.\" $OpenBSD: EVP_PKEY_decrypt.3,v 1.9 2024/07/21 08:10:17 tb Exp $
.\" full merge up to: OpenSSL 48e5119a Jan 19 10:49:22 2018 +0100
.\"
.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: July 21 2024 $
.Dt EVP_PKEY_DECRYPT 3
.Os
.Sh NAME
.Nm EVP_PKEY_decrypt_init ,
.Nm EVP_PKEY_decrypt
.Nd decrypt using a public key algorithm
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_PKEY_decrypt_init
.Fa "EVP_PKEY_CTX *ctx"
.Fc
.Ft int
.Fo EVP_PKEY_decrypt
.Fa "EVP_PKEY_CTX *ctx"
.Fa "unsigned char *out"
.Fa "size_t *outlen"
.Fa "const unsigned char *in"
.Fa "size_t inlen"
.Fc
.Sh DESCRIPTION
The
.Fn EVP_PKEY_decrypt_init
function initializes a public key algorithm context using key
.Fa ctx->pkey
for a decryption operation.
.Pp
The
.Fn EVP_PKEY_decrypt
function performs a public key decryption operation using
.Fa ctx .
The data to be decrypted is specified using the
.Fa in
and
.Fa inlen
parameters.
If
.Fa out
is
.Dv NULL
then the maximum size of the output buffer is written to the
.Fa outlen
parameter.
If
.Fa out
is not
.Dv NULL
then before the call the
.Fa outlen
parameter should contain the length of the
.Fa out
buffer.
If the call is successful, the decrypted data is written to
.Fa out
and the amount of data written to
.Fa outlen .
.Pp
After the call to
.Fn EVP_PKEY_decrypt_init ,
algorithm specific control operations can be performed to set any
appropriate parameters for the operation.
.Pp
The function
.Fn EVP_PKEY_decrypt
can be called more than once on the same context if several operations
are performed using the same parameters.
.Sh RETURN VALUES
.Fn EVP_PKEY_decrypt_init
and
.Fn 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.
.Sh EXAMPLES
Decrypt data using OAEP (for RSA keys):
.Bd -literal -offset indent
#include <openssl/evp.h>
#include <openssl/rsa.h>

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

/*
 * Assumes that key, in, and inlen are already set up
 * and that key is an RSA private key.
 */
ctx = EVP_PKEY_CTX_new(key, NULL);
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 = 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 */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_derive 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_meth_set_decrypt 3 ,
.Xr EVP_PKEY_sign 3 ,
.Xr EVP_PKEY_verify 3 ,
.Xr EVP_PKEY_verify_recover 3
.Sh HISTORY
.Fn EVP_PKEY_decrypt_init
and
.Fn EVP_PKEY_decrypt
first appeared in OpenSSL 1.0.0 and have been available since
.Ox 4.9 .