Provided by: libssl-doc_3.5.0-2ubuntu1_all bug

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 writen 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 wthe 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>.

3.5.0                                              2025-06-04                         EVP_PKEY_ENCAPSULATE(3SSL)