=pod

=head1 NAME

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

=head1 DESCRIPTION

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

KECCAK-KMAC is an Extendable Output Function (XOF), with a definition
similar to SHAKE, used by the KMAC EVP_MAC implementation (see
L<EVP_MAC-KMAC(7)>).

=head2 Identities

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

=over 4

=item KECCAK-KMAC-128

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 KECCAK-KMAC-256

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).

=item SHAKE-128

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

=item SHAKE-256

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

=back

=head2 Gettable Parameters

This implementation supports the common gettable parameters described
in L<EVP_MD-common(7)>.

=head2 Settable Context Parameters

These implementations support the following L<OSSL_PARAM(3)> entries,
settable for an B<EVP_MD_CTX> with L<EVP_MD_CTX_set_params(3)>:

=over 4

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

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

For backwards compatibility reasons the default xoflen length for SHAKE-128 is
16 (bytes) which results in a security strength of only 64 bits. To ensure the
maximum security strength of 128 bits, the xoflen should be set to at least 32.

For backwards compatibility reasons the default xoflen length for SHAKE-256 is
32 (bytes) which results in a security strength of only 128 bits. To ensure the
maximum security strength of 256 bits, the xoflen should be set to at least 64.

This parameter may be used when calling either EVP_DigestFinal_ex() or
EVP_DigestFinal(), since these functions were not designed to handle variable
length output. It is recommended to either use EVP_DigestSqueeze() or
EVP_DigestFinalXOF() instead.

=back

=head1 NOTES

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

=head1 SEE ALSO

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

=head1 COPYRIGHT

Copyright 2020-2023 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
