EVP_PKEY_ENCAPSULATE(3)

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

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);

DESCRIPTION
The EVP_PKEY_encapsulate_init() function initializes a public key algorithm context ctx for an encapsulation operation and then sets the params on the context in the same way as calling EVP_PKEY_CTX_set_params(3). Note that ctx is usually is produced using 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 authpriv authentication private key that is used during encapsulation.

The EVP_PKEY_encapsulate() function performs a public key encapsulation operation using ctx. The shared secret written to genkey can be used as an input for key derivation, typically for various symmetric algorithms. Its size is written to genkeylen, which must be initialised to the size of the provided buffer.

The ciphertext written to wrappedkey is an encapsulated form, which is expected to be only usable by the holder of the private key corresponding to the public key associated with ctx. This ciphertext is then communicated to the private-key holder, who can use EVP_PKEY_decapsulate(3) to securely recover the same shared secret.

If wrappedkey is NULL then the maximum size of the output buffer is written to the *wrappedkeylen parameter unless wrappedkeylen is NULL and the maximum size of the generated key buffer is written to *genkeylen unless genkeylen is NULL.

If wrappedkey is not NULL and the call is successful then the generated shared secret is written to genkey and its size is written to *genkeylen (which must be non-NULL). The encapsulated ciphertext is written to wrappedkey and its size is written to *wrappedkeylen (must also be non-NULL), The value pointed to by wrappedlen initially hold the size of the unwrapped buffer so that its size can be validated by the call, ensuring it is large enough to hold the result written to wrapped.

Absent detailed prior knowledge of the internals of the specific KEM algorithm, callers SHOULD NOT assume that the returned shared secret and ciphertext are necessarily of the maximum possible length. The lengths returned via *wrappedkeylen and *genkeylen SHOULD be used to determine the actual lengths of the outputs.

NOTES
After the call to EVP_PKEY_encapsulate_init(), algorithm-specific parameters for the operation may be set or modified using EVP_PKEY_CTX_set_params(3).

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.

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 */

SEE ALSO
EVP_PKEY_CTX_new_from_pkey(3), EVP_PKEY_decapsulate(3), EVP_KEM-RSA(7), EVP_KEM-X25519(7), EVP_KEM-EC(7), EVP_KEM-ML-KEM-512(7), EVP_KEM-ML-KEM-768(7), EVP_KEM-ML-KEM-1024(7)

HISTORY
The 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.

Support for ML-KEM was added in OpenSSL 3.5.

COPYRIGHT
Copyright 2020-2025 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 <https://www.openssl.org/source/license.html>.