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8

This scheme is not worth the name MAC; it is horribly weak. First and foremost, the tag/MAC is unchanged when two blocks of plaintext are exchanged (because of the commutativity and associativity of the $\oplus$ operation). If follows that from any message with at least two different blocks, we can make a different message for which we know the tag/MAC. ...


4

It has the disadvantages of any MAC-then-encrypt scheme, which I'm quoting from the linked answer below. In addition: It has the property that you need both a nonce and a hash, so for equivalent security it requires more message space. The nonce has to be random, so it requires strong random numbers for each message, unlike e.g. AES CTR + HMAC. Doesn't ...


3

Well, 32 bits is somewhat short, so one could just try ciphertexts. However, there is a much better attack. Choose M0 arbitrarily, let P be the CBC padding for Headers || CRC || M0, and choose M1 so that CRC( M0 || P || M1 ) = CRC(M0). Submit M0 || P || M1 to be encrypted, truncate the ciphertext to the length of encryptions of M0, and then output the ...


3

GMAC is quite simply GCM mode where all data is supplied as AAD (or additional authenticated data), or as NIST SP 800-38D puts it: If the GCM input is restricted to data that is not to be encrypted, the resulting specialization of GCM, called GMAC, is simply an authentication mode on the input data. If you don't have access to a cryptographic provider ...


3

As correctly pointed out in a comment, the authenticated encryption model assumes that the attacker knows the algorithm; the attacker can query the encryption oracle with any plaintext $P$ (and a unique nonce $N$) and get MAC-then-Encrypt ciphertext $C$; the attacker can query the decryption oracle with any string $C$ pretending to be a ciphertext. No ...


2

Usually. $\:$ A variable-length input keyed-PRF is a [PRF's output length]-bit MAC. In particular, a variable-length input keyed-PRF with long output is automatically a MAC.


2

If there exists an encryption scheme, then there exists an encryption schemes such that one can easily modify a single ciphertext so that whether or not that modifies the decryption result depends in a predictable-and-useful way on what the plaintext message was, such as: The modified encryption operation outputs a zero concatenated with the original ...



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