# All Questions

39 views

### Regex searchable word list for space-less monoalphabetic substitution

I am working on teaching myself basic cryptography, in the process I created a simple substitution cipher based on a single alphabet, and a statically defined offset (no key for now). Now that ...
74 views

### Perfect secrecy over Stirling numbers

For all $c_0\leftarrow m_0 \oplus k$ there exists a $k'$ such that $c_1 \leftarrow m_1 \oplus k'$, where $m_0 \neq m_1$ and $c_0 = c_1$. Assuming a truly random $k$, the first assignment is a ...
13 views

### How well does breaking multiple DH key exchanges over the same group scale?

Suppose you are running a server with the Diffie-Hellman parameters (so the group G and generator g) generated once. There are ...
125 views

### Keyed digest function with odds of collision below the birthday bound?

I wonder if it is possible to devise a function $F(K,S,R_S)\mapsto D$ where: $K$ is some key (I have freedom on $K$, it could even be generated by a trusted party); $S$ is in $\{0,1\}^s$, say ...
14 views

### Asymmetric Cryptography Confusion - 'Sign' a file to guarantee its source

What I want to do is use asymmetric crypto to guarantee that a file comes from a source. I have software that runs on lots of wireless devices, and each of those has a license file with an encrypted ...
57 views

### Slow one-way pseudorandom permutation?

I'm looking for a slow keyed trapdoor one-way pseudorandom permutation; or in other words a block cipher $E_K: P\in\{0,1\}^b\mapsto C\in\{0,1\}^b$ with moderate block size $b\approx 64$ bits, wide key ...
42 views

### Why should I use Authenticated Encryption instead of just encryption?

There are various different modes of operation for block cipher use, some of which provide "encryption" and some of which provide authenticated encryption. Why should I use an authenticated ...
343 views

### Attacks on the RSA Cryptosystem

I was reading some articles about attacks on RSA system and I wonder about some generalization of the following theorem. Theorem (Coppersmith). Let $N=pq$ be an $n$-bit RSA modulus, where ...
41 views

### How can I tell how many bits of security a secure hash function has?

How can I tell how many bits of security a secure hash function has? For example, how would I calculate or tell how many bits of security a secure hash function with 160 bits output would have? ...
23 views

### Will ecryptfs prompt for mount passphrase if configured to mount at fstab [on hold]

I am aware that encryption at boot is often accomplished with luks dm-crypt, however it would appear that dm-crypt can only handle fixed length containers or partitions. It would be convenient to ...
13 views

### Is it possible to change an ecryptfs mount/file passphrase? [on hold]

Is it possible change the mount passphrase of a ecryptfs mount? (not the wrapping passphrase of a ~/Private) I have a recollection of reading something about ...
14 views

### Where is the declaration of execution of ecryptfs for on user login automount? [on hold]

For a ~/Private mount generated by the ecryptfs helper utility, where is the declaration to (check for the automount file) execute ecryptfs on user login? Same ...
141 views

### Can insecure algorithms be combined to form a secure algorithm?

This is hypothetical as I can't think of any reason to do this, but out of curiosity.. Could I, for example, take the MD5 digest of a message and concatenate it with the SHA-1 digest (not quite ...
35 views

### What are the necessary things to consider when defining a game?

We generally use thought experiments or games to think about the security of any crypto scheme like encryption (indistinguishability games), or MACs (forging games). I am trying to come up with such a ...
47 views

### Is it possible to split a large AES encrypted string and decrypt the parts one by one?

Due to some platform restrictions our decryption algorithm can only handle up to 1 million bytes. The string we receive is larger, having been generated by AES in cipher block chaining (CBC) mode, ...
27 views

### Is there a Pairing-free ID-based AKE protocol that is secure under the eCK model?

As far as I know, most of ID-based authenticated key exchange (AKE) protocols are CK-secure, and there are few of them which are eCK-secure. All of the eCK-secure ID-based AKE protocols use the ...
86 views

