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9

AES is deemed secure because: Its building blocks and design principles are fully specified. It was selected as part of an open competition. It has sustained 15 years of attempted cryptanalysis from many smart people, in a high-exposure situation, and it came out relatively unscathed. Another reason, which is not as good but felt important by many ...


8

Randomness is not a property of strings of bits (or characters of any sort). Rather it is a property of the process that generates those strings. However, it is convenient to conflate the string with the thing that produced the string, and thus to speak about strings being 'random' or 'not random'. The string '00000', for example, is random if it was the ...


6

It fails to be a cryptographically-strong PRNG because it is predictable: given some outputs, you can predict the next outputs. For instance, if you observe the outputs at offsets 0, 1, and 4096, you can predict what the output will be at offset 4097. What it's missing: it's not that it's missing some little tweak (just change line 7 to use addition ...


4

The two primary techniques I'm familiar with is structuring a cryptographic primitive as a sequence of games and the universally composable security framework. Sequence of Games The idea here is to represent a protocol/primitive as a game played between an attacker and a challenger. You define a bad event and show through the game that the event happens ...


4

The encryption scheme in the experiment you describe does not have to be fixed-length. We simply require that the two messages the adversary sends to its oracle have the same length. The restriction is on the adversary, not on the encryption algorithm. So why do we put this requirement on the adversary? The reason is that in every practical encryption ...


4

To be concise, true randomness boils down to the selected data being causally unrelated. That is, if each piece of data is the result of no common cause, then there is no relation by which the rest of the data can be predicted or inferred. So being unpredictable is a consequence of being truly random, but it is the lack of causal relationship that is the ...


4

By a generic attack we also understand an attack that with minimal corrections would apply to every block cipher. For example, suppose you have a (plaintext,ciphertext) pair and test keys by exhaustive search: you apply the keyed cipher $E_K$ to plaintext $P$ for every $K$ and check if you get ciphertext $C$ in response. Quite often, the ciphertext bits ...


3

With this amount of information it is hard to advice. Previously when somebody asked about new algorithm he had produced, he was answered: Answer to Where could I submit my algorithm?. No matter what kind of cryptographic work, generally large part of that answer applies. If we knew anything about the work (like on which existing algorithms or problems you ...


3

Super-Pseudo-Random A function family is super-pseudo-random if no polynomial time adversary can tell the difference between a function from the family and a real random function, given oracle access to the function and its inverse. (As a practical example: block ciphers are typically modeled as super-pseudo-random permutations.) So, defining it a bit: a ...


3

An efficiently computable Permutation Ensemble is (Weakly) Pseudo-Random if and only if it is infeasible for an adversary with oracle access to [a function that was chosen either from then Ensemble or uniformly from the set of all permutations on bit-strings of the corresponding length] to distinguish between those two cases. An efficiently computable ...


3

Randomness is the information loss of any causal relationship between events. The universe needn't be a clockwork universe for the assumption of pervasive causality - if events are "sticky" and accrue localised causality in the same way that a molecular cloud accretes into stars and planets. The underlying cause of the speed of light might also be the prime ...


3

Does anyone have a reliable source for this? Well, you are asking about the definition of a CSPRNG, and whether this second criteria is a necessary part. Well, it comes down the to exact definition of the term 'CSPRNG'. If we define a CSPRNG as something that generates output which is indistinguishable from random (your first criteria), then a ...


3

Advantage and success probability are just words. Their meaning is in practice decided by how the speakers of the language use the words. You have observed that people use the terms advantage and probability in this way. One could probably argue that this is confusing or illogical or something like that, but such is language. About dividing by two: ...


3

There is no hard problem to which AES can be provably be reduced. It is believed to be difficult to break because lots of smart people have tried for more than a decade now, using the best (publically known) techniques, and so far the only successes have been marginal improvements compared to brute force.


3

You had your finger on it, you do know something about the encryption of two messages of different length before they are actually encrypted: the length of the corresponding ciphertexts. If the setting in which you're using your encryption scheme allows for a maximum message length then you can always pad to make every ciphertext the same size ...


3

The standard definition of existential forgery allows the adversary to ask and obtain the signature of any message she wants, and claim success if she can exhibit (with sizable odds) any acceptable (message, signature) pair, for any message for which she did not ask signature. Update: There is also strong existential unforgeability, where the adversary ...


