## 2 Related Research
### ABE and ABTD  
Attribute-based encryption is an extension of the concept of identity-based encryption, which [PERSON] and others first proposed. Attribute-based encryption refers to a method of decrypting a message when there is an attribute value sufficient to decrypt the message by comparing the attribute value of the user with the attribute value included in the ciphertext.  
Attribute-based encryption is classified into key-policy attribute-based encryption (KP-ABE) and ciphertext-policy attribute-based encryption (CP-ABE) according to the encryption policy. KP-ABE is a method in which an access tree is included in a private key issued to each user, and an attribute value is included in a ciphertext. The user who receives the private key decrypts the message when the attribute value included in the ciphertext satisfies the access tree of the private key. CP-ABE includes an attribute value of each user in the user's private key, and the ciphertext includes an access tree. Users can decrypt the message if their attribute values satisfy the access tree of the ciphertext [13].  
In the KP-ABE method, a hierarchical attribute-based encryption method was proposed for scalable access control [14; 15; 16].  
In the initial CP-ABE, the size of the ciphertext increases linearly according to the access policy. Therefore, as an efficiency improvement study, [PERSON] et al. proposed having a constant ciphertext size [17]. In addition, [PERSON] et al. proposed outsourced decryption that can be verified with a certain ciphertext length [18].  
[PERSON] et al. and [PERSON] et al. presented a hidden access policy in the ABE scheme [19; 20]. In addition, [PERSON] et al. proposed a searchable encryption scheme for the cloud [21; 22]. Methods for improving property cancellation and user collusion were also proposed [23]. [PERSON] et al. proposed a technique to improve the vulnerability of user collusion [24]. [PERSON] et al. proposed a traceable method to improve the vulnerability of leaking access vigilant proof [25; 26].  
Multiauthority ABE (MA-ABE) supports multiple attribute authorities and one central authority to improve the vulnerability of the existing ABE to the central authority [27].  
Figure 3: SDN-based VANET.  
MA-ABE has a vulnerability to collusion attacks by collecting user attributes from multiple organizations when obtaining a secret key by submitting the user's global identifier (GID) to the organization. Accordingly, [PERSON] and [PERSON] proposed the privacy-preserving MA-ABE (PPMA-ABE) by Certificate Authority (CA) [28]. In addition, [PERSON] et al. proposed a privacy-preserving decentralized CP-ABE (PPDCP-ABE) that keeps both user identifiers and attributes confidential [29]. This method requires the user's anonymous certificate, so there is a vulnerability in a collusion attack.  
Attribute-based encryption requires a method that prevents users who withdraw attribute values due to withdrawal from services from continuing to receive the services. In attribute-based encryption, techniques for withdrawing attribute values include a list of attribute-value withdrawers in the access tree. Most withdrawal techniques use a list of attribute-value withdrawers in KP-ABE. In CP-ABE, attribute value withdrawal techniques include placing an expiration date on attribute values [30; 31].  
In this paper, an attribute-enabled encryption algorithm capable of such withdrawal is used. It is intended to propose a model that prevents attacks such as user identity forgery in a smart traffic environment.