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.\" ========================================================================
.\"
.IX Title "EVP_PKEY_DECAPSULATE 3ossl"
.TH EVP_PKEY_DECAPSULATE 3ossl "2026-01-27" "3.5.5" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
EVP_PKEY_decapsulate_init, EVP_PKEY_auth_decapsulate_init, EVP_PKEY_decapsulate
\&\- Key decapsulation using a KEM algorithm with a private key
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fBEVP_PKEY_decapsulate_init()\fR function initializes a private key algorithm
context \fIctx\fR for a decapsulation operation and then sets the \fIparams\fR
on the context in the same way as calling \fBEVP_PKEY_CTX_set_params\fR\|(3).
Note that \fIctx\fR usually is produced using \fBEVP_PKEY_CTX_new_from_pkey\fR\|(3),
specifying the private key to use.
.PP
The \fBEVP_PKEY_auth_decapsulate_init()\fR function is similar to
\&\fBEVP_PKEY_decapsulate_init()\fR but also passes an \fIauthpub\fR authentication public
key that is used during decapsulation.
.PP
The \fBEVP_PKEY_decapsulate()\fR function performs a private key decapsulation
operation using \fIctx\fR. The data to be decapsulated is specified using the
\&\fIwrapped\fR and \fIwrappedlen\fR parameters (which must both non-NULL).
.PP
The \fIwrapped\fR 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 \fIunwrapped\fR is \s-1NULL\s0 then the size of the output shared secret buffer is
written to \fI*unwrappedlen\fR 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 \fIunwrapped\fR and the length of shared
secret is written to \fI*unwrappedlen\fR.
.PP
Note that the value pointed to by \fIunwrappedlen\fR (which must \s-1NOT\s0 be \fB\s-1NULL\s0\fR)
must be initialised to the length of \fIunwrapped\fR, so that the call can
validate it is of sufficient size to hold the result of the operation.
.PP
Absent detailed prior knowledge of the internals of the specific \s-1KEM\s0
algorithm, callers \s-1SHOULD NOT\s0 assume that the returned shared secret
is necessarily of the maximum possible length.
The length returned via \fI*unwrappedlen\fR \s-1SHOULD\s0 be used to determine the actual
length of the output.
.SH "NOTES"
.IX Header "NOTES"
After the call to \fBEVP_PKEY_decapsulate_init()\fR algorithm-specific parameters
for the operation may be set or modified using \fBEVP_PKEY_CTX_set_params\fR\|(3).
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBEVP_PKEY_decapsulate_init()\fR, \fBEVP_PKEY_auth_decapsulate_init()\fR and
\&\fBEVP_PKEY_decapsulate()\fR 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.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Decapsulate data using \s-1RSA:\s0
.PP
.Vb 1
\& #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 \*(AqRSASVE\*(Aq 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 */
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBEVP_PKEY_CTX_new_from_pkey\fR\|(3),
\&\fBEVP_PKEY_encapsulate\fR\|(3),
\&\s-1\fBEVP_KEM\-RSA\s0\fR\|(7),
\&\s-1\fBEVP_KEM\-X25519\s0\fR\|(7),
\&\s-1\fBEVP_KEM\-EC\s0\fR\|(7),
\&\s-1\fBEVP_KEM\-ML\-KEM\-512\s0\fR\|(7),
\&\s-1\fBEVP_KEM\-ML\-KEM\-768\s0\fR\|(7),
\&\s-1\fBEVP_KEM\-ML\-KEM\-1024\s0\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
The functions \fBEVP_PKEY_decapsulate_init()\fR and \fBEVP_PKEY_decapsulate()\fR were added
in OpenSSL 3.0.
.PP
The function \fBEVP_PKEY_auth_decapsulate_init()\fR was added in OpenSSL 3.2.
.PP
Support for \fBML-KEM\fR was added in OpenSSL 3.5.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2020\-2025 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
