I. Introduction
These days, due to the development of cloud computing, cloud environments are frequently used in companies, corporations, or public institutions with the advantage of safely storing or sharing data with others. However, when sharing data in a cloud environment, various security threats can occur. Therefore, in order to safely share data in a cloud environment, the security of shared data is important, and user authentication technology and access control technology are required to access data stored in the cloud. Among various security technologies, Ciphertext-Policy Attribute Based Encryption (CP-ABE) is one type of attribute-based encryption that is widely used in data sharing systems and data access control in a cloud environment[1]. In particular, CP-ABE allows a 1:N communication environment. When the data owner encrypts the data and uploads it to the storage, it has the feature that many unspecified users corresponding to the attributes can access and take the ciphertext and decrypt it. Various CP-ABE methods have been studied to date, and CPABE methods in which an outsourcing server is provided to support a user’s decryption computation burden has also been studied. However, when using the existing CP-ABE methods providing an outsourcing server, it is not known whether the ciphertext partial decryption was properly performed. It is not known whether the message obtained after the user performs the decryption is the message uploaded by the actual data owner or if the data value has been tampered with. Another problem is that the size of the ciphertext increases in proportion to the number of attributes specified in the access structure when the data is encrypted with attribute-based encryption. This causes a problem that can waste cloud storage[2][3][4].