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The following picture shows EAX: As you can see there is a OMAC calculation (or CMAC as it is usually called) over both N (the nonce) and H (the header / associated data). With regards to security it doesn't matter where you place the nonce and the other data in the header. I'll not go into the security of XOR'ing the calculated OMAC values for nonce, ...


Reusing an IV once opens you up to someone finding the XOR of those two plaintext, seriously compromising their confidentiality. Moreover, with GCM, a single IV reuse leaks significant information about the key used for authentication; if there are even a few pairs of reused IVs (not even one IV used many times; a few IVs each of which are used twice is ...

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