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.\"
.IX Title "EVP_PKEY_CTX_SET_HKDF_MD 3"
.TH EVP_PKEY_CTX_SET_HKDF_MD 3 "2023-09-11" "1.1.1w" "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_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt, EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info, EVP_PKEY_CTX_hkdf_mode \- HMAC\-based Extract\-and\-Expand key derivation algorithm
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/kdf.h>
\&
\& int EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
\&
\& int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
\&
\& int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
\&                                 int saltlen);
\&
\& int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
\&                                int keylen);
\&
\& int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
\&                                 int infolen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \s-1EVP_PKEY_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).
.PP
\&\fBEVP_PKEY_CTX_hkdf_mode()\fR sets the mode for the \s-1HKDF\s0 operation. There are three
modes that are currently defined:
.IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND\s0" 4
.IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND"
This is the default mode. Calling \fBEVP_PKEY_derive\fR\|(3) on an \s-1EVP_PKEY_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 or an error occurs.
.IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY\s0" 4
.IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY"
In this mode calling \fBEVP_PKEY_derive\fR\|(3) will just perform the extract
operation. The value returned will be the intermediate fixed-length pseudorandom
key K.
.Sp
The digest, key and salt values must be set before a key is derived or an
error occurs.
.IP "\s-1EVP_PKEY_HKDEF_MODE_EXPAND_ONLY\s0" 4
.IX Item "EVP_PKEY_HKDEF_MODE_EXPAND_ONLY"
In this mode calling \fBEVP_PKEY_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 or an
error occurs.
.PP
\&\fBEVP_PKEY_CTX_set_hkdf_md()\fR sets the message digest associated with the \s-1HKDF.\s0
.PP
\&\fBEVP_PKEY_CTX_set1_hkdf_salt()\fR sets the salt to \fBsaltlen\fR bytes of the
buffer \fBsalt\fR. Any existing value is replaced.
.PP
\&\fBEVP_PKEY_CTX_set1_hkdf_key()\fR sets the key to \fBkeylen\fR bytes of the buffer
\&\fBkey\fR. Any existing value is replaced.
.PP
\&\fBEVP_PKEY_CTX_add1_hkdf_info()\fR sets the info value to \fBinfolen\fR bytes of the
buffer \fBinfo\fR. If a value is already set, it is appended to the existing
value.
.SH "STRING CTRLS"
.IX Header "STRING CTRLS"
\&\s-1HKDF\s0 also supports string based control operations via
\&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3).
The \fBtype\fR parameter \*(L"md\*(R" uses the supplied \fBvalue\fR as the name of the digest
algorithm to use.
The \fBtype\fR parameter \*(L"mode\*(R" uses the values \*(L"\s-1EXTRACT_AND_EXPAND\*(R",
\&\*(L"EXTRACT_ONLY\*(R"\s0 and \*(L"\s-1EXPAND_ONLY\*(R"\s0 to determine the mode to use.
The \fBtype\fR parameters \*(L"salt\*(R", \*(L"key\*(R" and \*(L"info\*(R" use the supplied \fBvalue\fR
parameter as a \fBseed\fR, \fBkey\fR or \fBinfo\fR value.
The names \*(L"hexsalt\*(R", \*(L"hexkey\*(R" and \*(L"hexinfo\*(R" are similar except they take a hex
string which is converted to binary.
.SH "NOTES"
.IX Header "NOTES"
All these functions are implemented as macros.
.PP
A context for \s-1HKDF\s0 can be obtained by calling:
.PP
.Vb 1
\& EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
.Ve
.PP
The total length of the info buffer cannot exceed 1024 bytes in length: this
should be more than enough for any normal use of \s-1HKDF.\s0
.PP
The output length of an \s-1HKDF\s0 expand operation is specified via the length
parameter to the \fBEVP_PKEY_derive\fR\|(3) function.
Since the \s-1HKDF\s0 output length is variable, passing a \fB\s-1NULL\s0\fR buffer as a means
to obtain the requisite length is not meaningful with \s-1HKDF\s0 in any mode that
performs an expand operation. Instead, the caller must allocate a buffer of the
desired length, and pass that buffer to \fBEVP_PKEY_derive\fR\|(3) along with (a
pointer initialized to) the desired length. Passing a \fB\s-1NULL\s0\fR buffer to obtain
the length is allowed when using \s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.\s0
.PP
Optimised versions of \s-1HKDF\s0 can be implemented in an \s-1ENGINE.\s0
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
All these functions return 1 for success and 0 or a negative value for failure.
In particular a return value of \-2 indicates the operation is not supported by
the public key algorithm.
.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_PKEY_CTX *pctx;
\& unsigned char out[10];
\& size_t outlen = sizeof(out);
\& pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
\&
\& if (EVP_PKEY_derive_init(pctx) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
\&     /* Error */
\& if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
\&     /* Error */
\& if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
\&     /* Error */
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 5869\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBEVP_PKEY_CTX_new\fR\|(3),
\&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3),
\&\fBEVP_PKEY_derive\fR\|(3)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-2019 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (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>.