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The following table specifies a cryptosystem based around a very simple encryption algorithm with four different plaintexts A, B, C and D (one corresponding to each row) and four different ciphertexts A, B, C and D. The encryption algorithm has five different keys K1, K2, K3, K4, K5 (one corresponding to each column). By writing EK(P)=C to mean that the encryption of plaintext P using encryption key K is C, the entire cryptosystem is defined as follows:

EK1(A)=B   EK2(A)=B   EK3(A)=D   EK4(A)=A  EK5(A)=C
EK1(B)=C   EK2(B)=C   EK3(B)=B   EK4(B)=B  EK5(B)=D
EK1(C)=D   EK2(C)=A   EK3(C)=A   EK4(C)=D  EK5(C)=A
EK1(D)=A   EK2(D)=D   EK3(D)=C   EK4(D)=C  EK5(D)=B

I am getting conflicted on this question. To me the key space should be 5. But some articles I've read say you do do k^2. If this was the case it would be k^5 = 32. Does it change how you compute this based on the algorithm or? I'm stumped.

This is not school related. I'm just wanting to learn some security and failing lol

fgrieu
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  • I would guess, the "$k^2$" is a typo, and it should be $2^k$, to match the example (and $32$ doesn't have an integer square root). – tylo Jan 07 '20 at 16:20

1 Answers1

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the key space should be 5

Something is $5$, but that's not the key space.

The key space is the set $\{\mathtt{K1},\mathtt{K2},\mathtt{K3},\mathtt{K4},\mathtt{K5}\}$. We'll note it $\mathcal K$.

The size of the key space (or key space size for short) $\|\mathcal K\|$ is $5$.

The size of the key (or key size for short) in bits, that we'll note $k$, is the base-2 logarithm of the size of the key space, that is $k=\log_2(\|\mathcal K\|)$.
Here $k=\log_2(5)=\log(5)/\log(2)=2.3219\ldots$ bit.

But some articles I've read say you do do $k^2$.

Read again: probably the article says that the size $\|\mathcal K\|$ of the key space $\mathcal K$ is $2^k$, where $k$ is the size of the key in bits. That is, $\|\mathcal K\|=2^k$. Or equivalently, $k=\log_2(\|\mathcal K\|)$.

fgrieu
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  • I'm a bit confused. There can be 4! for any mapping. If the encryption uses a matrix for a key then it should be 4*4!? – kelalaka Jan 07 '20 at 14:19
  • @kelalaka: my reading of the question is that the key can take $5$ values, which is only a fraction of the $4!=24$ permutations of $4$ values (much like AES-256 keys can take $2^{256}$ values, which is only a fraction of the $2^{128}!$ permutations of $2^{128}$ values). I could be mistaken, especially since I have modified the question so that it better fits my understanding by exchanging the words "row" and "column". – fgrieu Jan 07 '20 at 15:23
  • I'm pretty sure that you'll answer if that is not the case. – kelalaka Jan 07 '20 at 16:09