EVP_PKEY_encapsulate.3ossl - Man Page

Key encapsulation using a public key algorithm


 #include <openssl/evp.h>

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


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).

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


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() 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.


Encapsulate an RSASVE key (for RSA keys).

 #include <openssl/evp.h>

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

 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(3), EVP_PKEY_decapsulate(3), EVP_KEM-RSA(7),


These functions were added in OpenSSL 3.0.

Referenced By

EVP_KEM-RSA.7ossl(7), EVP_PKEY_CTX_ctrl.3ossl(3), EVP_PKEY_decapsulate.3ossl(3), life_cycle-pkey.7ossl(7), provider-kem.7ossl(7).

The man page EVP_PKEY_encapsulate_init.3ossl(3) is an alias of EVP_PKEY_encapsulate.3ossl(3).

2021-09-09 3.0.0 OpenSSL