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=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 created with
EVP_MD_CTX_new() 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. The digest B<type> may be NULL if
the signing algorithm supports it.
Only EVP_PKEY types that support signing can be used with these functions. This
includes MAC algorithms where the MAC generation is considered as a form of
"signing." Built-in EVP_PKEY types supported by these functions are CMAC, DSA,
ECDSA, HMAC and RSA.
Not all digests can be used for all key types. The following combinations apply.
=over 4
=item DSA
Supports SHA1, SHA224, SHA256, SHA384 and SHA512
=item ECDSA
Supports SHA1, SHA224, SHA256, SHA384 and SHA512
=item RSA with no padding
Supports no digests (the digest B<type> must be NULL)
=item RSA with X931 padding
Supports SHA1, SHA256, SHA384 and SHA512
=item All other RSA padding types
Support SHA1, SHA224, SHA256, SHA384, SHA512, MD5, MD5_SHA1, MD2, MD4, MDC2,
RIPEMD160
=item HMAC
Supports any digest
=item CMAC
Will ignore any digest provided.
=back
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
using 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)>.
=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)>,
L<EVP_DigestInit(3)>,
L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,
L<MD5(3)>, L<MDC2(3)>, L<RIPEMD160(3)>,
L<SHA1(3)>, L<dgst(1)>
=head1 HISTORY
EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal()
were first added to OpenSSL 1.0.0.
=head1 COPYRIGHT
Copyright 2006-2018 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.
=cut
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