Wired Equivalent privacy

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report about wep , discussing the structure and the algorithm and the challenges and security tools, and simulation for the algorithm
  Wired Equivalent Privacy -WEP Nada Ibiary, Nesreen Bakr,Farah ThakebMay 10, 2013 1 Introduction Wired equivalent privacy (WEP) is a security protocol for wireless local area networks.It was definedin 802.11b Ethernet standard,and developed to introduce the same level of secrecy of wired LAN inthe wireless world,by encrypting the data transmitted over the radio waves.When it comes to security both wireless and wired systems are faced with the same problems which is the need of authentication,privacy and attacks resistance. Furthermore, compared with wired systems, wireless systems are morevulnerable having limited physical security to prevent unauthorised access, which makes securing thecommunication a bit of a challenge.WEP works over the physical and the data link layer of the OSI model,thus it doesn’t provide end toend security ,It represents one element of the network’s security strategy model,without employingWEP hackers can easily obtain data sent over the network as data is sent in clear text.Although it’s beenwidley used,several flaws were detected in the WEP algorithm.In 2004 IEEE announced that WEP failedto meet it’s security goals , So improvments were made in WPA and WPA-2later on to overcome WEP’svulnerabilities . 2 The Algorithm WEP covers authentication and data privacy,In authentication a challenge text is sent in order to avoidunauthorised access.to reach privacy data is encrypted to prevent eavesdropping. 2.1 Encryption and Decryption The encryption in WEP takes place depending on the RC4 algorithm. The initialization vector (IV) andthe secret key entered by the user are concatenated. The concatenated binary stream is the input of theRC4 algorithm.RC4 is symmetric stream cipher widely used to generate a pseudo random stream of bits using the keyentered by user. It consists of two stages, the Key Scheduling Algorithm (KSA) and the Pseudo RandomGeneration Algorithm (PRGA).First, a 256 bytes array called S is initialized and we run the KSA algorithm on it. The KSA is used toturn the key entered by user into an initial permutation of S. S is mixed with bytes of key entered byuser and permuted 256 times.Second, using the output of the KSA two counters i and j are initialized, swap the values of S[i] andS[j] and the key would be in the position of their xorâ˘A´Zd values of S[i] & S[j]. The output key is thenxorâ˘A´Zd with the plain text message that we need to encrypt. Finally, the encrypted binary stream isconcatenated with the IV and sent. Decryption in WEP is very similar to the encryption procedures1  Figure 1: [Fig 1 Encryption and decryption Block diagram because of the xor operation. The IV is extracted from the frame received and concatenated with thesecret key, the binary concatenated stream of bits is then entered as an input to the RC4 algorithmresulting into the encrypted key. The encrypted key is then xorâ˘A´Zd with received encrypted textresulting into the plain text. 2.1.1 Pseudo code of RC4KSA Stage for i from 0 to 255S[i] := iendforj := 0for i from 0 to 255j := (j + S[i] + key[i mod keylength]) mod 256swap values of S[i] and S[j]endfor PRSA Stage i := 0j := 0while GeneratingOutput:i := (i + 1) mod 256j := (j + S[i]) mod 256swap values of S[i] and S[j]K := S[(S[i] + S[j]) mod 256]output Kendwhile In the figure below the PRSA stage 2.1.2 WEP algorithm simulator We use a simple simulator to show how WEP works. The simulator is capable of generating the initial-ization vector (IV) randomly or the user can enter it. It uses IV of 24 bits in binary. These 24 bits arethen concatenated with a 40 bit key that we enter. The simulator allows for maximum of 5 characterskey entered by user. Both the IV and the WEP key are then concatenated and encrypted using the RC4algorithm. The message that is meant to be sent is converted to binary too and XORâ˘A´Zd with the RC4key. The frame sent contains the result of the XOR (Encrypted payload), the initialization vector and theMAC source and destination. The MAC addresses are only used to check whether the frame received2  Figure 2: [Fig2] 2PRSAis sent for this device or not. In decryption, the IV in the frame is retrieved and the user is asked toenter the key where both of them will be concatenated in the same manner as in the encryption. Theconcatenated stream will then be used to evaluate the RC4 key. The RC4 key will be XORâ˘A´Zd with thereceived encrypted payload found in the frame, and will give us back the plain text message that wassent in the first place. 2.2 Authentication Authentication in the wep is acheived by knowing of the pre-shared secret key ,that is in some casetransmitted through a secure channel between the communicating devices , or sometimes configuredmanually on the devices , authentcation is described as follows; the station send a request to the Accesspoint to be authenticated , the access point sends back a challenge text for the station to encrypt usingit’s key,when receiving the encrypted text the access point decrypt it using it’s key, if the decryptedtext is the same then the station is an authorised user,and an acknowldgment is sent to the station forsuccessful authentication.Figure 3: [Fig3] Authentication in WEP3  3 Flaws As mentioned though WEP is widley used , it’s security features can easily be defeated by numerousattacks some of these attacks are represented in this section. 3.1 Key sequence reuse Problems with using RC4 in WEP As previously mentioned the Initialization Vector IV is 24 bits, making it possible to have 2 24 differ-ent IV combination that might be reused if all the combinations are consumed resulting in a repeatedkeystream. In order to have unique keystreams, the secret share key must be changed which is verydifficult to accomplish. Keystream Attack When two different plaintext are encrypted with the same keystream and one of the plaintext is known,it is possible to derive the second plaintext.EX:1. C1 = P1 xor K2. C2 = P2 xor K3. C1 xor C2 = P1 xor P24. P2 = C1 xor C2 xor P1 3.2 Key Management -802.11 does not specify how the secret key is distributed among the stations in a network. This task isleft to an external system to accomplish it.-It is nearly impossible to change a key, as it will require that all stations to update their secret keysas well.-Leaving the secret key as it is will increase the probability of using the same IV and accordingly thesame key sequence will be used that subject the system to more attacks. 3.3 Decryption Dictionary IV  1 Keysequence 1 IV  2 Keysequence 2 ... .................... IV  2 24 Keysequence 2 24 When the plain text is known, it is easy to derive the key sequence, which in turn can decrypt any en-crypted message with the same IV.A decryption table can be built to map the IV with its correspondingsequence key.Ex: if the intruder knows the IV used in encryption, the key sequence can be mapped using the De-cryption Dictionary and the plaintext can be obtained:P1 = C1 xor Keysequence1 3.4 Message Injection An intruder may inject a number of encrypted text into the system,this is due to the keysquence attacksand the possibility of using the same IV for an unlimited number.4
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