### Why are these techniques not feasible to crack RSA?

For $d = e^{-1} \mod \phi(n)$ $$d\cdot e \operatorname{mod} \phi(n)=1$$ $$d = 1/(e \operatorname{mod} \phi(n))$$ With having $e$ and $n$, we can calculate the value of $d$ , the private key for ...
33 views

### Could anyone tell me some applications of attribute-based encryption?

I want to write a review of attribute-based encryption. Please give me some applications of ABE. Thank you!
94 views

### RSA performance

I was wondering what is the performance bottleneck in RSA algorithm? Is it the size of message, calculating the modular inverse, exponentiation? Say I have a fixed value of n to encrypt and decrypt ...
58 views

### How do I calculate the private key in RSA?

Given $n=pq$ for $p,q$ known, I can calculate $\phi(n)$. $e$ is selected such that $\gcd (e,\phi(n)) = 1$. Using this, how do I calculate the RSA private key? Example: I have $n = 35$, with ...
79 views

### Is there a name for this 'enhanced' caeser shift and if so, is it trivial to break?

This cipher shifts the letter that it will shift to, after each shift... I used to play with this when I was a kid. I was thinking about it recently and realized that it wasn't as simple as I ...
36 views

### Increasing the diffusion of the AES-CBC encryption algorithm in pycrypto for python

When encryption is done using the AES-CBC algorithm, the encryption can be thought of as chaining the cipher texts with the previous ones and an IV. However, if its on CBC mode, we give our cipher ...
25 views

### What is “serial concatenation”?

I have a third-party point-of-sale API (Speedflow Pay-N-Get) that I am trying to communicate with over HTTP, but I have a problem with it. In a nutshell, a request is encrypted using RSA, and the ...
32 views

### How do we arrive to the equation for solving D in RSA?

We have been shown that DE is congruent to 1 mod phi(n). How do we arrive to the real equation for figuring out d that is d = e^-1 mod phi(n). I want to know the entire process. I am just curious, ...
29 views

### A replay attack question

Which of the following protocols is susceptible to replay attack :- 1) A -----> B username, password 2) A -----> B username, h(timestamp,password) 3) A -----> B username timestamp, ...
51 views

### Enhance CSPRNG output [duplicate]

If I XOR the output from 2 CSPRNG will the result be better (more random) than the output from either one? Assume one of the PRNG is biased.
43 views

### Is there any analysis of freebsd's “geli” encrypted geometry provider?

I've done a few obvious searches on google and google scholar and not turned up any security analysis of FreeBSD's geli. Is there some out there, or has it gone unanalyzed since its introduction ...
44 views

### Need 32-bit mixing function that has perfect avalanche between octets

for my hobby tinkering project, I need a mixing function that takes 32-bit input and has 32-bit output (and will, most likely, run in a 32-bit C environment) and the following property (independent of ...
22 views

### Correlated or biased coin protocols?

There are a number of papers in the literature describing secure/fair coin flipping protocols. Unfortunately in my case, I am looking for coins that are epsilon-biased or epsilon-correlated. I am ...
161 views

### How hard is to invert the function that computes the middle-bits of (x^2)?

I'm designing a function f that should be moderately hard to invert and very fast to evaluate in a modern CPU. The function will be used in a proof-of-work function. I've read that the middle-bits of ...
49 views

### counting points not on elliptic curve

Given an curve with equation $y^2=x^3+ax+b$, I want to find the number of pairs $(a,b)\in \mathbb{F}_p \times \mathbb{F_p}$ NOT on the curve. How do I do it? I have an intuition that it is $p$, but ...
53 views

### Does it make sense to stretch non-keys?

I was thinking about stretching salt. Key-stretching is good against brute force guessing and assumes your system isn't compromised. Salting a one-way makes it so you have to compute * n for each ...
145 views

