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Jan
15
comment Comparison among algorithm based on key length
Ah, you're right... I'll correct that.
Jan
13
comment Can you help me with this Random Number Generator function?
In that case, yes. But in the question is an example of how he wants the output... and that is the full list of elements.
Jan
13
comment Can you help me with this Random Number Generator function?
The required memory of the algorithm is the same as the data of the output. If you can't handle that in memory, how can you handle it in the output... this just doesn't make any sense.
Jan
13
comment Difference between Hash function and Random Oracle
This answer misses the connection to the referenced paper: In that paper, they do not use random oracles to show anything. They use the collision resistance property of a hash function. The Proof of security is not in the random oracle model, if you don't use random oracles in the security proof. And they don't.
Jan
13
comment Can one deduce the symmetric key size used to encrypt a given a piece of ciphertext?
In that link there is plenty of information to answer your question, just read it again carefully. Anyway, usually the key size is fixed in symmetric encryption algorithms, and therefore we give that information to the attacker anyway. If you assume that this isn't the case, your attacker model is weaker than "ciphertext only"... which is already considered too weak for today's standards.
Jan
13
comment What is the importance of adding round key in AES
It is not about "complicated enough". If you are given a (fixed) single bit and then combine it with some other bit with XOR, you get a bijection: input equals output or input is unequal to the output. OR does not have this property, if one of its inputs is True. In general, XOR has much nicer properties than logical AND and OR.
Jan
13
answered Comparison among algorithm based on key length
Jan
12
comment Is it possible to use structures other than finite fields?
It is the other way around: In continous structures, there are no "hard to inverse" problems, as far as I know. In the mathematical sense, there "is no inverse" if either it does not exist or the inverse is not unique. But this is something else than "you cant compute the inverse". And then there is the fact, that we usually deal with discrete elements e.g. 0 and 1.
Jan
12
comment P = NP and current cryptographic systems
General statements about complexity classes should not be mixed with actual numbers. Even if breaking RSA was possible in $P$ and expressed in some polynomial in $O$ notion, this does not mean you can derive 1024 is unsafe and 3072 bit is safe. First, P contains any power over $n$. Second: This still does not put a relation to computation power, time required and bitsize. There are constants (or better: coefficients) to required, to get the actual relations.
Jan
12
comment Using hash of plaintext as nonce in NaCl secretbox- bad idea?
The context is crucial in this context. If you use the nonce in a challenge-response way, then it is quite bad to make it deterministic in dependency of something else. This is an example for the "do you need the only used once" property. Something like this can be weak against replay attacks, for example.
Jan
12
comment subgroup of quadratic residue
It would help if you tried to use the built-in math formating of the forum. So you got $\mathbb{Z}_{23}^*$. This has cardinality 22. And therefore, there can only be elements of order 1,2,11 and 22. I have no idea how you get to 10 there, and it is unclear to me, what you are aiming at... Besides, this is actually a purely mathematical question, and probably should be over at math.SE instead of here.
Jan
9
answered subgroup of quadratic residue
Dec
18
comment Does RSA operate over a Finite Field (Galois Field)?
While the correct answer to the question is "no", the RSA definitions also can work for those elements, which are not coprime to $n$. Since they are zero divisors, they dont have inverse elements, but it can still be true that $x^{ed} = x$, with $x$ not coprime to $n$. I cant recall the exact requirement there, but there are topics on this on crypto-SE.
Dec
18
comment Attacker in a key exchange
It looks like the notation for symmetric encryption in this is: $(X,K)$ for plaintext $X$ and key $K$. It is an awful notation tho, because it should either be something like $E(X,K)$ or $(X)_K$ to be less confusing. And the same notation is used to denote tuples.. guess what is what.
Dec
18
comment What would be the best plain text Cryptography method without the use of a computer?
Let me rephrase: People call all sorts of things "OTP", which aren't OTP but streamciphers. And if you use OTP, you can just store the plaintext instead of the key, because OTP does not consider key storage. Simply put: Using OTP is almost always entirely pointless. It cant be much terrible than that, because with RC4 you have weak security, but at least you could just store the short key for a longer message. Same goes for sending messages: If you can somehow transmit a key of equal length securely, then you could just send the real message securely instead.
Dec
15
comment What are the potential (major) flaws in this security scheme?
The checksum does not serve any security purpose in this example, unless Alice and Bob also have a shared key to use a Message Authentication Code (MAC; basically a cryptographic checksum with a key). As poncho pointed out in his answer: Your problem is, that Alice and Bob don't know anything about each other, and there is no available PKI for authentication. None of them will ever know, if they are talking to someone called "Alice", "Bob" or "Eve".
Dec
15
comment Why do we have fixed output length in the algorithm SHA1
SHA1 is a standard hash function. Hash functions are defined to have a fixed size output.
Dec
15
comment What would be the best plain text Cryptography method without the use of a computer?
It is unrealistic to expect today's standards to be met while not using a computer.
Dec
15
comment What would be the best plain text Cryptography method without the use of a computer?
OTP is quite terrible from a usage point of view. Because instead of storing the key, you can just store the plaintext (key has the same length as the plaintext). And it is crucial for OTP that keys are not used more than once. Shorter keys or repeating keys are not OTP - and you can not apply OTPs security definition.
Dec
15
comment Malleability of homomorphic encryption
In one direction, there is nothing to show: Being homomorphic implies already some meaningful relation (e.g. you can always compute $2 \cdot m,3 \cdot m,...$). And for the other direction: The expression "meaningful relation" is not specific enough to achieve a homomorphic relation between ciphertexts.