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1

If, as can be reasonably inferred from the question, a fresh random $k$ is chosen on each invocation of $G$, then $G$ is not a pseudorandom generator because it is not deterministic. (A pseudorandom generator by definition is a deterministic algorithm.) If $k$ is fixed, then more information would be needed. For example, is $k$ always the same, or is a ...


2

Note that, in Salsa20, the loads and additions of the key words ($x_1$, $x_2$, $x_3$, $x_4$, $x_{11}$, $x_{12}$, $x_{13}$, $x_{14}$) are critical for security, since the double-round function is trivially invertible. One might think that the remaining loads and additions could be skipped without sacrificing security, achieving almost half of the ...


3

Before answering the actual question, I will offer some general advice. It is important to pay attention, both in class and to the textbook you are reading. If learning how to solve such exercises is a key goal of the course, such solutions have very probably been discussed at length in class. Moreover, your textbook also has proof examples, and in this ...


1

$\big(\hspace{-0.03 in}$You don't need that. $\:$ $\operatorname{L}\hspace{-0.02 in}\operatorname{cm}\hspace{.02 in}(\hspace{.04 in}p\hspace{-0.04 in}-\hspace{-0.05 in}1,\hspace{-0.02 in}q\hspace{-0.04 in}-\hspace{-0.05 in}1)$ can be used instead of $\phi(N)$.$\hspace{-0.03 in}\big)$ $k$ is an integer which will make the quotient an integer. ...


0

I am planning to develop a more secure version of the RC4 algorithm. Since I´m not an encryption expert... Never roll your own crypto, especially not if you are no expert in that topic. There are so many traps, starting from implementation bugs to side channel attacks. Simply use existing crypto libraries of secure algorithms like AES.


4

Does the value of the key array(T) have to be in this range [0-255] if yes could you please specify why? Yes. RC4 operates on bytes. There are 256 possible values for an 8 bit (1 byte) number, that range from 0 to 255. RC4 treats the key as an array of bytes, so every entry in the key array is by definition in the range 0 to 255. Why did they use ...


0

Of course because all the calculation are done in residue Class modulo N. Example: $a^{'}=a * R \; mod \; N =(a+ \lambda .N)*R \; mod \; N$ Then the value of T is invariant modulo N.


9

Contrary to your assumption, this is done, and it is secure: For instance, the hash functions SHA-224 and SHA-384 are basically the same algorithms as SHA-256 and SHA-512! The only differences are in the initial values for the Merkle-Damgård construction used internally and, of course, in that only the first $224$ or $384$ bits of the resulting hash are ...


2

An algorithm which is secure even if the enemy acquires everything but the key may be regarded as a means of generating secure algorithms. If one presently has a secure channel for communicating with a correspondent, and will need to communicate securely in future when no secure channel is available, using some dice to generate a random key and conveying it ...


3

This question is based on opinion. At least kind-of. But the variants from which one can choose are quite a few. As for general construction the sponge construction (like Keccak / SHA-3 uses) are very versatile and can be used for many purposes, for example hashing, authenticating (= "MAC'ing"), authenticated encryption (see “General Overview of the ...


5

There are attacks on both blockciphers and hash functions that can exploit symmetry in the round functions. For example, completely identical round functions can permit Slide Attacks on Hash Functions, and rotational symmetries of the round function can permit rotational cryptanalysis. The round constant addition or 'iota' step of the Keccak Hash Function ...


2

Regarding points 2 and 3, cipher designers want to ensure that the relationship between the plaintext, the ciphertext, and the key are complex, so that no attacker can efficiently untangle them. If the ciphertext can be expressed as a linear or sufficiently low-degree system of functions of the plaintext and key then attackers can use efficient algebraic ...


2

I will specifically address your question 3; that is, quite a lot of block ciphers (and hash functions) consist of a regular round structure (where you repeatedly do the same thing over and over); why is this? Well, one incentive for doing that is that it makes the cipher easier to analyze; we can study the round function in depth; once we've done that, we ...


3

I can make a few comments regarding points 1 and 3: If you are going to encrypt only one block, your first assumption is not that misled. However, you will almost always need to encrypt a file longer (maybe way longer) than the key length (let's say 128 bits). Without considering encryption modes, that means that for every block of 128 bits, you will ...



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