3

Reductionist security In a reductionist security proof for some cryptographic protocol $\Pi$ to some alleged hard problem $P$ means, that we can build an algorithm $\cal B$ for solving $P$ if we have access to a hypothetical algorithm $\cal A$ that efficiently breaks the security definition for the protocol $\Pi$. In general, showing a polynomial time ...


3

As explained in a comment: A generic attack is one that works against all block-ciphers (with a given block and key size), without consideration about the structure of the block-cipher. One generic attack for a block cipher of a given block size $b$ bits builds a dictionary of input/output pairs (e.g. from past plaintext/ciphertext), for a fixed key. When ...


3

How secure is this cipher? At first glance, not very. It would appear to be vulnerable to a ciphertext-only attack, for example, the attacker can recover the plaintext given a ciphertext of about 10k (actually, he probably can deal with less), even assuming that all the attacker initially knows is that the plaintext is "ASCII English", and he has no ...


2

Suppose Alice has $x$ and Bob has $y$ in your scenario, and let $\pi =(\pi_A, \pi_B)$ be the protocol machines for Alice & Bob respectively. Here is how you would formally define security of the protocol against a corrupt Alice. Define the following algorithms / random variables: ${\sf Real}(\pi, y,\mathcal{A},1^k)$: Internally simulate an instance ...


2

You can probably prove the security against your game from the security in the IND-ID-CCA game of Boneh and Franklin (see http://courses.cs.vt.edu/cs6204/Privacy-Security/Papers/Crypto/IBE-Weil-Pairing.pdf). The idea is to create an adversary $\mathcal{B}$ against IND-ID-CCA from your adversary $\mathcal{A}$. Essentially $\mathcal{B}$ will play ...


2

Do you have a specific application domain in mind? I do not know of any formal definition that spans multiple application domains. A formal definition of Perfect Forward Secrecy for the domain of key exchange protocols is included in this paper: Beyond eCK: Perfect Forward Secrecy under Actor Compromise and Ephemeral-Key Reveal


2

In a signature scheme with appendix (such as RSASSA-PSS), the signature $s=Sign(M,PrivateKey)$ of the message $M$ is usually appended to the unmodified message $M$, forming $(M,s)$. This is effectively sent, and verified; the signature is an appendix to the message. Signature scheme with appendix opposes to signature scheme with message recovery (such as ...


2

In the context of the original question, what you're comparing your stream cipher to is a particular probability model. That model has each bit have probability 0.5 of being a 1, and has that probability be independent of the bit's position in the string and any surrounding bits. It's the kind of source you would get if you flipped a fair coin to determine ...


2

As with any research job, I can see two main ways that might allow you to find your work meaningful: Experts like it Public academics Company/state secrets The public uses it The "correct" answer is that if you have come up with a great new result, you should try and get it published in one of the major conferences or similar event. Alternatively, if ...


1

This sounds like a lot of work. I would break this down into parts. First, I would try to identify the random number generator being used. From that, my goal would be to determine the starting seed, which would let me accurately predict future results based on past results. I'd start with the NIST Random Number Generation statistical tests In them, ...


1

The problem with new cryptographic algorithms is the amount of time and effort it takes to convince people of its security. There are many factors working against new development: Existing ciphers have been subjected to intense scrutiny, and yet are barely trusted. Changes to existing ciphers or protocols are viewed with suspicion. Is there a back door ...


1

This depends on the encryption algorithms in use. If the encryption algorithm is a bijection, then every possible $C$ must have a preimage. Examples of this include block ciphers like AES. In these cases, the decryption algorithm will output a message $m$ such that $E_k(m)=c$. Note that if $c$ has been tampered with by an adversary but is still the same ...


1

Here's a couple of useful sources for definitions of CSPRNG or DRBGs (Deterministic Random Bit Generator). Check out the NIST documents for CSPRNG: http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf http://csrc.nist.gov/publications/drafts/800-90/draft-sp800-90b.pdf http://csrc.nist.gov/publications/drafts/800-90/draft-sp800-90c.pdf B and ...


1

I think there are two issues involved here: How do you tell if a ciphertext has the properties of a truly random stream? How do you tell if a stream actually is truly random? For the first part, there are many statistical techniques. But the basic question is whether there is any detectable relationship between the plaintext and the ciphertext. If ...



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