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Jan
16
comment Keyed-hash of a lot of small numbers
I'm definitely no expert here, but couldn't a stream cipher encrypted data be used instead of both (storing the data twice makes no sense to me)?
Nov
11
comment How can two (or more) parties share and agree upon a common random seed?
@nightcracker: I was thinking about generating large random numbers, as cutting them is trivial and they may be useful when the requirements change. And I was thinking about seeding Salsa20 where 256 bits fit perfectly (as the key).
Oct
18
comment Encryption of small messages
I was actually interested in a length preserving encyption, but I failed to state it, and your answer is interesting, too. Thanks.
Oct
18
comment Encryption of small messages
@Thomas Pornin: I didn't say that I wanted a length preserving encryption, but you understood it well. Thanks for the perfect answer.
Feb
14
comment How to best obtain bit sequences from throwing normal dice?
@Mok-Kong Shen: Yes, there's an efficiency drop for 3 rolls. You figures seem to be right.
Jan
17
comment LFSR for small numbers with large periods
@gl3829: No idea, what you're doing wrong, but fgrieu is obviously right. Try to pick the lower 1 bit. According to you you'll get a period of two, but it must be just like described in the second paragraph. Moreover, for any $m<n$ there's no number missing. I'd guess your LSFR is wrong.
Oct
28
comment How do ciphers change plaintext into numeric digits for computing?
@fgrieu: This is surely true (and I should have said it), but this goes far beyond "converting message into number" (which is probably what the OP wanted to know).
Oct
28
comment Is there an algorithm for factoring N, which is just as simple as this one, but faster?
@Thomas: Doesn't it? I'm basically saying that the choice of Ň gains nothing. So yes, there are better algorithms out there, and for number as small as the given one it's easy to implement.
Oct
28
comment Is there an algorithm for factoring N, which is just as simple as this one, but faster?
I'm no expert in cryptography and have spent only few minutes reading it, but it looks bad too. Unfortunately, whenever it looks like it needs no advanced math, it can't be right (unless it's one of the known algorithms). Note that something as simple as Pollard's rho can factor a number like the above in a few milliseconds.
Aug
31
comment Can you help me with this Random Number Generator function?
Can't any even-permutations-only generating cipher be made "perfect" by a trivial postprocessing conditionally switching two outputs (with a 50% probability based on the key)?
Aug
30
comment No SHA-1 Collision? Yet SHA1 is broken?
@Andrew Tomazos - Fathomling: Using the "birthday paradox", there's a brute force attack against any hash of length 160 bits (like SHA-1) taking $2^{80}$ operations. Anything faster is a break.
Aug
30
comment Do I have to have a different salt for each password?
What I meant wasn't the UI, but using the whole string the way the OP proposed, i.e., something like searching for PBKDF2(username + ":" + password).
Aug
30
comment Do I have to have a different salt for each password?
What about the user entering a single string composed of their username and their password (possibly with a separator)? Doesn't this schema have exactly the same strength as the classical one?
Aug
28
comment Secure Hash Function based on AES
For SHA-3 candidates using AES or similar parts see e.g. this paper.
Aug
28
comment Is there a way to make RC4 (ARCFOUR) secure, or is it completely broken?
@Kevin Jin: Do you really need to authenticate each tiny packet? This could get really expensive. For 128 bit security you need to send 128 bits... but if your program aborts after 10 failed attempts, you'd need less. That said, I'd look for a complete solution, running your own is too risky.
Aug
24
comment How do ciphers change plaintext into numeric digits for computing?
HTTPS uses a symmetric cipher, which one is subject to negotiation between the parties. It's all quite complicated, but for sure it doesn't use RSA for the whole communication as this would be extremely inefficient.
Aug
20
comment Trying to find an algorithm to share portions of a key with multiple people
For implementation, I'd suggest using GF(256) over prime fields. It has no size limitations, works bytewise, need no padding, no multiple precision arithmetic, and is probably faster and easy to program (even I did it:D). It limits the number of shares to 255, which should suffice.
Aug
19
comment What are the methods to construct a primitive binary nonlinear feedback shift register (NLFSR)?
@Bluemilk: I'm quite sure there's no such thing as primitive binary NLFSR, since the word primitive comes from "primitive polynomial over GF(2)" which makes sense for a LFSR only. That said, I can live with your definition. I'm afraid the search algorithm was just some smarter exhaustive search... given the prescribed form, it looks possible to me.