Abstract:
Methods and apparatus for improving the resilience of wireless packet-switched networks to Layer-2 attacks is provided via a lightweight mechanism for detecting spoofed frames. The mechanism enables a receiving node to detect spoofed frames from information contained in cookies sent with frames. A first cookie, containing initial information, is sent to the receiving station from the transmitting node along with the first frame of a frame set. For each received frame, spoofing detection includes applying a function to information received via a corresponding cookie received with the subject frame, the result of which function is compared with information received via a previous cookie. The validity of the subject frame is asserted if the result of applying the function to information received in the corresponding subject cookie correlates with previous or initial information received in a previous or the first cookie, respectively. An exemplary implementation includes using a one-way hashing function. Advantages are derived from a low computational overhead in effecting spoofed frame detection and from an ability of the proposed solution to co-exist with other standardized security mechanisms.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to wireless packet-switched communications, and in particular to methods of authenticating layer-2 frames in enhancing wireless communication security.  
       BACKGROUND OF THE INVENTION  
       [0002]     Solutions to wireless networking security have been, and are being sought. Proposals for enhanced wireless networking security have been put forward and some have been implemented with various measures of success between which:  
         [0003]     The state of the art in the field of wireless networking security includes an IEEE 802.11i standard. The IEEE 802.11i standard addresses wireless networking security issues. A subset of the IEEE 802.11i standard is also known as Wireless-LAN Protected Access (WPA).  
         [0004]     The IEEE 802.11i standard greatly increases the security of an 802.11 wireless communication network providing an authentication method for protecting access to the network, as well providing sound cryptographic mechanisms to ensure privacy in conveying content such as Temporal Key Integrity Protocol (TKIP), and Advanced Encryption System (AES). Nevertheless, in accordance with the IEEE 802.11i standard, management and control frames are exchanged in an unauthenticated fashion which constitutes an exposure to various “man-in-the-middle” and “Denial-of-Service” (DoS) attacks.  
         [0005]     The prior art includes a paper entitled “802.11 Denial-of-Service Attacks” published on the Internet at: http://ramp.ucsd.edu/˜bellardo/pubs/usenix-sec03-80211dos-html/aio.html by John Bellardo and Stefan Savage. In the paper, Bellardo and Savage present a thorough analysis of Media Access Control (MAC) layer vulnerabilities including “man-in-the-middle” and “denial-of-service” attacks. In the paper it is demonstrated that attacks based on 802.11 vulnerabilities can be mounted feasibly and efficiently in real world wireless networking environments. Such attacks are efficient and can easily lead to a loss of availability and reliability of a wireless network. The paper also shows that the level of expertise required to mount such attacks is low, as the tools required are widely available without need for specialized equipment.  
         [0006]     Another prior art attempt at improving wireless networking security includes a paper entitled “DoS and Authentication in Wireless Public Access Networks” published on the Internet at http://www.cse.cuhk.edu.hk/˜xqli/ by Daniel B. Faria and David R. Cheriton who propose a new set of protocols addressing security issues that are not addressed in the IEEE 802.11i standard. However the solution proposed by Faria and Cheriton requires making substantial changes to current standards, and requires extensive computational resources due to an extensive reliance on cryptographic mechanisms.  
         [0007]     Network operators seek to provide wireless networking services using IEEE 802.11 technologies to complement their existing offerings. Network operators are required to provide reliable communications services securely while adhering to existing standards and seek ways to do so with a minimal overhead.  
         [0008]     There therefore is a need to solve the above mentioned security issues.  
       SUMMARY OF THE INVENTION  
       [0009]     An object of the present invention is to address the above mentioned security issues.  
