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comment Yaos Millionaire Problem: Why distance >= 2?
I'm not familiar with this protocol, but looking at step 5, it seems likely that the reason why the $z$ values must differ by at least two is to avoid the possibility that $z_a = z_b+1$ for some $a$ and $b$, which could cause two of the numbers transmitted in step 5 to be identical. I'm not sure why that would actually be a problem, though.
Apr
9
comment Question about "toy: streamcipher
Is that scan from a textbook? Because if so, the mangled English and general lack of copyediting suggest that it's probably not very high quality. (If it's from an instructor's personal handouts, that could perhaps be excusable, at least if English isn't their native language. Although, even so, the broken grammar surely can't be making the material any easier to understand for the students.)
Apr
4
comment Decoding the Playfair cipher with a crib
This is, IMO, a perfect example of a useful duplicate question; the linked question does provide the answer, but there's no way you'd guess that from the title. Upvoted and voted to close. :)
Mar
14
comment Reverse Encryption Algorithm from Decryption code
The trick with SO is that "asking for code" is really more about how you ask your question than about what you ask. You want to stand out from the "gimme teh codez 4 my homework now" crowd, so you need to phrase your question as "please teach me how to do this myself" instead of "please give me code I can copy-paste". That's really all there is to it.
Mar
14
comment Reverse Encryption Algorithm from Decryption code
This is a decent question, and I've answered it below (although only partially so far, just in case it's an exercise you're supposed to complete on your own). However, I'm not 100% sure it really belongs on this site; it might be better suited for Stack Overflow, or perhaps for Reverse Engineering. If others feel the same way, we could always ask for a mod to migrate it.
Mar
13
comment How do I attack an RSA setup where e is even?
This isn't really RSA, but a variant of the Rabin cryptosystem. See e.g. this question for the difference.
Mar
3
comment Attack being kind of completely puzzled?
Honestly, I don't understand it either. Maybe it needs more context (I haven't actually read the paper), or maybe something got lost in translation?
Mar
3
comment Entropy calculation
@Mok-KongShen: If you only measure the entropy of the single-character frequencies, yes (because, by definition, that distribution ignores the order of the characters). If you instead measured, say, the distribution of consecutive groups of $n$ characters (where $n > 1$), the randomly shuffled sequence would have a much higher entropy than the sorted one.
Mar
2
comment How to publish a cipher (concept)?
Did your advisor provide any more specific reasons (besides "the review of a cipher [being] an ongoing and protracted process") why they consider your idea unsuitable for a bachelor's thesis?
Feb
29
comment How to decode a (Vigenère?) substitution cipher?
Alas, it turns out that your (and the OP's) initial assumption is wrong: the given message is not encrypted with a Vigenère cipher. That said, if it was, this would be a very good way to start solving it, and you have my upvote for that.
Feb
29
comment How to decode a (Vigenère?) substitution cipher?
@Biv: I suspect that, without the specific ciphertext, this would likely be a duplicate. (Alas, I'm in a slight hurry, and don't have time to look for dupes just now.) That said, merely shifting the emphasis of the questions from "decode this for me" to "teach me how to solve ciphers like this one" should be enough to make it on topic.
Feb
20
comment Is $f(f(x))$ a one way function?
Does the answer to this question clarify things for you?
Feb
11
comment AES128 Decryption in Objective-C just slightly off
@Journeyman: In CBC mode, the IV is simply XORed with the first block of plaintext before encryption / after decryption. (This is also why it's so important for security that the IV be random and unpredictable in CBC encryption.) So if you know the plaintext you should be getting, you can just XOR that with the plaintext you are getting, and then XOR that with the IV you're using to obtain the correct IV.
Feb
9
comment Steganographic embedding of an RSA-encrypted message into an image
This question has nothing to do with RSA specifically. Your real problem seems to be that you haven't understood how LSB steganography works.
Feb
7
comment Common Modulus Attack not reproducible
Um, no you can't. By definition, if $\gcd(m,n) = k$, then any multiple of $m$ modulo $n$ is also a multiple of $k$. The closest you can get is finding a pseudoinverse $m^*$ such that $m \times m^* = k \pmod n$ (and you can do that with the same extended Euclidean algorithm used to find normal modular inverses).
Feb
4
comment Find the minimum Adversary's Advantage
@stathisb: Are you missing the definition of advantage for MAC schemes, or the way to obtain a non-negligible advantage for this particular question (or something else)? If the former, your textbook (or lecture notes) should have it, assuming that it's using the term "advantage" for MACs in the first place; if the latter, see my answer below.
Feb
4
comment Find the minimum Adversary's Advantage
@stathisb: ...where you've now introduced three new undefined symbols ($K$, $M$ and $T$). I can guess what those mean (I bet they're the keyspace, message space and tag space respectively), but such notation isn't really unambiguous or universal enough that you could just assume that everybody will understand it (they way in math you can usually assume e.g. that $\pi \approx 3.14159265$), especially if they learned crypto from a different textbook than yours. But anyway, more important than all this notation is really that you still haven't told us what you've tried and where you're stuck.
Feb
4
comment Find the minimum Adversary's Advantage
@RickyDemer: I suspect it just means that they're representing their MAC scheme as a pair of functions $S$ and $V$, which probably stand for "signer" and "verifier". Wikipedia uses a similar notation. But yeah, the question should really mention that, or at least link to a definition.
Feb
4
comment Security of a parallelizable block cipher mode
Also, this construction looks vaguely similar to OCB mode. Are you familiar with that?
Feb
4
comment Security of a parallelizable block cipher mode
Maybe a silly question, but what's $\boxplus$? I assume $\oplus$ means bitwise XOR, but I don't really know a standard meaning for $\boxplus$.