# Tagged Questions

Security analysis is the examination and evaluation of the various factors affecting the value of a security solution and/or implementation .

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### What stops the Multiply-With-Carry Random Number Generator from being a Cryptographically Secure Pseudo-Random Number Generator?

Despite the fact that Marsaglia's MWC PRNG (multiply-with-carry random number generator) is considered to be "the mother of all RNGs", it does not seem to be considered to be a CSPRNG ...
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### How good is the RandomPool generator in Crypto++ library?

How good is the RandomPool generator in Crypto++ library? I never heard of it before… I wonder if there is a Fortuna C implementation that I could use in an Objective-C project. Also: How secure is ...
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### Is $q(n)=1/n$ a negligible function?

By definition - $q(n)$ is a negligible function if for every positive integer $c>0$ there exist an integer $N_c$ such that for all $x>N_c$ : $q(n)<1/x^c$ So for the function $1/x$, if we ...
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### Difference between computational and statistical indistinguishabilities

What is the difference between these two notions?
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### order between adversaries and type of resources given

We have a game $G0$ for which an adversary $A$ has access to a certain amount of ressources. Let us suppose that the maximum advantage for the adversary to win this game is $Adv_{G0}$. If we modify ...
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### Ideal system for an encryption scheme

What is the ideal system for an encryption scheme? For a pseudorandom permutation the ideal one is a random permutation, for a pseudorandom function the ideal one is a random function. For an ...
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### Dependence on Keyed Hash Function

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...
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### No Birthday Attack to TCR

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” , which compared TCR (Target Collision Resistant) and ACR (Any collision Resistant). It says we wish to stress ...
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### Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
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### Security based on PRF

I've seen that there is several kind of PRFs. Sometimes people speak about strong PRFs. When proofs of a protocol or algorithm are given based on the assumption of the use of a PRF, they speak ...
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### What's wrong with my security model - or how can it be attacked?

I'm working on a little personal project where security is very important, and I've thought for a while about the best way to do this. Now I am looking for a little outside help on good practices. The ...
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### Protocol/algorithms based on a variable-length input PRF

Are the proofs based on a PRF assumption still valid when using a variable-length input PRF ? The answer is maybe obvious but I have a doubt.
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### Verification of identity by certification authority

I have question "How user's identity by Certification Authority " is verified. While understanding, I am considering the following mathematics.. User generates his private key and with the help of ...
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### What are the pros and cons of deterministic site-specific password generation from a master pass?

I've been reading up a bit on deterministic password generators. All the ones I can find basically do something like this: Pick a master username and a master passphrase. The username could be your ...
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### A block cipher with independent keys for each round

I want to modify a standard block cipher in the following way. I replace each round key by a key picked at random. Is this block cipher as secure as the original one ? Thank you. EDIT Some missing ...
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### Cryptanalysis and weaknesses of SEED cipher

I've discovered that a client has SEED enabled in their SSL ciphers, and would like to know a bit more about it from a security perspective. The Wikipedia article doesn't mention any flaws, yet I've ...
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### Is this “layered” XOR cipher secure?

I am aware of the vulnerabilities of simple xor ciphers and recently I came across a block cipher that claims to solve those vulnerabilities. The cipher goes as follows with both the block and key ...
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### Is this EAX extension weakening the (provable) security of EAX?

I would like to insert a key deriving function into EAX mode, in order to hamper brute-force attacks for a key-size restricted cipher (56 bits). The modification inserts an identical multi-block ...
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### Repeated Encryption using same/different Encryption systems [duplicate]

I would like to know if there are any security risks/gains from encrypting twice, i.e. $$\text{plaintext}\xrightarrow{encrypt}\text{intermediate text}\xrightarrow{encrypt}\text{ciphertext}.$$ I would ...
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### how much trust can we place in protocol verifiers? [closed]

I read many papers on authenticated key exchange protocols. Most securitys proof are done by the authors. In this method, you can imagine that the efficiency is low. Moreover, even if you have proven ...
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### Is prime regeneration necessary for every new session using a Diffie-Hellman key exchange?

