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<title>CRYPTO_SIGN_INIT_FIRST_PASS(3MONOCYPHER)</title>
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<table class="head">
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<td class="head-ltitle">CRYPTO_SIGN_INIT_FIRST_PASS(3MONOCYPHER)</td>
<td class="head-vol">3MONOCYPHER</td>
<td class="head-rtitle">CRYPTO_SIGN_INIT_FIRST_PASS(3MONOCYPHER)</td>
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</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_sign_init_first_pass</b>,
<b class="Nm" title="Nm">crypto_sign_update</b>,
<b class="Nm" title="Nm">crypto_sign_final</b>,
<b class="Nm" title="Nm">crypto_sign_init_second_pass</b>,
<b class="Nm" title="Nm">crypto_check_init</b>,
<b class="Nm" title="Nm">crypto_check_update</b>,
<b class="Nm" title="Nm">crypto_check_final</b> &#x2014;
<span class="Nd" title="Nd">incremental public key signatures</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_sign_init_first_pass</b>(<var class="Fa" title="Fa">crypto_sign_ctx
*ctx</var>, <var class="Fa" title="Fa">const uint8_t secret_key[32]</var>,
<var class="Fa" title="Fa">const uint8_t public_key[32]</var>);
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_sign_update</b>(<var class="Fa" title="Fa">crypto_sign_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>
<br/>
<b class="Fn" title="Fn">crypto_sign_final</b>(<var class="Fa" title="Fa">crypto_sign_ctx
*ctx</var>, <var class="Fa" title="Fa">uint8_t signature[64]</var>);
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_sign_init_second_pass</b>(<var class="Fa" title="Fa">crypto_sign_ctx
*ctx</var>);
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_check_init</b>(<var class="Fa" title="Fa">crypto_check_ctx
*ctx</var>, <var class="Fa" title="Fa">const uint8_t signature[64]</var>,
<var class="Fa" title="Fa">const uint8_t public_key[32]</var>);
<div class="Pp"></div>
<var class="Ft" title="Ft">void</var>
<br/>
<b class="Fn" title="Fn">crypto_check_update</b>(<var class="Fa" title="Fa">crypto_check_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">int</var>
<br/>
<b class="Fn" title="Fn">crypto_check_final</b>(<var class="Fa" title="Fa">crypto_check_ctx
*ctx</var>);
<h1 class="Sh" title="Sh" id="DESCRIPTION"><a class="selflink" href="#DESCRIPTION">DESCRIPTION</a></h1>
These functions are variants of
<a class="Xr" title="Xr" href="crypto_sign.html">crypto_sign(3monocypher)</a>
and
<a class="Xr" title="Xr" href="crypto_check.html">crypto_check(3monocypher)</a>.
Prefer those simpler functions if possible.
<div class="Pp"></div>
The arguments are the same as those described in
<a class="Xr" title="Xr" href="crypto_sign.html">crypto_sign(3monocypher)</a>.
<div class="Pp"></div>
This incremental interface can be used to sign or verify messages too large to
fit in a single buffer. The arguments are the same as the direct interface
described in
<a class="Xr" title="Xr" href="crypto_sign.html">crypto_sign(3monocypher)</a>.
<div class="Pp"></div>
The direct and incremental interface produce and accept the same signatures.
<div class="Pp"></div>
Signing is done in two passes. This requires five steps:
<ul class="Bl-bullet">
<li class="It-bullet">Initialisation of the first pass with
<b class="Fn" title="Fn">crypto_sign_init_first_pass</b>(). The public key
is optional, and will be recomputed if not provided. This recomputation
doubles the execution time for short messages.</li>
<li class="It-bullet">The first pass proper, with
<b class="Fn" title="Fn">crypto_sign_update</b>().
