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An anonymous authenticated key-agreement scheme for multi-server infrastructure


Due to single-time registration, the multi-server authentication provides benefit for getting services from different servers through trusted agent. Generally, users feel hesitation for registering themselves individually with all service providers due to the problem of memorizing the multiple passwords. The multi-server authentication allows a quick access to services by real-time customer validation on public channel. Thereafter, hundreds of multi-server authentication protocols have been introduced. However, the more efficient and robust authentication schemes are being explored by the research academia. We introduce an anonymous scheme that resists the major security threats like impersonation attack, insider attack and password modification attacks in viable computing cost. We use random oracle model for formal security analysis of the proposed scheme. The performance analysis shows that the proposed scheme incurs less computation, energy, communication and storage cost as compared to related protocols. This analysis and comparison show that our proposed scheme is quite effective for the purpose of anonymous authentication and key agreement.


Multi-server authentication (MSA) allows the abrupt access to various online services as compared to single registration, in peer-to-peer environment. MSA architecture [1, 2] is suitable for both sides, i.e. customers and service providers. Because, due to one-time registration with registration center (RC), the customer does not need for remembering multiple passwords. Consequently, the MSA architecture facilitates the service providers to maintain verifier database for each authentic user, in order to avoid multiple registration. To get perk from these services from various servers, the customers rely on a single time registration with RC. The MSA environment involves various servers \((S_j)\), customers \((U_u)\) and a registration center (RC). The generic architecture of remote-user authentication is shown in Fig. 1. The registration of each user and server with RC take place on secure channel. Therefore, trust flows from RC to respective users and servers. Thereafter, the customers are able to get advantages of the services provided by service providers.

Already, plenty of schemes have been introduced so far to achieve better efficiency and security [3,4,5,6,7,8,9]. However, it is realized on the basis of frequent security attacks that more stronger protocols need to be developed [10,11,12,13,14]. Initially, a key agreement protocol for MSA framework is presented early in 2000 by Lee and Chang [15]. Later, the protocol is found susceptible to impersonation and anonymity violation attacks [16]. After that, MSA scheme engaged by using RSA\((Rivest-Shamir-Adleman)\)

Generic multi-server authentication environment

crypto-primitives and Lagrange interpolating polynomials, for a remote subscriber is presented by Tsaur [17]. The protocol [17] is compromised by password-guessing attack. Then, MSA protocol for a system of artificial neural network based on password is presented by Li et al. [18], which needs more time and higher cost. After that, ElGamal digital signature-dependent MSA protocol is presented by Lin et al. [1]. However, it is realized for smart card dependent applications that the scheme is too expensive in terms of memory requirement. Thereafter, MSA protocol based on symmetric crypto-system is presented by Juang [2], with an inherent scalability issue due to maintaining verifier-repository for each user at server end.

MSA protocol is then introduced by Chang and Lee [19], which is found to be vulnerable for privileged-insider and server impersonation attacks [20, 21]. Afterwards, another remote user verified key agreement protocol based on dynamic identity is presented by Liao and Wang [20] for MSA architecture. Hsiang and Shih [21] found that the protocol [20] is susceptible to spoofing and privileged insider attacks, and also introduced an enhanced protocol. Soon, Lee et al. [22] realized that the protocol [21] has no ability to attain agreement for mutual authentication and also introduced an modified scheme. However, Chen and Lee [23] observed that the protocol [24] is inadequate for smart card security and incompatible with two-factor authentication. Likewise, the protocol [24] also fails to prevent masquerading attack. Moreover, password change phase becomes more complex due to involvement of RC. Then, Irshad et al. [25] proves that the protocol [23] is insecure against smart card stolen attack that helps to reveal the session key and password. The protocol [23] is also vulnerable to trace and spoofing attacks. Later, due to the inefficiency and insecurity of the above mentioned schemes, many researchers have made improvements to the authentication method [26,27,28]. Additionally, some protocols have begun to use biometrics to ensure security [29]. The above discussion shows that designing the protocol for multi-server infrastructures to meet security requirements is a serious task. All current solutions are neither immune to all known attacks, nor they can guarantee the consumption of their own computations. Section III demonstrates our proposed scheme. Security and performance analysis are illustrated in Section IV and V, respectively. Presented work is concluded in last Section VI.

Our contribution

An anonymous three factor authentication protocol is introduced in this paper and the authentication of users with the help of biometric impression is enhanced. We encompass our contributions as follows.

