Updated: 2025/Nov/16

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EVP_PKEY_decrypt(3)                 OpenSSL                EVP_PKEY_decrypt(3)


NAME
       EVP_PKEY_decrypt_init, EVP_PKEY_decrypt_init_ex, EVP_PKEY_decrypt -
       decrypt using a public key algorithm

SYNOPSIS
        #include <openssl/evp.h>

        int EVP_PKEY_decrypt_init(EVP_PKEY_CTX *ctx);
        int EVP_PKEY_decrypt_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
        int EVP_PKEY_decrypt(EVP_PKEY_CTX *ctx,
                             unsigned char *out, size_t *outlen,
                             const unsigned char *in, size_t inlen);

DESCRIPTION
       The EVP_PKEY_decrypt_init() function initializes a public key algorithm
       context using key pkey for a decryption operation.

       The EVP_PKEY_decrypt_init_ex() function initializes a public key
       algorithm context using key pkey for a decryption operation and sets
       the algorithm specific params.

       The EVP_PKEY_decrypt() function performs a public key decryption
       operation using ctx. The data to be decrypted is specified using the in
       and inlen parameters. If out is NULL then the minimum required size of
       the output buffer is written to the *outlen parameter.

       If out is not NULL then before the call the *outlen parameter must
       contain the length of the out buffer. If the call is successful the
       decrypted data is written to out and the amount of the decrypted data
       written to *outlen, otherwise an error is returned.

NOTES
       After the call to EVP_PKEY_decrypt_init() algorithm specific control
       operations can be performed to set any appropriate parameters for the
       operation.  These operations can be included in the
       EVP_PKEY_decrypt_init_ex() call.

       The function EVP_PKEY_decrypt() can be called more than once on the
       same context if several operations are performed using the same
       parameters.

RETURN VALUES
       EVP_PKEY_decrypt_init(), EVP_PKEY_decrypt_init_ex() and
       EVP_PKEY_decrypt() 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.

WARNINGS
       In OpenSSL versions before 3.2.0, when used in PKCS#1 v1.5 padding,
       both the return value from the EVP_PKEY_decrypt() and the outlen
       provided information useful in mounting a Bleichenbacher attack against
       the used private key. They had to be processed in a side-channel free
       way.

       Since version 3.2.0, the EVP_PKEY_decrypt() method when used with
       PKCS#1 v1.5 padding as implemented in the default provider implements
       the implicit rejection mechanism (see
       OSSL_ASYM_CIPHER_PARAM_IMPLICIT_REJECTION in provider-asym_cipher(7)).
       That means it doesn't return an error when it detects an error in
       padding, instead it returns a pseudo-randomly generated message,
       removing the need of side-channel secure code from applications using
       OpenSSL.  If OpenSSL is configured to use a provider that doesn't
       implement implicit rejection, the code still needs to handle the
       returned values using side-channel free code.  Side-channel free
       handling of the error stack can be performed using either a pair of
       unconditional ERR_set_mark(3) and ERR_pop_to_mark(3) calls or by using
       the ERR_clear_error(3) call.

EXAMPLES
       Decrypt data using OAEP (for RSA keys):

        #include <openssl/evp.h>
        #include <openssl/rsa.h>

        EVP_PKEY_CTX *ctx;
        ENGINE *eng;
        unsigned char *out, *in;
        size_t outlen, inlen;
        EVP_PKEY *key;

        /*
         * NB: assumes key, eng, in, inlen are already set up
         * and that key is an RSA private key
         */
        ctx = EVP_PKEY_CTX_new(key, eng);
        if (!ctx)
            /* Error occurred */
        if (EVP_PKEY_decrypt_init(ctx) <= 0)
            /* Error */
        if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING) <= 0)
            /* Error */

        /* Determine buffer length */
        if (EVP_PKEY_decrypt(ctx, NULL, &outlen, in, inlen) <= 0)
            /* Error */

        out = OPENSSL_malloc(outlen);

        if (!out)
            /* malloc failure */

        if (EVP_PKEY_decrypt(ctx, out, &outlen, in, inlen) <= 0)
            /* Error */

        /* Decrypted data is outlen bytes written to buffer out */

SEE ALSO
       EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_sign(3),
       EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), EVP_PKEY_derive(3)

HISTORY
       These functions were added in OpenSSL 1.0.0.

COPYRIGHT
       Copyright 2006-2025 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>.

3.5.1                             2025-07-01               EVP_PKEY_decrypt(3)