<b class="Sy" title="Sy">Under no circumstances must you forget the first
pass</b>: Forgetting to call
<b class="Fn" title="Fn">crypto_sign_update</b>() will appear to work in
that it produces valid signatures, but also loses all security because
attackers may now recover the secret key.</li>
<li class="It-bullet">Initialisation of the second pass with
<b class="Fn" title="Fn">crypto_sign_init_second_pass</b>().</li>
<li class="It-bullet">The second pass proper, with
<b class="Fn" title="Fn">crypto_sign_update</b>(). The same update
function is used for both passes.</li>
<li class="It-bullet">Signature generation with
<b class="Fn" title="Fn">crypto_sign_final</b>(). This also wipes the
context.</li>
</ul>
<div class="Pp"></div>
Verification requires three steps:
<ul class="Bl-bullet">
<li class="It-bullet">Initialisation with
<b class="Fn" title="Fn">crypto_check_init</b>().</li>
<li class="It-bullet">Update with
<b class="Fn" title="Fn">crypto_check_update</b>().</li>
<li class="It-bullet">Signature verification with
<b class="Fn" title="Fn">crypto_check_final</b>().</li>
</ul>
<h1 class="Sh" title="Sh" id="RETURN_VALUES"><a class="selflink" href="#RETURN_VALUES">RETURN
VALUES</a></h1>
<b class="Fn" title="Fn">crypto_sign_init_first_pass</b>(),
<b class="Fn" title="Fn">crypto_sign_init_second_pass</b>(),
<b class="Fn" title="Fn">crypto_sign_update</b>(),
<b class="Fn" title="Fn">crypto_sign_final</b>(),
<b class="Fn" title="Fn">crypto_check_init</b>() and
<b class="Fn" title="Fn">crypto_check_update</b>() return nothing.
<div class="Pp"></div>
<b class="Fn" title="Fn">crypto_check_final</b>() returns 0 for legitimate
messages and -1 for forgeries.
<h1 class="Sh" title="Sh" id="EXAMPLES"><a class="selflink" href="#EXAMPLES">EXAMPLES</a></h1>
Sign a message:
<div class="Pp"></div>
<div class="Bd" style="margin-left: 5.00ex;">
<pre class="Li">
uint8_t sk [ 32]; /* Secret key */
const uint8_t pk [ 32]; /* Public key (optional) */
const uint8_t message [500]; /* Message to sign */
uint8_t signature[ 64]; /* Output signature */
crypto_sign_ctx ctx;
crypto_sign_init_first_pass((crypto_sign_ctx_abstract*)&amp;ctx, sk, pk);
/* Wipe the secret key if no longer needed */
crypto_wipe(sk, 32);
for (size_t i = 0; i &lt; 500; i += 100) {
crypto_sign_update((crypto_sign_ctx_abstract*)&amp;ctx, message + i, 100);
}
crypto_sign_init_second_pass((crypto_sign_ctx_abstract*)&amp;ctx);
for (size_t i = 0; i &lt; 500; i += 100) {
crypto_sign_update((crypto_sign_ctx_abstract*)&amp;ctx, message + i, 100);
}
crypto_sign_final((crypto_sign_ctx_abstract*)&amp;ctx, signature);
</pre>
</div>
<div class="Pp"></div>
Check the above:
<div class="Pp"></div>
<div class="Bd" style="margin-left: 5.00ex;">
<pre class="Li">
const uint8_t pk [ 32]; /* Public key */
const uint8_t message [500]; /* Message to sign */
const uint8_t signature[ 64]; /* Signature to check */
crypto_check_ctx ctx;
crypto_check_init((crypto_sign_ctx_abstract*)&amp;ctx, signature, pk);
for (size_t i = 0; i &lt; 500; i += 100) {
crypto_check_update((crypto_sign_ctx_abstract*)&amp;ctx, message + i, 100);
}
if (crypto_check_final((crypto_sign_ctx_abstract*)&amp;ctx)) {
/* Message is corrupted, abort processing */
} else {
/* Message is genuine */
}
</pre>
</div>
<div class="Pp"></div>
This interface can be used to mitigate attacks that leverage power analysis and
fault injection (glitching) &#x2013; both of which require physical access and
appropriate equipment &#x2013; by injecting additional randomness (at least 32
bytes) and padding (to the hash function's block size, which is 128 bytes for
all hash functions supported by Monocypher), of which 32 bytes are already
inserted into the buffer by
<b class="Fn" title="Fn">crypto_sign_init_first_pass</b>(). Access to a
