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Feb
4
comment How many polynomials are used in Secure Multiparty Computation?
The question is too broad, in the sense that the only answer can be "as many as your algorithm or protocol requires". It is like asking "how many stones do you need to build a house?" without stating the size or the number of appartments.
Jan
13
comment ElGamal with elliptic curves
You're right, it doesn't work for every $x$. It was a simplified version of the paper's function $(2)$ in section $3$. Basically they suggest for $m < q/1000 - 1$ and then look for $x$ with $1000m \leq x < 1000(m+1)<q$ and a solution for the curve equation. It's a probabilitstic embedding, which still has a chance that there is no sulition in the interval. The inverse can then be calculated by dropping the last 3 decimal digits of x..
Nov
30
comment Using a SHA512 hash to encrypt data
There is evidence for frequency analysis in the 9th century by Al-kindi. Vigenere was broken in 1863. And that breakthrough was how to find out the key length. The rest was just using techniques, which were a millenium old already. That was the problem, even if it isn't any more today with our current knowledge.
Nov
30
comment Using a SHA512 hash to encrypt data
I've never seen the term "crip" used in any serious cryptanalytic publication. Mostly, because it is such a imprecise expression. It is much better to talk about frequency analysis (uni-, bi, trigrams), frequently used (or expected) words and phrases (like in the enigma attacks), which is a form of known plaintext attack (with high probability).
Nov
18
comment Using a SHA512 hash to encrypt data
Vigenere would imply that the keylength is unknown, which was for a long time the main problem, when frequency analysis could already be used to break Caesar.
Nov
17
comment Are all encryption tools made equal?
Well, "uses SHA256" is a good start. But if it is just a single hash operation, dictionary attacks are still on the table. If you use a password or passphrase, you have to use a construction which is meant for low entropy input, such as passwords. And PBKDF, bcrypt, scrypt, etc. are such constructions.
Nov
17
comment Is it interesting to design stream ciphers parametrized by a CSPRNG?
The security definitions for stream ciphers and CSPRNG are pretty much the same: The attacker should not be able to predict the next bit according to previous keystream he should not be able to guess the internal state, etc. The differentiation CSPRNG and stream cipher is mostly just a different point of view in the application/protocol/goal. Security-wise, they are the same. So your trivial answer is "no, it would not be interesting".
Nov
17
comment Factorizing N to derive D
"Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime." Generally, for homework questions like this pointers towards the correct way are much more useful than just handing over the result.
Nov
17
comment Prevent replay attack without storage
Okay, maybe 8 bit cpu's and crypto work together, but he also said low power sensor unit. And considering that he doesn't want to save session information because of limited memory, doing public key crypto in memory is unrealistic. For a simple square and multiply for RSA you need at least two elements in memory, so that's at least 300 bytes. Quite a lot if you can't afford 32 bytes for a session key.
Nov
16
comment Does complicating a flawed algorithm make it secure?
I doubt, most of these comments and answers make much sense for the author of the question. In short: If you're not an expert in cryptography, then most likely don't improve security at all - not even a tiny bit. "It looks different" is not a suitable measure for security. Even classical ciphers achieve that. The most basic rule for using cryptography: "Don't implement it yourself". If you're a software engineer, the best way to handle this is by designing the system s.t. you can exchange your cryptographic library if required.
Nov
9
comment What is the difference between “securely realizes” and “securely implements”?
The quoted part is exactly what it is in the contxt of MPC. It is two different points of view of (mostly) the same thing. Internal vs external.
Nov
9
comment Why do cryptographic hashes need to be fixed length?
Fixed length is no special property from the cryptographically secure part. It comes from the more general specification of hash functions. In fact, it is the defining property of hash functions: "A hash function is any function that can be used to map data of arbitrary size to data of fixed size" (Wikipedia)
Nov
9
comment Three party coin flipping protocol with only 2 active participants
This is unachievable. From Carol's point of view: She isn't involved in the protocol execution (except possibily some initial input). She doesn't trust Alice and Bob (they could in fact be just one party). And she has to trust the result of the coin toss somehow, while having no influence or ability to verify anything. Without further specification this is impossible.
Oct
28
comment ElGamal Generator g problem
If ElGamal would be defined today, it would require a Schnorr group, with "find $h$, s.t. $h^r \neq 1$". In the original paper it actually was a generator, if I recall correctly. Anyway: $1$ as $g$ is always a bad choice, because every public key will be $1$ then. And not just if you choose the elements order (or a multiple of it) as private exponent.
Oct
28
comment How to toggle a bit homomorphically ?
OR is not an addition. The addition is XOR. In order to ensure that true and true returns true, you need to have something like $A + B + AB$. Which also contains a multiplication. NOT gates are easy: Just use XOR and the constant 1
Oct
23
comment How bad is it to use the identity function as hash for ECDSA?
"...that your customer is stupid???" Request on mailing list doesn't imply customer. And then no one said stupid, which sounds quite harsh. But as a hint to the sad truth about cryptography: Most people don't understand it, and unless studying crypto extensively, they also have no clue what damage can be done by easy beginner's mistakes. There are countless self-proclaimed "experts", which come up with new schemes and basically re-invent things, which have been dismissed for 30 years, maybe from a different point of view, but with the same weaknesses.
Oct
23
comment Is there an additively homomorphic encryption scheme that supports calculating a square root on the ciphertext?
There is a substantial difference in using square roots over reals (with approximations, etc.) and square roots in finite rings and fields. $2^{1/2}$ mod $7$ equals both $3$ and $4$. It has nothing to do with $\sqrt{2}\approx 1.414$
Oct
23
comment Is there a format preserving cryptographically secure hash?
This is not about the construction of FPE. It is about creating a cryptographic hash function (and utilizing FPE to do so). Cryptographically secure hash functions don't protect against brute force (of small message spaces) at all. It is about collision resistance and preimage resistance. If short message spaces are a problem, you should use a construction with a key anyway (e.g. MAC).
Oct
19
comment Can it ever be impossible to invert a PRNG?
The entire argument for "impossible to uniquely invert" is not relevant for the cryptographic strength with emphasis on "uniquely": If an attacker can exclude only a portion of the possible states, he might already be able to predict the next bit with a non-negligible probability, breaking the algorithm in the cryptographic sense. Finding out the exact internal state is not required for that. As a rule of thumb: CSPRNGs and PRNGs are very different things, and the similar name is very misleading. Don't mix them up, it doesn't work.
Oct
19
comment Deciphering “easy” ciphers without hints
Well, without the actual challenge, it's quite difficult to point in the right direction. If the only "flashy" value is 81, and 3,9,27 are just average like the rest, then it is unlikely that 3,9 and 27 are correct key sizes (Factors of the correct key size don't show deviating values, just the multiples). A Vigenere cipher with a key of length 81 sounds unlikely, but not impossible. But anyway, you have found some characteristic, that means you're on the right track.