EAX is a combination of a type of CBC-MAC and CTR mode encryption

EAX is a combination of a type of CBC-MAC and CTR mode encryption, since with known problems with CBC-MAC mode i.e. length of all messages are required to be same fixed length and length to be positive multiples of n, OMAC (one-key CBC MAC) was approve for authentication. EAX takes as input a key K, nonce N, header to be authenticated H and Message M.
In the first-pass, EAX Mode encrypts the plaintext message in CTR mode using Nonce “Number used once”: a non-random IV (Initialization Vector) value of a counter (Counter, Counter + 1, … , Counter + N -1). Essentially the CTR mode take each plaintext and the associated encrypted counter and XOR it with pi to get ci (ci = pi ? E(K, i) 6.
The tag is generated in a second pass, where the header and encrypted parts are authenticated by OMAC. i.e. OMAC takes as input a key the message and performs a CBC encryption on a specific transform of t concatenated with the message. This is referred to as a pad, and is essentially an XOR of the message and one of two constants created encrypted with the key. Once the pad is applied to the message, CBC encryption is performed and the last block is output. In the decryption, EAX first separates the tag from the encrypted in CTR part and then verifies the newly calculated tag with the sent one. If the verification passes the message is decrypt.


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