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.\" ========================================================================
.\"
.IX Title "EVP_KDF_SSHKDF 7"
.TH EVP_KDF_SSHKDF 7 "2023-12-18" "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_SSHKDF \- The SSHKDF EVP_KDF implementation
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Support for computing the \fB\s-1SSHKDF\s0\fR \s-1KDF\s0 through the \fB\s-1EVP_KDF\s0\fR \s-1API.\s0
.PP
The \s-1EVP_KDF_SSHKDF\s0 algorithm implements the \s-1SSHKDF\s0 key derivation function.
It is defined in \s-1RFC 4253,\s0 section 7.2 and is used by \s-1SSH\s0 to derive IVs,
encryption keys and integrity keys.
Five inputs are required to perform key derivation: The hashing function
(for example \s-1SHA256\s0), the Initial Key, the Exchange Hash, the Session \s-1ID,\s0
and the derivation key type.
.SS "Numeric identity"
.IX Subsection "Numeric identity"
\&\fB\s-1EVP_KDF_SSHKDF\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_MD\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_MD"
.PD 0
.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_SET_SSHKDF_XCGHASH\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_SSHKDF_XCGHASH"
.PD 0
.IP "\fB\s-1EVP_KDF_CTRL_SET_SSHKDF_SESSION_ID\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_SSHKDF_SESSION_ID"
.PD
These controls expect two arguments: \f(CW\*(C`unsigned char *buffer\*(C'\fR, \f(CW\*(C`size_t length\*(C'\fR
.Sp
They set the respective values to the first \fBlength\fR bytes of the buffer
\&\fBbuffer\fR. If a value is already set, the contents are replaced.
.Sp
\&\fBEVP_KDF_ctrl_str()\fR takes two type strings for these controls:
.RS 4
.ie n .IP """xcghash""" 4
.el .IP "``xcghash''" 4
.IX Item "xcghash"
.PD 0
.ie n .IP """session_id""" 4
.el .IP "``session_id''" 4
.IX Item "session_id"
.PD
The value string is used as is.
.ie n .IP """hexxcghash""" 4
.el .IP "``hexxcghash''" 4
.IX Item "hexxcghash"
.PD 0
.ie n .IP """hexsession_id""" 4
.el .IP "``hexsession_id''" 4
.IX Item "hexsession_id"
.PD
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_SSHKDF_TYPE\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_SSHKDF_TYPE"
This control expects one argument: \f(CW\*(C`int mode\*(C'\fR
.Sp
Sets the type for the \s-1SSHHKDF\s0 operation. There are six supported types:
.RS 4
.IP "\s-1EVP_KDF_SSHKDF_TYPE_ININITAL_IV_CLI_TO_SRV\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_ININITAL_IV_CLI_TO_SRV"
The Initial \s-1IV\s0 from client to server.
A single char of value 65 (\s-1ASCII\s0 char 'A').
.IP "\s-1EVP_KDF_SSHKDF_TYPE_ININITAL_IV_SRV_TO_CLI\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_ININITAL_IV_SRV_TO_CLI"
The Initial \s-1IV\s0 from server to client
A single char of value 66 (\s-1ASCII\s0 char 'B').
.IP "\s-1EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV"
The Encryption Key from client to server
A single char of value 67 (\s-1ASCII\s0 char 'C').
.IP "\s-1EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI"
The Encryption Key from server to client
A single char of value 68 (\s-1ASCII\s0 char 'D').
.IP "\s-1EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV"
The Integrity Key from client to server
A single char of value 69 (\s-1ASCII\s0 char 'E').
.IP "\s-1EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI"
The Integrity Key from client to server
A single char of value 70 (\s-1ASCII\s0 char 'F').
.RE
.RS 4
.Sp
\&\fBEVP_KDF_ctrl_str()\fR type string: \*(L"type\*(R"
.Sp
The value is a string of length one character. The only valid values
are the numerical values of the \s-1ASCII\s0 caracters: \*(L"A\*(R" (65) to \*(L"F\*(R" (70).
.RE
.SH "NOTES"
.IX Header "NOTES"
A context for \s-1SSHKDF\s0 can be obtained by calling:
.PP
.Vb 1
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_SSHKDF);
.Ve
.PP
The output length of the \s-1SSHKDF\s0 derivation is specified via the \f(CW\*(C`keylen\*(C'\fR
parameter to the \fBEVP_KDF_derive\fR\|(3) function.
Since the \s-1SSHKDF\s0 output length is variable, calling \fBEVP_KDF_size()\fR
to obtain the requisite length is not meaningful. The caller must
allocate a buffer of the desired length, and pass that buffer to the
\&\fBEVP_KDF_derive\fR\|(3) function along with the desired length.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
This example derives an 8 byte \s-1IV\s0 using \s-1SHA\-256\s0 with a 1K \*(L"key\*(R" and appropriate
\&\*(L"xcghash\*(R" and \*(L"session_id\*(R" values:
.PP
.Vb 7
\& EVP_KDF_CTX *kctx;
\& unsigned char key[1024] = "01234...";
\& unsigned char xcghash[32] = "012345...";
\& unsigned char session_id[32] = "012345...";
\& unsigned char out[8];
\& size_t outlen = sizeof(out);
\& kctx = EVP_KDF_CTX_new_id(EVP_KDF_SSHKDF);
\&
\& if (EVP_KDF_CTX_set_md(kctx, EVP_sha256()) <= 0)
\&     /* Error */
\& if (EVP_KDF_CTX_set1_key(kctx, key, 1024) <= 0)
\&     /* Error */
\& if (EVP_KDF_CTX_set1_sshkdf_xcghash(kctx, xcghash, 32) <= 0)
\&     /* Error */
\& if (EVP_KDF_CTX_set1_sshkdf_session_id(kctx, session_id, 32) <= 0)
\&     /* Error */
\& if (EVP_KDF_CTX_set_sshkdf_type(kctx,
\&                    EVP_KDF_SSHKDF_TYPE_ININITAL_IV_CLI_TO_SRV) <= 0)
\&     /* Error */
\& if (EVP_KDF_derive(kctx, out, &outlen) <= 0)
\&     /* Error */
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 4253\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 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>.
