>() is recommended over
+ <b class="Fn" title="Fn">crypto_chacha20</b>(). The ability to use random
+ nonces makes it easier to use securely, and the performance hit is often
+ negligible in practice.
+<div class="Pp"></div>
+The <b class="Fn" title="Fn">crypto_chacha20</b>() and
+ <b class="Fn" title="Fn">crypto_xchacha20</b>() encrypt
+ <var class="Fa" title="Fa">plain_text</var> by XORing it with a pseudo-random
+ stream of numbers, seeded by the provided <var class="Fa" title="Fa">key</var>
+ and <var class="Fa" title="Fa">nonce</var>.
+<div class="Pp"></div>
+Since XOR is its own inverse, decryption is the same operation as encryption. To
+ decrypt the cipher text, &#x201C;encrypt&#x201D; it again with the same key
+ and nonce. You will likely want to wipe the key when you are done with
+ encryption or decryption. Use
+ <a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>
+ to wipe them.
+<div class="Pp"></div>
+The <var class="Fa" title="Fa">plain_text</var> pointer is allowed to be
+ <code class="Dv" title="Dv">NULL</code>, in which case it will be interpreted
+ as an all zero input. This is useful as a user space random number generator.
+ While <b class="Sy" title="Sy">this should not be used as a random number
+ generator for secrets</b>, for which the operating system random number
+ generator should be preferred, it can be handy outside of a security context.
+ Deterministic procedural generation and reproducible property-based tests come
+ to mind. Additionally, it <i class="Em" title="Em">can</i> be used to generate
+ large amounts of random-looking data quickly, for example to generate padding.
+<div class="Pp"></div>
+The <b class="Fn" title="Fn">crypto_chacha20_ctr</b>() and
+ <b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() perform a Chacha20 or
+ XChacha20 encryption, respectively, starting the stream at the block
+ <var class="Fa" title="Fa">ctr</var> (which is the byte
+ &#x2018;<code class="Li">ctr &#x00D7; 64</code>&#x2019;). This can be used to
+ encrypt (or decrypt) part of a long message, or to implement some AEAD
+ constructions such as the one described in RFC 8439. Be careful when using
+ this not to accidentally reuse parts of the random stream as that would
+ destroy confidentiality.
+<h1 class="Sh" title="Sh" id="RETURN_VALUES"><a class="selflink" href="#RETURN_VALUES">RETURN
+ VALUES</a></h1>
+<b class="Fn" title="Fn">crypto_chacha20</b>() and
+ <b class="Fn" title="Fn">crypto_xchacha20</b>() return nothing.
+ <b class="Fn" title="Fn">crypto_chacha20_ctr</b>() and
+ <b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() functions return the next
+ <var class="Fa" title="Fa">ctr</var> to use with the same key and nonce
+ values; this is always <var class="Fa" title="Fa">text_size</var> divided by
+ 64; plus one if there was a remainder.
+<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>
+Simple encryption:
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+uint8_t key [ 32]; /* Secret random key */
+uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
+uint8_t plain_text [500] = {1}; /* Secret message */
+uint8_t cipher_text[500]; /* Encrypted message */
+arc4random_buf(key, 32);
+arc4random_buf(nonce, 24);
+crypto_xchacha20(cipher_text, plain_text, 500, key, nonce);
+/* Wipe secrets if they are no longer needed */
+crypto_wipe(key, 32);
+crypto_wipe(plain_text, 500);
+</pre>
+</div>
+<div class="Pp"></div>
+To decrypt the above:
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+uint8_t key [ 32]; /* Same key as above */
+const uint8_t nonce [ 24]; /* Same nonce as above */
+uint8_t plain_text [500]; /* Message to decrypt */
+uint8_t cipher_text[500]; /* Secret message */
+crypto_xchacha20(cipher_text, plain_text, 500, key, nonce);
+/* Wipe secrets if they are no longer needed */
+crypto_wipe(key, 32);
+/* The plain text likely needs to be processed before you wipe it */
+crypto_wipe(plain_text, 12);
+</pre>
+</div>
+<div class="Pp"></div>
+Incremental encryption (in blocks of 64 bytes):
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+uint8_t key [ 32]; /* Secret random key */
+uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
+uint8_t plain_text [500]; /* Secret message */
+uint8_t cipher_text[500]; /* Encrypted message */
+uint64_t ctr = 0; /* Block counter */
+unsigned int i;
+arc4random_buf(key, 32);
+arc4random_buf(nonce, 24);
+for(i = 0; i &lt; 500; i += 64) {
+ ctr = crypto_xchacha20_ctr(cipher_text+i, plain_text+i, 64,
+ key, nonce, ctr);
+}
+/* Process data that didn't fit into 64 byte pieces */
+crypto_xchacha20_ctr(cipher_text+500-(i-64),
+ plain_text+500-(i-64),
+ 500-(i-64),
+ key, nonce, ctr);
+/* Wipe secrets if they are no longer needed */
