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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_certificate(self, common_name, public_key_algorithm='rsa', signature_algorithm='rsa_sha_512', key_length=2048, signing_ca=None):
 """ Generate an internal gateway certificate used for VPN on this engine. Certificate request should be an instance of VPNCertificate. :param: str common_name: common name for certificate :param str public_key_algorithm: public key type to use. Valid values rsa, dsa, ecdsa. :param str signature_algorithm: signature algorithm. Valid values dsa_sha_1, dsa_sha_224, dsa_sha_256, rsa_md5, rsa_sha_1, rsa_sha_256, rsa_sha_384, rsa_sha_512, ecdsa_sha_1, ecdsa_sha_256, ecdsa_sha_384, ecdsa_sha_512. (Default: rsa_sha_512) :param int key_length: length of key. Key length depends on the key type. For example, RSA keys can be 1024, 2048, 3072, 4096. See SMC documentation for more details. :param str,VPNCertificateCA signing_ca: by default will use the internal RSA CA :raises CertificateError: error generating certificate :return: GatewayCertificate """

        return GatewayCertificate._create(self, common_name, public_key_algorithm,
            signature_algorithm, key_length, signing_ca)