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BASIC SECURITY CONCEPTS
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requiring limited overhead One such approach is based on the use of HMAC In this case, the two parties share a secret which is then used in the HMAC to verify the authenticity of the message Thus, the receiver can verify the originator of the data This approach can be extended to the broadcast domain, where a single sender shares a MAC key with several receivers The sender then calculates the HMAC of every message before sending it and attaches to the message Each of the receivers of the message can check the MAC and verify the authenticity of the message Unfortunately there is a problem with applying this approach directly to the broadcast/ multicast domain Any receiver in the group can also use the shared key to provide an HMAC value on a message The node can transmit this message claiming that the message was sent from another node Therefore, a receiver does not have a guarantee that the source has indeed created the message This implies that symmetric schemes cannot be used for providing individual authentication This is because users share the key and therefore it is not possible to verify which one of the users sharing the secret actually created the message Of course, an alternative here is to have the source share pairwise keys with every receiver and use these pairwise keys to authenticate the message However, such an approach is highly inef cient Therefore, more ef cient schemes to ensure broadcast authentication are needed One such scheme is TESLA (timed ef cient stream loss-tolerant authentication), [10], which makes use of only symmetric cryptographic primitives TESLA requires loose time synchronization among the entities participating in the network Further it achieves its objective by making use of delayed key disclosure as we explain next To understand TESLA, consider Figure 211 We consider a single sender and multiple receivers The sender in the group determines a secret key K, which is then hashed to get the hashed value H(K ) The sender discloses the hashed value authentically (using a digital signature for example) to all the receivers The sender then transmits all packets P such that the transmitted packet contains the HMAC of P in addition to P The HMAC calculation is done using the key K The receivers on receiving these packets cannot verify the HMAC since they do not have the key K, so receivers will have to buffer all the packets
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Figure 211 Basic operation of TESLA
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24 MISCELLANEOUS PROPERTIES
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After a certain duration, the sender discloses the key K to all the receivers At this point every receiver can check if the disclosed key K is indeed valid This can be done by checking if the hash of K corresponds to the hashed value sent authentically by the sender earlier If this is true, then every receiver can verify the authenticity of every packet Note that the receiver must not accept any packet the HMAC of which has been calculated using key K after the disclosure of the key K This is achieved by the sender disclosing the timeout schedule to the receivers and by having loose time synchronization between the receivers and the sender The approach given above can be easily extended (and is in fact more ef cient) when considering multiple time intervals In this case, each sender splits time into multiple intervals It then chooses a secret key and generates a one-way key chain through repeated use of the one-way hash function property The last value of the hash chain is then transferred to the receivers authentically Each generated key that is one of the values of the hash chain will be used in one time interval The keys are used in the reverse order of generation Thus, the message authentication keys used for packets in the previous interval are derived (via hashing) from the message authentication keys used in the current interval During each interval the sender calculates the HMAC of each packet using the key corresponding to that interval The transmitted packet contains the original contents of the packet, the calculated HMAC value over the original packet and the most recent one-way chain value that can be disclosed Thus, the sender discloses the keys used after the time interval of their use The receivers must know the key disclosure schedule, so when the receiver receives a packet, it checks that the key used to compute the HMAC on the received packet is still secret It can do so using its knowledge of the key disclosure schedule since the receivers and the sender are assumed to be loosely synchronized in time As long as this key is still secret, the receiver buffers the packet If the key is not secret then the receiver has to drop the packets When the key is disclosed, the receiver checks the correctness of the key (using the one way property of hash functions) and then authenticates the buffered packets This operation of TESLA is illustrated in Figure 212 Thus, TESLA achieves asymmetry needed for broadcast authentication through clock synchronization and a delayed key disclosure Note also that TESLA is robust to packet loss A drawback, though, is the need for receiver to buffer packets transmitted during an
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