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135 .IX Title "EVP_PKEY_CTX_SET_HKDF_MD 3"
136 .TH EVP_PKEY_CTX_SET_HKDF_MD 3 "2019-09-10" "1.1.1d" "OpenSSL"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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142 EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt, EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info, EVP_PKEY_CTX_hkdf_mode \- HMAC\-based Extract\-and\-Expand key derivation algorithm
144 .IX Header "SYNOPSIS"
146 \& #include <openssl/kdf.h>
148 \& int EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
150 \& int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
152 \& int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
155 \& int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
158 \& int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
162 .IX Header "DESCRIPTION"
163 The \s-1EVP_PKEY_HKDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function.
164 \&\s-1HKDF\s0 follows the \*(L"extract-then-expand\*(R" paradigm, where the \s-1KDF\s0 logically
165 consists of two modules. The first stage takes the input keying material
166 and \*(L"extracts\*(R" from it a fixed-length pseudorandom key K. The second stage
167 \&\*(L"expands\*(R" the key K into several additional pseudorandom keys (the output
170 \&\fBEVP_PKEY_CTX_hkdf_mode()\fR sets the mode for the \s-1HKDF\s0 operation. There are three
171 modes that are currently defined:
172 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND\s0" 4
173 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND"
174 This is the default mode. Calling \fBEVP_PKEY_derive\fR\|(3) on an \s-1EVP_PKEY_CTX\s0 set
175 up for \s-1HKDF\s0 will perform an extract followed by an expand operation in one go.
176 The derived key returned will be the result after the expand operation. The
177 intermediate fixed-length pseudorandom key K is not returned.
179 In this mode the digest, key, salt and info values must be set before a key is
180 derived or an error occurs.
181 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY\s0" 4
182 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY"
183 In this mode calling \fBEVP_PKEY_derive\fR\|(3) will just perform the extract
184 operation. The value returned will be the intermediate fixed-length pseudorandom
187 The digest, key and salt values must be set before a key is derived or an
189 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXPAND_ONLY\s0" 4
190 .IX Item "EVP_PKEY_HKDEF_MODE_EXPAND_ONLY"
191 In this mode calling \fBEVP_PKEY_derive\fR\|(3) will just perform the expand
192 operation. The input key should be set to the intermediate fixed-length
193 pseudorandom key K returned from a previous extract operation.
195 The digest, key and info values must be set before a key is derived or an
198 \&\fBEVP_PKEY_CTX_set_hkdf_md()\fR sets the message digest associated with the \s-1HKDF.\s0
200 \&\fBEVP_PKEY_CTX_set1_hkdf_salt()\fR sets the salt to \fBsaltlen\fR bytes of the
201 buffer \fBsalt\fR. Any existing value is replaced.
203 \&\fBEVP_PKEY_CTX_set1_hkdf_key()\fR sets the key to \fBkeylen\fR bytes of the buffer
204 \&\fBkey\fR. Any existing value is replaced.
206 \&\fBEVP_PKEY_CTX_add1_hkdf_info()\fR sets the info value to \fBinfolen\fR bytes of the
207 buffer \fBinfo\fR. If a value is already set, it is appended to the existing
210 .IX Header "STRING CTRLS"
211 \&\s-1HKDF\s0 also supports string based control operations via
212 \&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3).
213 The \fBtype\fR parameter \*(L"md\*(R" uses the supplied \fBvalue\fR as the name of the digest
215 The \fBtype\fR parameter \*(L"mode\*(R" uses the values \*(L"\s-1EXTRACT_AND_EXPAND\*(R",
216 \&\*(L"EXTRACT_ONLY\*(R"\s0 and \*(L"\s-1EXPAND_ONLY\*(R"\s0 to determine the mode to use.
217 The \fBtype\fR parameters \*(L"salt\*(R", \*(L"key\*(R" and \*(L"info\*(R" use the supplied \fBvalue\fR
218 parameter as a \fBseed\fR, \fBkey\fR or \fBinfo\fR value.
219 The names \*(L"hexsalt\*(R", \*(L"hexkey\*(R" and \*(L"hexinfo\*(R" are similar except they take a hex
220 string which is converted to binary.
223 All these functions are implemented as macros.
225 A context for \s-1HKDF\s0 can be obtained by calling:
228 \& EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
231 The total length of the info buffer cannot exceed 1024 bytes in length: this
232 should be more than enough for any normal use of \s-1HKDF.\s0
234 The output length of an \s-1HKDF\s0 expand operation is specified via the length
235 parameter to the \fBEVP_PKEY_derive\fR\|(3) function.
236 Since the \s-1HKDF\s0 output length is variable, passing a \fB\s-1NULL\s0\fR buffer as a means
237 to obtain the requisite length is not meaningful with \s-1HKDF\s0 in any mode that
238 performs an expand operation. Instead, the caller must allocate a buffer of the
239 desired length, and pass that buffer to \fBEVP_PKEY_derive\fR\|(3) along with (a
240 pointer initialized to) the desired length. Passing a \fB\s-1NULL\s0\fR buffer to obtain
241 the length is allowed when using \s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.\s0
243 Optimised versions of \s-1HKDF\s0 can be implemented in an \s-1ENGINE.\s0
245 .IX Header "RETURN VALUES"
246 All these functions return 1 for success and 0 or a negative value for failure.
247 In particular a return value of \-2 indicates the operation is not supported by
248 the public key algorithm.
250 .IX Header "EXAMPLES"
251 This example derives 10 bytes using \s-1SHA\-256\s0 with the secret key \*(L"secret\*(R",
252 salt value \*(L"salt\*(R" and info value \*(L"label\*(R":
255 \& EVP_PKEY_CTX *pctx;
256 \& unsigned char out[10];
257 \& size_t outlen = sizeof(out);
258 \& pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
260 \& if (EVP_PKEY_derive_init(pctx) <= 0)
262 \& if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
264 \& if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
266 \& if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
268 \& if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
270 \& if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
274 .IX Header "CONFORMING TO"
277 .IX Header "SEE ALSO"
278 \&\fBEVP_PKEY_CTX_new\fR\|(3),
279 \&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3),
280 \&\fBEVP_PKEY_derive\fR\|(3)
282 .IX Header "COPYRIGHT"
283 Copyright 2016\-2019 The OpenSSL Project Authors. All Rights Reserved.
285 Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
286 this file except in compliance with the License. You can obtain a copy
287 in the file \s-1LICENSE\s0 in the source distribution or at
288 <https://www.openssl.org/source/license.html>.