=pod

=head1 NAME

EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal,
EVP_DigestSign - 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);

 int EVP_DigestSign(EVP_MD_CTX *ctx, unsigned char *sigret,
                    size_t *siglen, const unsigned char *tbs,
                    size_t tbslen);

=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<e> 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. Note that any existing value in
B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be freed
directly by the application if B<ctx> is not assigned an EVP_PKEY_CTX value before
being passed to EVP_DigestSignInit() (which means the EVP_PKEY_CTX is created
inside EVP_DigestSignInit() and 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.

No B<EVP_PKEY_CTX> will be created by EVP_DigsetSignInit() if the passed B<ctx>
has already been assigned one via L<EVP_MD_CTX_set_ctx(3)>. See also L<SM2(7)>.

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,
Poly1305, DSA, ECDSA, HMAC, RSA, SipHash, Ed25519 and Ed448.

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, SHA512 and SM3

=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,
SHA3-224, SHA3-256, SHA3-384, SHA3-512

=item Ed25519 and Ed448

Support no digests (the digest B<type> must be NULL)

=item HMAC

Supports any digest

=item CMAC, Poly1305 and SipHash

Will ignore any digest provided.

=back

If RSA-PSS is used and restrictions apply then the digest must match.

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> and 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>.

EVP_DigestSign() signs B<tbslen> bytes of data at B<tbs> and places the
signature in B<sig> and its length in B<siglen> in a similar way to
EVP_DigestSignFinal().

=head1 RETURN VALUES

EVP_DigestSignInit(), EVP_DigestSignUpdate(), EVP_DigestSignaFinal() and
EVP_DigestSign() 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.

EVP_DigestSign() is a one shot operation which signs a single block of data
in one function. For algorithms that support streaming it is equivalent to
calling EVP_DigestSignUpdate() and EVP_DigestSignFinal(). For algorithms which
do not support streaming (e.g. PureEdDSA) it is the only way to sign data.

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_free() 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 added in 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
