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<p class="source-line" data-source-line="2">We are currently working
on the integration of PQC algorithms in Libgcrypt based on <a
href="https://datatracker.ietf.org/doc/draft-wussler-openpgp-pqc/">draft-wussler-openpgp-pqc</a>
and will also add KMAC to Libgcrypt since this algorithm is used
for the key derivation inside the key combiner.</p>
<p class="source-line" data-source-line="5"><a
href="https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-185.pdf#page=16">KMAC</a>
is based on <a
href="https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-185.pdf#page=13">cSHAKE</a>,
which is variant of SHAKE that requires a
different final bit padding than SHAKE and is currently not
implemented in Libgcrypt.
cSHAKE is defined as</p>
<pre><code>cSHAKE(X, L, N, S):
1. If N = "" and S = "":
return SHAKE256(X, L);
2. Else:
return KECCAK[256](bytepad(encode_string(N) || encode_string(S), 168) || X || 00, L)
</code></pre>
<p class="source-line" data-source-line="17">In order to support the
additional arguments N and S, I propose the following approach:</p>
<ul>
<li class="source-line" data-source-line="19">
<p class="source-line" data-source-line="19">cSHAKE is added as
an XOF message digest like SHAKE</p>
</li>
<li class="source-line" data-source-line="20">
<p class="source-line" data-source-line="20">For the purpose of
providing the additional arguments N and S we add</p>
<pre><code>typedef enum
{
GCRY_MD_ADDIN_CSHAKE_N = 1,
GCRY_MD_ADDIN_CSHAKE_S = 2
} gcry_md_add_input_t;
gcry_error_t
gcry_md_set_add_input (gcry_md_hd_t *h,
gcry_md_add_input_t addin_type,
const void* v, size_t v_len)
</code></pre>
<p class="source-line" data-source-line="34">In order to invoke
cSHAKE with non-empty N and S parameters, after the call to <code>_gcry_md_open()</code>,
two calls to <code>gcry_md_set_add_input()</code> have to be
made to set N and S in that order. If data is added without
having made these calls,
then it will behave as normal SHAKE as required by the
specification.</p>
</li>
</ul>
<p class="source-line" data-source-line="37">Does anyone have any
thoughts on this?</p>
<p class="source-line" data-source-line="37">- Falko<br>
</p>
<p></p>
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