Computes the ECDSA signature of the given hash value using
the supplied private key and returns the created signature.
\param dgst pointer to the hash value
\param dgst_len length of the hash value
\param eckey EC_KEY object containing a private EC key
\return pointer to a ECDSA_SIG structure or NULL if an error occurred
Computes the ECDSA signature of the given hash value using the supplied private key and returns the created signature. \param dgst pointer to the hash value \param dgst_len length of the hash value \param eckey EC_KEY object containing a private EC key \return pointer to a ECDSA_SIG structure or NULL if an error occurred