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bio website en.wikipedia.org/wiki/…
location United Kingdom
age 26
visits member for 2 years, 10 months
seen Sep 2 at 7:44

Pentester, ex-developer, security researcher, reverse engineer, electronics tinkerer, internet activist, zombie eradicator, promulgator of useless facts, shrubbery inspector, bacon aficionado.

Strengths: Security, Crypto, Win32 API, C#, .NET, PHP, x86 assembly

All answers and comments are encrypted with ROT256-ECB.

Opinions are my own. Advice provided with no warranty.


Nov
24
comment Security of simple xor and s-box cipher?
Hmm, do we compute each $S[x]$ for $x$ on the fly, using some function $f$, rather than "caching" the entire $S$ field in an array somewhere?
Nov
24
comment Security of simple xor and s-box cipher?
@PaŭloEbermann - Thanks for the extra explanation. This is mainly as a hobby; I'm not exactly trying to produce the next AES. I understand most of the mathematical principles behind crypto, but I was never taught any formal notation, which can make things a little awkward. I learn fast, though! :)
Nov
24
comment Security of simple xor and s-box cipher?
Wow. This answer is precisely what I was looking for. The only part I don't quite get is the S-box section - are you implying that a $S[x]$ is meant to exist for every $n$-bit $x$, where $n$ is the block size? Wouldn't this result in ludicrously sized S-boxes? I know you mentioned something about huge S-boxes that can't fit in the universe (yay for the $10^{89}$ particle limit!) but I'm not sure how this works practically.
Nov
24
comment Desirable S-box properties
Could you explain criteria R5 and R7, please? And is criteria R2 essentially "No S[x] must exist for x where the result is a rotation of x, e.g. 01010010 -> 10010100"?
Nov
24
comment Is a RSA-signature of some identifying data a safe way to implement a license key?
@MichaelKjörling - The motherboard and hard disk serial numbers are unique to each item, not unique to the product or brand. Using WMI you can query Win32_BIOS for SerialNumber on the first instance (there should only be one), which will give you the serial number of the BIOS device. You can do the same with Win32_Baseboard.SerialNumber, which gives you the serial number of the motherboard. You can grab WMI Explorer from here to take a look around: ks-soft.net/hostmon.eng/wmi/index.htm
Nov
23
comment Security of simple xor and s-box cipher?
Same issues for all the stuff like $E : K \times M \to M$. It's completely opaque to me without a description.
Nov
23
comment Security of simple xor and s-box cipher?
Buh? Any chance you could clear up your explanation to avoid use of stuff like $\mathbb{Z}_m$, or at least explain the meaning of them? I'm not versed in such formal notation. I didn't understand a single part of anything you typed in LaTeX.
Nov
23
revised Security of simple xor and s-box cipher?
deleted 241 characters in body
Nov
23
comment Security of simple xor and s-box cipher?
@Ninefingers - I was happy with it the way it was, but the responses relied heavily on my original question, so I put the original implementation back. Wasn't sure what else to do. Feel free to revert.
Nov
23
comment Security of simple xor and s-box cipher?
Yes, it splits each block and each round key into 16 blocks of 16 bits, then computes $S[M \oplus k]$ for each of those blocks. It might technically be described as a 16-bit block size and 256 rounds, with each round key being a subsection of the actual key.
Nov
23
comment Security of simple xor and s-box cipher?
For every single block there are 16 rounds, i.e. $C_n = M_n$, then loop $C_n = S[K_r \oplus C_n]$ for $r = 0 \to 15$ for each block $n$.
Nov
23
revised Security of simple xor and s-box cipher?
re-added the original scheme
Nov
23
comment Security of simple xor and s-box cipher?
Sorry, must've missed it off my re-write. If you take a look at the edit history, you'll see my original design. It specifies 16 rounds of $C_n = S[K_r \oplus M_n]$ on each block. Would this result in any significant changes in the feasibility of the attacks you described?
Nov
23
awarded  Commentator
Nov
23
comment Security of simple xor and s-box cipher?
Does the known plaintext not get prevented by the use of multiple rounds? Since it's essentially $C_n = S[S[S[K_n \oplus M_n] \oplus M_{n+1}] \oplus M_{n+2} ... ]]]$
Nov
23
comment Desirable S-box properties
It doesn't really explain why they made the choices they did, though. It just says "this is the S-box and these are the choices we made". I'm really looking for answers that provide both an explanation of the facts and the reasoning behind making the choices.
Nov
23
comment Desirable S-box properties
The output of the cipher has the avalanche property and appears random, but the construction of the S-box is not random. It's a case of not allowing any correlation, rather than specifying that a particular output is not allowed.
Nov
23
comment Is a RSA-signature of some identifying data a safe way to implement a license key?
Windows product IDs are NOT unique. Mass user OEM licenses have the same key for every machine. I'd suggest fetching the motherboard and C drive serial numbers instead (you can do this with WMI).
Nov
23
comment Desirable S-box properties
My rational is that S[a] = a provides no benefit and S[a] = !a will always maintain the same bit "pattern" as its input. Since the idea of an S-box is to provide "confusion" (as defined by Shannon), it seems reasonable to ensure that neither of these cases are allowed. I may, however, be incorrect.
Nov
23
awarded  Supporter