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.\" ========================================================================
.\"
.IX Title "EVP_KDF-TLS13_KDF 7ossl"
.TH EVP_KDF-TLS13_KDF 7ossl "2026-01-16" "3.5.1" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
EVP_KDF\-TLS13_KDF \- The TLS 1.3 EVP_KDF implementation
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Support for computing the \s-1TLS 1.3\s0 version of the \fB\s-1HKDF\s0\fR \s-1KDF\s0 through
the \fB\s-1EVP_KDF\s0\fR \s-1API.\s0
.PP
The \s-1EVP_KDF\-TLS13_KDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function
as used by \s-1TLS 1.3.\s0
.PP
The output is considered to be keying material.
.SS "Identity"
.IX Subsection "Identity"
\&\*(L"\s-1TLS13\-KDF\*(R"\s0 is the name for this implementation; it
can be used with the \fBEVP_KDF_fetch()\fR function.
.SS "Supported parameters"
.IX Subsection "Supported parameters"
The supported parameters are:
.ie n .IP """properties"" (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4
.el .IP "``properties'' (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4
.IX Item "properties (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>"
.PD 0
.ie n .IP """digest"" (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4
.el .IP "``digest'' (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4
.IX Item "digest (OSSL_KDF_PARAM_DIGEST) <UTF8 string>"
.ie n .IP """key"" (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) <octet string>" 4
.el .IP "``key'' (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) <octet string>" 4
.IX Item "key (OSSL_KDF_PARAM_KEY) <octet string>"
.ie n .IP """salt"" (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) <octet string>" 4
.el .IP "``salt'' (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) <octet string>" 4
.IX Item "salt (OSSL_KDF_PARAM_SALT) <octet string>"
.PD
These parameters work as described in \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3).
.ie n .IP """prefix"" (\fB\s-1OSSL_KDF_PARAM_PREFIX\s0\fR) <octet string>" 4
.el .IP "``prefix'' (\fB\s-1OSSL_KDF_PARAM_PREFIX\s0\fR) <octet string>" 4
.IX Item "prefix (OSSL_KDF_PARAM_PREFIX) <octet string>"
This parameter sets the label prefix on the specified \s-1TLS 1.3 KDF\s0 context.
For \s-1TLS 1.3\s0 this should be set to the \s-1ASCII\s0 string \*(L"tls13 \*(R" without a
trailing zero byte.  Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details.
.ie n .IP """label"" (\fB\s-1OSSL_KDF_PARAM_LABEL\s0\fR) <octet string>" 4
.el .IP "``label'' (\fB\s-1OSSL_KDF_PARAM_LABEL\s0\fR) <octet string>" 4
.IX Item "label (OSSL_KDF_PARAM_LABEL) <octet string>"
This parameter sets the label on the specified \s-1TLS 1.3 KDF\s0 context.
Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details.
.ie n .IP """data"" (\fB\s-1OSSL_KDF_PARAM_DATA\s0\fR) <octet string>" 4
.el .IP "``data'' (\fB\s-1OSSL_KDF_PARAM_DATA\s0\fR) <octet string>" 4
.IX Item "data (OSSL_KDF_PARAM_DATA) <octet string>"
This parameter sets the context data on the specified \s-1TLS 1.3 KDF\s0 context.
Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details.
.ie n .IP """mode"" (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or <integer>" 4
.el .IP "``mode'' (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or <integer>" 4
.IX Item "mode (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>"
This parameter sets the mode for the \s-1TLS 1.3 KDF\s0 operation.
There are two modes that are currently defined:
.RS 4
.ie n .IP """\s-1EXTRACT_ONLY""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR" 4
.el .IP "``\s-1EXTRACT_ONLY''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR" 4
.IX Item "EXTRACT_ONLY or EVP_KDF_HKDF_MODE_EXTRACT_ONLY"
In this mode calling \fBEVP_KDF_derive\fR\|(3) will just perform the extract
operation. The value returned will be the intermediate fixed-length pseudorandom
key K.  The \fIkeylen\fR parameter must match the size of K, which can be looked
up by calling \fBEVP_KDF_CTX_get_kdf_size()\fR after setting the mode and digest.
.Sp
The digest, key and salt values must be set before a key is derived otherwise
an error will occur.
