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.\" ========================================================================
.\"
.IX Title "EVP_KDF_HKDF 7"
.TH EVP_KDF_HKDF 7 "2026-07-13" "1.1.1k" "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_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 "Numeric identity"
.IX Subsection "Numeric identity"
\&\fB\s-1EVP_KDF_HKDF\s0\fR is the numeric identity for this implementation; it
can be used with the \fBEVP_KDF_CTX_new_id()\fR function.
.SS "Supported controls"
.IX Subsection "Supported controls"
The supported controls are:
.IP "\fB\s-1EVP_KDF_CTRL_SET_SALT\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_SALT"
.PD 0
.IP "\fB\s-1EVP_KDF_CTRL_SET_MD\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_MD"
.IP "\fB\s-1EVP_KDF_CTRL_SET_KEY\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_KEY"
.PD
These controls work as described in \*(L"\s-1CONTROLS\*(R"\s0 in \s-1\fBEVP_KDF_CTX\s0\fR\|(3).
.IP "\fB\s-1EVP_KDF_CTRL_RESET_HKDF_INFO\s0\fR" 4
.IX Item "EVP_KDF_CTRL_RESET_HKDF_INFO"
This control does not expect any arguments.
.Sp
Resets the context info buffer to zero length.
.IP "\fB\s-1EVP_KDF_CTRL_ADD_HKDF_INFO\s0\fR" 4
.IX Item "EVP_KDF_CTRL_ADD_HKDF_INFO"
This control expects two arguments: \f(CW\*(C`unsigned char *info\*(C'\fR, \f(CW\*(C`size_t infolen\*(C'\fR
.Sp
Sets the info value to the first \fBinfolen\fR bytes of the buffer \fBinfo\fR.  If a
value is already set, the contents of the buffer are appended to the existing
value.
.Sp
The total length of the context info buffer cannot exceed 1024 bytes;
this should be more than enough for any normal use of \s-1HKDF.\s0
.Sp
\&\fBEVP_KDF_ctrl_str()\fR takes two type strings for this control:
.RS 4
.ie n .IP """info""" 4
.el .IP "``info''" 4
.IX Item "info"
The value string is used as is.
.ie n .IP """hexinfo""" 4
.el .IP "``hexinfo''" 4
.IX Item "hexinfo"
The value string is expected to be a hexadecimal number, which will be
decoded before being passed on as the control value.
.RE
.RS 4
.RE
.IP "\fB\s-1EVP_KDF_CTRL_SET_HKDF_MODE\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_HKDF_MODE"
This control expects one argument: \f(CW\*(C`int mode\*(C'\fR
.Sp
Sets the mode for the \s-1HKDF\s0 operation. There are three modes that are currently
defined:
.RS 4
.IP "\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0" 4
.IX Item "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.
.IP "\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0" 4
.IX Item "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 \f(CW\*(C`keylen\*(C'\fR parameter must match the size of K, which can be looked
up by calling \fBEVP_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.
.IP "\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0" 4
.IX Item "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
.Sp
\&\fBEVP_KDF_ctrl_str()\fR type string: \*(L"mode\*(R"
.Sp
The value string is expected to be one of: \*(L"\s-1EXTRACT_AND_EXPAND\*(R", \*(L"EXTRACT_ONLY\*(R"\s0
or \*(L"\s-1EXPAND_ONLY\*(R".\s0
.RE
.SH "NOTES"
.IX Header "NOTES"
A context for \s-1HKDF\s0 can be obtained by calling:
.PP
.Vb 1
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);
.Ve
.PP
The output length of an \s-1HKDF\s0 expand operation is specified via the \f(CW\*(C`keylen\*(C'\fR
parameter to the \fBEVP_KDF_derive\fR\|(3) function.  When using
\&\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 the \f(CW\*(C`keylen\*(C'\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_size()\fR
after setting the mode and digest on the \f(CW\*(C`EVP_KDF_CTX\*(C'\fR.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
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 2
\& EVP_KDF_CTX *kctx;
\& unsigned char out[10];
\&
\& kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);
\&
\& if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_MD, EVP_sha256()) <= 0) {
\&     error("EVP_KDF_CTRL_SET_MD");
\& }
\& if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SALT, "salt", (size_t)4) <= 0) {
\&     error("EVP_KDF_CTRL_SET_SALT");
\& }
\& if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, "secret", (size_t)6) <= 0) {
\&     error("EVP_KDF_CTRL_SET_KEY");
\& }
\& if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_ADD_HKDF_INFO, "label", (size_t)5) <= 0) {
\&     error("EVP_KDF_CTRL_ADD_HKDF_INFO");
\& }
\& if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 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-1EVP_KDF_CTX\s0,
\&\fBEVP_KDF_CTX_new_id\fR\|(3),
\&\fBEVP_KDF_CTX_free\fR\|(3),
\&\fBEVP_KDF_ctrl\fR\|(3),
\&\fBEVP_KDF_size\fR\|(3),
\&\fBEVP_KDF_derive\fR\|(3),
\&\*(L"\s-1CONTROLS\*(R"\s0 in \s-1\fBEVP_KDF_CTX\s0\fR\|(3)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-2018 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>.
