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=head1 NAME

EVP_PKEY_encapsulate_init, EVP_PKEY_auth_encapsulate_init, EVP_PKEY_encapsulate
- Key encapsulation using a public key algorithm

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_PKEY_encapsulate_init(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
 int EVP_PKEY_auth_encapsulate_init(EVP_PKEY_CTX *ctx, EVP_PKEY *authpriv,
                                   const OSSL_PARAM params[]);
 int EVP_PKEY_encapsulate(EVP_PKEY_CTX *ctx,
                          unsigned char *out, size_t *outlen,
                          unsigned char *genkey, size_t *genkeylen);

=head1 DESCRIPTION

The EVP_PKEY_encapsulate_init() function initializes a public key algorithm
context I<ctx> for an encapsulation operation and then sets the I<params>
on the context in the same way as calling L<EVP_PKEY_CTX_set_params(3)>.

The EVP_PKEY_auth_encapsulate_init() function is similiar to
EVP_PKEY_encapsulate_init() but also passes an I<authpriv> authentication private
key that is used during encapsulation.

The EVP_PKEY_encapsulate() function performs a public key encapsulation
operation using I<ctx> with the name I<name>.
If I<out> is B<NULL> then the maximum size of the output buffer is written to the
I<*outlen> parameter and the maximum size of the generated key buffer is written
to I<*genkeylen>. If I<out> is not B<NULL> and the call is successful then the
internally generated key is written to I<genkey> and its size is written to
I<*genkeylen>. The encapsulated version of the generated key is written to
I<out> and its size is written to I<*outlen>.

=head1 NOTES

After the call to EVP_PKEY_encapsulate_init() algorithm specific parameters
for the operation may be set or modified using L<EVP_PKEY_CTX_set_params(3)>.

=head1 RETURN VALUES

EVP_PKEY_encapsulate_init(), EVP_PKEY_auth_encapsulate_init() and
EVP_PKEY_encapsulate() 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.

=head1 EXAMPLES

Encapsulate an RSASVE key (for RSA keys).

 #include <openssl/evp.h>

 /*
  * NB: assumes rsa_pub_key is an public key of another party.
  */

 EVP_PKEY_CTX *ctx = NULL;
 size_t secretlen = 0, outlen = 0;
 unsigned char *out = NULL, *secret = NULL;

 ctx = EVP_PKEY_CTX_new_from_pkey(libctx, rsa_pub_key, NULL);
 if (ctx = NULL)
     /* Error */
 if (EVP_PKEY_encapsulate_init(ctx, NULL) <= 0)
     /* Error */

 /* Set the mode - only 'RSASVE' is currently supported */
  if (EVP_PKEY_CTX_set_kem_op(ctx, "RSASVE") <= 0)
     /* Error */
 /* Determine buffer length */
 if (EVP_PKEY_encapsulate(ctx, NULL, &outlen, NULL, &secretlen) <= 0)
     /* Error */

 out = OPENSSL_malloc(outlen);
 secret = OPENSSL_malloc(secretlen);
 if (out == NULL || secret == NULL)
     /* malloc failure */

 /*
  * The generated 'secret' can be used as key material.
  * The encapsulated 'out' can be sent to another party who can
  * decapsulate it using their private key to retrieve the 'secret'.
  */
 if (EVP_PKEY_encapsulate(ctx, out, &outlen, secret, &secretlen) <= 0)
     /* Error */

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_decapsulate(3)>,
L<EVP_KEM-RSA(7)>, L<EVP_KEM-X25519(7)>, L<EVP_KEM-EC(7)>

=head1 HISTORY

These functions EVP_PKEY_encapsulate_init() and EVP_PKEY_encapsulate() were
added in OpenSSL 3.0.
The function EVP_PKEY_auth_encapsulate_init() was added in OpenSSL 3.2.

=head1 COPYRIGHT

Copyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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