=pod

=head1 NAME

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

=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 *wrappedkey, size_t *wrappedkeylen,
                          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)>.
Note that I<ctx> is usually is produced using L<EVP_PKEY_CTX_new_from_pkey(3)>,
specifying the public key to use.

The EVP_PKEY_auth_encapsulate_init() function is similar 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>.
The symmetric secret generated in I<genkey> can be used as key material.
The ciphertext in I<wrappedkey> is its encapsulated form, which can be sent
to another party, who can use L<EVP_PKEY_decapsulate(3)> to retrieve it
using their private key.
If I<wrappedkey> is NULL then the maximum size of the output buffer
is written to the I<*wrappedkeylen> parameter unless I<wrappedkeylen> is NULL
and the maximum size of the generated key buffer is written to I<*genkeylen>
unless I<genkeylen> is NULL.
If I<wrappedkey> is not 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<wrappedkey> and its size is written to I<*wrappedkeylen>.  Note that if
I<wrappedlen> is not NULL, then the value it points to must initially hold the size of
the  I<unwrapped> buffer so that its size can be validated by the call, ensuring
it is large enough to hold the result written to I<wrapped>.

=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_from_pkey(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 2020-2023 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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