This paper describes a new concept of security measurement in quantum cryptography (QC). The most popular quantum key distribution protocol BB84 and the key distillation process are briefly introduced first. Next, a new concept of entropy of security in QC is proposed, and a unique measure of security is defined. Using this quantitative approach to security, it is possible to manage security and personalize services based on QC. Two different security levels are defined: the basic security level and the advanced security level. This differentiation of security enables us to choose the appropriate security level for specific end-users' requirements and needs. The last section presents the results of simulation experiments which verified the proposed solution.