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.\" ========================================================================
.\"
.IX Title "EVP_PKEY_derive 3"
.TH EVP_PKEY_derive 3 "2019-12-20" "1.0.2u" "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_PKEY_derive_init, EVP_PKEY_derive_set_peer, EVP_PKEY_derive \- derive public key algorithm shared secret.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/evp.h>
\&
\& int EVP_PKEY_derive_init(EVP_PKEY_CTX *ctx);
\& int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *ctx, EVP_PKEY *peer);
\& int EVP_PKEY_derive(EVP_PKEY_CTX *ctx, unsigned char *key, size_t *keylen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fBEVP_PKEY_derive_init()\fR function initializes a public key algorithm
context using key \fBpkey\fR for shared secret derivation.
.PP
The \fBEVP_PKEY_derive_set_peer()\fR function sets the peer key: this will normally
be a public key.
.PP
The \fBEVP_PKEY_derive()\fR derives a shared secret using \fBctx\fR.
If \fBkey\fR is \fB\s-1NULL\s0\fR then the maximum size of the output buffer is written to
the \fBkeylen\fR parameter. If \fBkey\fR is not \fB\s-1NULL\s0\fR then before the call the
\&\fBkeylen\fR parameter should contain the length of the \fBkey\fR buffer, if the call
is successful the shared secret is written to \fBkey\fR and the amount of data
written to \fBkeylen\fR.
.SH "NOTES"
.IX Header "NOTES"
After the call to \fBEVP_PKEY_derive_init()\fR algorithm specific control
operations can be performed to set any appropriate parameters for the
operation.
.PP
The function \fBEVP_PKEY_derive()\fR can be called more than once on the same
context if several operations are performed using the same parameters.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBEVP_PKEY_derive_init()\fR and \fBEVP_PKEY_derive()\fR return 1 for success and 0
or a negative value for failure. In particular a return value of \-2
indicates the operation is not supported by the public key algorithm.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
Derive shared secret (for example \s-1DH\s0 or \s-1EC\s0 keys):
.PP
.Vb 2
\& #include <openssl/evp.h>
\& #include <openssl/rsa.h>
\&
\& EVP_PKEY_CTX *ctx;
\& unsigned char *skey;
\& size_t skeylen;
\& EVP_PKEY *pkey, *peerkey;
\& /* NB: assumes pkey, peerkey have been already set up */
\&
\& ctx = EVP_PKEY_CTX_new(pkey);
\& if (!ctx)
\&        /* Error occurred */
\& if (EVP_PKEY_derive_init(ctx) <= 0)
\&        /* Error */
\& if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
\&        /* Error */
\&
\& /* Determine buffer length */
\& if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
\&        /* Error */
\&
\& skey = OPENSSL_malloc(skeylen);
\&
\& if (!skey)
\&        /* malloc failure */
\& 
\& if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
\&        /* Error */
\&
\& /* Shared secret is skey bytes written to buffer skey */
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBEVP_PKEY_CTX_new\fR\|(3),
\&\fBEVP_PKEY_encrypt\fR\|(3),
\&\fBEVP_PKEY_decrypt\fR\|(3),
\&\fBEVP_PKEY_sign\fR\|(3),
\&\fBEVP_PKEY_verify\fR\|(3),
\&\fBEVP_PKEY_verify_recover\fR\|(3),
.SH "HISTORY"
.IX Header "HISTORY"
These functions were first added to OpenSSL 1.0.0.