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Figure 28 CFB mode of operation
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Figure 29 OFB mode of operation
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Figure 210 CTR mode of operation
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explicit self-synchronization is required This implies that sender and receiver must be synchronized during transmission, which may increase the overhead associated with this scheme We next consider the CTR mode of operation, shown in Figure 210 This mode encrypts a counter value and the output of the encryption is then chained to the plaintext Further, the counter value is changed for every plaintext block Thus, the counter value is never reused Typically, this mode is used to ensure high-speed network encryptions This is because the key stream can be generated in advance and encryptions or decryptions done in parallel In addition, this mode also allows random access to encrypted data blocks Thus, decryption can start at any point rather than being forced to start at the beginning (assuming the value of the counter is known for the speci c block of interest) In terms of error recovery, this mode has the property that, if a ciphertext block is garbled, then only the corresponding plaintext is garbled When using this mode, it is important to ensure that the counter values are not reused assuming that the key does not change Otherwise, the attacker can obtain extra information by using the XOR of two ciphertext blocks By doing so, the attacker will obtain the value corresponding to the XOR of the matching plaintext blocks
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In addition to the fundamental properties discussed earlier, there are several other properties that are made use of in the various security mechanisms discussed throughout the book In this section, we explain these properties We start off by looking at hash chains More speci cally, we consider the pre-image resistance property of hashing
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algorithms and describe this again brie y We do this because this property is used in several security mechanisms 241 One-Way Property of Hash Chains
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One application of the pre-image property of hash algorithms is in the context of one time passwords Note, however, that this property is also used in other schemes such as TESLA, which we discuss later in this chapter The one-time password problem consists of a party making use of passwords in order to prove his or her identity to the other party Further, every password is used only once We next explain this property Consider two parties, Alice and Bob Alice is considered to be the claimant and is expected to verify her identity to the veri er Bob We next explain how this can be done using the one-way hash function H Initially Alice chooses a secret w as well as a constant t The constant t indicates the number of times veri cation can be done Then Alice hashes the secret wt times using the hash function H to get the value denoted as H t(w) Then Alice transfers H t(w) securely to Bob This secure transfer can be accomplished through the transmission of a digital signature by Alice on the hash value transferred This is the initial shared secret between Alice and Bob Now during the operation of the protocol, when Alice has to identify herself to Bob for the ith time (let us say during the initiation of the ith session), Alice computes H t2i(w) and transmits the value to Bob Bob checks that that the value is correct by calculating H i[H t2i(w)] and verifying whether the outcome equals the initial value H t(w) transferred securely by Alice If the veri cation holds, Bob accepts the claim of Alice and otherwise not Note that this scheme requires that both Alice and Bob be synchronized in terms of the number of identi cation sessions completed Further, this scheme also limits the number of identi cation sessions to t Once this number of sessions expires, the whole process is repeated again A scheme based on this idea, called Lamport s one-way function, based on one-time passwords, has been proposed [7] In addition, this concept of hash chains also arises in several places when we look at the various functions in ad hoc networks 242 TESLA
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We next consider another primitive that is widely used, especially in the context of broadcast authentication Broadcasting and multicasting are used widely in a variety of applications in wireless ad hoc networks Example applications include IP multicast, situational awareness applications in the battle eld environment, and emergency response in ad hoc networks Such broadcast/multicast applications also need security However, security solutions designed for point-to-point communication are not often applicable to broadcast/multicast communication This is particularly true regarding authentication To understand this, consider point-to-point communication, also referred to as unicast communication Message authentication in such a case can be achieved through the use of symmetric, asymmetric schemes or message digests, as discussed earlier A typical authentication approach using asymmetric cryptography is based on the use of digital signatures As discussed earlier, though, the digital signature approach is typically expensive, particularly in terms of the computational overhead The generation, veri cation, and communication costs associated with digital signatures are very high This might make this approach impractical in resource-constrained networks such as ad hoc networks The other alternative in such a case is to make use of symmetric key concepts The use of message digests can provide individual authentication in point-to-point communication
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