.ie n .IP """\s-1EXPAND_ONLY""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR" 4
.el .IP "``\s-1EXPAND_ONLY''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR" 4
.IX Item "EXPAND_ONLY or EVP_KDF_HKDF_MODE_EXPAND_ONLY"
In this mode calling \fBEVP_KDF_derive\fR\|(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.
.Sp
The digest, key and info values must be set before a key is derived otherwise
an error will occur.
.RE
.RS 4
.RE
.PP
The OpenSSL \s-1FIPS\s0 provider also supports the following parameters:
.ie n .IP """fips-indicator"" (\fB\s-1OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR\s0\fR) <integer>" 4
.el .IP "``fips-indicator'' (\fB\s-1OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR\s0\fR) <integer>" 4
.IX Item "fips-indicator (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer>"
A getter that returns 1 if the operation is \s-1FIPS\s0 approved, or 0 otherwise.
This may be used after calling EVP_KDF_derive. It returns 0 if any \*(L"***\-check\*(R"
related parameter is set to 0 and the check fails.
.ie n .IP """digest-check"" (\fB\s-1OSSL_KDF_PARAM_FIPS_DIGEST_CHECK\s0\fR) <integer>" 4
.el .IP "``digest-check'' (\fB\s-1OSSL_KDF_PARAM_FIPS_DIGEST_CHECK\s0\fR) <integer>" 4
.IX Item "digest-check (OSSL_KDF_PARAM_FIPS_DIGEST_CHECK) <integer>"
The default value of 1 causes an error during \fBEVP_KDF_CTX_set_params()\fR if
used digest is not approved.
Setting this to zero will ignore the error and set the approved
\&\*(L"fips-indicator\*(R" to 0.
This option breaks \s-1FIPS\s0 compliance if it causes the approved \*(L"fips-indicator\*(R"
to return 0.
.Sp
According to \s-1RFC 8446,\s0 the following are approved digest algorithms: \s-1SHA2\-256,
SHA2\-384.\s0
.ie n .IP """key-check"" (\fB\s-1OSSL_KDF_PARAM_FIPS_KEY_CHECK\s0\fR) <integer>" 4
.el .IP "``key-check'' (\fB\s-1OSSL_KDF_PARAM_FIPS_KEY_CHECK\s0\fR) <integer>" 4
.IX Item "key-check (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer>"
The default value of 1 causes an error during \fBEVP_KDF_CTX_set_params()\fR if the
length of used key-derivation key (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) is shorter than 112
bits.
Setting this to zero will ignore the error and set the approved
\&\*(L"fips-indicator\*(R" to 0.
This option breaks \s-1FIPS\s0 compliance if it causes the approved \*(L"fips-indicator\*(R"
to return 0.
.SH "NOTES"
.IX Header "NOTES"
This \s-1KDF\s0 is intended for use by the \s-1TLS 1.3\s0 implementation in libssl.
It does not support all the options and capabilities that \s-1HKDF\s0 does.
.PP
The \fI\s-1OSSL_PARAM\s0\fR array passed to \fBEVP_KDF_derive\fR\|(3) or
\&\fBEVP_KDF_CTX_set_params\fR\|(3) must specify all of the parameters required.
This \s-1KDF\s0 does not support a piecemeal approach to providing these.
.PP
A context for a \s-1TLS 1.3 KDF\s0 can be obtained by calling:
.PP
.Vb 2
\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13\-KDF", NULL);
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
.Ve
.PP
The output length of a \s-1TLS 1.3 KDF\s0 expand operation is specified via the
\&\fIkeylen\fR parameter to the \fBEVP_KDF_derive\fR\|(3) function.  When using
\&\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 the \fIkeylen\fR 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
\&\fBEVP_KDF_CTX_get_kdf_size()\fR after setting the mode and digest on the
\&\fB\s-1EVP_KDF_CTX\s0\fR.
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 8446\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\s-1\fBEVP_KDF\s0\fR\|(3),
\&\fBEVP_KDF_CTX_new\fR\|(3),
\&\fBEVP_KDF_CTX_free\fR\|(3),
\&\fBEVP_KDF_CTX_get_kdf_size\fR\|(3),
\&\fBEVP_KDF_CTX_set_params\fR\|(3),
\&\fBEVP_KDF_derive\fR\|(3),
\&\*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3),
\&\s-1\fBEVP_KDF\-HKDF\s0\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
This functionality was added in OpenSSL 3.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2021\-2024 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
