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.\" ========================================================================
.\"
.IX Title "EVP_KDF-TLS13_KDF 7"
.TH EVP_KDF-TLS13_KDF 7 "2023-05-07" "3.0.9" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.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
.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
.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 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>.