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21h
comment prepaid meters that rely on a disconnected system
The "without knowing the algorithm" is a classical (and today wrong) point of view. One reason for making that assumption might be, that security understanding of the banking sector is most of the time decades behind current standards (which are also a few years behind current research). Knowing the algorithm involved is the very basic ability we grant any serious attacker. Any weaker attacker is irrelevant today.
21h
comment How to find the encryption method and password?
If you already decrypted it... then it was the algorithm you used to decrypt it?
23h
comment Alice and Bob's crush
Or you could just use a different algorithm, namely to compute $a \wedge b$ in two-party-computation, instead of making it unnecessarily complex.
May
19
comment Can spatial filters be used to factor composite numbers?
"otherwise they have to behave like functions that depend on brute force searching" That's the part telling you how (in)efficient it is. And brute search is very, very slow, compared to state-of-the-art factorization algorithms.
May
19
comment Is the hash function defined in this exercise collision-resistant?
You're right. I adapted the answer to make up for my earlier error.
May
8
comment How can I map arbitrary group elements to unique integers without using Hash functions?
$\mathbb{Z}_p$ does not have order $p$, unless you use the addition as group operation. The multiplicative group $\mathbb{Z}_p^*$ has order $p-1$.
May
7
comment Why does Diffie-Hellman need be a cyclic group?
@poncho From the notation up there and the lack of definition of the group, you could assume that to be $\mathbb{Z}$ or a subgroup of that and consider the factorization problem as hard - which is only true if you don't publish one of factors previously. And in that case division is easy. Regarding the question: If you go with the standard DH notation ($g^a,g^b,g^{ab}$) and base the chosen elements on a single generating element $g$, you operate on a cyclic group if it is finite. If there are multiple generating elements, you either have a problem with soundness or end up with standard DH.
May
7
comment Why does Diffie-Hellman need be a cyclic group?
Or multiplies like this: $(ac)\cdot(bc)/(c)$ to get the shared "secret"
May
6
comment Calculating RSA private exponent when given public exponent and the modulus factors using extended euclid
The error is in the line before already, unless that equal sign should be a minus sign and he left out the $1=$ on the left side. Otherwise, that line reads as $17 = 18$.
May
6
comment Mutual verification of shared secret
Maybe add a reference to fair exchange protocols to tackle the part about one party learning the result and refusing to let the other party know.
May
6
comment How difficult is homomorphic encryption?
FHE is a very specific research topic, not a career path (in the business world). Right now, FHE does not exist outside of research programs. And it comes with the territory, that any serious contribution to the field is surely at the doctoral level (PhD students at the very least). If you don't plan to go that far in academia, this goal might not be ideal. About the math: It is far more complex than anything you would learn up to your bachelor degree.
May
6
comment Brute-force attack given small search-space and hash prefix
One more question would be: What is the actual goal of this? If the attacker can only see a partial hash, why is that? Is this used in some scheme, and only the first 32 bits are checked? This would be pretty terrible. What happens if an attacker finds a matching candidate, which is not the actual secret?
May
6
comment How to calculate if probability is negligible or not
Negligible probability is not a fixed threshold but a function, which has to decrease faster than any polynomial. Most commonly this references to exponential decrease, but it also could be $\frac{1}{g(x)}$, where $g(x)$ is super-polynomial. If specific thresholds for probability are mentioned, most commonly $\frac{1}{2^{80}}$ is considered the upper limit of improbable.
May
6
comment Taking advantage of one-time pad key reuse?
As far as I understand, this "crib dragging" is just a fancy name for pattern matching. It is neither something new nor a term used in science. Adding to that, it is probably the most basic and inefficient way of performing pattern matching. I wouldn't recommend using that blog as a reference.
May
4
comment Alice and Bob's crush
Sociallist's millionaire problem is wrong in this case, because it answers the function $x=y$. If both input no, they shouldn't be informed that their input was equal. Even worse: This reveals both inputs to both parties: They know their own input, and they learn if their input was equal. So in either case, the other input can be derived.
May
4
comment How to decrypt unusual Many Time Pad
Both $C_0$ and $C_1$ serve no purpose at all. In order to decrypt and retrieve $P_0$, all you need is $R_1$. So if we assume the key contains $R_1$, I can decrypt even if I only get $C_2$ and not $C_1,C_2$. Sending random things alongside actual information does not mean it is any different than the original scheme.
May
4
comment Is there a format preserving cryptographically secure hash?
As noted in the last sentence, it should be a cryptographically secure one. I am not entirely sure if it is strictly needed, but it wouldn't hurt.
May
4
comment Why are there no OTP ciphers?
@cpast That's why I said "almost". In very specific circumstances, it can be useful. For example if you have the possibility to exchange arbitrary amounts of data now safely, and want to exchanges messages later. However, key management and secure storage are still major issues, and many of the "oh let's use OTP" ideas disregard this entirely.
May
4
comment Integer encryption preserving equality
Probably I would just use AES in OFB mode. When encrypting and decrypting, just pad it with zeros and throw away that part of the result afterwards.
May
4
comment Zero knowledge / proof of knowledge sudoku solution
One of the important details is: What can Bob actually observe? If Alice can swap out cards before or during the shuffling, that would destroy the proof.