### Using the same RSA keypair to sign and encrypt

The RSA signature operation is basically the same as encrypting with the private key. In particular, both operations use the same kind of keys. Is it safe to use the same RSA keypair both for ...
75 views

### Is an elliptic curve over $\mathbb{F}_p$ order preserving for the points $(x,y) \in \mathbb{Z}_p$?

Let $E\colon y^2=x^3+ax+b$ be an elliptic curve, and consider its realisation over the finite field of prime order $p$: $E(\mathbb{F}_{p})$. Then if $0<a,b$ is the following true? \forall ...
75 views

### Why doesn't this dummy mutual authentication protocol provide mutual authentication?

I am a student taking a cryptography course so forgive me if this comes off as a silly question. This is an assignment question: ...
54 views

### If H(m) = 0 for some m, how can a DSA signature be forged?

If we know for some message $m$ that $H(m) = 0$, how can we forge a DSA signature with only the public key? I got that $g^s*r = (g^x)^r$ where $x$ is the private key, but that's one equation with 2 ...
66 views

### How widely is Perfect Forward Secrecy in HTTPS clients? [migrated]

We are hosting an SaaS website and API with the https protocol, both of which are accessed by numerous users with a lot of different browsers and client-side apps. What would happen if I made PFS ...
22 views

### Why doesn't this authentication protocol work? [duplicate]

I am a student taking a cryptography course so forgive me if this comes off as a silly question. This is an assignment question: Alice ------R--------> Bob Alice <-----[R]B------ Bob Alice ...
64 views

### Differences between working of Tor and onion routing [migrated]

I am learning about Tor and Onion routing and I'm trying to compare both services. From the wiki link, I can see that Diffie Hellman (DH) handshake is used and when look into the “tor-design.pdf” for ...
64 views

### counting points on elliptic curve

Given an elliptic curve with equation $y^2=x^3+ax+b$, and i want to find the number of points $(a,b)\in E(\mathbb{F}_p)$ where the polynomial has repeated roots, how do i do it? I have an intuition it ...
83 views

### Is a simple stream cipher “partially homomorphic” if no integrity check is applied?

My understanding is that, simply put, a stream cipher is just a CSPRNG such that $R(i,k)$ will produce a deterministic but statistically random sequence, where $i$ is an IV, and $k$ is the session ...
90 views

### Weak Stream Cipher cryptanalysis

Hi I have a stream cipher which uses a keystream of: F(r) || F(r+1) || F(r+2) || ... Suppose F(r) is the RSA encryption algorithm: ...
145 views

### What is the difference between a 'cipher' and a 'mode of operation'?

What is the difference between the term cipher (a name like RIJNDAEL) and mode of operation (like ECB)? Aren't these both terms for the encryption/decryption technique?
35 views

### unknown Transposition ciphertext [on hold]

What cryptanalysis can be made to find the transposition code for a cipher like this one: it's not railfence, slidefair, classical rectangular by rows or columns, or double rectangular, or Ubchi. For ...
111 views

### Why can an RSA signature be authenticated ONLY with the signer's public key?

I'm an undergraduate Math student currently taking a cryptology course. I have a question about the RSA signature authentication mechanism. An RSA signature can be authenticated with the public key ...
181 views

### Are there any bijective one-way functions not based on number-theoretic hardness assumptions?

I'm trying to find a bijective function $y=F(x)$ which should be easy to compute in one direction but hard to compute in the other, where the one-way property is not based on a number theoretic ...
8k views

### Who uses Dual_EC_DRBG?

Recent news articles have suggested that the NSA may be involved in trying to influence the cryptography in public standards or commercially deployed software, to enable the NSA to decrypt the ...
39 views

### The real-life meaning of proving over a group that doesn't support the oracle?

If I proved a scheme's security under GDH assumption, in real-life, if this DDH oracle does exist, then it's good, but what about other side ? In real-life, if this DDH oracle doesn't exist, then ...