As I understand it, bruteforcing a Diffie-Hellman generated secret key takes $P-1$ attempts to crack the shared secret, where $P$ is a very large prime used for modulus. If your $P$ is quite large, ...
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### Which platforms are most vulnerable to Simple Power Analyse Attack

In a "Simple Power Analyse Attack", the difference in power consumption during multiply or square operation is used. In the RSA decryption process with the "Square and Multiply" algorithm, both ...
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### Is this really the correct way to do an IND-CPA test?

In a description of IND-CPA (indistinguishability under the chosen plaintext attack), I have been reading the following, simple test: The adversary can generate as many messages as he wants. Then, ...
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### Can padding length, and thus plaintext length, be considered public information when using aes-cbc?

Right now I'm using a simple padding system that takes the plaintext, hashes it with sha512, and appends x bytes of the hash as padding where x is the delta between the plaintext and nearest blocksize ...
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### Is there an automated security protocol verification tool?

I want a tool that for automated verification of a protocol in the eCK security model as described in Microsoft Research's paper "Stronger Security of Authenticated Key Exchange" . Does anyone know ...
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### Hash function based on block cipher (and proof of security in the PRP model)

I wonder if there exists proofs of security of primitives like hash function (based on block cipher) in the PRP model. I often see proofs in the random oracle model (for hash function based on ...
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### Problems about Security proofs? [closed]

When it comes to designing new PKI-based key-exchange protocols, why are security proofs needed? Without them, can we show a protocol's security? Does there exist a PKI-based key-exchange protocol ...
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### What is it meant by a “hybrid argument”?

Can anyone explain (or point to a reference) what is a hybrid argument in a security proof, and when it's convenient or preferable to use it? Among some of the places where I've seen it mentioned, ...
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### The specification of modern, non-communicating cipher machinery [closed]

I'm looking for references, papers or guidance on a class of system which I have not seen widely discussed. Consider powerful electronic machinery which remains in the custody of communicating ...
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### Concerns about using a hash to determine if specific input data has already been encrypted

A system is used to securely store account numbers. When an account number is added to the system it is securely encrypted and stored in a database along with the IV and a reference to the encrypted ...
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### Hash function based on pseudorandom functions and security

Are there hash functions that make use of pseudorandom functions. Precisely, I'm looking for a specification of a hash function based on PRF (and based on the security of such a primitive).
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### Pseudorandom functions

The usual case to distinguich a pseudorandom function from a random function is to assume that the adversary can choose the plaintext blocks. Is there another case (game) in which the adversary can ...
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### Is a small size block cipher usable?

Consider a block cipher with a key of size 128 bits but a small block size, say 32 bits. Is this kind of block cipher secure? I would like to know to what extent I can use a small block cipher, like ...
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### Have these compositions of block ciphers the same security?

I'm interested by the compositions of the block cipher DES, instantiated with independent keys $k_1$, $k_2$ and $k_3$. Are these 3 compositions equivalent in terms of security? ...
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### Proofs of security methodologies

I'm looking for course material on the subject of proofs, reductions, and games, as used to prove cryptographic schemes secure. What are the methodologies? What are the preferred ones? In what cases ...
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### Composition of block ciphers and 3DES

There is some intriguing things about DES and 3DES. Now, I know that DES is weak and 3DES was an attempt to construct a more secure block-cipher from a deprecated one. Having this in mind, what ...
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### Secure use-cases of block cipher with 64-bit block size

In what cases can we use a weak block cipher like DES ? More precisely, Are there specific situation in which a weaken block cipher can still be used, for instance for certain types of plaintext ?
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### Acceptable assumptions when prooving security

Considering the output of a cryptographic primitive, like an encryption scheme (CBC, ...), a hash function or even the output of any schemes based on number theoretic assumptions, is it reasonable ...
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### Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
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### Initialize a PRNG with a password

Let's assume that we have a secure PRNG. Is it "safe" to initialize it with password, or seed based on a password like SHA256(password). If yes, is it "safe" to generate as RSA or DSA key from it? If ...