332 lines
17 KiB
HTML
332 lines
17 KiB
HTML
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8"/>
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<style>
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table.head, table.foot { width: 100%; }
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td.head-rtitle, td.foot-os { text-align: right; }
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td.head-vol { text-align: center; }
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div.Pp { margin: 1ex 0ex; }
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</style>
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<link rel="stylesheet" href="style.css" type="text/css" media="all"/>
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<title>CRYPTO_CHACHA20(3MONOCYPHER)</title>
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</head>
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<body>
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<table class="head">
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<tr>
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<td class="head-ltitle">CRYPTO_CHACHA20(3MONOCYPHER)</td>
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<td class="head-vol">3MONOCYPHER</td>
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<td class="head-rtitle">CRYPTO_CHACHA20(3MONOCYPHER)</td>
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</tr>
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</table>
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<div class="manual-text">
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<h1 class="Sh" title="Sh" id="NAME"><a class="selflink" href="#NAME">NAME</a></h1>
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<b class="Nm" title="Nm">crypto_chacha20</b>,
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<b class="Nm" title="Nm">crypto_chacha20_ctr</b>,
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<b class="Nm" title="Nm">crypto_xchacha20</b>,
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<b class="Nm" title="Nm">crypto_xchacha20_ctr</b> —
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<span class="Nd" title="Nd">Chacha20 and XChacha20 encryption functions</span>
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<h1 class="Sh" title="Sh" id="SYNOPSIS"><a class="selflink" href="#SYNOPSIS">SYNOPSIS</a></h1>
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<b class="In" title="In">#include
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<<a class="In" title="In">monocypher.h</a>></b>
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<div class="Pp"></div>
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<var class="Ft" title="Ft">void</var>
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<br/>
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<b class="Fn" title="Fn">crypto_chacha20</b>(<var class="Fa" title="Fa">uint8_t
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*cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
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*plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
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<var class="Fa" title="Fa">const uint8_t key[32]</var>,
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<var class="Fa" title="Fa">const uint8_t nonce[8]</var>);
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<div class="Pp"></div>
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<var class="Ft" title="Ft">void</var>
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<br/>
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<b class="Fn" title="Fn">crypto_xchacha20</b>(<var class="Fa" title="Fa">uint8_t
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*cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
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*plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
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<var class="Fa" title="Fa">const uint8_t key[32]</var>,
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<var class="Fa" title="Fa">const uint8_t nonce[24]</var>);
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<div class="Pp"></div>
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<var class="Ft" title="Ft">uint64_t</var>
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<br/>
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<b class="Fn" title="Fn">crypto_chacha20_ctr</b>(<var class="Fa" title="Fa">uint8_t
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*cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
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*plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
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<var class="Fa" title="Fa">const uint8_t key[32]</var>,
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<var class="Fa" title="Fa">const uint8_t nonce[8]</var>,
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<var class="Fa" title="Fa">uint64_t ctr</var>);
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<div class="Pp"></div>
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<var class="Ft" title="Ft">uint64_t</var>
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<br/>
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<b class="Fn" title="Fn">crypto_xchacha20_ctr</b>(<var class="Fa" title="Fa">uint8_t
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*cipher_text</var>, <var class="Fa" title="Fa">const uint8_t
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*plain_text</var>, <var class="Fa" title="Fa">size_t text_size</var>,
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<var class="Fa" title="Fa">const uint8_t key[32]</var>,
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<var class="Fa" title="Fa">const uint8_t nonce[24]</var>,
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<var class="Fa" title="Fa">uint64_t ctr</var>);
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<h1 class="Sh" title="Sh" id="DESCRIPTION"><a class="selflink" href="#DESCRIPTION">DESCRIPTION</a></h1>
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These functions provide an interface for the Chacha20 encryption primitive.
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<div class="Pp"></div>
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Chacha20 is a low-level primitive. Consider using authenticated encryption,
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implemented by
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<a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>.
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<div class="Pp"></div>
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The arguments are:
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<dl class="Bl-tag">
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">key</var></dt>
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<dd class="It-tag">A 32-byte secret key.</dd>
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">nonce</var></dt>
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<dd class="It-tag">An 8-byte or 24-byte number, used only once with any given
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key. It does not need to be secret or random, but it does have to be
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unique. Repeating a nonce with the same key reveals the XOR of two
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different messages, which allows decryption. 24-byte nonces can be
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selected at random. 8-byte nonces <i class="Em" title="Em">cannot</i>.
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They are too small, and the same nonce may be selected twice by accident.
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See <a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a> for
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advice about generating random numbers (use the operating system's random
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number generator).</dd>
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">plain_text</var></dt>
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<dd class="It-tag">The message to encrypt. It is allowed to be
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<code class="Dv" title="Dv">NULL</code>, in which case it will be
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interpreted as an all zero input.
