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.\" ========================================================================
.\"
.IX Title "EVP_KDF_KRB5KDF 7"
.TH EVP_KDF_KRB5KDF 7 "2023-12-18" "1.1.1k" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
EVP_KDF_KRB5KDF \- The RFC3961 Krb5 KDF EVP_KDF implementation
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Support for computing the \fB\s-1KRB5KDF\s0\fR \s-1KDF\s0 through the \fB\s-1EVP_KDF\s0\fR \s-1API.\s0
.PP
The \fB\s-1EVP_KDF_KRB5KDF\s0\fR algorithm implements the key derivation function defined
in \s-1RFC 3961,\s0 section 5.1 and is used by Krb5 to derive session keys.
Three inputs are required to perform key derivation: a cipher, (for example
\&\s-1AES\-128\-CBC\s0), the initial key, and a constant.
.SS "Numeric identity"
.IX Subsection "Numeric identity"
\&\fB\s-1EVP_KDF_KRB5KDF\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_CIPHER\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_CIPHER"
.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_KRB5KDF_CONSTANT\s0\fR" 4
.IX Item "EVP_KDF_CTRL_SET_KRB5KDF_CONSTANT"
This control expects two arguments: \f(CW\*(C`unsigned char *constant\*(C'\fR, \f(CW\*(C`size_t constantlen\*(C'\fR
.Sp
This control sets the \fIconstant\fR value for the \s-1KDF.\s0
If a value is already set, the contents are replaced.
.SH "NOTES"
.IX Header "NOTES"
A context for \s-1KRB5KDF\s0 can be obtained by calling:
.PP
.Vb 1
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_KRB5KDF);
.Ve
.PP
The output length of the \s-1KRB5KDF\s0 derivation is specified via the \fIkeylen\fR
parameter to the \fBEVP_KDF_derive\fR\|(3) function, and \s-1MUST\s0 match the key
length for the chosen cipher or an error is returned. Moreover the
\&\fIconstant\fR's length must not exceed the block size of the cipher.
Since the \s-1KRB5KDF\s0 output length depends on the chosen cipher, calling
\&\fBEVP_KDF_size()\fR to obtain the requisite length returns the correct length
only after the cipher is set. Prior to that \fB\s-1EVP_MAX_KEY_LENGTH\s0\fR is returned.
The caller must allocate a buffer of the correct length for the chosen
cipher, and pass that buffer to the \fBEVP_KDF_derive\fR\|(3) function along
with that length.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
This example derives a key using the \s-1AES\-128\-CBC\s0 cipher:
.PP
.Vb 5
\& EVP_KDF_CTX *kctx;
\& unsigned char key[16] = "01234...";
\& unsigned char constant[] = "I\*(Aqm a constant";
\& unsigned char out[16];
\& size_t outlen = sizeof(out);
\&
\& kctx = EVP_KDF_CTX_new_id(EVP_KDF_KRB5KDF);
\&
\& EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_CIPHER, EVP_aes_128_cbc());
\& EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, key, (size_t)16);
\& EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KRB5KDF_CONSTANT, constant, strlen(constant));
\& if (EVP_KDF_derive(kctx, out, outlen) <= 0)
\&     /* Error */
\& EVP_KDF_CTX_free(kctx);
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 3961\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\s-1\fBEVP_KDF_CTX\s0\fR\|(3),
\&\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 "HISTORY"
.IX Header "HISTORY"
This functionality was added to OpenSSL 3.0.
.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>.