  1. 1.

    First, we introduce an ECC based three-factor user authenticated key-agreement protocol.

  2. 2.

    Second, if smart card can be forged by an adversary, then the environment of user cannot be secure. In our introduced protocol, the verification of biometric impression of users can be done by the client as well as by the server; in some specific applications it can provide security protection for specific requirements. RC and server have separate responsibilities, as RC is involved in authentication phase. RC retains the privacy of registration and server validates the client for further service providing; it can make the protocol more scalable for multi-server the architecture.

  3. 3.

    Finally, our protocol offers the mutual authentication for each pair of three participants (server, user and RC) for providing strong protection by identifying as possible replay messages.


The hash functions, elliptic curve cryptography, adversarial model which is used in this paper are stated in this section. Whereas, Table 1 is presenting the common notations, used in rest of the article.

Hash functions

By taking an input string \(O=H(String)\) of random size, a fixed size output is generated by hash. Generated output is called hash code. A little change in the value of string can cause a huge difference. Whereas, a secure one way hash function has following specifications:

  • If the string is described, it is easy to find \(O=H(String)\).

  • It is impossible to find out the string, if \(O=H(String)\) is illustrated.

  • It is mundane task to distinguish input of \(String_{1}\) and \(String_{2}\) so that \(H(String_{1})=H(String_{2})\). This feature is called collision resistance.

Definition 1

(Characteristics of collision Resistance) Secure hash function H(.) is predetermined for collision resistance. The possibility that an attacker \(\mathcal {A}\) can find a pair \((String_{1}\ne String_{2})\) as \(H(String_{1})=H(String_{2})\) is separated as \(Advs^{HASH}_{\mathcal {A}}(t)=Prob[(String_{1},String_{2})\Leftarrow _{r}\mathcal {A}:(String_{1}\ne String_{2}), H(String_{1})=H(String_{2})]\), where attacker is allowed to select a pair \((String_{1},String_{2})\) randomly. Attacker’s perk is calculated against the randomly selections taken up with-in polynomial time (t). The collision resistance conclude that \(Advs^{HASH}_{\mathcal {A}}(t)\le \in\), whereas \(\in > 0\), is an enough tiny value.

Elliptic-curve cryptography(ECC)

The Elliptic-curve equation is defined in the form \(E_{p}(e,f)\): \(c^{2}=d^{3}+ed+f\) over a prime finite field \((d,c)\in W^*_{P}\times W_{P},e,f\) and \(4e^{3}+27f^{2} \ne 0\) (mod P). Where P is a selected huge prime number, the size of P is \(\ge\) 160 bits. Scalar product is gained by repeated addition e.g. \(nP=P+P+P+...+P(n times)\), over a determined t which a point on \(E_{P}(e,f)\) and the multiplier n. The variables (eftPn) should be a part of limited field \(F_{P}\). E is supposed to be the abelian group. Whereas O, is stated as the ID’s infinity point.

Definition 2

(Logarithmic issues in ECDLP) ECDLP: is given two specified points over \(R,V\in E_{P}(e,f)\), calculate n a scalar so that \(R=nV\). The chances that attacker \(\mathcal {A}\) can compute n in polynomial time(T) are described as \(Advs^{ECDLP}_{X}(T)=prob[(X(R,V)=x:xx\in W_{P})]\). ECDLP assumption concludes that \(Advs^{ECDLP}_{x}(T)\le \in\).

Adversarial model

The familiar adversarial model is deliberated in this paper, as specified in [2, 30]. Where the following considerations are followed as per the expertise of the adversary Advs:

  1. 1.

    Advs have full control over the public communication channel. Advs is adept to eliminate, amend, rerun, interrupt or can send a new replicated message.

  2. 2.

    The information stored in the smart card can be excerpted by Advs, by doing power analysis.

  3. 3.

    Advs can be a deceitful or intruder user or service provider of the system.

  4. 4.

    The identities of registered servers and users are not private but familiar to insiders.

  5. 5.

    The attack on server cannot be launched by Advs because the server is assumed to be secured.

