Beatboxer Discussion [LINEAR CRYPTANALYSIS]

CryptoHack
1

Let’s discuss Beatboxer. Please do not share any flags or writeups.

2

s-box is linear.

sage: S.linearity()
256

but how to continue?

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3

Given that the S-Box is linear, the cipher in the challenge is an affine function because the S-Box is the non-linear part

4

Given that the S-Box is linear, the cipher in the challenge is an affine function because the S-Box is the non-linear part.

So, AES(x) = A*x + B, where A is a 16x16 matrix and B is a 16x1 vector. We can find A and B by solving the system of equations. Note that the plus sign represents the XOR operation because the field is GF(2). A depends on the S-Box and B depends on the key.

The challenge provides only one attempt to encrypt a plaintext of our choice.

We can use the fact that AES(0) = 0 to find B and then it’s easy to compute A locally.

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