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.\" ========================================================================
.\"
.IX Title "EVP_KDF-HKDF 7"
.TH EVP_KDF-HKDF 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\-HKDF \- The HKDF EVP_KDF implementation
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Support for computing 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\-HKDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function.
\&\s-1HKDF\s0 follows the \*(L"extract-then-expand\*(R" paradigm, where the \s-1KDF\s0 logically
consists of two modules. The first stage takes the input keying material
and \*(L"extracts\*(R" from it a fixed-length pseudorandom key K. The second stage
\&\*(L"expands\*(R" the key K into several additional pseudorandom keys (the output
of the \s-1KDF\s0).
.SS "Identity"
.IX Subsection "Identity"
\&\*(L"\s-1HKDF\*(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 """info"" (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) <octet string>" 4
.el .IP "``info'' (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) <octet string>" 4
.IX Item "info (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 \s-1HKDF.\s0
.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-1HKDF\s0 operation.
There are three modes that are currently defined:
.RS 4
.ie n .IP """\s-1EXTRACT_AND_EXPAND""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR" 4
.el .IP "``\s-1EXTRACT_AND_EXPAND''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR" 4
.IX Item "EXTRACT_AND_EXPAND or EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND"
This is the default mode.  Calling \fBEVP_KDF_derive\fR\|(3) on an \s-1EVP_KDF_CTX\s0 set
up for \s-1HKDF\s0 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.
.Sp
In this mode the digest, key, salt and info values must be set before a key is
derived otherwise an error will occur.
.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"
A context for \s-1HKDF\s0 can be obtained by calling:
.PP
.Vb 2
\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
.Ve
.PP
The output length of an \s-1HKDF\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 "EXAMPLES"
.IX Header "EXAMPLES"
This example derives 10 bytes using \s-1SHA\-256\s0 with the secret key \*(L"secret\*(R",
salt value \*(L"salt\*(R" and info value \*(L"label\*(R":
.PP
.Vb 4
\& 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);
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 5869\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\-TLS13_KDF\s0\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
This functionality was added in OpenSSL 3.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-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>.