Table 1 Common used notations

Proposed scheme

We propose an anonymous multi-server authentication protocol in this section. Although, proposed protocol brings more computation at server side, but server is usually assumed to have sufficient resources. Therefore, server can easily manages these extra computations in order to lower the computation cost on user side. The scheme is based on multi-server architecture which involves user\((U_{u})\), server\((S_{j})\) and registration center(RC). RC provides facility for user registration and further helps to give services from server. RC selects its master secret key x to register all users. Like former schemes, the proposed scheme has also three stages: the authentication, registration and password change stage. The proposed protocol is shown in Fig. 2 and described in the below subsections.

Server registration phase

To become legitimate server \(S_j\), the server needs to register with RC by following these steps.

  1. SR Step1:

    \(S_j\) selects his identity \(ID_{j}\) and sends to RC through secure channel.

  2. SR Step2:

    After receiving \(ID_{j}\), RC calculates \(s=h(ID_j\Vert x)\), \(pk_{S_j}=sP\) and \(pk_{RC}=xP\) where x is secret key maintained by RC.

  3. SR Step3:

    After that, RC sends s, \(pk_{S_j}\), \(pk_{RC}\) to server \(S_j\) and aborts the registration.

User registration phase

\(U_{u}\) performs the following operations with RC to become the legal user of the network.

  1. UR Step1:

    User selects his identity \(ID_{u}\), password \(PW_{u}\), biometric impression \(B_u\) and generates an arbitrary nonce a. Then user determines \(M=H(ID_{u} \Vert B_{u})\), \(TW=h(a \oplus H(B_{u}\Vert PW_{u}))\) and sends \(ID_{u}, M, TW\) to RC for completing the registration.

  2. UR Step2:

    After that RC determines \(X_{u}=h(ID_u\Vert pk_{RC})\), \(Y_{u}=X_{u} \oplus h(M\Vert TW)\) and \(F_{u}=h(h(ID_{u}\Vert TW)),\) then RC stores \(h(), Y_{u}, F_{u}\) in smart card and sends \((SC_{u})\) towards \(U_u\).

  3. UR Step3:

    \(U_u\) further adds a number a into \(SC_u\). Now smart card have \(\{h(), Y_{u}, F_{u}, a\}\).

Fig. 1

Generic multi-server authentication environment

Login and authentication phase

Fig. 2

Proposed Scheme

In this phase, authenticated access is granted to user \(U_u\) for accessing service providers \(S_j\). \(U_u\) and \(S_j\) authenticate themselves in following steps.

  1. LAP Step1:

    \(U_u\) inputs \(ID_u\), password \(PW_u\) and scan biometric impression in scanner. Then smart card determines \(TW=h(a\oplus H(B_{u}\Vert PW_{u}))\) and checks whether \(F_{u}{\mathop {=}\limits ^{?}}h(h(ID_{u}\Vert TW))\). If yes, then determines \(M=H(ID_{u}\Vert B_{u})\), \(U_u\) creates a random number \(C_u\) and computes \(O_p = C_{u}pk_{S_j} = C_{u}sP\), \(PID_{u}=C_{u}P\oplus ID_{u}\), \(X^{'}_u=Y_u\oplus h(M\Vert TW)\) and \(DID_u=h(ID_u \Vert X^{'}_u\Vert C_uP)\). Then \(U_u\) sends \(M_1 =PID_u, DID_{u}, O_p\) to \(S_j\).

  2. LAP Step2:

    After receiving \(M_1 =PID_u, DID_{u}, O_p\), \(S_j\) using his secret key s computes \(s^{-1}O_p=C_uP\), \(ID_u= C_uP \oplus PID_u\) and \(X_u=h(ID_u\Vert pk_{RC})\). After that \(S_j\) checks \(DID_u {\mathop {=}\limits ^{?}} h(ID_u \Vert X_u \Vert C_uP)\). If it holds true, then RC creates arbitrary nonce \(D_j\) and determines \(T_u =h(ID_u\Vert X_u)\), \(V_j=D_j\oplus X_u\) and \(Q_{uj}=h(ID_u\Vert T_u\Vert C_uP\Vert D_j\Vert X_u\Vert ID_j)\). Subsequently, \(S_j\) sends a message \(M_2=Q_{uj}, V_j\) to \(U_u\).

  3. LAP Step3:

    \(U_u\) determines \(D_j=V_j\oplus X_u\) after receiving \(M_2\) and checks \(h(ID_u\Vert h(ID_u\Vert X^{'}_{u})\Vert C_uP\Vert D_j\Vert X^{'}_u\Vert ID_j) {\mathop {=}\limits ^{?}} Q_{uj}\).

