Block cipher decryption

hasin

11/13/23, 10:14 PM

I have a course work for university.

The question is:

My solution to the question is:

    P||R = D(K,C)

However, in the question it doesn't say we are given R so I'm not sure how to get P from P||R.

This could be because my understanding of concatenation is incorrect as I see it as simply adding the nonce to the plaintext.

If what I have done looks correct please let me know, otherwise any help on anything I can read to help me understand this question would be greatly appreciated.

120

1 + 3

block-cipher

nonce

kelalaka

11/13/23, 10:20 PM

The size is given, why don't you use it?

Marc Ilunga

11/13/23, 10:22 PM

To me, the question is a bit under specified. My interpretation of it is that $u$ is a fixed value for the nonce length. So what you have to do is to precise what $R||P$ means. Remembering that $D(k,c)$ outpus a n-long bit string (call it $M$ for example).

hasin

11/13/23, 10:28 PM

@kelalaka how is the size relevant for this question?

Score:0

Crypto

Maarten Bodewes

11/14/23, 4:47 AM

You'd have to separate the nonce after the decryption, so you need the size of the nonce, or the nonce itself.

However, the size is a configuration parameter, while the nonce is an input parameter. Configuration parameters are considered known in advance, and nothing in the question indicates that you are given the nonce.

So in the end you have to perform the decryption, i.e. $X = D(K,C)$, where $X = R \| P$. Then you need to extract the plaintext, $P = X_u \dots X_{n-1}$.

+ 0

Elon Musk

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EN: Block cipher decryption

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