# All Questions

1k views

### How do you find a cheater in Shamir Secret Sharing?

If there are 4 people involved, and every two of them should be able to know the secret (the polynomial is just a line) and you are given f(x) and x for each of those people, and you know one of them ...
516 views

### Is it reasonable to assure that p-1 and q-1 aren't smooth?

I came across the requirement that, in RSA, $p-1$ and $q-1$ shouldn't be smooth, shouldn't consist of lots of small factors. Therefore my question: How complicated is it to check whether $p-1$ is ...
853 views

### How can I improve a password generation scheme based on a shared secret and URL?

I currently use the following method to generate a different password on every website I have to login: password = SHA1 ( mainPassword . domainName . number ) ...
265 views

### Will varying plaintext compensate for a fixed initialisation vector?

This is a follow-up question to Relative merits of AES ECB and CBC modes for securing data at rest. I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option ...
476 views

### Why does SRP-6a use k = H(N, g) instead of the k = 3 in SRP-6?

I've been reading up on the Secure Remote Pasword protocol (SRP). There are a couple different versions of the protocol (the original published version being designated SRP-3, with two subsequent ...
2k views

### Why not use CTR with a randomized IV?

I'm currently reading the chapter of Cryptographic Engineering (Ferguson, Schneier, Kohno 2010) about block cipher modes of operation. They have recommended CBC with random IV instead of CTR due to ...
723 views

### Does AES have any fixed-points?

Is there any pair of 128-bit strings M and K such that AES$_K$(M) = M? If yes, how do I go about (efficiently) finding such a pair?
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### How many bits of hash are realistically needed for key verification?

Say I'm connecting to a web server secured with TLS but with a self-signed certificate. Accordingly, I call the owners of the server and ask them what SHA1 fingerprint they have. They start reading ...
3k views

### Encryption with “constant” initialization vector considered harmful

I try to get the full reasoning behind the above statement. First, after reading articles here and at wikipedia i understand that using an IV only once is good practice. For stream ciphers not doing ...
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### Is it possible to weaken a bitcoin private key by “using” it elsewhere?

What are the increased possibilities (if any) of being able to crack a private key given the following: The associated bitcoin (ECDSA Secp256k1-based) public key is known. The private key has been ...
631 views

### Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
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Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod ... 0answers 430 views ### How can I find two strings$m_1$and$m_2$, knowing that I know$m_1 \oplus m_2$? [duplicate] Possible Duplicate: How does one attack a two-time pad (i.e. one time pad with key reuse)? I recently started to follow the cryptography class of Dan Boneh on coursera.org and the first ... 1answer 3k views ### How large should a Diffie-Hellman p be? In a Diffie-Hellman exchange, the parties need to agree on a prime p and a base g in order to continue. Assuming some ... 3answers 330 views ### Designing a key expander out of ciphers Is it possible to compose or combine$n$ciphers ($cipher_0, cipher_1, ... cipher_n$) in such a way that they generate$n$ciphertexts from the same key,$k$, and that recovering the key,$k$, from ... 1answer 822 views ### Luby-Rackoff theorem confusion The Luby-Rackoff theorem states that if a round function is a secure pseudorandom function (PRF) then 3 rounds are sufficient to make the block cipher a pseudorandom permutation (PRP). PRPs are ... 2answers 466 views ### Quantum key distribution simulation I'm looking for a simulation of the Quantum Key Distribution protocol. Does anyone here have one that they could recommend? 2answers 375 views ### Why does CTR mode XOR the plaintext into the output of the block cipher rather than XORing the plaintext into the input of the block cipher? As I understand it, CTR mode essentially turns a block cipher into a stream cipher like so: ... 3answers 502 views ### P = NP and current cryptographic systems I've recently heard some people claiming that if the fact that P = NP is proven, most (all?) the current cryptographic algorithm considered secure like RSA will be unusable in secure systems. My ... 1answer 332 views ### Why is feed-forward mechanism used in hash functions? The compression function of SHA-1 when used in Davies-Meyer mode adds its input to the chaining values at the final step. For the first message block, the IV is used as the input and in the next step, ... 1answer 200 views ### Why are there$ signs in my passwd file?

I am trying to get access to my eReader and I managed to get the passwd file. ...
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First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
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### Composing two one-way functions such that the result is not a one-way function

Is it possible to have two distinct one-way functions (called, say, $h$ and $g$) such that their composition $h \circ g = [\, x \mapsto h(g(x)) \,]$ is not one-way?
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### Build set with PKI from reliable broadcast / random beacon

I am trying to build a protocol (or find an existing one) for creating a set $S_2$ with PKI from a set of parties $S_1$ that initially does not know anything about the other parties. We assume ...
188 views

### ECDSA: How to retrieve a non-random k

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
739 views

### Why does Openssh use only SHA1 for signing and verifying of digital signatures?

I am learning SSH protocol. With my current understanding of SSH protocol, I think that message digest algorithms for using in digital signature should be derived from Key Exchange. But Openssh ...
172 views

### ElGamal in private set intersection: how to handle negative numbers?

I am trying to implement the Efficient Robust Private Set Intersection using Additive ElGamal. For this, from the client side, my inputs are $C = \{1, 2\}$. So, the polynomial is $(x-1)(x-2)$ which ...
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### What does this Authentication protocol achieve and what information is shared?

