.\" $OpenBSD: EVP_PKEY_derive.3,v 1.7 2018/02/15 14:52:16 schwarze Exp $
.\" full merge up to: OpenSSL 48e5119a Jan 19 10:49:22 2018 +0100
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.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: February 15 2018 $
.Dt EVP_PKEY_DERIVE 3
.Os
.Sh NAME
.Nm EVP_PKEY_derive_init ,
.Nm EVP_PKEY_derive_set_peer ,
.Nm EVP_PKEY_derive
.Nd derive public key algorithm shared secret
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_PKEY_derive_init
.Fa "EVP_PKEY_CTX *ctx"
.Fc
.Ft int
.Fo EVP_PKEY_derive_set_peer
.Fa "EVP_PKEY_CTX *ctx"
.Fa "EVP_PKEY *peer"
.Fc
.Ft int
.Fo EVP_PKEY_derive
.Fa "EVP_PKEY_CTX *ctx"
.Fa "unsigned char *key"
.Fa "size_t *keylen"
.Fc
.Sh DESCRIPTION
The
.Fn EVP_PKEY_derive_init
function initializes a public key algorithm context using key
.Fa ctx->pkey
for shared secret derivation.
.Pp
The
.Fn EVP_PKEY_derive_set_peer
function sets the peer key: this will normally be a public key.
.Pp
The
.Fn EVP_PKEY_derive
function derives a shared secret using
.Fa ctx .
If
.Fa key
is
.Dv NULL ,
then the maximum size of the output buffer is written to the
.Fa keylen
parameter.
If
.Fa key
is not
.Dv NULL
then before the call the
.Fa keylen
parameter should contain the length of the
.Fa key
buffer.
If the call is successful, the shared secret is written to
.Fa key
and the amount of data written to
.Fa keylen .
.Pp
After the call to
.Fn EVP_PKEY_derive_init ,
algorithm specific control operations can be performed to set any
appropriate parameters for the operation.
.Pp
The function
.Fn EVP_PKEY_derive
can be called more than once on the same context if several operations
are performed using the same parameters.
.Sh RETURN VALUES
.Fn EVP_PKEY_derive_init
and
.Fn EVP_PKEY_derive
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 EXAMPLES
Derive shared secret (for example DH or EC keys):
.Bd -literal -offset indent
#include <openssl/evp.h>
#include <openssl/rsa.h>

EVP_PKEY_CTX *ctx;
ENGINE *eng;
unsigned char *skey;
size_t skeylen;
EVP_PKEY *pkey, *peerkey;

/* Assumes that pkey, eng, and peerkey have already been set up. */
ctx = EVP_PKEY_CTX_new(pkey, eng);
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 = malloc(skeylen);

if (!skey)
	/* malloc failure */

if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
	/* Error */

/* Shared secret is skey bytes written to buffer skey */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_decrypt 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_meth_set_derive 3 ,
.Xr EVP_PKEY_sign 3 ,
.Xr EVP_PKEY_verify 3 ,
.Xr EVP_PKEY_verify_recover 3 ,
.Xr X25519 3
.Sh HISTORY
These functions were first added to OpenSSL 1.0.0.