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Apr
12
comment Will the contents of a password protected Zip 2.0 file be safe for a week
You removed "Baring hypothesis change or progress in some of the above" before my statement; among the perhaps unfounded hypothesis are: a single file is encrypted, and nothing is known about its content (it is random, most importantly its beginning), and PKZIP 2.04g was used, and no weakness is discovered about its RNG (or appears, e.g. due to running it in some VM context). All in all, I see a problem with direct use of my statement in the context of this question.
Apr
12
revised ZIP 2.0 encryption bruteforce attack
Addition following partial quotation
Apr
12
comment Secret construction
By an entropy argument, the added requirement "The string $S$ should be smaller in length than all the lengths of $s_i$ combined", together with the initial requirement "we should be able to deconstruct the $S$ back to initial set of $w =\{s_1,s_2,\dots,s_n\}$", implies that at least one of the $s_i$ has entropy lower than its length in bit, or the $s_i$ are not independent.
Apr
11
comment Calculate if a digital signature is valid
The RSA private key operation is not a logarithm; RSA is not: exponentiation is easy but logarithm is hard.
Apr
11
comment Calculate if a digital signature is valid
Fact: $870099^{87953}\bmod1562501$ is easy to compute, and is $161153$; but computing $870099$ from the other three values is harder, and requires factoring $1562501$, or comparable effort [revised].
Apr
11
comment Does the SHA-256 sum of a 256-bit key help you crack it?
The hash of a password (in the sense hash has in the present question) often allows to recover the password! There are rainbow tables for that.
Apr
10
awarded  Popular Question
Apr
8
comment How to encrypt an image with logistic chaotic map?
Correction: the drawing is not quite CFB, even if you notice that the top feekback is cancelled: there are 2 bytes of key input from a 256-byte circular buffer at each step. I'm leaning towards unclear and unsafe. @Yuriy N.: you may need to encrypt a color image; but why would you need to use a logistic chaotic map for that purpose? If that's an assignment, can you politely repel the "logistic chaotic map" part of it as unacademic, and replace it by something that is?
Apr
8
comment How to encrypt an image with logistic chaotic map?
@E. Rose: the paper contains: "The proposed (sic) is a simple block cipher with block size of 8-bit and 256-bit secret key". The drawing below that shows something that could be block cipher in CFB mode, and an 8-bit block cipher in CFB mode can't be safe. So the system in the paper is unsafe, OR the paper is unclear (with OR being used in it's inclusive form).
Apr
7
comment Homomorphic Encryption with Addition and Exponentiation
As of practicality of the leading edge two years ago, the introduction of Shai Halevi and Victor Shoup's Bootstrapping for HElib is very telling.
Apr
6
comment Question about "toy: streamcipher
Exactly: the black box represents the Clock (or/and Control) Bit of each LFSR. The complex-looking table just tells that a LFSR remains unclocked if, and only if, its own CB disagrees with both other CBs. The CBs otherwise operate just like any other LFSR bit. The clocking controlled by CBs is A5/1's defining feature.
Apr
6
comment Filling in MD5 input variables
If the input string is long, and the unknown part is near the end, there is an obvious shortcut to brute force, giving an improvement about proportional to (length of the input string in bytes)/64. $\;$ John the Ripper or a small variation might help.
Apr
6
comment Question about "toy: streamcipher
This problem is perfectly understandable as is, without reference. You are not supposed to get internet access on exams. A busy engineer solves many problems a week which solution is nowhere on the internet. That said, this toy stream cipher is A5/1 with shorter LFSRs.
Apr
6
revised Is TrueCrypt's multiple/cascading encryption safe?
remove an unsubstanciated addition
Apr
5
revised How can I figure out a hash algorithm?
polish
Apr
5
revised How to reverse engineer a cryptography algorithm if some input-output pair is known.
affine includes linear
Apr
5
revised How can I figure out a hash algorithm?
Use proper terminology: affine!
Apr
5
comment Why is CRC said to be linear?
Ah, that corrects a long-standing terminology problem I have had, with (wrongly) using linear where affine was meant in a cryptanalytic context! I'll have to scrub my earlier answers..
Apr
5
revised How to use the Extended Euclidean algorithm to invert a finite field element?
typo
Apr
5
revised How to use the Extended Euclidean algorithm to invert a finite field element?
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