Meir Maor
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In 2020, SHA-1 practically broken in chosen-prefix collision (CP-collision). Can double SHA-1 hashing prevent CP-collision?
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29 votes

a. No such double hashing doesn't do a bit of good. Anything which collides after a single hash will definetly collide after a double hash. It preserves all collisions and adds new ones. We might ...

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Why can't they just fix SHA-1 if it's broken?
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26 votes

I have 3 answers: We can't fix SHA-1, we shouldn't fix SHA-1 and we already did fix SHA-1. SHA-1 is a hash standard; many different people can and have implemented it and they all get the same results....

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Make a Strong, Easy-to-Remember Password Using Classical Cryptography?
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21 votes

I fail to see why one would want to use classical or pencil and paper tools for derivation. For anyone attacking your technique it will make no difference. An attacker with a modern computer will only ...

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What is hardened SHA-1, how does it work and how much protection does it offer?
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21 votes

Hardened SHA-1 detects collisions built of a certain form, If someone were to find a collision using brute-force birthday attack (currently not feasible) the detection would not work. The vectors ...

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Cryptography elements needed for a story
16 votes

Though quantum computers fit the requirements, I'm not sure they are the best option. A general purpose quantum computer capable of attacking modern encryption (RSA, AES) would have serious ...

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Deep Learning application in decryption?
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16 votes

There is no evidence of deep learning breaking modern cryptography. Deep learning is simply glorified gradient descent. With a reasonable cipher you get no indication of almost finding the key, so I ...

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Is it possible to recover the seed used by Python's pseudo-random number generator?
15 votes

Python uses a Mersenne twister PRNG, and though it is not secure it does have a large state. You have here 40 numbers, the first one gives you 1 bit and each subsequent number has an extra bit for a ...

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Are there any benefits to using ECB over CBC?
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13 votes

ECB benefits: It's a tiny bit easier to implement. It allows for parallel encryption and decryption (CBC only decryption). A single corrupted cipher block corrupts only one block of plain text(in ...

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Which is the simplest cryptographic algorithm which is close to commercial-level security?
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11 votes

I will throw tiny encryption algorithm into the mix: https://en.m.wikipedia.org/wiki/Tiny_Encryption_Algorithm It's a very respectable block cipher. It really works as a block cipher with convenient ...

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Why can every prime number be written as 6k±1?
9 votes

I am not sure if this question should be considered on topic here, but I will answer anyway. Theorem: All prime numbers larger than $3$ can be written as $6k+1$ or $6k-1$ for some natural number $k$. ...

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Can the same One-Time-Pad be re-used with some tricks
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9 votes

Based on comments it seems the question asks if this is as secure as OTP as opposed to finding a practical attack. It is clearly not as secure as OTP. It is not information thetorical secure. With ...

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What password hash function for the next 50 years?
9 votes

As Maarten writes you should use specialized password hashing algorithms and not generic hash functions. But I would like to discuss the futility of planning for 50 years into the future. It's really ...

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PhD in cryptography using elliptic curves
9 votes

If you want to end up in the industry, I strongly doubt a PhD is a good investment of your time, regardless of the rest of this discussion. I believe a general purpose quantum computer, the kind that ...

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S-box basic question
9 votes

DES uses a Feistel network. The S-box results gets XOR-ed with the other half so no information is lost. It doesn't need to be invertible.

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Can we pick which key is private or public in asymmetric encryption? Do the keys actually encrypt and decrypt a cipher text?
8 votes

In RSA encryption and decryption are similar. If you chose e randomly and calculate matching d you could then chose to swap their roles and pick either as the public key. Usually we don't do this, we ...

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Soft question: What are examples of beautiful proofs in cryptography?
8 votes

My favorite crypto proof, is how to prove a graph coloring exists with zero knowledge. I like it because it doesn't require any cryptography. Let's say we have a graph which I know of a coloring ...

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How many trials does it take to break HMAC-MD5?
8 votes

We can find collisions in MD5 much easier than via a generic birthday attack, due to flaws discovered in it's construction. However no practical preimage attack is known, and no practical attack on ...

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Understanding WannaCrypt's hybrid encryption scheme
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7 votes

The server will never release Spriv, it protects all ransomed clients. Assuming the server wants to allow a given client to decrypt it's file the server will use Spriv to decrypt Cpriv and send that ...

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Is it possible to find the product of two numbers without knowing the two numbers?
7 votes

In the way you setup the problem the answer is No. As Charlie can perform $C(A(1),B(b))$.

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Is it possible to compress a random sequence with high entropy?
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7 votes

Random data can not be compressed. Good pseudo random data can not be compressed(with generic algorithm). As Paul commented if you use an inefficient encoding such as hexadecimal or Decimal ASCII ...

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Academic breach revealed too late
6 votes

Differential cryptanalysis In 1990 Eli Biham and Adi Shamir discovered a powerful technique capable of breaking a wide range of ciphers. When they applied it to the DES cipher developed by IBM and ...

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Smallest Guaranteed hash collision cycle length
6 votes

There is a big difference between what we know for sure and what we expect. We don't know for sure almost anything, We don't know what is the cycle structure of sha256 We don't know how big any cycle ...

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What conditions does a provable secure cryptosystem satisfy?
6 votes

Provably secure doesn't actually mean secure. Plenty of provably secure cryptosystems were broken and not because the proof had a flaw. When something is provably secure, it just meant some statement ...

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Can public key cryptography survive quantum computers?
6 votes

Current commonly used public key cryptography systems are based on the hardness assumption of factorization and/or discrete lograrithm. Both these problems are solved efficiently using Shor's ...

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Why are $\lceil 1/\operatorname{entropy-per-bit} \rceil$ number of bits not sufficient to generate an unbiased bit?
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5 votes

I will take a slightly different approach, a side step to a simpler problem to gain intuition. Let's say we have a fair 6 sided die. And we wish to draw a number uniformly from 1 to 4. It can't be ...

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Why do memory-hard functions rely on a time-space trade-off?
5 votes

In order to create such a function you fill up memory with results of some computation; the memory-hard function then reads these values to further the computation later on. Rather than saving the ...

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Does the double-hash H(H(x)) have greater collision probability than H(x)?
5 votes

I'm not sure what the question here is, but obviously applying the hash function twice can never decrease the number/probability of collision as all collisions in the first invocation are maintained. ...

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Why is it inadvisable to increase the number of rounds or using a larger block size to a published(standardized) block cipher such as AES?
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5 votes

Just in general messing with well studied cryptographic standards is inadvisable. Especially if there is no clear benefit. Adding rounds to AES is fairly safe, before it became AES the number of ...

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What are the weaknesses of Shamir's Secret Sharing?
5 votes

Shamir's secret sharing does not split a secret into smaller ones nor does it have weaknesses. It is information theoretically secure, and does not rely on limited computing strength of attacker. (...

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An unbreakable book cipher?
5 votes

Book ciphers are well understood and this seems like a fairly minor variation. Following Kerchoff's Principle we separate the key being the book and the arithmetic operator and numbers and the rest. ...

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