Reputation
14,185
Next tag badge:
109/100 score
19/20 answers
Badges
1 14 61
Newest
 Curious
Impact
~295k people reached

Mar
14
comment How do you break encryption on a message?
Your questions seems really broad, perhaps too broad to be answered well on a Q&A site like this. What you really need is an introductory book on cryptography and cryptanalysis. Here's a few choices to get you started.
Mar
12
comment Beginner question about secure communication with one user using DES
Is that really the exact question asked? I suppose the users could implement a Diffie-Hellman key exchange, if that's allowed, and then derive a DES (or better yet, triple-DES) key from the DH result. But without some kind of a prior shared secret (or asymmetric key pair), there's no way for the users to know they're talking with each other, and not with some middle-man.
Mar
12
comment Implementing modular reductions (n*n)
Reduce modulo $n$, subtract remainder from input, divide by $n$, reduce again? You now effectively have the last two base-$n$ digits of the input.
Mar
11
comment Repair AES-128 decrypted file
Can you tell us the exact openssl command you used? In particular, which cipher mode did you use, and did you use a salt?
Mar
11
comment Encrypt-then-MAC: Do I need to authenticate the IV?
Congrats for hitting the Hot Network Questions list, by the way. :)
Mar
11
comment Is the 1st Encrypted Block Less Secure Than Subsequent Ones?
Related: crypto.stackexchange.com/questions/5421/…
Mar
11
comment Composing two one-way functions such that the result is not a one-way function
Ps. If you really must have two distinct one-way functions, you could always, say, use $1^{n/2}$ instead of $0^{n/2}$ for one of them.
Mar
11
comment Is $f(x)\oplus x$ a one-way function?
Just FYI, I borrowed your construction for a (community wiki) answer to a related question.
Mar
10
comment Encryption scheme that allows compare ciphertexts based on the clear text
I've voted to close this as a duplicate (since the literal answer to your question is basically "this is called order-preserving encryption"), but +1 for a well-asked question.
Mar
10
comment Known plaintext, unknown 128 bit block cipher
Ps. Related, possibly duplicate: crypto.stackexchange.com/questions/997/…
Mar
10
comment Key length requirement in a simple XOR implementation
Yes, it should work, because of the % klen part. (Exercise: what does that part actually do?) You could've also simply tried it yourself on some test data. (In any case, I'm voting to close this question as off topic, since it's really more about basic programming rather than cryptography. It would be a better fit for Stack Overflow.)
Mar
10
comment Known plaintext, unknown 128 bit block cipher
Alas, the one thing the paper does not seem to include is any kind of demonstration that their method actually works. In fact, for modern block ciphers with the same block size, I can fairly confidently assert that it doesn't -- if it did, the authors would be famous. (They do suggest that their program may also use side channel information, so it's somewhat plausible that it could e.g. be able to distinguish DES from Blowfish based on the time needed to encrypt a message; but that'll only work if such timing information is actually available.)
Mar
4
comment Is CBC theoretically harder to brute force when compared with ECB?
You're talking about a scenario where the attacker can obtain many messages encrypted with different keys, and only needs to break one key, right? I agree, ECB is (slightly) weaker than CBC in that case, at least as long as the IVs for CBC are properly chosen (i.e. unpredictable by the attacker).
Mar
4
comment AES mode scheme feedback
For the second part, see crypto.stackexchange.com/questions/3229/…
Mar
4
comment Koblitz encoding a message to a point, what is the “associated auxiliary base parameter”?
That said, I took a closer look at the Kodali & Sarma paper to see which EC cryptosystem they're actually using, and I couldn't make any sense of it -- it looks as if they're effectively just running a symmetric Caesar cipher over an elliptic curve (after first doing ECDH key agreement). If so, that still makes absolutely no sense to me; it's not semantically secure, and anyway they'd be much better off just feeding the ECDH secret (computed over a secure curve, not the tiny one they seem to be using) to a KDF and using it to key a standard symmetric cipher, like normal people do.
Mar
4
comment Koblitz encoding a message to a point, what is the “associated auxiliary base parameter”?
Yeah, sorry, ignore that bit about padding, that was basically a brain fart. I was thinking (if I may use the word) about RSA-like schemes, but EC schemes (like the related discrete logarithm based schemes) generally don't need it, because they have their randomization built in.
Mar
3
comment Predicting Google Authenticator OTP Codes
I think this question would need a lot of editing to be a good question for Crypto.SE. For one thing, it definitely needs to lose the "infinite amount of time" part: almost every cryptosystem is trivially breakable by an attacker with infinite computing time. The real question is, can it be broken by a feasible attacker bound by the known laws of physics? In any case, given the age of the question and the lack of attention it has received, I've simply voted to close it.
Mar
3
comment Vigenere ciphers : Need help for math analysis
On a tangent, the method you describe appears to be equivalent to the autokey cipher (a form of which was actually described in the writings of Blaise de Vigenère, unlike the cipher nowadays commonly bearing his name).
Mar
3
comment How to choose the integer m in the general number field sieve (GNFS)?
While this question would be on topic for Mathematics (as it does not use any crypto-specific terminology), I'd say it's also sufficiently closely related to crypto (seeing as it's essentially about optimizing a cryptanalytic attack on RSA) to be borderline on topic here as well.
Mar
3
comment How to compare between two cryptographic algorithms in terms of security?
@SHdotCom: I've edited your question to clean up the grammar a bit, and to include some information from your comment above. However, if you'd like to see it reopened, it would help if you could edit it yourself to clearly answer Maarten's questions above (and to correct any mistakes I might have accidentally introduced). In particular, based on your comments, and the use of the word "hash" in the original question, I'm assuming that you're specifically asking about hash algorithms, but it would be good for you to explicitly state that.