=pod

=head1 NAME

EVP_SealInit, EVP_SealUpdate, EVP_SealFinal - EVP envelope encryption

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_SealInit(EVP_CIPHER_CTX *ctx, EVP_CIPHER *type, unsigned char **ek,
		int *ekl, unsigned char *iv,EVP_PKEY **pubk, int npubk);
 void EVP_SealUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 void EVP_SealFinal(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl);

=head1 DESCRIPTION

The EVP envelope routines are a high level interface to envelope
encryption. They generate a random key and then "envelope" it by
using public key encryption. Data can then be encrypted using this
key.

EVP_SealInit() initialises a cipher context B<ctx> for encryption
with cipher B<type> using a random secret key and IV supplied in
the B<iv> parameter. B<type> is normally supplied by a function such
as EVP_des_cbc(). 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. B<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
B<ek[i]> must have room for B<EVP_PKEY_size(pubk[i])> bytes. The actual
size of each encrypted secret key is written to the array B<ekl>. B<pubk> is
an array of B<npubk> public keys.

EVP_SealUpdate() and EVP_SealFinal() have exactly the same properties
as the EVP_EncryptUpdate() and EVP_EncryptFinal() routines, as 
documented on the L<EVP_EncryptInit(3)|EVP_EncryptInit(3)> manual
page. 

=head1 RETURN VALUES

EVP_SealInit() returns -1 on error or B<npubk> if successful.

EVP_SealUpdate() and EVP_SealFinal() do not return values.

=head1 NOTES

Because a random secret key is generated the random number generator
must be seeded before calling EVP_SealInit().

The public key must be RSA because it is the only OpenSSL public key
algorithm that supports key transport.

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.

=head1 SEE ALSO

L<evp(3)|evp(3)>,L<rand(3)|rand(3)>
L<EVP_EncryptInit(3)|EVP_EncryptInit(3)>,
L<EVP_OpenInit(3)|EVP_OpenInit(3)>

=head1 HISTORY

=cut
