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EVP_PKEY_DECAPSULATE(3ossl)         OpenSSL        EVP_PKEY_DECAPSULATE(3ossl)



NAME

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


SYNOPSIS

        #include <openssl/evp.h>

        int EVP_PKEY_decapsulate_init(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
        int EVP_PKEY_auth_decapsulate_init(EVP_PKEY_CTX *ctx, EVP_PKEY *authpub,
                                          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);


DESCRIPTION

       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_auth_decapsulate_init() function is similar to
       EVP_PKEY_decapsulate_init() but also passes an authpub authentication
       public key that is used during decapsulation.

       The EVP_PKEY_decapsulate(3) function performs a private key
       decapsulation operation using ctx. The data to be decapsulated is
       specified using the wrapped and wrappedlen parameters (which must both
       non-NULL).

       The wrapped parameter is an output argument, to which the decapsulated
       shared secret is written.  The shared secret may not match the peer's
       value even when decapsulation returns success.  Instead, the shared
       secret must be used to derive a key that is used to authenticate data
       subsequently received from the peer.  If unwrapped is NULL then the
       size of the output shared secret buffer is written to *unwrappedlen and
       no decapsulation is performed, this makes it possible to determine the
       required buffer size at run time.  Otherwise, the decapsulated secret
       data is written to unwrapped and the length of shared secret is written
       to *unwrappedlen.

       Note that the value pointed to by unwrappedlen (which must NOT be NULL)
       must be initialised to the length of unwrapped, so that the call can
       validate it is of sufficient size to hold the result of the operation.

       Absent detailed prior knowledge of the internals of the specific KEM
       algorithm, callers SHOULD NOT assume that the returned shared secret is
       necessarily of the maximum possible length.  The length returned via
       *unwrappedlen SHOULD be used to determine the actual length of the
       output.


NOTES

       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).


RETURN VALUES

       EVP_PKEY_decapsulate_init(), EVP_PKEY_auth_decapsulate_init() and
       EVP_PKEY_decapsulate(3) 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.


EXAMPLES

       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.
         */

        EVP_PKEY_CTX *ctx = NULL;
        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_decapsulate(ctx, secret, &secretlen, in, inlen) <= 0)
            /* Error */


SEE ALSO

       EVP_PKEY_CTX_new_from_pkey(3), EVP_PKEY_encapsulate(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_decapsulate(3)
       were added in OpenSSL 3.0.

       The function EVP_PKEY_auth_decapsulate_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>.

3.5.0                             2025-04-10       EVP_PKEY_DECAPSULATE(3ossl)

openssl 3.5.0 - Generated Sun Apr 20 13:46:16 CDT 2025
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