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<var class="Fa" title="Fa">cipher_text</var> will then contain the raw
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Chacha20 stream.</dd>
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">cipher_text</var></dt>
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<dd class="It-tag">The encrypted message.</dd>
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">text_size</var></dt>
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<dd class="It-tag">Length of both <var class="Fa" title="Fa">plain_text</var>
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and <var class="Fa" title="Fa">cipher_text</var>, in bytes.</dd>
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<dt class="It-tag"> </dt>
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<dd class="It-tag"> </dd>
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<dt class="It-tag"><var class="Fa" title="Fa">ctr</var></dt>
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<dd class="It-tag">The number of 64-byte blocks since the beginning of the
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stream.</dd>
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</dl>
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<div class="Pp"></div>
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The <var class="Fa" title="Fa">key</var> and
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<var class="Fa" title="Fa">nonce</var> buffers may overlap.
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<var class="Fa" title="Fa">plain_text</var> and
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<var class="Fa" title="Fa">cipher_text</var> must either be the same buffer
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(for in-place encryption), or non-overlapping.
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<div class="Pp"></div>
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<b class="Fn" title="Fn">crypto_chacha20</b>() performs a Chacha20 operation. It
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uses an 8-byte nonce, which is too small to be selected at random. Use a
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message counter as a nonce instead.
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<div class="Pp"></div>
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<b class="Fn" title="Fn">crypto_xchacha20</b>() performs an XChacha20 operation.
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It uses a 24-byte nonce, which is large enough to be selected at random.
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<div class="Pp"></div>
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<b class="Fn" title="Fn">crypto_xchacha20</b>() is recommended over
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<b class="Fn" title="Fn">crypto_chacha20</b>(). The ability to use random
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nonces makes it easier to use securely, and the performance hit is often
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negligible in practice.
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<div class="Pp"></div>
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The <b class="Fn" title="Fn">crypto_chacha20</b>() and
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<b class="Fn" title="Fn">crypto_xchacha20</b>() encrypt
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<var class="Fa" title="Fa">plain_text</var> by XORing it with a pseudo-random
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stream of numbers, seeded by the provided <var class="Fa" title="Fa">key</var>
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and <var class="Fa" title="Fa">nonce</var>.
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<div class="Pp"></div>
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Since XOR is its own inverse, decryption is the same operation as encryption. To
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decrypt the cipher text, “encrypt” it again with the same key
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and nonce. You will likely want to wipe the key when you are done with
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encryption or decryption. Use
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<a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>
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to wipe them.
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<div class="Pp"></div>
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The <var class="Fa" title="Fa">plain_text</var> pointer is allowed to be
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<code class="Dv" title="Dv">NULL</code>, in which case it will be interpreted
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as an all zero input. This is useful as a user space random number generator.
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While <b class="Sy" title="Sy">this should not be used as a random number
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generator for secrets</b>, for which the operating system random number
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generator should be preferred, it can be handy outside of a security context.
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Deterministic procedural generation and reproducible property-based tests come
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to mind. Additionally, it <i class="Em" title="Em">can</i> be used to generate
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large amounts of random-looking data quickly, for example to generate padding.
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<div class="Pp"></div>
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The <b class="Fn" title="Fn">crypto_chacha20_ctr</b>() and
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<b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() perform a Chacha20 or
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XChacha20 encryption, respectively, starting the stream at the block
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<var class="Fa" title="Fa">ctr</var> (which is the byte
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‘<code class="Li">ctr × 64</code>’). This can be used to
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encrypt (or decrypt) part of a long message, or to implement some AEAD
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constructions such as the one described in RFC 8439. Be careful when using
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this not to accidentally reuse parts of the random stream as that would
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destroy confidentiality.
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<h1 class="Sh" title="Sh" id="RETURN_VALUES"><a class="selflink" href="#RETURN_VALUES">RETURN
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VALUES</a></h1>
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<b class="Fn" title="Fn">crypto_chacha20</b>() and
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<b class="Fn" title="Fn">crypto_xchacha20</b>() return nothing.
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<b class="Fn" title="Fn">crypto_chacha20_ctr</b>() and
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<b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() functions return the next
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<var class="Fa" title="Fa">ctr</var> to use with the same key and nonce
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values; this is always <var class="Fa" title="Fa">text_size</var> divided by
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64; plus one if there was a remainder.
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<h1 class="Sh" title="Sh" id="EXAMPLES"><a class="selflink" href="#EXAMPLES">EXAMPLES</a></h1>
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The following examples assume the existence of
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<b class="Fn" title="Fn">arc4random_buf</b>(), which fills the given buffer
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with cryptographically secure random bytes. If
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<b class="Fn" title="Fn">arc4random_buf</b>() does not exist on your system,
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see <a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a> for
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advice about how to generate cryptographically secure random bytes.