         [0010]     In accordance with an aspect of the invention, methods and apparatus for improving the resilience of IEEE 802.11 networks to Layer-2 attacks is provided via a lightweight mechanism for detecting spoofed frames. The mechanism enables a receiving station to detect spoofed frames from information contained in cookies sent with valid frames. A first cookie, containing initial information, is sent to the receiving station from the transmitting station at the start of a session. For each received frame, spoofing detection includes applying a system-wide function to information received via a corresponding cookie received with the subject frame, the result of which function is compared with information received via a previous cookie. The validity of the subject frame is asserted if the result of applying the function to information received in the corresponding subject cookie correlates with previous or initial information received in a previous or the first cookie, respectively.  
         [0011]     In accordance with another aspect of the invention, a method of conveying Layer-2 frames of a frame set used to provision packet-switched communications in a wireless communications network between a transmitting node and at least one of multitude of receiving nodes is provided. A commitment cookie value is transmitted along with a first frame in the frame set from the transmitter node towards the receiving node. The commitment cookie value is received at the receiving node along with the first frame. A corresponding unique message cookie value of a multitude of message cookie values generated sequentially by a repeated application of a generating function to a first seed value is transmitted along with each subsequent frame of the frame set. The sequence of message cookies includes the commitment cookie value. A message cookie value is received at the receiving node along with each subsequent received frame of the frame set. And, the authenticity of each subsequent received frame is asserted if a previously received cookie value can be derived from the message cookie value corresponding to the received frame.  
         [0012]     In accordance with a further aspect of the invention, a method of authenticating a multitude of Layer-2 frames of a frame set used to provision broadcast communications in a packet-switched communications network between a transmitting node and a multitude of receiving nodes is provided. Transmitting frames to each receiving node includes: obtaining a first seed value; generating a unique sequence of message cookie values at the transmitting node by a repeated application of a generating function to the first seed value, the unique sequence of message cookie values including the commitment cookie value; and transmitting a unique message cookie value along with each transmitted frame of the frame set, the commitment cookie value being transmitted along with the first frame of the frame set, a unique subsequent message cookie value in the sequence of message cookie values being transmitted along with a subsequent frame in the frame set.  
         [0013]     In accordance with yet another aspect of the invention, a method of verifying the authenticity of a multitude of Layer-2 frames of a frame set used to provision broadcast communications in a packet-switched communications network between a transmitting node and a multitude of receiving nodes is provided. Receiving frames at each receiving node includes: receiving a commitment cookie value along with a fist frame of the frame set; receiving a message cookie value along with each subsequent received frame of the frame set; and selectively asserting the authenticity of each subsequent received frame, if a previously received cookie value can be derived from the message cookie value corresponding to the received frame.  
         [0014]     Advantages are derived from a low computational overhead in effecting spoofed frame detection and from an ability of the proposed solution to co-exist with other standardized security mechanisms. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The features and advantages of the invention will become more apparent from the following detailed description of the exemplary embodiments with reference to the attached diagrams wherein:  
         [0016]      FIG. 1  is a schematic diagram showing, in accordance with an exemplary implementation of an exemplary embodiment of the invention, method steps implementing secure exchange of messages between a transmitting network node and a receiving network node;  
         [0017]      FIG. 2  is a schematic diagram showing, in accordance with another exemplary implementation of the exemplary embodiment of the invention, method steps implementing secure exchange of messages between a transmitting network node and a receiving network node;  
         [0018]      FIG. 3  provides an overview of exemplary relationships between cookie values employed in authenticating broadcast frames, in accordance with the exemplary embodiment of the invention; and  
         [0019]      FIG. 4  is a schematic diagram showing, in accordance with another exemplary embodiment of the invention, method steps implementing secure exchange of messages between a transmitting network node and a multitude of receiving network nodes. 
     
