# All Questions

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### Is the security of an HMAC equally important as the security of the cipher itself?

I'm a little unclear on how hash based MAC works... So I would like to know, is the security of your hash function as important as the security of the overall cipher? Basically, if you can crack the ...
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### Is (1 bit HW signal) XOR (8 bits PRNG output) sufficient for a HWRNG?

This is really a follow up question to a comment made regarding How to roll a HWRNG In the situation of whitening a hardware generated random bit stream, "XORing the two bitstreams should be fine ...
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### How is the ciphertext labeled with a set of attributes in Key-Policy Attribute-based Encryption (KP-ABE)

My question is about Key-Policy ABE (KP-ABE). What does it mean when a "Ciphertext is Labeled with set of attribute"? How is it labeled?
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### Attack on textbook RSA with small messages

We learned in class that if Alice wants to send a small message $m$, such that $m < N^{\frac{1}{e}}$, so we can break the system using $e$-th square of $m^e$. But what if Alice knows this flaw and ...
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### NMaster key from n secret key

I have to derive master key from N secret key. With the help of master key, i can collect all the secret keys which is used to derive master key. key1, key2,..keyN ==> single master key and single ...
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### Message security and also program copy detection

Imagine I have client and server running on different machines, each as a separate software. I want to ensure: message authenticity, integrity and confidentiality at least of messages coming from ...
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### Modular Arithmetic in RSA

Consider the following the following RSA public key $pk = (N, e) = (1457, 1307)$. (a) Knowing that $187^2 \equiv 1 \pmod {1457}$ find the factorization of $N$. (b) Given the factorization of $N$ ...
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### Implicit authentication aritcle

First, I want to know what is the logarithm used in the following system of equations (from the paper Flexible and Robust Privacy-Preserving Implicit Authentication - Domingo-Ferrer, Wu, Blanco-...
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### How to force non-negative int64_t output for blowfish encryption with int32_t input?

I would like to encrypt a series of small integers one by one. The range of them is about 0~10^8. The output must be non-negative int64_t integers. Since Blowfish is 64-bit cipher, I prefer to use it ...
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### Is this MAC scheme secure?

Say we have a computationally secure Message Authentication Code scheme (Gen, Tag, Ver). Let $Tag'_k(m):=$ first half bits of $Tag_k(m)$. Let's assume the range of tags is $n$bits. Is (Gen, Tag', ...
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### Key Derivation Function (KDF): Can a key derived from KDF be considered as a secure key?

Consider a case where we have a master key MS that is used in pseudo-random function to generate a set of pseudo-random values. Then we use key derivation function to derive a key from each of pseudo-...
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### State of the art techniques for Anonymous Authentication?

What are the state of the art techniques for Anonymous Authentication? What are the most popular ones? The problem domain I am considering is with respect to different users accessing cloud content; ...
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### Is a hash “reset sequence” possible?

Is there such thing as "hash reset sequence" that brings hash to the same known state? A hash function has a state. In MD5 and others vulnerable to the length extension attack, the attack works by ...
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### Confused with 128-bits AES round numbers

The key size used for an AES cipher specifies the number of repetitions of transformation rounds that convert the input, called the plaintext, into the final output, called the ciphertext. The number ...
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### Easy hash function collision

I'm designing a cryptography-breaking assignment for a college-level introductory security course, and I'm looking for a hash function which is reasonably easy (but not too easy) to generate ...
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### Symmetric-like cyphers with several steps

Are there any good symmetric-ish cyphers that involve three or more steps with independent key material for each step? Formally, there should be a function $K(s)$ that returns a tuple $(k_1,..,k_n)$ ...
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### Does Mersenne Twister generate homogeneous numbers?

I have been assigned to generate 15000+ homogeneous and uniform random numbers for a work. I have been searching if Mersenne Twister generate homogeneous distribution of numbers but I don't find ...
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### Proving the security of a one-way function with partially known input

Let's say we have a construction like in this question: We have a OWF $h(.)$, a secret salt TXT, and a counter starting at 1, and we compute ...
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### Weaknesses of RFC6628

In RFC6628 section 2.2, they say: w = The password remembered by the user. W = The password verifier registered in the ...
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### Inverting One-Way Functions

One of the conditions that a one-way function has to satisfy is the following: $$Pr[A(f(x))\in f^{-1}(f(x))] \leq negl(n)$$ Now, suppose that we have the following function that's not one way: f(x)...
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### Secret only known on consensus

Does a method exist where a secret is generated so that is is unknown until 3 or more people (predetermined) reach consensus that it should be revealed?
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### How does perfect hash function works?

According to wikipedia, a perfect hash function is a hash function that uses algorithms that has a certain random aspect to their logic. It is suppose to be collision-free. However due to the pigeon ...
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### Is my protocol that uses hybrid cryptography and AES-GCM secure?

I'm having a lot of trouble trying to understand how to implement AES-GCM. My usecase: Person A sent me a file with a gauranteed unique name. I want to be assured it was from person A, and that no ...
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### Why is the tweak length 64bit in BPS FPE algorithm?

In the algorithm for BPS Format Preserving Encryption Is this tweak always the same length (64bit)? Why did they choose the length 64bit for the tweak?
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### What are the number of output bits of the internal function F used in BPS algorithm?

In the BPS specification document it says We denote by f the number of output bits of the internal function F What is this if the internal function is AES? Would it be the block size (e.g. 128)?
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### Forward-secure static-ephemeral ECDH key agreement protocol

The question is whether the following simple key agreement protocol design has good security properties, and how it can or should be improved. Assumptions Alice is a persistent entity with a static ...
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### Is it not secure if we use weak passwords to protect exported PKCS #12 files with private keys?

Consider a practical scenario as described in Microsoft's Expand Export a Certificate with the Private Key. We can see a certificate with private key can be exported as a PKCS #12 file, where the ...
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### Encryption on the cloud with versioning

Can file-versioning, which is available for instance in Dropdox, weaken the security of encrypted files if someone is able to see two or more snapshots of the ciphertext at different times? To be ...
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### Subkey generation in OMAC1/CMAC with XTEA?

All implementations of CMAC that I have found so far use a block cipher with the same key and block size. However, XTEA enciphers a 64 bit block by using a 128 bit key. If ...
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### Reverse output of general Fibonacci LFSR

Suppose we have some Fibonacci LFSR, and it outputs some sequence. How to change starting LFSR so, that it outputs exactly same sequence, but in reverse order?
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