# Tag Info

9

I'll assumme All ciphered blocks means the same as ciphertext for CBC-Encryption with implicit zero IV, while CBC-MAC is the last block of that. All ciphered blocks is unsafe as a message authenticator for messages longer than one block, for it succumbs to a trivial attack (here with two blocks): Eve intercepts message $M=M_0||M_1$ and its authenticator ...

5

Don't believe every claim ever made in any paper ever written, particularly when the paper provides little or no justification for the claim; not everything you read reflects the cryptographic consensus. This is particularly true for a paper written in 2002, which is a time our understanding of authenticated encryption and security engineering was still in ...

3

Does it negatively affect security to calculate a hash value of the ciphertext before MAC calculation? Like exchanging step 2. with this: HMAC-SHA256(SHA256(ciphertext)). Technically, yes, but not significantly. In order to attack the scheme you propose, the attacker would have to be able to do at least one of two things: (1) Find an attack on the ...

3

If the receiver can wait for all the packets before decrypting: This case is simple, since your final goal is to ensure that the plaintext you decrypt was the exact same plaintext you encrypted. (Trivially, this includes rejecting re-ordered plaintext.) Use an Authenticated Encryption (AE) scheme (eg, CCM, GCM, etc) across all the packets, treating the ...

2

There aren't any known attacks on the PRFness of HMAC-SHA256 better than brute force. (So you can truncate that MAC to length L where $\:\:\frac1{2^L}+\epsilon\:\:$ is an acceptable risk of forgery.) To reduce the impact of a forgery without making the ciphertext any longer, one should use a format-preserving encryption (FPE) scheme that is secure against ...

1

The verifier and logger start with a seed for a forward-secure pseudo-random number generator. To denote a valid ending of a log, append the string of the next $b$ bits of the PRNG's output to the end of the log. $\;\;$ To add a log entry, get the next $\:b\hspace{-0.03 in}+\hspace{-0.03 in}k\:$ bits of the PRNG's output, use the last $k$ of those bits to ...

1

You asked the same question over on the IT Security site. Please don't cross-post. It is frowned upon, under the rules these sites operate. Here is what you should be doing: Truncate the MAC tag to an acceptable length. You will need to choose a length that provides a suitable tradeoff between packet size vs. security against forgery. I suggest you use ...

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