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 3.1.0, 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 processed in a side-
       channel free way.

       Since version 3.1.0, the EVP_PKEY_decrypt() method when used with PKCS#1 v1.5 padding
       doesn't return an error in case 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.

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 2006-2022 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>.