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First, a fact. For some polynomial $f_x$ and some random polynomial of the same degree, say $t$, an adversary given only $f_x+t$, knows no additional information about $f_x$. Basically (due to the finite ring), this operation is the same thing as the one-time-pad. On to the problem at hand. Let $f_3=f_2\cdot(s+1)$. Since $f_2$ and $s$ have the same ...


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A good white-box implementation does not just implement AES. Rather, it implements $G(AES(F^-1(x)))$ for a plaintext $x$. These functions $F$ and $G$ are called external encodings and are baked in the white-box. Now, to use this cryptographic part, you need to undo / apply the encodings elsewhere in your code (usually that code is terribly obfuscated). ...


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The non-linearity of an s-box is related to its linear approximation table. For an $n$-bit input x $m$-bit ouput s-box the LA table is a $2^n$ x $2^m$ table. In the case of AES this is a 256x256 table. The entries of this table represent the difference between how many matches are found and how many are expected for a linear expression of the s-box boolean ...


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Python is a scripting language, so if you've got the program, you usually also have the source code. So you don't even have to reverse-engineer. That doesn't matter much for two reasons: other languages are pretty easy to reverse engineer (or they are complex for both the programmer and the attacker); the algorithm does not have to be kept safe anyway, due ...


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A long message is a message that, when padded, is longer than the block size of the hash function. That means that the hash function has to process the message in parts and keep track of state somehow, which may allow for attacks. Such attacks would not apply to messages shorter than the block size, and may additionally require a large number of blocks to ...


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The point of cryptography is having algorithms that are secure even when the attacker knows them. Google security by obscurity to see why it's bad. I'll add the following based on otus comment. Python can be reverse engineered, so you can't hide your algorithms. Basically, if someone can run your code, they can reverse engineer the algorithms. The point of ...


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Without the specific reference I can't be sure this is what you are talking about, but generally a "long message" attack is a way to defeat second preimage resistance with less complexity than expected. It uses a time-space tradeoff to find a second preimage with complexity $2^{n/2}$ for a $n$-bit hash function (normally you would expect $2^n$). In the ...


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You should not just "pick the highest frequency character and assume it should be E" because it will probably fail most of the time, except if your ciphertext is really long. If your alphabet is small enough (usually either 26 or 255), it would be wiser to try all the possibilities for each group, and to check if the output looks like real english ...



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