=pod

=head1 NAME

EVP_SignInit, EVP_SignUpdate, EVP_SignFinal - EVP signing functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);
 void EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
 int EVP_SignFinal(EVP_MD_CTX *ctx,unsigned char *sig,unsigned int *s, EVP_PKEY *pkey);

 int EVP_PKEY_size(EVP_PKEY *pkey);

=head1 DESCRIPTION

The EVP signature routines are a high level interface to digital
signatures.

EVP_SignInit() initialises a signing context B<ctx> to using digest
B<type>: this will typically be supplied by a function such as
EVP_sha1().

EVP_SignUpdate() hashes B<cnt> bytes of data at B<d> into the
signature context B<ctx>. This funtion can be called several times on the
same B<ctx> to include additional data.

EVP_SignFinal() signs the data in B<ctx> using the private key B<pkey>
and places the signature in B<sig>. If the B<s> parameter is not NULL
then the number of bytes of data written (i.e. the length of the signature)
will be written to the integer at B<s>, at most EVP_PKEY_size(pkey) bytes
will be written.  After calling EVP_SignFinal() no additional calls to
EVP_SignUpdate() can be made, but EVP_SignInit() can be called to initialiase
a new signature operation.

EVP_PKEY_size() returns the maximum size of a signature in bytes. The actual
signature returned by EVP_SignFinal() may be smaller.

=head1 RETURN VALUES

EVP_SignInit() and EVP_SignUpdate() do not return values.

EVP_SignFinal() returns 1 for success and 0 for failure.

EVP_PKEY_size() returns the maximum size of a signature in bytes.

The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 NOTES

The B<EVP> interface to digital signatures should almost always be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the algorithm used and much more flexible.

Due to the link between message digests and public key algorithms the correct
digest algorithm must be used with the correct public key type. A list of
algorithms and associated public key algorithms appears in 
L<EVP_DigestInit(3)|EVP_DigestInit(3)>.

When signing with DSA private keys the random number generator must be seeded
or the operation will fail. The random number generator does not need to be
seeded for RSA signatures.

=head1 BUGS

Several of the functions do not return values: maybe they should. Although the
internal digest operations will never fail some future hardware based operations
might.

=head1 SEE ALSO

L<EVP_VerifyInit(3)|EVP_VerifyInit(3)>,
L<EVP_DigestInit(3)|EVP_DigestInit(3)>, L<err(3)|err(3)>,
L<evp(3)|evp(3)>, L<hmac(3)|hmac(3)>, L<md2(3)|md2(3)>,
L<md5(3)|md5(3)>, L<mdc2(3)|mdc2(3)>, L<ripemd(3)|ripemd(3)>,
L<sha(3)|sha(3)>, L<digest(1)|digest(1)>

=head1 HISTORY

EVP_SignInit(), EVP_SignUpdate() and EVP_SignFinal() are
available in all versions of SSLeay and OpenSSL.

=cut