  4. LAP Step4:

    If the \(h(ID_u\Vert h(ID_u\Vert X^{'}_{u})\Vert C_uP\Vert D_j\Vert X^{'}_u\Vert ID_j){\mathop {=}\limits ^{?}}Q_{uj}\) holds true, \(U_u\) further determines \(SK_{uj}=h(ID_u\Vert C_uP\Vert D_j\Vert X^{'}_u\Vert ID_j)\) and computes \(Z_{uj}=h(SK_{uj}\Vert ID_u\Vert D_j\Vert X^{'}_u\Vert ID_j)\). \(U_u\) sends \(M_3 =Z_{uj}\) towards \(S_j\) so that it can check the challenge based on \(D_j\).

  5. LAP Step5:

    After getting \(M_3\), the server \(S_j\) determines \(SK_{uj}= h(ID_u\Vert C_uP\Vert D_j\Vert X_u\Vert ID_j)\). After that, it justifies the equation i.e. \(h(SK_{uj}\Vert ID_u\Vert C_uP\Vert X_u\Vert ID_j){\mathop {=}\limits ^{?}}Z_{uj}\). Finally, on successful justification, server exchanges the session-key SK with user as \(h(ID_u\Vert C_uP\Vert D_j\Vert X_u\Vert ID_j)\). The description of this protocol can be endorsed from Fig. 2.

Password changing phase

\(U_u\) may change his password into another new password \((PW_u^{n})\) by using these steps.These steps are as follows:

  1. PC Step1:

    Initially, user input identity \(ID_u^*\), password\((PW_u^*)\) and scan biometric impression factor after inserting smart card (SC) into reader. After that, SC determines \(TW=h(a \oplus h(B_{u}\Vert PW_{u}))\) and justify \(F_{u}{\mathop {=}\limits ^{?}} h(h(ID_{u}\Vert TW))\). If it holds true then user will follow next step.

  2. PC Step2:

    Subsequently, SC determines \(TW=h(a \oplus h(B_{u}\Vert PW_{u}))\) and calculates \(X_u=h(ID_u\Vert TW)\), \(Y_u* =X_u \oplus h(M\Vert TW)\).

  3. PC Step3:

    Afterwards, when user will change password (\(PWi^{n}\)). The smart card then determines \(TW=h(a \oplus h(B_{u}\Vert PW_u^{n}))\), \(Y_u^{n}=X_u \oplus h(M\Vert TW')\), and \(F_{u}^{n}= h(h(ID_{u}\Vert TW))\).

  4. PC Step4:

    Then the values \(X_u\), \(Y_u\), and \(F_u\) are changed by \(X_u^{n}, Y_u^{n}, F_u^{n}\) in smart card.

Revocation/re-registration phase

In this section, we show that if \(U_{u}\)’s smart card has been stolen or his account has been revoked then he can request for reregistration. For this purpose he must follow subsequent steps:

  1. RP Step1:

    \(\mathcal ({U}_u)\) engenders a random number \(a^{*}\), a new password \(PW^{*}_{u}\), and biometric \(B^{*}_{u}\) of his/her own choice. Then calculates \(M^{*}=H(ID_{u}\Vert B^{*}_{u})\) and \(TW^{*}=h(a^{*} \oplus H(B^{*}_{u}\Vert PW^{*}_{u}))\) and submits request message \(\{ID_{u}, M^{*}, TW^{*}\}\) to the registration centre \(\mathcal {(RC)}\) through a secure path.

  2. RP Step2:

    On receiving request message \(\{ID_{u}, M^{*}, TW^{*}\}\) from \(\mathcal ({U}_u)\), \(\mathcal {RC}\) will first verify whether \(\mathcal ({U}_u)\) is already a registered user or not from the verifier table. If a match is not found in the database, the \(\mathcal {RC}\) will reject the request.

  3. RP Step3:

    \(\mathcal {RC}\) then embeds the security parameters \(\{h(),Y^{*}_{u},F^{*}_{u}\}\ \ in \ a\ new \ SC^{*}_{u}\) into the smart card and sends the new smart card to the user \(\mathcal ({U}_u)\) through secure path.

  4. RP Step4:

    \(U_{u}\) takes new smart card \(SC^{*}_{u}\) and embeds \(a^{*}\) into it. The phase is shown in Fig. 3.

Security analysis

In this section, informal and formal security analysis are presented. The security analysis highlights that the proposed scheme is safe and secure against various possible attacks.

