.\"	$OpenBSD: EVP_SealInit.3,v 1.2 2016/11/06 15:52:50 jmc Exp $
.\"
.Dd $Mdocdate: November 6 2016 $
.Dt EVP_SEALINIT 3
.Os
.Sh NAME
.Nm EVP_SealInit ,
.Nm EVP_SealUpdate ,
.Nm EVP_SealFinal
.Nd EVP envelope encryption
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_SealInit
.Fa "EVP_CIPHER_CTX *ctx"
.Fa "const EVP_CIPHER *type"
.Fa "unsigned char **ek"
.Fa "int *ekl"
.Fa "unsigned char *iv"
.Fa "EVP_PKEY **pubk"
.Fa "int npubk"
.Fc
.Ft int
.Fo EVP_SealUpdate
.Fa "EVP_CIPHER_CTX *ctx"
.Fa "unsigned char *out"
.Fa "int *outl"
.Fa "unsigned char *in"
.Fa "int inl"
.Fc
.Ft int
.Fo EVP_SealFinal
.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
encryption.
They generate a random key and IV (if required) then "envelope" it by
using public key encryption.
Data can then be encrypted using this key.
.Pp
.Fn EVP_SealInit
initializes a cipher context
.Fa ctx
for encryption with cipher
.Fa type
using a random secret key and IV.
.Fa type
is normally supplied by a function such as
.Fn EVP_aes_256_cbc 3 ;
see
.Xr EVP_EncryptInit 3
for details.
The secret key is encrypted using one or more public keys.
This allows the same encrypted data to be decrypted using any of
the corresponding private keys.
.Fa ek
is an array of buffers where the public key encrypted secret key will be
written.
Each buffer must contain enough room for the corresponding encrypted
key: that is
.Fa ek[i]
must have room for
.Fn EVP_PKEY_size pubk[i]
bytes.
The actual size of each encrypted secret key is written to the array
.Fa ekl .
.Fa pubk
is an array of
.Fa npubk
public keys.
.Pp
The
.Fa iv
parameter is a buffer where the generated IV is written to.
It must contain enough room for the corresponding cipher's IV, as
determined by (for example)
.Fn EVP_CIPHER_iv_length type .
.Pp
If the cipher does not require an IV then the
.Fa iv
parameter is ignored and can be
.Dv NULL .
.Pp
.Fn EVP_SealUpdate
and
.Fn EVP_SealFinal
have exactly the same properties as the
.Xr EVP_EncryptUpdate 3
and
.Xr EVP_EncryptFinal 3
routines.
.Pp
The public key must be RSA because it is the only OpenSSL public key
algorithm that supports key transport.
.Pp
Envelope encryption is the usual method of using public key encryption
on large amounts of data.
This is because public key encryption is slow but symmetric encryption
is fast.
So symmetric encryption is used for bulk encryption and the small random
symmetric key used is transferred using public key encryption.
.Pp
It is possible to call
.Fn EVP_SealInit
twice in the same way as
.Xr EVP_EncryptInit 3 .
The first call should have
.Fa npubk
set to 0 and (after setting any cipher parameters) it should be called
again with
.Fa type
set to NULL.
.Sh RETURN VALUES
.Fn EVP_SealInit
returns 0 on error or
.Fa npubk
if successful.
.Pp
.Fn EVP_SealUpdate
and
.Fn EVP_SealFinal
return 1 for success and 0 for failure.
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_EncryptInit 3 ,
.Xr EVP_OpenInit 3 ,
.Xr rand 3
.Sh HISTORY
.Fn EVP_SealFinal
did not return a value before OpenSSL 0.9.7.