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<div class="Pp"></div>
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Simple encryption:
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<div class="Pp"></div>
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<div class="Bd" style="margin-left: 5.00ex;">
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<pre class="Li">
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uint8_t key [ 32]; /* Secret random key */
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uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
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uint8_t plain_text [500] = {1}; /* Secret message */
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uint8_t cipher_text[500]; /* Encrypted message */
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arc4random_buf(key, 32);
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arc4random_buf(nonce, 24);
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crypto_xchacha20(cipher_text, plain_text, 500, key, nonce);
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/* Wipe secrets if they are no longer needed */
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crypto_wipe(key, 32);
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crypto_wipe(plain_text, 500);
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</pre>
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</div>
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<div class="Pp"></div>
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To decrypt the above:
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<div class="Pp"></div>
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<div class="Bd" style="margin-left: 5.00ex;">
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<pre class="Li">
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uint8_t key [ 32]; /* Same key as above */
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const uint8_t nonce [ 24]; /* Same nonce as above */
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uint8_t plain_text [500]; /* Message to decrypt */
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uint8_t cipher_text[500]; /* Secret message */
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crypto_xchacha20(cipher_text, plain_text, 500, key, nonce);
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/* Wipe secrets if they are no longer needed */
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crypto_wipe(key, 32);
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/* The plain text likely needs to be processed before you wipe it */
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crypto_wipe(plain_text, 12);
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</pre>
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</div>
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<div class="Pp"></div>
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Incremental encryption (in blocks of 64 bytes):
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<div class="Pp"></div>
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<div class="Bd" style="margin-left: 5.00ex;">
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<pre class="Li">
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uint8_t key [ 32]; /* Secret random key */
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uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
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uint8_t plain_text [500]; /* Secret message */
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uint8_t cipher_text[500]; /* Encrypted message */
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uint64_t ctr = 0; /* Block counter */
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unsigned int i;
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arc4random_buf(key, 32);
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arc4random_buf(nonce, 24);
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for(i = 0; i < 500; i += 64) {
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ctr = crypto_xchacha20_ctr(cipher_text+i, plain_text+i, 64,
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key, nonce, ctr);
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}
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/* Process data that didn't fit into 64 byte pieces */
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crypto_xchacha20_ctr(cipher_text+500-(i-64),
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plain_text+500-(i-64),
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500-(i-64),
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key, nonce, ctr);
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/* Wipe secrets if they are no longer needed */
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crypto_wipe(key, 32);
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crypto_wipe(plain_text, 500);
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</pre>
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</div>
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<div class="Pp"></div>
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Encryption by jumping around (do not do that, this is only meant to show how
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<b class="Fn" title="Fn">crypto_xchacha20_ctr</b>() works):
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<div class="Pp"></div>
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<div class="Bd" style="margin-left: 5.00ex;">
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<pre class="Li">
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uint8_t key [ 32]; /* Secret random key */
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uint8_t nonce [ 24]; /* Unique nonce (possibly random) */
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uint8_t plain_text [500] = {1}; /* Message to be encrypted */
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uint8_t cipher_text[500]; /* Will be the encrypted message */
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arc4random_buf(key, 32);
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arc4random_buf(nonce, 24);
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/* Encrypt the second part of the message first... */
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crypto_xchacha20_ctr(cipher_text + (3 * 64),
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plain_text + (3 * 64),
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500 - (3 * 64),
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key, nonce, 3);
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/* ...then encrypt the first part */
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crypto_xchacha20_ctr(cipher_text, plain_text, 3 * 64, key, nonce, 0);
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/* Wipe secrets if they are no longer needed */
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crypto_wipe(key, 32);
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crypto_wipe(plain_text, 500);
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</pre>
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</div>
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<h1 class="Sh" title="Sh" id="SEE_ALSO"><a class="selflink" href="#SEE_ALSO">SEE
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ALSO</a></h1>
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<a class="Xr" title="Xr" href="crypto_ietf_chacha20.html">crypto_ietf_chacha20(3monocypher)</a>,
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<a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>,
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<a class="Xr" title="Xr" href="crypto_wipe.html">crypto_wipe(3monocypher)</a>,
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<a class="Xr" title="Xr" href="intro.html">intro(3monocypher)</a>
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<h1 class="Sh" title="Sh" id="STANDARDS"><a class="selflink" href="#STANDARDS">STANDARDS</a></h1>
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These functions implement Chacha20 and XChacha20. Chacha20 is described in:
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<cite class="Rs" title="Rs"><span class="RsA">Daniel J. Bernstein</span>,
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<span class="RsT">ChaCha, a variant of Salsa20</span>, <i class="RsJ">SASC
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2008 – The State of the Art of Stream Ciphers</i>,
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||
|
|
<span class="RsP">pp. 273–278</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>
|