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137 .IX Title "EVP_PKEY_derive 3"
138 .TH EVP_PKEY_derive 3 "2013-02-05" "1.0.1n" "OpenSSL"
139 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
140 .\" way too many mistakes in technical documents.
141 .if n .ad l
142 .nh
143 .SH "NAME"
144 EVP_PKEY_derive_init, EVP_PKEY_derive_set_peer, EVP_PKEY_derive \- derive public key algorithm shared secret.
145 .SH "LIBRARY"
146 libcrypto, -lcrypto
147 .SH "SYNOPSIS"
148 .IX Header "SYNOPSIS"
149 .Vb 1
150 \& #include <openssl/evp.h>
151 \&
152 \& int EVP_PKEY_derive_init(EVP_PKEY_CTX *ctx);
153 \& int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *ctx, EVP_PKEY *peer);
154 \& int EVP_PKEY_derive(EVP_PKEY_CTX *ctx, unsigned char *key, size_t *keylen);
155 .Ve
156 .SH "DESCRIPTION"
157 .IX Header "DESCRIPTION"
158 The \fIEVP_PKEY_derive_init()\fR function initializes a public key algorithm
159 context using key \fBpkey\fR for shared secret derivation.
160 .PP
161 The \fIEVP_PKEY_derive_set_peer()\fR function sets the peer key: this will normally
162 be a public key.
163 .PP
164 The \fIEVP_PKEY_derive()\fR derives a shared secret using \fBctx\fR.
165 If \fBkey\fR is \fB\s-1NULL\s0\fR then the maximum size of the output buffer is written to
166 the \fBkeylen\fR parameter. If \fBkey\fR is not \fB\s-1NULL\s0\fR then before the call the
167 \&\fBkeylen\fR parameter should contain the length of the \fBkey\fR buffer, if the call
168 is successful the shared secret is written to \fBkey\fR and the amount of data
169 written to \fBkeylen\fR.
170 .SH "NOTES"
171 .IX Header "NOTES"
172 After the call to \fIEVP_PKEY_derive_init()\fR algorithm specific control
173 operations can be performed to set any appropriate parameters for the
174 operation.
175 .PP
176 The function \fIEVP_PKEY_derive()\fR can be called more than once on the same
177 context if several operations are performed using the same parameters.
178 .SH "RETURN VALUES"
179 .IX Header "RETURN VALUES"
180 \&\fIEVP_PKEY_derive_init()\fR and \fIEVP_PKEY_derive()\fR return 1 for success and 0
181 or a negative value for failure. In particular a return value of \-2
182 indicates the operation is not supported by the public key algorithm.
183 .SH "EXAMPLE"
184 .IX Header "EXAMPLE"
185 Derive shared secret (for example \s-1DH\s0 or \s-1EC\s0 keys):
186 .PP
187 .Vb 2
188 \& #include <openssl/evp.h>
189 \& #include <openssl/rsa.h>
190 \&
191 \& EVP_PKEY_CTX *ctx;
192 \& unsigned char *skey;
193 \& size_t skeylen;
194 \& EVP_PKEY *pkey, *peerkey;
195 \& /* NB: assumes pkey, peerkey have been already set up */
196 \&
197 \& ctx = EVP_PKEY_CTX_new(pkey);
198 \& if (!ctx)
199 \& /* Error occurred */
200 \& if (EVP_PKEY_derive_init(ctx) <= 0)
201 \& /* Error */
202 \& if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
203 \& /* Error */
204 \&
205 \& /* Determine buffer length */
206 \& if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
207 \& /* Error */
208 \&
209 \& skey = OPENSSL_malloc(skeylen);
210 \&
211 \& if (!skey)
212 \& /* malloc failure */
213 \&
214 \& if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
215 \& /* Error */
216 \&
217 \& /* Shared secret is skey bytes written to buffer skey */
218 .Ve
219 .SH "SEE ALSO"
220 .IX Header "SEE ALSO"
221 \&\fIEVP_PKEY_CTX_new\fR\|(3),
222 \&\fIEVP_PKEY_encrypt\fR\|(3),
223 \&\fIEVP_PKEY_decrypt\fR\|(3),
224 \&\fIEVP_PKEY_sign\fR\|(3),
225 \&\fIEVP_PKEY_verify\fR\|(3),
226 \&\fIEVP_PKEY_verify_recover\fR\|(3),
227 .SH "HISTORY"
228 .IX Header "HISTORY"
229 These functions were first added to OpenSSL 1.0.0.