Informal security

In this section, a comprehensive informal security analysis of contributed protocol is presented.

Correct notion of user anonymity

In several authentication schemes for multi-server environment, the server is usually unable to identify the identity of a user requesting for login. In our view, such notion of perfect anonymity is erroneous and not desirable in any environment, because if the server is unable to know a user’s identity, he will be unable to provide the specific services to the user. In fact in this, any user can continue to get the services provided by the service provider even if he is not registered to the network or his lease has been expired. However, in proposed protocol, instead of user’s identity \(ID_{u}\), a dynamic-pseudo identity \(PID_u\) is sent during each authentication request message, to \(S_j\). Furthermore, user’s identity \(ID_u\) can only be extracted using server’s private key s. In addition, by analyzing two different session, an adversary will remain unable to guess whether the same user has initiated session. Hence, in this way our introduced protocol provides user’s anonymity and untraceability.

Replay attack

In this flaw, the retrieved messages are restored without endure transformation to deceive any legitimate user [31,32,33,34]. Adversary can get the parameters \({PID_u, DID_{u}, O_p, Q_{uj}, T_u, V_j}\) and try to endure these parameters in request to forge the legal member. However, if an adversary retrieves contents, he cannot initiate an attack because \(C_{u}\) and \(D_{j}\) is created by legitimate member for every session. Similarly, if an adversary endeavors to replay \(M_1\)= \({PID_u, DID_{u},O_p}\) toward server, server verifies the validity of user in \(M_3\), in reply to the challenge based on \(D_{j}\). Synchronously, the legitimate user validates \(S_j\) in \(M_2\) to response to the \(M_1\) based challenge \(C_{u}\). Hence the contributed protocol thwart replay attack.

Stolen smart card attack with offline dictionary

In stolen smart card attack with offline dictionary, the attacker tries different sequences of dictionary ingredients using stolen SC credentials [35,36,37]. An attacker may attempts to exploit with its feasible parameters of SC i.e \(h(), Y_{u}, F_{u}\). For estimating the \(PW_{u}\) from \(Y_{u}\) and \(F_{u}\) parameters, adversary needs to perceive \(ID_{u}\), a and \(B_{u}\) to estimate \(PW_u\) from TW where \(TW=h(a \oplus h(B_{u}\Vert PW_{u}))\). Furthermore, this attack cannot initiate in polynomial time using smart card.

Known-key security

Known-key security provides the confidentiality of private keys even with exposed session key for a particular session [38, 39]. Given that the specific session-key \(SK_{uj}= h(ID_u\Vert C_uP\Vert D_j\Vert X_u\Vert ID_j)\) does not hold \(U_{u}'s\) password \(PW_{u}'s\) as a parameter. Owing it to, the adversary may not discover the parameters from derived session key. Hence, the contributed protocol offers known-key security.

Mutual authentication

Mutual authentication is provided by the enhanced scheme because the legitimate participants verify each other and thus it ensure mutual authentication strongly. This property makes our protocol secure and provides the early detection of possible attacks like replay attacks.

Masquerading attack

According to this attack, an attacker can masquerade one member of a specific session, if it reveals another member’s key of the current session. The contributed protocol is immune to key-compromise impersonation threat in contrary to scheme, [23] as the contents of stolen card will not help the attacker to get other constructive parameters, such as \(X_{u}\). Hence, the attacker cannot obtain newly generated \(Q_{uj}\) factor and ultimately impersonation attack cannot be initiated.

Stolen verifier attack

The adversary misuses valued data which is stored at server’s side and user’s privates like passwords or other parameter, masquerade as legal users. The contributed protocol offers mutual authentication without maintaining repository on \(S_j\) and RC’s side. This shows that our scheme is withstand stolen verifier attack.

Password guessing attack

The guessing attack is applicable, if an adversary accesses the parameters \({PID_u, DID_{u}, O_p, Q_{uj}, T_u, V_j}\) on little analysis of any open channel. Nonetheless, an adversary cannot extract the password, after all it is not use as a factor for the computation of any contents, hence it minimizes the chances of estimating the consistent factors.

Modification attacks

The adversary changes the retrieving parameters and submit to promise party. In case, the scheme is designed to resist against modification threat. If the adversary attempts to change the public contents \({PID_u, DID_{u}, O_p, Q_{uj}, T_u, V_j}\), adversary will not able reassemble following parameters \({PID_u, DID_{u}, O_p}\) by introducing recent session arbitrary variables, since to assemble these parameters acquires the information of secret key and \(X_u\) which knows to legal member. Consequently, the legal member can expose any venomous member easily. So, the enhanced scheme can easily discourage this attack.