    
       [0020]     It will be noted that in the attached diagrams like features bear similar labels.  
       DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]     In the present description, a transmitting node is a wireless communications network node that transmits a frame, also known as a Protocol Data Unit (PDU), to a receiving node. The receiving node is a wireless communications network node that receives the frame. The receiving node verifies the authenticity of each received frame. In practice, each communications network node acts as a transmitting node and as a receiving node in providing bi-directional connectivity therebetween, wherein each bi-directional connection may be understood as a pair of interrelated reciprocal unidirectional connections. Wireless communications network nodes include wireless communications network node stations participating in “ad-hoc wireless networks” established therebetween, and service communications network nodes such an access points employed in implementing “wireless infrastructure networks” in which multiple wireless communications network node stations communicate with a wireless communications network access point. The invention described herein may also be applied to wireless repeater nodes which relay frames in a communications network.  
         [0022]     In accordance with an exemplary embodiment of the invention, wireless network nodes are provided with detection means for identifying spoofed Layer-2 frames. Cookies produced by transmitting nodes and sent along with corresponding protected frames in order to provide receiving nodes with the means to verify that the frames have been sent by the expected transmitting nodes.  
         [0023]     Accordingly a lightweight (not resource intensive) cookie-based method is proposed for mitigating MAC frame spoofing attacks such as, but not limited to, man-in-the-middle and denial-of-service attacks. The method enables a receiving network node to detect and reject frame encapsulated messages spoofed with the physical address (MAC) of a different transmitting network node than the transmitting node the receiving network node had been previously communicating with. As a result, an attacker is prevented from impersonating trusted network nodes by forging MAC/Layer-2 frames.  
         [0024]     In accordance with an exemplary embodiment of the invention, two types of cookies are proposed for use: a Commitment Cookie (CC) and a Message Cookie (MC). Cookies are understood to be an integral amount of information generated, stored, and optionally conveyed in a communications network, the cookie information being specific to a particular application. In the present description, the application is the protection of Layer-2 frames, wherein the cookie information may include as little as a single value without limiting the invention thereto. For this reason the terms “cookie” and “cookie value” may be used interchangeably herein.  
         [0025]     Each CC cookie is generated by a transmitting node at the initiation of connection establishment with a counterpart receiving node, and typically sent along with the first transmitted frame. In order to provide adequate/desired security/protection, the CC cookie and/or the CC value is selected to be valid only for the given receiving node, and may further be chosen to be valid only for a specific connection. Implementations of methods presented herein may be applied to any set of frames which is to be protected including, but not limited to: data, control, management frame sets.  
         [0026]     Each MC cookie is generated by the transmitting node, sent to the receiving node along with a corresponding subsequent frame in the frame set to be protected, and can be verified by the receiving node using the corresponding CC cookie. Each MC cookie is generated and used only once in conveying an associated frame between the transmitting node and the receiving node. Should a frame in the frame set require retransmission, a different unique MC cookie is associated with each retransmitted frame.  
         [0027]     In accordance with the exemplary embodiment of the invention, the receiving node includes authenticity validation means for validating whether each MC cookie has been generated by the expected transmitting node having regard to the previously received CC cookie. Failure to validate the authenticity of an MC cookie is understood to indicate that the corresponding frame has not been sent by the expected transmitting node pointing to an attempt to forge/spoof frames.  
         [0028]     In accordance the exemplary embodiment of the invention, MC cookies are generated in such a way that disclosing the CC cookie and an arbitrary set of MC cookies does not allow one to easily predict the next valid MC cookie. MC cookies are generated in sequences which include the CC cookie.  
         [0029]     A related prior art paper entitled “Two Simple Micropayment Schemes” published on the Internet at http://theory.lcs.mit.edu/˜rivest/RivestShamir-mpay.pdf by Ronald L. Rivest and Adi Shamir, incorporated herein by reference, describes examples of usage of hash chains to provide security in micropayment schemes. This is an example of a mechanism providing security at a low computational cost, and similar techniques have been applied for online auction, authentication schemes, etc. However, to date, such mechanisms are not known to have been applied to authenticate IEEE 802 Layer-2 frames as proposed herein.  
         [0030]     In accordance with an exemplary implementation of the exemplary embodiment of the invention, properties of exemplary one-way hashing functions such as, but not limited to: SHA-1 or MD5, are employed in generating unique hash chains used in generating CC and MC cookies used for protecting transmitted frames in order to improve security of packet-switched wireless communications. The choice of the one-way hashing function employed relates to a balance between computational complexity and the level of security provided. Each network node may include computing means, typically in the form of hardware logic, for efficiently computing a group of one-way hashing functions. The selection of the one-way hashing function used may be configurable on the equipment or may be negotiated between transmitting nodes and receiving nodes.  
         [0031]     In accordance with the exemplary embodiment of the invention, exemplary method steps  100  are shown in  FIG. 1 . For each (data, control, management) set of frames  106  to be protected, the transmitting node performs initialization steps including: 
        Let N be the maximum number of message cookies MC to be associated to a given commitment cookie CC cookie, and therefore, the maximum number of frames that can be protected based on the CC cookie. The value of N is chosen  108  by the transmitting node;     Select  110  a seed value, typically a Random Value (RV) generated by a random value generator;     (step  112 ) Let HN equal the seed value RV;     (step  114 ) Iterate N times H i =H(H i+1 ), for each i between 0≦i≦N using the computing means;     (optionally) Store  116  each H i ; and     Let  118  the cookie value CC=H 0 , 
 
