=pod

=head1 NAME

EVP_KDF-HKDF - The HKDF EVP_KDF implementations

=head1 DESCRIPTION

Support for computing the B<HKDF> KDF through the B<EVP_KDF> API.

The EVP_KDF-HKDF algorithm implements the HKDF key derivation function.
HKDF follows the "extract-then-expand" paradigm, where the KDF logically
consists of two modules. The first stage takes the input keying material
and "extracts" from it a fixed-length pseudorandom key K. The second stage
"expands" the key K into several additional pseudorandom keys (the output
of the KDF).

The output is considered to be keying material.

=head2 Identity

The following algorithms are available for this implementation; they
can be used with the EVP_KDF_fetch() function.

In this list, names are grouped together to signify that they are the same
algorithm having multiple names.  This also includes the OID in canonical
decimal form (which means that they are possible to fetch if the caller has a
mere OID which came out in this form after a call to L<OBJ_obj2txt(3)>).

=over 4

=item "HKDF"

The B<OSSL_KDF_PARAM_DIGEST> parameter must be set for B<HKDF> before it can
be used.

=item "HKDF-SHA256", "id-alg-hkdf-with-sha256", "1.2.840.113549.1.9.16.3.28"

=item "HKDF-SHA384", "id-alg-hkdf-with-sha384", "1.2.840.113549.1.9.16.3.29"

=item "HKDF-SHA512", "id-alg-hkdf-with-sha512", "1.2.840.113549.1.9.16.3.30"

B<HKDF-SHA256>, B<HKDF-SHA384> and B<HKDF-SHA512> are fixed-digest versions
of B<HKDF> with the appropriate digest already configured.
L<EVP_KDF_CTX_reset(3)> will not reset the context's digest for fixed-digest
versions.

=back

=head2 Supported parameters

The supported parameters are:

=over 4

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

=item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string>

Attempting to set the digest on a fixed-digest B<HKDF> will result in an error.

=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>

=item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string>

These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.

=item "info" (B<OSSL_KDF_PARAM_INFO>) <octet string>

This parameter sets the info value.
The length of the context info buffer cannot exceed 1024 bytes;
this should be more than enough for any normal use of HKDF.

=item "mode" (B<OSSL_KDF_PARAM_MODE>) <UTF8 string> or <integer>

This parameter sets the mode for the HKDF operation.
There are three modes that are currently defined:

=over 4

=item "EXTRACT_AND_EXPAND" or B<EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND>

This is the default mode.  Calling L<EVP_KDF_derive(3)> on an EVP_KDF_CTX set
up for HKDF will perform an extract followed by an expand operation in one go.
The derived key returned will be the result after the expand operation. The
intermediate fixed-length pseudorandom key K is not returned.

In this mode the key, salt and info values must be set before a key is
derived otherwise an error will occur. For non-fixed mode (B<HKDF>) the digest
must also be set.

=item "EXTRACT_ONLY" or B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY>

In this mode calling L<EVP_KDF_derive(3)> will just perform the extract
operation. The value returned will be the intermediate fixed-length pseudorandom
key K.  The I<keylen> parameter must match the size of K, which can be looked
up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.

The key and salt values must be set before a key is derived otherwise
an error will occur. For non-fixed mode (B<HKDF>) the digest
must also be set.

=item "EXPAND_ONLY" or B<EVP_KDF_HKDF_MODE_EXPAND_ONLY>

In this mode calling L<EVP_KDF_derive(3)> will just perform the expand
operation. The input key should be set to the intermediate fixed-length
pseudorandom key K returned from a previous extract operation.

The key and info values must be set before a key is derived otherwise
an error will occur. For non-fixed mode (B<HKDF>) the digest
must also be set.

=back

=back

The OpenSSL FIPS provider also supports the following parameters:

=over 4

=item "fips-indicator" (B<OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR>) <integer>

A getter that returns 1 if the operation is FIPS approved, or 0 otherwise.
This may be used after calling EVP_KDF_derive. It returns 0 if "key-check"
is set to 0 and the check fails.

=item "key-check" (B<OSSL_KDF_PARAM_FIPS_KEY_CHECK>) <integer>

The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the
length of used key-derivation key (B<OSSL_KDF_PARAM_KEY>) is shorter than 112
bits.
Setting this to zero will ignore the error and set the approved
"fips-indicator" to 0.
This option breaks FIPS compliance if it causes the approved "fips-indicator"
to return 0.

=back

=head1 NOTES

A context for HKDF can be obtained by calling:

 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of an HKDF expand operation is specified via the I<keylen>
parameter to the L<EVP_KDF_derive(3)> function.  When using
EVP_KDF_HKDF_MODE_EXTRACT_ONLY the I<keylen> parameter must equal the size of
the intermediate fixed-length pseudorandom key otherwise an error will occur.
For that mode, the fixed output size can be looked up by calling EVP_KDF_CTX_get_kdf_size()
after setting the mode and digest on the B<EVP_KDF_CTX>.

=head1 EXAMPLES

=head2 HKDF Algorithm

This example derives 10 bytes using SHA-256 with the secret key "secret",
salt value "salt" and info value "label":

 EVP_KDF *kdf;
 EVP_KDF_CTX *kctx;
 unsigned char out[10];
 OSSL_PARAM params[5], *p = params;

 kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
 kctx = EVP_KDF_CTX_new(kdf);
 EVP_KDF_free(kdf);

 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                         SN_sha256, strlen(SN_sha256));
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                          "secret", (size_t)6);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                          "label", (size_t)5);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                          "salt", (size_t)4);
 *p = OSSL_PARAM_construct_end();
 if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
     error("EVP_KDF_derive");
 }

 EVP_KDF_CTX_free(kctx);

=head2 HKDF-SHA256 Algorithm

This example derives 10 bytes using HKDF-SHA256 with the secret key "secret",
salt value "salt" and info value "label":

 EVP_KDF *kdf;
 EVP_KDF_CTX *kctx;
 unsigned char out[10];
 OSSL_PARAM params[4], *p = params;

 kdf = EVP_KDF_fetch(NULL, "HKDF-SHA256", NULL);
 kctx = EVP_KDF_CTX_new(kdf);
 EVP_KDF_free(kdf);

 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                          "secret", (size_t)6);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                          "label", (size_t)5);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                          "salt", (size_t)4);
 *p = OSSL_PARAM_construct_end();
 if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
     error("EVP_KDF_derive");
 }

 EVP_KDF_CTX_free(kctx);

=head1 CONFORMING TO

RFC 5869 and RFC 8619

=head1 SEE ALSO

L<EVP_KDF(3)>,
L<EVP_KDF_CTX_new(3)>,
L<EVP_KDF_CTX_free(3)>,
L<EVP_KDF_CTX_get_kdf_size(3)>,
L<EVP_KDF_CTX_set_params(3)>,
L<EVP_KDF_derive(3)>,
L<EVP_KDF(3)/PARAMETERS>,
L<EVP_KDF-TLS13_KDF(7)>

=head1 HISTORY

HKDF-SHA256, HKDF-SHA384 and HKDF-SHA512 were added in OpenSSL 3.6.

All other functionality was added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2016-2025 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
