context, must have been created with EVP_MD_CTX_new()
if not null, then the pkey context will be copied here
the message digest algorithm that will be used
the digest engine to use. May be null for some algorithms
the private key
1 on success, 0 or negative for failure, -2 if the operation is not supported by the publuc key algorithm
Sets up the digest context for generating a signature
For some key types and parameters the random number generator must be seeded or the operation will fail.