 where determining the number of frames to be exchanged and determining the number N of function iterations needed may be performed via a lookup table depending on the set of frames to be protected. For example, in protecting connection control messages, both connection setup messages and tear down messages are to be accounted for. The expected number of frames N to be exchanged may account for frame retransmissions providing a fraction of spares. 
       
 
         [0038]     The transmitting node generates  120  the CC cookie and sends  122  the initial CC cookie, and therefore the CC value, to the receiving node along with the first frame of the frame set to be protected.  
         [0039]     The receiving node receives  122  the initial frame and the CC cookie, and stores  124  the CC value, in this case H 0 .  
         [0040]     For each subsequent frame sent by the transmitting node, the transmitting node generates  128  an MC i  cookie for authenticating the a corresponding frame in the frame set, such that 0&lt;i≦N, by setting  126  the value of MC i =H i  and incrementing i before sending  130  each subsequent frame. If no frame retransmissions are necessary, cookie value MC i  would correspond to the i&#39;th frame of the frame set.  
         [0041]     The receiving node validates  132  the authenticity of each MC cookie received with a corresponding frame thereby verifying that the frame has been sent by the expected transmitting node. A LastMC register is initialized ( 124 ) with the CC value upon reception  122  of the CC cookie. The following pseudo code shows exemplary steps of a received MC cookie authentication verification process ( 132 ). The principle is that applying the same one-way hashing function H to a received correct MC cookie value would result to a previously received MC cookie value or the initial CC cookie value:  
                                                                                           TempMC = MC;           Verification = false;           For x = 1 to T {                TempMC = H(TempMC)           If (TempMC == LastMC) {                LastMC = MC           Verification = True           Break the loop                }                }           if (Verification == True){Accept_Frame( )}           else {Reject_Frame( )}                      
 
 where the value T is used to provide tolerance to a possible loss of synchronization between the transmitting and receiving nodes that may otherwise lead to an inconsistent LastMC value. Note that once the frame is validated, the LastMC value is updated  134  with the newly validated MC value ensuring that the previous MC value in the hash chain would be rejected if received again. 
 
         [0042]     Note that in accordance with the exemplary implementation described above, all hash chain values H i  have to be computed  114  in advance and stored  116  at the transmitting node as the cookie values are used in a reverse order than the order in which the cookie value were generated. Depending on the set of frames to be protected and the number of concurrent sessions supported by transmitting node, a significant amount of storage may be required at the transmitting node. In accordance with an exemplary implementation, in order to reduce the storage requirements needed at the transmitting node, storing  116  only every 10 th  H i  value, the storage space required may be reduced by a factor of 10. However, the transmitting node must iterate  114  on average 4.5 times to compute any given H i  value.  
         [0043]     In accordance with another exemplary implementation of the exemplary embodiment of the invention, an exemplary method of generating CC and MC cookies which does not require MC cookie values to be computed in advance, nor stored, is presented. The method is based on a well known computationally intractable problem in number theory which relates to functions which are infeasible to invert. Exemplary method steps  200  shown in  FIG. 2  and are performed by the transmitting node in authenticating a (data, control, management) frame set  202  to be protected, the initialization steps including: 
        Selecting  204  two distinct large prime numbers p and q, such that p≡q≡3 mod 4;     (step  206 ) Setting n=p*q, such a number n is known as a Blum-integer;     (step  210 ) Selecting s, a random number in the range [1, n−1], such that s and n are relatively prime (i.e. GCD(s, n)=1, where GCD stands for Greatest Common Divisor);     Computing  212  the CC cookie value: MC 0 =s 2  mod n using computing means at the transmitting node; and     In generating  214  the CC cookie, letting CC=(MC 0 , n).        
 