Formal security analysis

We have described model of security for presented protocol in this section. Furthermore, using given model of security the presented protocol is proved safe against known attacks. At the end, the proposed protocol is described to fulfill all the necessary requirements that relates to the security of the presented protocol.

Theorem THM1

Consider\(D_i\)as a uniformly distributed dictionary consists of various possible passwords. |D| denotes the size of\(D_i\). Consider A as an adversary against semantic security within a time bound t. Suppose a ECCDHproblem stands, then we have

\(D_i\) is considered as evenly distributed dictionary which consists of numerous passwords that can be possible. The size of \(D_i\) is denoted by |D|. A is considered as an adversary against syntactic security in a time bound t. If a ECCDH problem occurs, then we have

$$\begin{aligned}A_{\Pi ,D}(A) &\le \frac{(q_{hsh}+q_{exe})^2 }{2p}+\frac{q^2_{hsh}}{p}\\&\quad +\frac{q_{hsh}}{p}+{q_{hsh}A^{ECCDH}_{\Pi }(A)}\\&\quad +\frac{q_{hsh}}{p}+\frac{q^2_{snd}}{D}. \end{aligned}$$

where the possibility of solving the ECCDH problem by A, is denoted by \(A^{ECCDH}_{\Pi }\). The number of Execute, Random-oracle and Send query are \(\{q_{exe}, q_{hsh}, q_{snd}\}\), respectively.


In order to give the proof of Theorem THM1, six composite games are considered from game \(G_1\) to \(G_6\). The game will be started where the real attack is simulated and a game will be ended where adversary A has no advantage. The possibility of successfully guessing the random bit b in test-query by A is denoted by \(Suc_i\) for each game \(G_i\), where \(1 {\mathop {\le }\limits ^{}} i {\mathop {\le }\limits ^{}} 6\).


In this random oracle model, the real attacks are simulated with the help of this game. In game \(G_1\), every instance like \(U_u\), \(S_j\) and RC will be modeled as authentic executions. As per the definition of Suc1, we get following equation.

$$\begin{aligned} \begin{aligned} A^{ECCDH}_{\Pi ,D}(A)=2Pr(Suc1)-1. \end{aligned} \end{aligned}$$

Multiple oracles like hash oracle h Execute, Corrupt, Reveal, Send and Test are simulated with \(G_2\). Hash oracle is simulated by game \(G_2\) by maintaining a hash list \(h_{list}\), \(h_{list}\) comprises on queries entries as (input, output). When a hash query is answered by hash oracle, then it returns the corresponding output if there is any existing query (input, output) in \(h_{list}\), else it will return value from 0, 1. Moreover, corrupt, reveal, send and Test queries will be run as real attacks. Thread model is used to specify the actual actions of all these queries. This simulation indicates that game \(G_2\) is perfectly secured from the real attacks. Thus, we have

$$\begin{aligned} \begin{aligned} Pr(Suc2)=Pr(Suc1) \end{aligned} \end{aligned}$$

This game consists on all possible executions of ROM as elaborated in game \(G_2\) except that it will be discarded when some collision occured in the simulation of all hash queries, that are inquired by the adversary A. So, this game helps to avoid from collision to be occurred in ciphertext, password and output of Send-queries. By the definition of birthday paradox, the chances of occuring collision in hash oracle is \(\frac{q^2_{exe}}{2p}\). Thats why the chances of occuring collision in game \(G_3\) is \(\frac{(q_{hsh}+q_{exe})^2}{2p}\). For this simulations, we achieved following equation

$$\begin{aligned} \begin{aligned} |Pr(Suc3)- Pr(Suc2)| {\mathop {\le }\limits ^{}} \frac{(q_{hsh}+q_{exe})^2}{2p}+ \frac{q_{exe}}{2p}. \end{aligned} \end{aligned}$$

This game consists on all possible executions of ROM as elaborated in game \(G_3\) but it will be discarded after the successful guessing of \(X_u\) by adversary A without asking the hash oracle h. This game is similar all previous games unless the instances \(\Pi ^i_U\) and \(\Pi ^j_{S}\)\(S_j\) reject the actual authentication value. From game \(G_4\), we get following equation

$$\begin{aligned} \begin{aligned} |Pr(Suc4)- Pr(Suc3)| {\mathop {\le }\limits ^{}} \frac{q_{hsh}}{p} \end{aligned} \end{aligned}$$

