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131 .IX Title "EVP_PKEY_CTX_SET_HKDF_MD 3"
132 .TH EVP_PKEY_CTX_SET_HKDF_MD 3 "2018-09-11" "1.1.1" "OpenSSL"
133 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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138 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
140 .IX Header "SYNOPSIS"
142 \& #include <openssl/kdf.h>
144 \& int EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
146 \& int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
148 \& int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
151 \& int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
154 \& int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
158 .IX Header "DESCRIPTION"
159 The \s-1EVP_PKEY_HKDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function.
160 \&\s-1HKDF\s0 follows the \*(L"extract-then-expand\*(R" paradigm, where the \s-1KDF\s0 logically
161 consists of two modules. The first stage takes the input keying material
162 and \*(L"extracts\*(R" from it a fixed-length pseudorandom key K. The second stage
163 \&\*(L"expands\*(R" the key K into several additional pseudorandom keys (the output
166 \&\fIEVP_PKEY_CTX_hkdf_mode()\fR sets the mode for the \s-1HKDF\s0 operation. There are three
167 modes that are currently defined:
168 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND\s0" 4
169 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND"
170 This is the default mode. Calling \fIEVP_PKEY_derive\fR\|(3) on an \s-1EVP_PKEY_CTX\s0 set
171 up for \s-1HKDF\s0 will perform an extract followed by an expand operation in one go.
172 The derived key returned will be the result after the expand operation. The
173 intermediate fixed-length pseudorandom key K is not returned.
175 In this mode the digest, key, salt and info values must be set before a key is
176 derived or an error occurs.
177 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY\s0" 4
178 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY"
179 In this mode calling \fIEVP_PKEY_derive\fR\|(3) will just perform the extract
180 operation. The value returned will be the intermediate fixed-length pseudorandom
183 The digest, key and salt values must be set before a key is derived or an
185 .IP "\s-1EVP_PKEY_HKDEF_MODE_EXPAND_ONLY\s0" 4
186 .IX Item "EVP_PKEY_HKDEF_MODE_EXPAND_ONLY"
187 In this mode calling \fIEVP_PKEY_derive\fR\|(3) will just perform the expand
188 operation. The input key should be set to the intermediate fixed-length
189 pseudorandom key K returned from a previous extract operation.
191 The digest, key and info values must be set before a key is derived or an
194 \&\fIEVP_PKEY_set_hkdf_md()\fR sets the message digest associated with the \s-1HKDF.\s0
196 \&\fIEVP_PKEY_CTX_set1_hkdf_salt()\fR sets the salt to \fBsaltlen\fR bytes of the
197 buffer \fBsalt\fR. Any existing value is replaced.
199 \&\fIEVP_PKEY_CTX_set_hkdf_key()\fR sets the key to \fBkeylen\fR bytes of the buffer
200 \&\fBkey\fR. Any existing value is replaced.
202 \&\fIEVP_PKEY_CTX_add1_hkdf_info()\fR sets the info value to \fBinfolen\fR bytes of the
203 buffer \fBinfo\fR. If a value is already set, it is appended to the existing
206 .IX Header "STRING CTRLS"
207 \&\s-1HKDF\s0 also supports string based control operations via
208 \&\fIEVP_PKEY_CTX_ctrl_str\fR\|(3).
209 The \fBtype\fR parameter \*(L"md\*(R" uses the supplied \fBvalue\fR as the name of the digest
211 The \fBtype\fR parameter \*(L"mode\*(R" uses the values \*(L"\s-1EXTRACT_AND_EXPAND\*(R",
212 \&\*(L"EXTRACT_ONLY\*(R"\s0 and \*(L"\s-1EXPAND_ONLY\*(R"\s0 to determine the mode to use.
213 The \fBtype\fR parameters \*(L"salt\*(R", \*(L"key\*(R" and \*(L"info\*(R" use the supplied \fBvalue\fR
214 parameter as a \fBseed\fR, \fBkey\fR or \fBinfo\fR value.
215 The names \*(L"hexsalt\*(R", \*(L"hexkey\*(R" and \*(L"hexinfo\*(R" are similar except they take a hex
216 string which is converted to binary.
219 All these functions are implemented as macros.
221 A context for \s-1HKDF\s0 can be obtained by calling:
224 \& EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
227 The total length of the info buffer cannot exceed 1024 bytes in length: this
228 should be more than enough for any normal use of \s-1HKDF.\s0
230 The output length of an \s-1HKDF\s0 expand operation is specified via the length
231 parameter to the \fIEVP_PKEY_derive\fR\|(3) function.
232 Since the \s-1HKDF\s0 output length is variable, passing a \fB\s-1NULL\s0\fR buffer as a means
233 to obtain the requisite length is not meaningful with \s-1HKDF\s0 in any mode that
234 performs an expand operation. Instead, the caller must allocate a buffer of the
235 desired length, and pass that buffer to \fIEVP_PKEY_derive\fR\|(3) along with (a
236 pointer initialized to) the desired length. Passing a \fB\s-1NULL\s0\fR buffer to obtain
237 the length is allowed when using \s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.\s0
239 Optimised versions of \s-1HKDF\s0 can be implemented in an \s-1ENGINE.\s0
241 .IX Header "RETURN VALUES"
242 All these functions return 1 for success and 0 or a negative value for failure.
243 In particular a return value of \-2 indicates the operation is not supported by
244 the public key algorithm.
247 This example derives 10 bytes using \s-1SHA\-256\s0 with the secret key \*(L"secret\*(R",
248 salt value \*(L"salt\*(R" and info value \*(L"label\*(R":
251 \& EVP_PKEY_CTX *pctx;
252 \& unsigned char out[10];
253 \& size_t outlen = sizeof(out);
254 \& pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
256 \& if (EVP_PKEY_derive_init(pctx) <= 0)
258 \& if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
260 \& if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
262 \& if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
264 \& if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
266 \& if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
270 .IX Header "CONFORMING TO"
273 .IX Header "SEE ALSO"
274 \&\fIEVP_PKEY_CTX_new\fR\|(3),
275 \&\fIEVP_PKEY_CTX_ctrl_str\fR\|(3),
276 \&\fIEVP_PKEY_derive\fR\|(3)
278 .IX Header "COPYRIGHT"
279 Copyright 2016\-2018 The OpenSSL Project Authors. All Rights Reserved.
281 Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
282 this file except in compliance with the License. You can obtain a copy
283 in the file \s-1LICENSE\s0 in the source distribution or at
284 <https://www.openssl.org/source/license.html>.