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+.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.