.\"	$OpenBSD: EVP_OpenInit.3,v 1.9 2023/11/16 20:27:43 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: November 16 2023 $
.Dt EVP_OPENINIT 3
.Os
.Sh NAME
.Nm EVP_OpenInit ,
.Nm EVP_OpenUpdate ,
.Nm EVP_OpenFinal
.Nd EVP envelope decryption
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_OpenInit
.Fa "EVP_CIPHER_CTX *ctx"
.Fa "EVP_CIPHER *type"
.Fa "unsigned char *ek"
.Fa "int ekl"
.Fa "unsigned char *iv"
.Fa "EVP_PKEY *priv"
.Fc
.Ft int
.Fo EVP_OpenUpdate
.Fa "EVP_CIPHER_CTX *ctx"
.Fa "unsigned char *out"
.Fa "int *outl"
.Fa "unsigned char *in"
.Fa "int inl"
.Fc
.Ft int
.Fo EVP_OpenFinal
.Fa "EVP_CIPHER_CTX *ctx"
.Fa "unsigned char *out"
.Fa "int *outl"
.Fc
.Sh DESCRIPTION
The EVP envelope routines are a high level interface to envelope
decryption.
They decrypt a public key encrypted symmetric key and then decrypt data
using it.
.Pp
.Fn EVP_OpenInit
initializes a cipher context
.Fa ctx
for decryption with cipher
.Fa type .
It decrypts the encrypted symmetric key of length
.Fa ekl
bytes passed in the
.Fa ek
parameter using the private key
.Fa priv .
The IV is supplied in the
.Fa iv
parameter.
.Pp
.Fn EVP_OpenUpdate
and
.Fn EVP_OpenFinal
have exactly the same properties as the
.Xr EVP_DecryptUpdate 3
and
.Xr EVP_DecryptFinal 3
routines.
.Pp
It is possible to call
.Fn EVP_OpenInit
twice in the same way as
.Xr EVP_DecryptInit 3 .
The first call should have
.Fa priv
set to
.Dv NULL
and (after setting any cipher parameters) it should be
called again with
.Fa type
set to
.Dv NULL .
.Pp
If the cipher passed in the
.Fa type
parameter is a variable length cipher then the key length will be set to
the value of the recovered key length.
If the cipher is a fixed length cipher then the recovered key length
must match the fixed cipher length.
.Pp
.Fn EVP_OpenUpdate
is implemented as a macro.
.Sh RETURN VALUES
.Fn EVP_OpenInit
returns 0 on error or a non-zero integer (actually the recovered secret
key size) if successful.
.Pp
.Fn EVP_OpenUpdate
returns 1 for success or 0 for failure.
.Pp
.Fn EVP_OpenFinal
returns 0 if the decrypt failed or 1 for success.
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_EncryptInit 3 ,
.Xr EVP_SealInit 3
.Sh HISTORY
.Fn EVP_OpenInit ,
.Fn EVP_OpenUpdate ,
and
.Fn EVP_OpenFinal
first appeared in SSLeay 0.5.1 and have been available since
.Ox 2.4 .