         [0049]     The CC cookie, including both MC 0 , and n values, is sent  216  to the receiving node along with the first transmitted frame in the protected frame set. The receiving node stores  218  the CC cookie including both MC 0 , and n values. Note that value s is only known to the transmitting node.  
         [0050]     For generating  232  each subsequent MC i  cookie in the chain, to send  234  along with a subsequent protected frame, where i is the sequence number of the frame to be authenticated such that i≧1, the transmitting node computes  230  the MC i  value to be the quadratic residue of √{square root over (MC i−1 )} mod n, which has the property of being a quadratic residue itself. For each Blum-integer n, this function is uniquely defined. The entire chain of MC cookie values is unique based on the CC cookie value, for this reason the CC cookie value is also known as a seed value.  
         [0051]     The MC i  cookie is sent  234  to the receiving node along with a corresponding frame to be protected. The value i is incremented before sending each subsequent message.  
         [0052]     Security is achieved as calculating the quadratic residue is computationally feasible only when p, q and s are known and as the transmitting node is the only node knowing the p, q and s values. It would be computationally infeasible for another rogue transmitting node to generate MC i  cookie values despite the fact that the CC cookie value and n were essentially published via the first frame transmission.  
         [0053]     The receiving node validates  236  the authenticity of each MC cookie received along with a corresponding frame verifying that the frame has been sent by the expected transmitting node.  
         [0054]     The following pseudo code shows exemplary steps of a received MC cookie authentication verification process  236 . A LastMC register is initialized ( 218 ) to the MC 0  value upon receiving  216  of the CC=(MC 0 , n) cookie. The principle is that applying the inverse of the generating function, the square function modulo n to a correct received MC value would result to a previously received cookie value:  
                                                                                           TempMC = MC;           Verification = false;           For x = 1 to T {                TempMC = TempMC 2  mod n           If (TempMC == LastMC) {                LastMC = MC           Verification = True           Break the loop                }                }           if (Verification == True){Accept_Frame( )}           else {Reject_Frame( )},                      
 
 where the T value provides tolerance to a possible loss of synchronization between the transmitting and receiving node that may lead to an inconsistent LastMC value. Note that once the authenticity of the frame is validated, the LastMC value is updated  238  with the newly validated MC value ensuring that the previous MC value in the sequence would be rejected if received again. 
 
