xref: /freebsd-11-stable/crypto/openssl/doc/crypto/EVP_DigestSignInit.pod

=pod

=head1 NAME

EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal - EVP signing functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_DigestSignInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
			const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
 int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
 int EVP_DigestSignFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t *siglen);

=head1 DESCRIPTION

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

EVP_DigestSignInit() sets up signing context B<ctx> to use digest B<type> from
ENGINE B<impl> and private key B<pkey>. B<ctx> must be initialized with
EVP_MD_CTX_init() before calling this function. If B<pctx> is not NULL, the
EVP_PKEY_CTX of the signing operation will be written to B<*pctx>: this can
be used to set alternative signing options. Note that any existing value in
B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be freed
directly by the application (it will be freed automatically when the EVP_MD_CTX
is freed). The digest B<type> may be NULL if the signing algorithm supports it.

EVP_DigestSignUpdate() hashes B<cnt> bytes of data at B<d> into the
signature context B<ctx>. This function can be called several times on the
same B<ctx> to include additional data. This function is currently implemented
usig a macro.

EVP_DigestSignFinal() signs the data in B<ctx> places the signature in B<sig>.
If B<sig> is B<NULL> then the maximum size of the output buffer is written to
the B<siglen> parameter. If B<sig> is not B<NULL> then before the call the
B<siglen> parameter should contain the length of the B<sig> buffer, if the
call is successful the signature is written to B<sig> and the amount of data
written to B<siglen>.

=head1 RETURN VALUES

EVP_DigestSignInit() EVP_DigestSignUpdate() and EVP_DigestSignaFinal() 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.

The error codes can be obtained from 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.

In previous versions of OpenSSL there was a link between message digest types
and public key algorithms. This meant that "clone" digests such as EVP_dss1()
needed to be used to sign using SHA1 and DSA. This is no longer necessary and
the use of clone digest is now discouraged.

For some key types and parameters the random number generator must be seeded
or the operation will fail. 

The call to EVP_DigestSignFinal() internally finalizes a copy of the digest
context. This means that calls to EVP_DigestSignUpdate() and
EVP_DigestSignFinal() can be called later to digest and sign additional data.

Since only a copy of the digest context is ever finalized the context must
be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak
will occur.

The use of EVP_PKEY_size() with these functions is discouraged because some
signature operations may have a signature length which depends on the
parameters set. As a result EVP_PKEY_size() would have to return a value
which indicates the maximum possible signature for any set of parameters.

=head1 SEE ALSO

L<EVP_DigestVerifyInit(3)|EVP_DigestVerifyInit(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<dgst(1)|dgst(1)>

=head1 HISTORY

EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal() 
were first added to OpenSSL 1.0.0.

=cut