## 2 Related Work
### Dynamic Encryption Key of OTP (S/KEY) Method  
The OTP (SKEY) method [28] is an authentication key generation method developed by Bell Communications Research and is used for authentication in UNIX based operating systems. In this method, the generation algorithm sends a random secret key determined by the client to the server. The secret key received from the client is used as the first value. Then, the task of obtaining the hash value for the previous result value is repeated N times based on the hash chain method. The generated N OTPs are stored in the server. The hash function determined by the client is applied n-i times and sent to the server. The server applies the hash function to the value received from the client once and checks whether the result matches the n-I + stOTP stored in the server. In the study of Limited-Used Key Generation Scheme [29], a method is proposed for generating a one-time password key by applying the OTP (S/KEY) method. This scheme generates an encryption key sequence in advance to generate a dynamic key. Because each encryption key is used to encrypt only one message, every message uses a different encryption key. This scheme uses the predefined hash function and the shared K\({}_{\text{AB}}\) master key. In this scheme, a dynamic encryption key is generated as shown in Figure 1.  
The authentication server generates distribution key (DK) and sends it using the authenticated key exchange protocol. The client and the authentication server generate the basic setting keys, K set (\(\{\text{K}_{1}=\text{hash(DK, K${}_{\text{AB}}$)}\), \(\text{K}_{2}=\text{h(DK, K${}_{1}$)...}\)\(\text{K}_{\text{m}}=\text{h(DK, K${}_{\text{m-1}}$)}\}\)) through iterative hash of DK and K\({}_{\text{AB}}\). Subsequently, the authentication server generates random number r and sends it to the client,  
Figure 1: Limited-Used Key Generation Scheme.  
then w = r mod n is derived from the random number r. The authentication server and the client select one Kmid1 among {K1, K2, K3....Kw} from w, and select Kmid2 among {K1, K2, K3....Kmid1}. The authentication server and the client obtain SIK = h(kmid, Kmid2) and generate the session key (SK) sets as follows through iterative hash of SIK and DK: SK1 = h(SIK,DK), SK2 = h(SIK,SK2).... SKn = h(SIK,SKn-1).  
In the OTP (S/KEY) method, there is no recovery plan protocol for solving the encryption key synchronization problem. In the current method, two or more communication steps of synchronization method is required if the key synchronization fails due to packet loss. A and B should send the request/reply messages to inform the encryption key mismatch and attempt to match the current counter. In the case of OTP method, because the initial seed value of encryption key is stored in the OTP server, it is vulnerable to hijacking attacks [24]. If a malicious attacker steals the seed value of OTP server and reproduces the mechanism of encryption key generation, the key can be predicted. Furthermore, the OTP method employs a method of generating the encryption keys that will be used in advance. Therefore, encryption keys have to be regenerated when the encryption keys are all exhausted.