         [0055]     It is noted that in accordance with the exemplary implementation described above, besides the advantage of not requiring MC cookie values to be precomputed, no restriction is imposed on the number of messages of the protected frame set.  
         [0056]     Implementations are envisioned wherein both exemplary methods  100  and  200  described above are employed. For example, the first method  100 , specifically taking advantage of the fact that a specific number of valid hash values H i  are computed in advance, is used in respect of control/management frames specifically as a known number of such frames are transmitted between a transmitting node and a receiving node, while the second method  200  may be used to protect data frames transmitted between the transmitting node and receiving node in respect of an established connection which persists for an unknown extended duration.  
         [0057]     In accordance with the exemplary implementation of the above described exemplary embodiment of the invention, the transmitting node is the only node able to generate valid MC cookies efficiently. Knowing the initial CC cookie value and/or previously sent MC cookie values, does not allow rogue nodes other than the legitimate transmitting node to easily generate subsequent valid MC cookie values, while knowing the CC cookie value or a previously sent MC cookie value, the receiving node is able to verify that subsequent MC cookie values received along with corresponding frames are authentic, and as a consequence, generated by the expected transmitting node.  
         [0058]     While two exemplary methods have been presented above in respect of the exemplary embodiment of the invention, the invention is not limited to the two methods presented above, as any other methods that fulfill the same above stated properties could be employed.  
         [0059]     In accordance with a second exemplary embodiment of the invention, frame validation is provided in support of unicast and/or broadcast scenarios/deployments typically suited for an access point transmitting node used to broadcast frames in a wireless infrastructure network.  FIG. 4  shows steps of an exemplary method steps  400  performed by a transmitting node (access point) including: 
        Computing  402  receiving node specific seed values RNV 1 , RNV 2 , RNV 3 , . . . such that if RNV J  is published, any receiving node (network node station) can compute values RNV I , for I&lt;J.     Using one of the RNV I  values at the transmitting node as a random seed value in generating  404  a unique list of P cookies (MC I,0 , MC I,1 , . . . , MC I,P+1 ) for the receiving node I.     In initiating exchange of frames of a frame set (data, control, management), the transmitting node sends  406  the MC I,0  value in a commitment cookie CC I  to the corresponding receiving node I.     In authenticating a single frame for the receiving node I, the transmitting node proceeds in accordance with one of the frame authentication method  100  and the frame authentication method  200  described herein above using the appropriate MC cookie list for the receiving node I.     The receiving node I can validate the authenticity of each MC cookie by deriving the corresponding a previously received MC cookie or the commitment cookie CC I .     If the transmitting node wants to authenticate a unique broadcast message intended for and transmitted to all receiving nodes I such that I&lt;J, the transmitting node uses the cookie RNV J  as the seed value, each receiving node I then being able to retrieve the corresponding commitment cookie CC I . from the respective value RNV I . Broadcast frames are flagged as such.        
 
         [0066]     In order to use the broadcast frame authentication method, the transmitting node needs to: 
        Build  402  the main list of RNV&#39;s; and     Compute  404  the list of message cookies MC I  and the corresponding commitment cookie CC I  for each receiving node I.  FIG. 3  provides an overview of the relationships between cookie values.        
 
         [0069]     Two exemplary methods for performing the above task  402  are presented herein below, the methods being loosely derived from the frame authentication methods  100  and  200  presented in respect of the first exemplary embodiment. The list of methods presented herein is not exhaustive as any methods that fulfill the same properties could also be used. A combination of methods can be used to achieve implementation tradeoffs:  
         [0070]     In accordance with an exemplary implementation of the second exemplary embodiment of the invention, where building  402  the main list of RVs includes computing a one-way hash chain, where H is a one-way hashing function, the hash chain being generated as follows: 
 
RNV N =random seed value 
 
 RNV   I   =H ( RNV   i+1 ) for 0 ≦i≦N  
 
 where N relates to the number of receiving nodes supported. 
 
         [0071]     In accordance with another exemplary implementation of the second exemplary embodiment of the invention, a method used to build  402  the list of RNVs which does not require RNVs to be computed in advance and includes: 
        The transmitting node selecting two distinct large prime numbers such that p≡q≡3 mod 4;     Letting n=p*q, such a number n is known as a Blum-integer;     Selecting a random number s in [1, n−1] such as the GCD (s, n)=1     Using computing means compute RNV 0 =s 2  mod n; and     Computing RNV i =√{square root over (RV i−1 )} mod n which is a unique quadratic residue. 
 
 Notice there is no limitation on the number of receiving nodes supported. 
       
 
         [0077]     In transmitting frames, message cookies MC I,J  are computed  404  for each receiving node:  
         [0078]     In accordance with an exemplary implementation of the second embodiment of the invention, the method of generating  404  a list of receiving node specific message cookies MC I,J  includes computing hash value chains: 
        Let G be a one-way hashing function known to the transmitting node and all receiving nodes I, the chain of cookie values for receiving node I is generated by: 
 
 MC   I,P+1   =RNV   I  which was generated specifically for receiving node  I  
 
 MC   I,J   =G ( MC   I,J+1 ) for 0 ≦J≦P  
 
CC I =MC I,0  
    where, CC I  value is transmitted in the commitment cookie CC I  to the receiving node I. Then, the list of the P successive cookies is given by MC I,1 , MC I,2 , . . . , MC I,P . 
            When the receiving node I receives the cookie MC I,J , along with a frame, the receiving node I can validate the cookie by applying G to value MC I,J  and determining if the result corresponds to a previous valid cookie MC I,K  for K&lt;J or the commitment cookie CC I .    
               
