# Tag Info

## New answers tagged authentication

3

The authentication tag is defined as an output parameter in GCM (see section 7, step 7 of NIST SP 800-38D). In all the API's I've encountered it's appended to the ciphertext. Where it is actually placed is up to the protocol designer. The protocol designer may well consider the place behind the ciphertext as ad hoc default though. The name "tag" of course ...

-1

It seems that you do want to be given the answer and not just hints, so I will do that. But I'll go step by step so that you can stop reading if you want to finish by yourself. It seems to me that both solutions satisfy the first requirement consisting in authenticity. Breaking this property would consist in changing the content of $R$ to fool $A$ (this ...

6

First the theoretical explanations: Integrity and authenticity are different goals to achieve, but both are achieved (for symmetric encryption) with a MAC. You should probably be using encrypt-than-MAC or an authenticated cipher unless you have very good reasons not to. No blanket statements can be made though. HMAC: HMAC is a often used construct. It ...

2

When we have regions of the packet that we only authenticate but not encrypt, that happens because we have data that we want to bind to the encrypted region, but we don't need to include within the encrypted region. Examples of this are: For IPsec, we include the sequence number (as a part of the ESP header). We include that within the authentication ...

0

It's the same situation as for RSA encryption. If you accidentally pick a bad $s$, i.e. one that's not coprime to $N$ then the protocol fails. How likely is this to happen? If you hit a bad $s$, then you can find the factors of $N$, hence you can break the protocol - put another way, finding a bad $s$, whether deliberately or by accident is no easier than ...

2

The way a stream cipher works, traditionally, is that $E_k$ produces a pseudorandom bitstream (the keystream) based solely on the key $k$. The message is then encrypted by XORing the message with the keystream. This has a number of consequences, notably that if you know both the plaintext and ciphertext, it's trivial to compute the keystream (if \$C=M\oplus ...

2

I have found one source that claims to have invented the word "pepper". -- But this doesn't mean that they were the first or the only inventors. I guess it may have been invented and reinvented several times. 1999 Paper Kedem, G., & Ishihara, Y. (1999, August 23-26). Brute Force Attack on UNIX Passwords with SIMD Computer. Paper presented at the 8th ...

1

No, it is not. There are several misconceptions with your scheme. First, your question/answer pairing does simply not contain enough entropy. This is much less entropy than your common "please choose a password" input, because you can't just use all words/numbers/common symbols. You have to stick to answers that make sense. This is what Ricky Demer hinted ...

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