I'm having trouble with the following question from my class: "What is the purpose of the protocol, and how does it achieve this purpose and what shared information does A and B end up after ...
381 views

### Definition of a CSPRNG

I am interested in what conditions are necessary and sufficient to define a cryptographically secure pseudo-random number generator (CSPRNG). Wikipedia lists two defining characteristics: It ...
658 views

### What does an RSA signature look like?

I am using OpenSSL libs to generate signatures. Internally, I learn that a signature is a hash of the message with some padding added to it. I am trying to understand the structure of a signature. If ...
553 views

### How to attack a fixed LCG with partial output?

To show some colleague programmers exactly how broken C's rand() is (at least on Windows) I decided to break it. So everyone knows the exact parameters, MSVC's ...
229 views

### Parallel-resistant proof-of-work scheme?

I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function $f()$, a target ...
507 views

### Are RSA signatures deterministic?

If I sign the word HELLO with the mechanism "NONEwithRSA" with the same private key, do I always will have the same signature? A Java example always return ...
213 views

### Non-cryptographic hash function as MAC for stream ciphers

I understand that for a stream cipher to be useful, there must be a way to verify that the message was not tampered with (bits were flipped by an attacker). So, instead of using some cryptographic ...
365 views

### CPA Secure Chosen plaintext scheme

The example for IND-CPA secure schemes given is generally: for a random r, Enc_k(m) =(r|| E_k(r) XOR m) where E is a PRF But does the role of r and k really matter--i.e. isn't this equally ...
892 views

### Why do we need in RSA the modulus to be product of 2 primes?

I think I roughly understand how the RSA alorithm is working. However, I don't understand why we need the $N$, which we use as a modulus, to be $pq$ for some large primes $p, q$. I vaguely know it ...
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### Are there secure stream ciphers that cannot be parallelized?

Are there any stream ciphers (or a deterministic random number generators, that should work as well I guess?) that cannot be parallelized? So for example if I seed it with a specific value, and then ...
814 views

### What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
301 views

### Security considerations for partially shared password databases

Programs like KeyPass and 1Password store password database files encrypted by a single password. If someone knows the protecting password ("Vault Key"), they can read the entire database ("secrets"). ...
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### Can I pre-define the points in Shamir's Secret Sharing algorithm

With Shamir's secret sharing is it possible to pre-define the points returned by the algorithm? For simplicity if I have (k, n) where k=2, and n=4, and I have the points X,Y,Z, and Q. Can I create ...
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### How can I design a cryptographically secure, key-dependent s-box?

I would like to design a cryptographically secure, key-dependent s-box, but I am not sure how. Mainly, because I am unsure about the difference in design compared to a random s-box. What is the ...
101 views

### Roots in modulo field

I have a point $(X,Y)$ on an elliptical curve $E(a,b)$ where $a=-3$ and $B$ is a large number that is in hexadecimal from -51BD. To compress this point oficially in a program, we know that every $X$ ...
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### Is there a way to systematically calculate the public exponent $e$ in RSA?

I'm learning RSA in one of my classes and we were given a problem: $p = 5$, $q = 11$ I have done the following steps: $n = 5 \cdot 11 = 55$ $\phi = (5-1)\cdot(11-1) = 40$ I know that to ...
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### How could Fully Homomorphic Encryption support power operations?

Fully Homomorphic Encryption (FHE) enables arbitrary functions computed on encrypted data, because it supports both addition and multiplication. But I wonder if FHE supports power operations. For ...
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### How do I convert the definition of E-521 into a curve definition a la Bouncy Castle?

I am currently trying to create an ECCCurve for E-521. Unfortunately, it is not currently a named curve in the library I am using, so I will have to define it manually. I am using the definition of ...
2k views

### How to calculate the maximum output size for data encrypted with a RSA Private Key?

I have an an encryption algorithm I am working with that looks like this: prv_key_enc(sha1_hash(data)) Where, the RSA parameters are: RSA/ECB/PKCS1Padding 1024 ...
385 views

### How is HKDF-Expand better than a simple hash?

Why do we need HKDF-Expand if we can simply hash the pseudorandom key to make it longer? The docs (in the link) say: The second stage "expands" the pseudorandom key to the desired length... ...
63 views

### 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 $K$ ...
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 ...