cryptographically secure pseudo-random generator is a requirement for
effective side channel mitigation. Signing a message with increased
power-related side channel mitigations:
<div class="Pp"></div>
<div class="Bd" style="margin-left: 5.00ex;">
<pre class="Li">
const uint8_t message [ 500]; /* Message to sign */
uint8_t sk [ 32]; /* Secret key */
const uint8_t pk [ 32]; /* Public key (optional) */
uint8_t signature[ 64]; /* Output signature */
uint8_t buf [128-32] = {0}; /* Mitigation buffer */
crypto_sign_ctx ctx;
crypto_sign_ctx_abstract *actx = (crypto_sign_ctx_abstract *)&amp;ctx;
arc4random_buf(buf, 32);
/* The rest of buf MUST be zeroes. */
crypto_sign_init_first_pass(actx, sk, pk);
crypto_sign_update (actx, buf, sizeof(buf));
crypto_sign_update (actx, message, 500);
crypto_sign_init_second_pass(actx);
crypto_sign_update (actx, message, 500);
crypto_sign_final (actx, signature);
crypto_wipe(buf, 32);
/* Wipe the secret key if no longer needed */
crypto_wipe(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_blake2b.html">crypto_blake2b(3monocypher)</a>,
<a class="Xr" title="Xr" href="crypto_key_exchange.html">crypto_key_exchange(3monocypher)</a>,
<a class="Xr" title="Xr" href="crypto_lock.html">crypto_lock(3monocypher)</a>,
<a class="Xr" title="Xr" href="crypto_sign.html">crypto_sign(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 PureEdDSA with Curve25519 and Blake2b, as described in
RFC 8032. This is the same as Ed25519, with Blake2b instead of SHA-512.
<div class="Pp"></div>
The example for side channel mitigation follows the methodology outlined in
I-D.draft-mattsson-cfrg-det-sigs-with-noise-02.
<h1 class="Sh" title="Sh" id="HISTORY"><a class="selflink" href="#HISTORY">HISTORY</a></h1>
The <b class="Fn" title="Fn">crypto_sign_init_first_pass</b>(),
<b class="Fn" title="Fn">crypto_sign_update</b>(),
<b class="Fn" title="Fn">crypto_sign_final</b>(),
<b class="Fn" title="Fn">crypto_sign_init_second_pass</b>(),
<b class="Fn" title="Fn">crypto_check_init</b>(),
<b class="Fn" title="Fn">crypto_check_update</b>(), and
<b class="Fn" title="Fn">crypto_check_final</b>() functions first appeared in
Monocypher 1.1.0.
<div class="Pp"></div>
<b class="Sy" title="Sy">A critical security vulnerability</b> that caused
all-zero signatures to be accepted was introduced in Monocypher 0.3; it was
fixed in Monocypher 1.1.1 and 2.0.4.
<h1 class="Sh" title="Sh" id="SECURITY_CONSIDERATIONS"><a class="selflink" href="#SECURITY_CONSIDERATIONS">SECURITY
CONSIDERATIONS</a></h1>
Messages are not verified until the call to
<b class="Fn" title="Fn">crypto_check_final</b>(). Messages may be stored
before they are verified, but they cannot be
<i class="Em" title="Em">trusted</i>. Processing untrusted messages increases
the attack surface of the system. Doing so securely is hard. Do not process
messages before calling <b class="Fn" title="Fn">crypto_check_final</b>().
<div class="Pp"></div>
When signing messages, the security considerations documented in
<a class="Xr" title="Xr" href="crypto_sign.html">crypto_sign(3monocypher)</a>
also apply. In particular, if power-related side channels are part of your
threat model, note that there may still be other power-related side channels
(such as if the CPU leaks information when an operation overflows a register)
that must be considered.
<h1 class="Sh" title="Sh" id="IMPLEMENTATION_DETAILS"><a class="selflink" href="#IMPLEMENTATION_DETAILS">IMPLEMENTATION
DETAILS</a></h1>
EdDSA signatures require two passes that cannot be performed in parallel. There
are ways around this limitation, but they all lower security in some way. For
this reason, Monocypher does not support them.</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>