This game indicates that if adversary guesses the session key directly without knowing and inquiring about hash oracle h then this game will be terminated. It enables the session key to be independent with \(\{PW_u, B_u\}\) and random numbers as well as point multiplication \(C_u, D_j, P\). \(G_4\). This game will be aborted after the inquiring common value \(X_u\). Thus, \(A^{ECCDH}_{\Pi }(A){\mathop {\le }\limits ^{}} \frac{1}{q_{hsh}}|Pr(Suc5)- Pr(Suc4)|-\frac{1}{p}\) and we have

$$\begin{aligned} \begin{aligned} |Pr(Suc5)- Pr(Suc4)| {\mathop {\le }\limits ^{}} q_{h}A^{ECCDH}_{\Pi }(A)+\frac{q_{hsh}}{p}. \end{aligned} \end{aligned}$$

This game consists on all possible executions of ROM as elaborated in game \(G_5\) except the rule if follow in Test query. \(G_5\) will be aborted when A queries about hash oracle that is identical values \(C_u, D_j, P\). The chance of adversary A getting the correct session-key by hash-query is at most \(\frac{q^2_{hsh}}{2p}\). Thus, we have

$$\begin{aligned} \begin{aligned} |Pr(Suc6)- Pr(Suc5)| {\mathop {\le }\limits ^{}} \frac{q^2_{hsh}}{2p}. \end{aligned} \end{aligned}$$

Until adversary A does not enter correct value into the random oracle h, the random oracle will remain indistinguishable against real attack. That’s why A does not have any advantage of identifying the legal session key from random oracle attempt. Furthermore, when corrupt query is performed, not more than 3 queries can be performed simultaneously. It means that if smart card corrupt and biometric corrupt (\(\Pi ^{i}_U\),3), (\(\Pi ^{i}_U\),4) are performed then password corrupt ((\(\Pi ^{i}_U\),2)) cannot be performed this is the reason that success rate of off-line password guessing attack is \(\frac{q^2_{snd}}{D}\). By combining all the equations from \(G_1\) to \(G_6\), we get following equation

$$\begin{aligned} \begin{aligned}A_{\Pi ,D}(A)&\le \frac{(q_{hsh}+q_{exe})^2}{2p}+\frac{q^2_{hsh}}{p}\\&\quad +\frac{q_{hsh}}{p}+q_{h}A^{ECCDH}_{\Pi }(A) \\&\quad +\frac{q_{hsh}}{p}+\frac{q^2_{snd}}{D}. \end{aligned} \end{aligned}$$

Performance analysis

In this section, the robustness of proposed protocol is assessed with respect to other schemes [20,21,22,23, 25] based on multi server architecture. The security traits and the scrutiny of defending to numerous attacks for different schemes are described in Table 2, in which the proposed protocol is signified as a strong corroborated key-agreement in contrast to former schemes. Table 2 presents the analysis of our schemes with related schemes [20,21,22,23, 25]. As per the analysis, we can conclude that our protocol is more secure than [20,21,22,23, 25]. All these protocols depend upon hash-based symmetric cryptography and similar in nature.

Table 2 Comparison of security parameters

Later, the performance analysis of our authenticated protocol in terms of cost has been analyzed. The specification and description for the implementation is as follows; the implementation of the cryptographic functions \((T_{\oplus },T_\Vert ,T_{h(.)},P_{m})\) is done by using py-crypto library inside ubuntu 19.04, with 16.0 GB RAM and 3.60 GHz processor core i7 with the help of python programming language. The execution of authentication scheme is done under same assumptions for 10 times by averaging. Some functions like \((T_\Vert ,T_{\oplus })\) have not been considered because they acquires negligible execution time. The execution time for h(.), H(.) and point multiplication operations is 0.0120 ms, 0.015 ms and 0.02957 ms, respectively. The communication, energy requirements, storage and computation cost of our scheme with respect to related protocols is presented in Table 3. The time for execution of considered cryptographic functions are assumed as follows:

  • Execution time for one way hash function is \(E_{h}=0.0120 ms\).

  • Execution time for one way bio-hash function is \(E_{H}=0.015 ms\).

