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    Home Unlabelled Cryptography Asking for comments on a candidate computationally hard problem (of my own). Any advice is welcomed.

    Cryptography Asking for comments on a candidate computationally hard problem (of my own). Any advice is welcomed.

    May 08, 2019

    Cryptography Asking for comments on a candidate computationally hard problem (of my own). Any advice is welcomed.

    Asking for comments on a candidate computationally hard problem (of my own). Any advice is welcomed.

    Posted: 07 May 2019 02:09 AM PDT

    Hi, I want to know if the algorithm that follows leads to a computationally hard problem, whom a public key criptosystem can be build. A link to the full description is at the end of this post. By now I will want to center the discussion in the problem itself. I'm looking for attack strategies as I'm unable to find any.

    ​

    The algorithm is intended for use in public key criptography, while using a round mixing symmetric-like structure to get a plaintext encrypted with a key.

    ​

    So, the description:

    ​

    First let's define a basic function:

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    a,b are complex numbers with coefficients in Z/pZ, and C(a) is the complex conjugate of a. i is the imaginary unit, square root of -1. Then the f function is defined as:

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    f(a,b)=(C(a)+i)(C(b)+i)+i

    ​

    This function is non-associative, so forget about being able to do a sort of exponentiation by squaring to find the result of many applications of f. The null element that must be avoided as parameter is i. There's no neutral element for this function.

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    An example of a valid prime is p=2^32-17=4294967279.

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    Next, lets define the round function g and the mixing function m. A and B are lists of N complex numbers as defined previously, for p defined above and to get 256-bit lists, N=4

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    function g(A,B) for i in 0..N-1 R[i]=f(A[i],B[i]) R[0]=f(R[N-1],R[0]) for i in 1..N-1 R[i]=f(R[i-1],R[i]) return R function m(T,K) R=T for 64 rounds R=g(R,K) return R

    ​

    The computational hard problem is in the formula R=m(T,K), knowing T and R, find K.

    ​

    In the following document is described the full criptosystem:

    ​

    https://drive.google.com/file/d/115Xwo7uL9P3V2upIqDSYdbTr8s4Ni15-/view?usp=sharing

    ​

    And an implementation in c99 is at:

    ​

    https://drive.google.com/file/d/136PT6iLfvuG0hc7kAd41lMwAo_JzbhTq/view?usp=sharing

    ​

    Greets

    Daniel Nager - [daniel.nager@gmail.com](mailto:daniel.nager@gmail.com)

    submitted by /u/DanielNgr
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    How to decrypt a monoalphabetic substitution cipher?

    Posted: 07 May 2019 06:25 AM PDT

    I'm playing a security CTF where I have to decrypt some ciphertext. The ciphertext key changes when the page is refreshed but the words never change in length and letter frequency stays the same. For example, it might be

    "Pss htktc id hxm, ixbth iwpi cjbqtg qn udjg, iwtc wpakt iwpi cjbqtg. Lwpi sd ndj vti?"

    and then when I refresh it's

    "Xaa pbsbk ql pfu, qfjbp qexq krjybo yv clro, qebk exisb qexq krjybo. Texq al vlr dbq?"

    I have no plain text but this level has a wizard appear throughout it multiple times, so the word "wizard" might be in there. I've been taking shots in the dark for hours and don't know what I'm missing. I'm 90% sure it is a monoalphabetic substitution cipher because while it's marked as "hard" the other "hard" ones have been relatively simple and don't require toooooo much work.

    submitted by /u/frostedflaker1
    [link] [comments]

    I wonder if someone, who has some knowledge in cryptography, could suggest a way to solve this, without giving me the answer

    Posted: 07 May 2019 08:36 AM PDT
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