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.\" ========================================================================
.\"
.IX Title "EVP_KDF-KRB5KDF 7"
.TH EVP_KDF-KRB5KDF 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\-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 \s-1EVP_KDF\-KRB5KDF\s0 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 "Identity"
.IX Subsection "Identity"
\&\*(L"\s-1KRB5KDF\*(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 """cipher"" (\fB\s-1OSSL_KDF_PARAM_CIPHER\s0\fR) <\s-1UTF8\s0 string>" 4
.el .IP "``cipher'' (\fB\s-1OSSL_KDF_PARAM_CIPHER\s0\fR) <\s-1UTF8\s0 string>" 4
.IX Item "cipher (OSSL_KDF_PARAM_CIPHER) <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 """constant"" (\fB\s-1OSSL_KDF_PARAM_CONSTANT\s0\fR) <octet string>" 4
.el .IP "``constant'' (\fB\s-1OSSL_KDF_PARAM_CONSTANT\s0\fR) <octet string>" 4
.IX Item "constant (OSSL_KDF_PARAM_CONSTANT) <octet string>"
This parameter sets the constant 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 2
\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
.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
constant'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_CTX_get_kdf_size\fR\|(3) 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 7
\& EVP_KDF *kdf;
\& 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);
\& OSSL_PARAM params[4], *p = params;
\&
\& kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
\& kctx = EVP_KDF_CTX_new(kdf);
\& EVP_KDF_free(kdf);
\&
\& *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
\&                                         SN_aes_128_cbc,
\&                                         strlen(SN_aes_128_cbc));
\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
\&                                          key, (size_t)16);
\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
\&                                          constant, strlen(constant));
\& *p = OSSL_PARAM_construct_end();
\& if (EVP_KDF_derive(kctx, out, outlen, params) <= 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\s0\fR\|(3),
\&\fBEVP_KDF_CTX_free\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\-2021 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>.