  Digital signature scheme is commonly employed in modern electronic commerce
and quantum digital signature (QDS) offers the information-theoretical security
by the laws of quantum mechanics against attackers with unreasonable
computation resources. The focus of previous QDS was on the signature of 1-bit
message. In contrast, based on quantum ghost imaging with security test, we
propose a scheme of QDS in which a multi-bit message can be signed at a time.
Our protocol is no more demanding than the traditional cases for the
requirements of quantum resources and classical communications. The multi-bit
protocol could simplify the procedure of QDS for long messages in practical
applications.
