Cryptography and SSL/TLS Toolkit



EVP_PKEY_decapsulate_init, EVP_PKEY_decapsulate - Key decapsulation using a KEM algorithm with a private key


#include <openssl/evp.h>

int EVP_PKEY_decapsulate_init(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
int EVP_PKEY_decapsulate(EVP_PKEY_CTX *ctx,
                         unsigned char *unwrapped, size_t *unwrappedlen,
                         const unsigned char *wrapped, size_t wrappedlen);


The EVP_PKEY_decapsulate_init() function initializes a private key algorithm context ctx for a decapsulation operation and then sets the params on the context in the same way as calling EVP_PKEY_CTX_set_params(3). Note that ctx usually is produced using EVP_PKEY_CTX_new_from_pkey(3), specifying the private key to use.

The EVP_PKEY_decapsulate() function performs a private key decapsulation operation using ctx. The data to be decapsulated is specified using the wrapped and wrappedlen parameters. If unwrapped is NULL then the maximum size of the output secret buffer is written to *unwrappedlen. If unwrapped is not NULL and the call is successful then the decapsulated secret data is written to unwrapped and the amount of data written to *unwrappedlen.


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


EVP_PKEY_decapsulate_init() and EVP_PKEY_decapsulate() 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 private key algorithm.


Decapsulate data using RSA:

#include <openssl/evp.h>

 * NB: assumes rsa_priv_key is an RSA private key,
 * and that in, inlen are already set up to contain encapsulated data.

size_t secretlen = 0;
unsigned char *secret = NULL;;

ctx = EVP_PKEY_CTX_new_from_pkey(libctx, rsa_priv_key, NULL);
if (ctx = NULL)
    /* Error */
if (EVP_PKEY_decapsulate_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_decapsulate(ctx, NULL, &secretlen, in, inlen) <= 0)
    /* Error */

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

/* Decapsulated secret data is secretlen bytes long */
if (EVP_PKEY_decapsulaterctx, secret, &secretlen, in, inlen) <= 0)
    /* Error */


EVP_PKEY_CTX_new_from_pkey(3), EVP_PKEY_encapsulate(3), EVP_KEM-RSA(7),


These functions were added in OpenSSL 3.0.

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