.Dd $Mdocdate: November 3 2016 $
.Dt EVP_DIGESTSIGNINIT 3
.Os
.Sh NAME
.Nm EVP_DigestSignInit ,
.Nm EVP_DigestSignUpdate ,
.Nm EVP_DigestSignFinal
.Nd EVP signing functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_DigestSignInit
.Fa "EVP_MD_CTX *ctx"
.Fa "EVP_PKEY_CTX **pctx"
.Fa "const EVP_MD *type"
.Fa "ENGINE *e"
.Fa "EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_DigestSignUpdate
.Fa "EVP_MD_CTX *ctx"
.Fa "const void *d"
.Fa "unsigned int cnt"
.Fc
.Ft int
.Fo EVP_DigestSignFinal
.Fa "EVP_MD_CTX *ctx"
.Fa "unsigned char *sig"
.Fa "size_t *siglen"
.Fc
.Sh DESCRIPTION
The EVP signature routines are a high level interface to digital
signatures.
.Pp
.Fn EVP_DigestSignInit
sets up the signing context
.Fa ctx
to use the digest
.Fa type
from
.Vt ENGINE
.Fa e
and private key
.Fa pkey .
.Fa ctx
must be initialized with
.Xr EVP_MD_CTX_init 3
before calling this function.
If
.Fa pctx
is not
.Dv NULL ,
the
.Vt EVP_PKEY_CTX
of the signing operation will be written to
.Pf * Fa pctx :
this can be used to set alternative signing options.
.Pp
.Fn EVP_DigestSignUpdate
hashes
.Fa cnt
bytes of data at
.Fa d
into the signature context
.Fa ctx .
This function can be called several times on the same
.Fa ctx
to include additional data.
This function is currently implemented using a macro.
.Pp
.Fn EVP_DigestSignFinal
signs the data in
.Fa ctx
and places the signature in
.Fa sig .
If
.Fa sig
is
.Dv NULL ,
then the maximum size of the output buffer is written to
.Pf * Fa siglen .
If
.Fa sig
is not
.Dv NULL ,
then before the call
.Fa siglen
should contain the length of the
.Fa sig
buffer.
If the call is successful, the signature is written to
.Fa sig
and the amount of data written to
.Fa siglen .
.Pp
The 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.
.Pp
In previous versions of OpenSSL, there was a link between message digest
types and public key algorithms.
This meant that "clone" digests such as
.Xr EVP_dss1 3
needed to be used to sign using SHA1 and DSA.
This is no longer necessary and the use of clone digest is now
discouraged.
.Pp
The call to
.Fn EVP_DigestSignFinal
internally finalizes a copy of the digest context.
This means that
.Fn EVP_DigestSignUpdate
and
.Fn EVP_DigestSignFinal
can be called later to digest and sign additional data.
.Pp
Since only a copy of the digest context is ever finalized, the context
must be cleaned up after use by calling
.Xr EVP_MD_CTX_cleanup 3 ,
or a memory leak will occur.
.Pp
The use of
.Xr EVP_PKEY_size 3
with these functions is discouraged because some signature operations
may have a signature length which depends on the parameters set.
As a result,
.Xr EVP_PKEY_size 3
would have to return a value which indicates the maximum possible
signature for any set of parameters.
.Sh RETURN VALUES
.Fn EVP_DigestSignInit ,
.Fn EVP_DigestSignUpdate ,
and
.Fn EVP_DigestSignFinal
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.
.Pp
The error codes can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ERR 3 ,
.Xr evp 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_DigestVerifyInit 3
.Sh HISTORY
.Fn EVP_DigestSignInit ,
.Fn EVP_DigestSignUpdate ,
and
.Fn EVP_DigestSignFinal
were first added to OpenSSL 1.0.0.