+crypto_wipe(key, 32);
+crypto_wipe(plain_text, 500);
+</pre>
+</div>
+<div class="Pp"></div>
+Encryption by jumping around (do not do that, this is only meant to show how
+ <b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() works):
+<div class="Pp"></div>
+<div class="Bd" style="margin-left: 5.00ex;">
+<pre class="Li">
+uint8_t key [ 32]; /* Secret random key */
+uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
+uint8_t plain_text [500] = {1}; /* Message to be encrypted */
+uint8_t cipher_text[500]; /* Will be the encrypted message */
+arc4random_buf(key, 32);
+arc4random_buf(nonce, 24);
+/* Encrypt the second part of the message first... */
+crypto_xchacha20_ctr(cipher_text + (3 * 64),
+ plain_text + (3 * 64),
+ 500 - (3 * 64),
+ key, nonce, 3);
+/* ...then encrypt the first part */
+crypto_xchacha20_ctr(cipher_text, plain_text, 3 * 64, key, nonce, 0);
+/* Wipe secrets if they are no longer needed */
+crypto_wipe(key, 32);
+crypto_wipe(plain_text, 500);
+</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_ietf_chacha20.html">crypto_ietf_chacha20(3monocypher)</a>,
+ <a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>,
+ <a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(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 Chacha20 and XChacha20. Chacha20 is described in:
+ <cite class="Rs" title="Rs"><span class="RsA">Daniel J. Bernstein</span>,
+ <span class="RsT">ChaCha, a variant of Salsa20</span>, <i class="RsJ">SASC
+ 2008 &#x2013; The State of the Art of Stream Ciphers</i>,
+ <span class="RsP">pp. 273&#x2013;278</span>.</cite> The nonce and counter
+ sizes were modified in RFC 8439. XChacha20 derives from Chacha20 the same way
+ XSalsa20 derives from Salsa20, and benefits from the same security reduction
+ (proven secure as long as Chacha20 itself is secure).
+<h1 class="Sh" title="Sh" id="HISTORY"><a class="selflink" href="#HISTORY">HISTORY</a></h1>
+<b class="Fn" title="Fn">crypto_chacha20</b>(),
+ <b class="Fn" title="Fn">crypto_chacha20_ctr</b>(),
+ <b class="Fn" title="Fn">crypto_xchacha20</b>(), and
+ <b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() were added in Monocypher
+ 3.0.0. They replace <b class="Fn" title="Fn">crypto_chacha20_encrypt</b>(),
+ <b class="Fn" title="Fn">crypto_chacha20_init</b>(),
+ <b class="Fn" title="Fn">crypto_chacha20_stream</b>(),
+ <b class="Fn" title="Fn">crypto_chacha20_x_init</b>(), and
+ <b class="Fn" title="Fn">crypto_chacha20_set_ctr</b>() that were deprecated in
+ Monocypher 3.0.0.
+<h1 class="Sh" title="Sh" id="SECURITY_CONSIDERATIONS"><a class="selflink" href="#SECURITY_CONSIDERATIONS">SECURITY
+ CONSIDERATIONS</a></h1>
+<h2 class="Ss" title="Ss" id="Encrypted_does_not_mean_secure"><a class="selflink" href="#Encrypted_does_not_mean_secure">Encrypted
+ does not mean secure</a></h2>
+Chacha20 only protects against eavesdropping, not forgeries. Most applications
+ need protection against forgeries to be properly secure. To ensure the
+ integrity of a message, use Blake2b in keyed mode, or authenticated
+ encryption; see
+ <a class="Xr" title="Xr" href="crypto_blake2b.html">crypto_blake2b(3monocypher)</a>
+ and
+ <a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>.
+<h2 class="Ss" title="Ss" id="Nonce_reuse"><a class="selflink" href="#Nonce_reuse">Nonce
+ reuse</a></h2>
+Repeating a nonce with the same key exposes the XOR of two or more plain text
+ messages, effectively destroying confidentiality.
+<div class="Pp"></div>
+For the same reason, <b class="Sy" title="Sy">do not select small nonces at
+ random</b>. The <b class="Fn" title="Fn">crypto_chacha20</b>() nonce spans
+ only 64 bits, which is small enough to trigger accidental reuses. A message
+ counter should be used instead. If multiple parties send out messages, Each
+ can start with an initial nonce of 0, 1 .. n-1 respectively, and increment
+ them by n for each new message. Make sure the counters never wrap around.
+<h2 class="Ss" title="Ss" id="Secure_random_number_generation"><a class="selflink" href="#Secure_random_number_generation">Secure
+ random number generation</a></h2>
+Do not use these functions as a cryptographic random number generator. Always
+ use the operating system's random number generator for cryptographic purposes,
+ see <a class="Xr" title="Xr" href="intro.html">intro(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>
+Secrets should not dwell in memory longer than needed. Use
+ <a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>
+ to erase secrets you no longer need. For Chacha20, this means the key and in
+ some cases the plain text itself.</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>
+</table>
+</body>
+</html>