 
         [0082]     On the other hand, if a receiving node L, for L&lt;I, receives the cookie RNV I , the receiving node L can verify the cookie by determining if the corresponding commitment cookie CC L  or any other authenticated cookie MC L,K  for K&gt;0 can be derived from the cookie RNV I .  
         [0083]     In accordance with another exemplary implementation of the second embodiment of the invention, an method of generating  404  message cookies MC I,J  is used which does not require MC I,J &#39;s to be stored as follows: 
 
 MC   I,P+1   =RNV   I  
 
 MC   I,J   =RNV   I   2     (P−J+1)    mod  n   I  for 0 ≦J≦P  
 
 where n I  is the product of two distinct prime numbers p I ≡q I ≡3 mod 4 chosen randomly by the transmitting node, and where the commitment cookie CC I  for the receiving node I includes values n I  and M I,0 . A receiving node specific random number s I  in [1, n I −1] such as the GCD (s I , n I )=1 is employed by the transmitting node in the computation. 
 
         [0084]     When the receiving node I receives cookie MC I,J , in validating the MC I,J  cookie, the receiving node I employs computation means to determine if a previously validated cookie MC I,K  for K&lt;J, or the value M I,0  of the commitment cookie CC I  can be derived by repetitively squaring MC I,J  modulo n I  as described herein above.  
         [0085]     On the other hand, if a receiving node L, for L&lt;I, receives the cookie RNV I  with a broadcast message frame, the receiving node verifies the authentication of the cookie by determining if the commitment cookie CC L  or any other authenticated cookie MC L,K  for K&gt;0, can be derived from the RNV I  value. This can be done by first deriving RNV L  from RNV I , and then deriving CC L  (or MC L,K ) from RNV L .  
         [0086]     It is pointed out that, in accordance with the above described methods, once a communication session has been established, and hence a CC cookie has been sent to the receiving node, subsequent protected messages cannot be spoofed by an attacker. Methods of authenticating the transmitting node, described elsewhere, are employed to separate valid transmitting nodes from rogue transmitting nodes.  
         [0087]     It is understood, although not shown, that each value tracked by the transmitting node and/or the receiving node may be stored in a corresponding register at the transmitting node and/or the receiving node respectively. Mention was made to computing means both at the transmitting node and the receiving node, computing means which is understood to include logic for carrying out needed computations.  
         [0088]     The above proposed solutions therefore incur only a relatively low computational overhead avoiding reliance on arithmetic processor consuming operations that could lead to an inability for the subject network nodes to operate normally reducing exposure to other forms of denial-of-service attacks based on arithmetic processor exhaustion.  
         [0089]     The above proposed solutions therefore do not incur a management overhead as there are no requirements for the installation and the management of cryptographic key material on the participating network nodes and therefore no dependence on a Public Key Infrastructure.  
         [0090]     The above proposed solutions therefore does not replace other security mechanisms including proposed IEEE 802.11i and IEEE 802.1X mechanisms, but rather complement existing mechanisms in order to fill identified Layer-2 security gaps enabling deployment of implementations of the proposed solution along existing technologies.  
         [0091]     While the invention has been described making reference to the IEEE 802.11 wireless standard, the invention is not limited thereto, being equally applicable to other wireless networking protocols such as exemplary described in the IEEE 802.15, IEEE 802.16, and IEEE 802.20, all of which are incorporated herein by reference.  
         [0092]     Advantageously the proposed solution increases the resilience of wireless packet-switched networks to Layer-2 attacks and hence contributes to improve the reliability and availability of such wireless networks without requiring changes to existing standards.  
         [0093]     While the invention has been described making reference to exemplary wireless communications networks and network elements, such reference is not intended to limit the invention thereto, as the solutions described herein may be implemented in respect of wired communications networks and wired network elements.  
         [0094]     The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the above described embodiments may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.