=pod

=head1 NAME

EVP_MD-SHAKE, EVP_MD-CSHAKE, EVP_MD-CSHAKE-KECCAK,
- The SHAKE / KECCAK family EVP_MD implementations

=head1 DESCRIPTION

Support for computing SHAKE, CSHAKE or CSHAKE-KECCAK digests through the
B<EVP_MD> API.

CSHAKE is an Extendable Output Function (XOF), that allows custom strings
"n" and "s". If these strings are both empty then it is identical to SHAKE,
otherwise it uses CSHAKE-KECCAK.

CSHAKE-KECCAK is an Extendable Output Function (XOF), with a definition
similar to SHAKE but appends 2 extra zero bits. It is used internally by CSHAKE.

=head2 Identities

This implementation is available in the FIPS provider as well as the default
provider, and includes the following varieties:

=over 4

=item SHAKE-128

Known names are "SHAKE-128" and "SHAKE128".

=item SHAKE-256

Known names are "SHAKE-256" and "SHAKE256".

=item CSHAKE-128

Known names are "CSHAKE-128" and "CSHAKE128".

=item CSHAKE-256

Known names are "CSHAKE-256" and "CSHAKE256".

=item CSHAKE-KECCAK-128

Other known names are "KECCAK-KMAC-128" and "KECCAK-KMAC128".  This is used
by L<EVP_MAC-KMAC128(7)>.  Using the notation from NIST FIPS 202
(Section 6.2), we have S<KECCAK-KMAC-128(M, d)> = S<KECCAK[256](M || 00, d)>
(see the description of KMAC128 in Appendix A of NIST SP 800-185).

=item CSHAKE-KECCAK-256

Other known names are "KECCAK-KMAC-256" and "KECCAK-KMAC256".  This is used
by L<EVP_MAC-KMAC256(7)>.  Using the notation from NIST FIPS 202
(Section 6.2), we have S<KECCAK-KMAC-256(M, d)> = S<KECCAK[512](M || 00, d)>
(see the description of KMAC256 in Appendix A of NIST SP 800-185).

=back

=head2 Parameters

This implementation supports the following L<OSSL_PARAM(3)> entries:

=over 4

=item "xoflen" (B<OSSL_DIGEST_PARAM_XOFLEN>) <unsigned integer>

Sets or Gets the digest length for extendable output functions.
The length of the "xoflen" parameter should not exceed that of a B<size_t>.

The SHAKE-128 and SHAKE-256 implementations do not have any default digest
length.

The CSHAKE-128 and CSHAKE-256 implementations have default digest lengths of
32 and 64 bytes respectively (which correspond to security strengths of 128 and
256 bits respectively).

For SHAKE this parameter must be set before calling either EVP_DigestFinal_ex()
or EVP_DigestFinal(), since these functions were not designed to handle
variable length output. If it is not set CSHAKE will use the default value.
It is recommended to either use EVP_DigestSqueeze() or
EVP_DigestFinalXOF() instead.

=item "size" (B<OSSL_DIGEST_PARAM_SIZE>) <unsigned integer>

An alias of "xoflen".

=item "function-name" (B<OSSL_DIGEST_PARAM_FUNCTION_NAME>)<UTF8 string>

Sets the function name string used by CSHAKE that can be set to one of
"", "TupleHash", "ParallelHash" or "KMAC". The default value is "".

=item "customization" (B<OSSL_DIGEST_PARAM_CUSTOMIZATION>)<UTF8 string>

Sets a customisation string used by CSHAKE.
It is an optional value with a length of at most 512 bytes, and is
empty by default.

=item "properties" (B<OSSL_DIGEST_PARAM_PROPERTIES>) <UTF8 string>

An optional property used internally by CSHAKE when fetching either
SHAKE or KECCAK algorithms.

=back

See L<EVP_DigestInit(3)/PARAMETERS> for further information related to parameters

=head1 NOTES

For SHAKE-128 and CSHAKE-128, to ensure the maximum security strength of
128 bits, the output length passed to EVP_DigestFinalXOF() should be at least 32.

For SHAKE-256 and CSHAKE-256, to ensure the maximum security strength of
256 bits, the output length passed to EVP_DigestFinalXOF() should be at least 64.

The SHA3 specification allows bit strings to be used, but OpenSSL only allows
byte strings for inputs, and outputs.

=head1 CONFORMING TO

=over 4

=item FIPS 202 (SHA3 and SHAKE)

=item SP800-185 Section 3 cSHAKE

=item SP800-185 Section 4 KMAC

=back

=head1 SEE ALSO

L<EVP_MD_CTX_set_params(3)>, L<provider-digest(7)>, L<OSSL_PROVIDER-default(7)>

=head1 HISTORY

Since OpenSSL 3.4 the SHAKE-128 and SHAKE-256 implementations have no default
digest length.

CSHAKE-128 and CSHAKE-256 were added in OpenSSL 4.0

=head1 COPYRIGHT

Copyright 2020-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