  • Execution time for point multiplication \(E_{pm}=0.02957 ms\).

It is observed the computation cost of our proposed scheme is higher than [20,21,22,23, 25] schemes but it offers aided security features. Furthermore, the mandatory security objectives are achieved by our protocol in less cost than Hsiang and Shih’s scheme. Moreover, the proposed protocol (contrasting with former protocols) is secure to smart card stolen, password guessing and insider attacks.

Fig. 3

Revocation phase

Table 3 Comparison of energy and computational, communication and storage costs

We have determined cost comparison in Table 3, which are later elaborated by drawing Figs. 4, 5, 6 and 7. The cost of computation for proposed and relevant schemes is showcased in Fig. 4. The number of verifiers of our proposed and existing protocols are shown horizontally and required computation time according to the number of verifiers is shown vertically in the graph. It can be observed that computation cost of our protocol is far less than the related schemes.

Fig. 4

Computation cost comparison

Fig. 5

Analysis of energy of proposed and other protocols

Fig. 6

Analysis of communication cost between proposed and related protocols

Fig. 7

Analysis of storage cost of proposed and other protocols


Energy consumption can be calculated as \(E_c=T_{cc}P_{CPU}\), where \(T_{cc}\) is the total computation cost for a single hash function (0.054 mJ), \(P_{CPU}\) is the maximum power (65 W) of CPU and \(E_c\) is the energy consumption [40]. Power consumption can be used to give a rough estimate of energy consumed during computation. Moreover, we have examined the protocol with respect to energy consumption by considering computation cost of energy for SHA-1 as 0.54 mJ for single byte [41] shown in Fig. 5. By Considering this, the consumption of energy for the [20,21,22,23, 25] and our scheme amounts to 1.64 mJ, 0.98mJ, 1.10 mJ, 1.05 mJ, 1.24 mJ and 0.91mJ, respectively. The final energy consumption determined values of proposed and related schemes are given in Table 3. Hence, it can be calculated that the energy consumption of proposed scheme is less than related schemes.

The assumptions made for determining the communication and storage cost are as follows: 160 bits are reserved for random nonce, timestamps, password and identity, 256 bits are for one way hash function and for public key, 512 bits. The calculations of storage and communication cost of our and related schemes on the basis of above mentioned assumptions are presented in Table 3.

The cost of communication for proposed and relevant schemes is presented in Fig. 6. The proposed and related schemes are given horizontally, while the required number of communication bits are shown vertically in the graph. It is observed that the number of communication bits of proposed scheme is slightly greater than related schemes but our scheme provides more security traits. The storage cost of proposed and related schemes is displayed in Fig. 7. The vertically labeled values on the graph are for the required number of storage bits, while proposed and related schemes are listed horizontally.

The storage bits of our scheme is slightly greater than the related protocol. This is just because of providing more security features for making secure protocol. After analyzing Tables 2 and 3, we can say that the computation time of our scheme is less than the related schemes and also provides more security traits with slightly higher communication and storage costs.


The robustness of multi-server authentication is observed as an important requisite for the current remote based authentication paradigm. Recently, extensive research has been conducted for developing robust authentication protocols for multi-server authentication environment. In this paper, we proposed an anonymous multi-server authentication scheme. The flaws of previous schemes are kept in mind in order to develop the proposed scheme with enhanced security features. The analysis of performance evaluation and formal security is also described in this paper against various schemes. This analysis also shows that our scheme provides more security features.

Availability of data and materials

It is theoretical work no data is needed.


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MAA, ZG and KM analyzed the requirement of the security for Multi-server Infrastructure, designed the framework, conducted the experiment and drafted the manuscript. Also, KM gave full support in conducting the experiment and assisted in draft work and revised the manuscript. SK contributed by reviewing the work done and in revising the content of the manuscript. In addition, all the work is done in SK’s supervision. KA has helped us in major revision according to the comments of reviewers. CMC coordinated the whole study. All authors read and approved the final manuscript.

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Correspondence to Chien-Ming Chen.

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Akram, M.A., Ghaffar, Z., Mahmood, K. et al. An anonymous authenticated key-agreement scheme for multi-server infrastructure. Hum. Cent. Comput. Inf. Sci. 10, 22 (2020).

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  • Multi-server
  • User anonymity
  • Authentication
  • Impersonation
  • Key-agreement protocol
  • Internet of Things
  • Cloud computing