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.\" ========================================================================
.\"
.IX Title "EVP_KDF-SSHKDF 7"
.TH EVP_KDF-SSHKDF 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\-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 "Identity"
.IX Subsection "Identity"
\&\*(L"\s-1SSHKDF\*(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>"
.PD
These parameters work as described in \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3).
.ie n .IP """xcghash"" (\fB\s-1OSSL_KDF_PARAM_SSHKDF_XCGHASH\s0\fR) <octet string>" 4
.el .IP "``xcghash'' (\fB\s-1OSSL_KDF_PARAM_SSHKDF_XCGHASH\s0\fR) <octet string>" 4
.IX Item "xcghash (OSSL_KDF_PARAM_SSHKDF_XCGHASH) <octet string>"
.PD 0
.ie n .IP """session_id"" (\fB\s-1OSSL_KDF_PARAM_SSHKDF_SESSION_ID\s0\fR) <octet string>" 4
.el .IP "``session_id'' (\fB\s-1OSSL_KDF_PARAM_SSHKDF_SESSION_ID\s0\fR) <octet string>" 4
.IX Item "session_id (OSSL_KDF_PARAM_SSHKDF_SESSION_ID) <octet string>"
.PD
These parameters set the respective values for the \s-1KDF.\s0
If a value is already set, the contents are replaced.
.ie n .IP """type"" (\fB\s-1OSSL_KDF_PARAM_SSHKDF_TYPE\s0\fR) <\s-1UTF8\s0 string>" 4
.el .IP "``type'' (\fB\s-1OSSL_KDF_PARAM_SSHKDF_TYPE\s0\fR) <\s-1UTF8\s0 string>" 4
.IX Item "type (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string>"
This parameter sets the type for the \s-1SSHKDF\s0 operation.
There are six supported types:
.RS 4
.IP "\s-1EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_INITIAL_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_INITIAL_IV_SRV_TO_CLI\s0" 4
.IX Item "EVP_KDF_SSHKDF_TYPE_INITIAL_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
.RE
.SH "NOTES"
.IX Header "NOTES"
A context for \s-1SSHKDF\s0 can be obtained by calling:
.PP
.Vb 2
\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
.Ve
.PP
The output length of the \s-1SSHKDF\s0 derivation is specified via the \fIkeylen\fR
parameter to the \fBEVP_KDF_derive\fR\|(3) function.
Since the \s-1SSHKDF\s0 output length is variable, calling \fBEVP_KDF_CTX_get_kdf_size\fR\|(3)
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 "EXAMPLES"
.IX Header "EXAMPLES"
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 9
\& EVP_KDF *kdf;
\& EVP_KDF_CTX *kctx;
\& char type = EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV;
\& 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);
\& OSSL_PARAM params[6], *p = params;
\&
\& kdf = EVP_KDF_fetch(NULL, "SSHKDF", 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,
\&                                          key, (size_t)1024);
\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_XCGHASH,
\&                                          xcghash, (size_t)32);
\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_SESSION_ID,
\&                                          session_id, (size_t)32);
\& *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_SSHKDF_TYPE,
\&                                         &type, sizeof(type));
\& *p = OSSL_PARAM_construct_end();
\& if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
\&     /* Error */
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 4253\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_set_params\fR\|(3),
\&\fBEVP_KDF_CTX_get_kdf_size\fR\|(3),
\&\fBEVP_KDF_derive\fR\|(3),
\&\*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
This functionality was added in OpenSSL 3.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-2022 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>.