# All Questions

104 views

### Implementing AugPAKE over ECC

The AugPAKE spec says it can be implemented over elliptic curves. This sounds very promising, but they don't actually back that claim. Can this really be achieved? If so, how would one go about ...
58 views

### Can CBC encryption have separate keys for separate blocks of plain text?

Can we have separate key for separate blocks which we get while operating a plain text message using CBC encryption model? If yes, what are it's advantage? Please enlighten me.
174 views

### Use cases for “online” authenticated encryption?

What is the the point of an "online" mode for an authenticated cipher? I understand what "online" means in this context. However, I have trouble coming up with applications that would benefit from ...
92 views

### Times of nested algorithms in proofs of security

Proofs of security may be constructed such that an adversary $A$ is used to construct an adversary $A'$. The reduction/algorithm which uses $A$ has to perform a number of computations in order to ...
79 views

### Is it secure to choose d in a RSA key pair?

An RSA key pair consists of the private key $(n,d)$ and a public key $(n,e)$ such that $de \equiv 1 \bmod{\lambda(n)}$. Usually one chooses a small $e$ and computes $d$ by inverting it modulo ...
92 views

### Any field in a PKI certificate where some text info can be stored?

I want to store 5-10 lines of Text Info in a PKI Certificate. All I want is that when using common tools like openssl command line or certutil from Microsoft, this text info should be displayed as is. ...
246 views

### Why does Fortuna RNG use double SHA-256?

In the paper for Fortuna the authors say that you can use any good digest algorithm (obviously as long as its output is 256 bit) and then they recommend double SHA-256. Why? What's the benefit? What ...
84 views

### Near preimages, applicable to Bitcoin?

Bitcoin mining relies on generating a smaller hash than the so-called target (a function of the so-called difficultly), thus is vulnerable to a truncated preimage attack (you just need to obtain a ...
222 views

### Side-channel attacks against ECDH for Weierstrass normal form curves

I hear a lot about why Montgomery curves are used in ECC, and one reason is that the same algorithm can be used to do both point addition and doubling (this is not true for the Weierstrass normal ...
124 views

### Do we need symmetric cryptosystems?

One question I had to answer in my crypto exam today was: Do we need symmetric cryptosystems? As it stands, that's probably a debatable question, so I'd like to reformulate this as: Are ...
106 views

### Is substitution with random prefix codes secure? [closed]

The following paper interests me very much: D. W. Gillman, M. Mohtashemi, R. I. Rivest, On breaking a Huffman code. IEEE Trans. Inf. Th. Vol.42 (1996), pp 972-976. However, unfortunately due to my ...
171 views

### m ∈ Zn \Z*n, RSA works but not secure

If you happen happen to have a message m ∈ Zn \ Z*n, RSA works but not secure. How likely is it going to happen? |n|=1024 bits |p| = 512 bits |q| = 512 bits.
113 views

### Encryption of numeric value using playfair

I am studying in CSE and in my recent exam paper, A question was asked as: Construct a playfair cipher for Plaintext: semester5 and key:technology. Generally as we are taught yet, there is no ...
290 views

### AES CBC MAC splicing attack

I'm trying to implement AES CBC MAC splicing attack in Python, the idea is: given a message M, its tag Tm (MAC(M) = T), a new message N and its tag Tn we build a message such as: M||N = (M1, ..., Mn, ...
207 views

### Is there a time-space tradeoff attack for breaking symmetrical cryptos?

Is there any known techniques for using time-space tradeoff for speeding up symmetrical crypto breaking? Kind of like rainbow tables speed up breaking hashes by using huge precomputed tables. Is ...
223 views

### Security proof of FO(Fujisaki-Okamoto) hybrid encryption

The proof of FO hybrid encryption is hard to understand. $\:$ Especially, how does the challenger respond to the decryption queries when the challenger can only have some encryption queries? Can ...
927 views

### LFSR get output from characteristic polynomial?

Say you have a characteristic polynomial of an LFSR: $$f(X) = X^4 + X^3 + 1$$ How can I use this function f to get the output of the LFSR, given some initial state? Obviously I can create the LFSR ...
756 views

### How useful is NIST's Randomness Beacon for cryptographic use?

NIST have just launched a new service called the NSANIST Randomness Beacon. It has been met with some initial skepticism. Perhaps the cryptography community would have used it before June 2013 when ...
185 views

### Defending hybrid encryption schemes against padding oracle attacks

I intend to use a generic integrated/hybrid encryption scheme for transmitting information between a client and a server. Key encapsulation: a 128-bit symmetric key is generated and asymmetrically ...
2k views

### Is PBKDF2-HMAC-SHA1 really broken?

I just read through this article which demonstrates practical (and seemingly trivial) collisions in PBKDF2-HMAC-SHA1, and provides a few examples of collisions. Am I missing something here? Is ...
206 views

### derangements and permutations in cryptography

Given a partial key for a mono-alphabetic substitution cipher (missing 11 letters), calculate the number $N$ of possible keys, given that no plaintext letter can be mapped to itself. ...
138 views

### Challenge–response authentication which can be done in head?

Say we have door where you have to say a password to pass. If theres someone standing nearby he would hear the password and could also use it. So therefore the doorman gives you a different number ...
882 views

### Compare RFC 5246 SessionID re-use versus RFC 5077 Session Resumption?

Can you help me understand the differences, both algorithmic and practical, between RFC 5246 SessionID re-use and RFC 5077 Session Resumption? Both appear to be ways to nail up a second TLS session ...
245 views

### Asymmetric key derivation – Who derives the new pub key can't know the new private key

I'm looking for a peer reviewed protocol or algorithm that can do this, preferentially I need something based on elliptic-curves cryptography. Alice know the main public key of Bob (B-PUB-1). Alice ...
778 views

### How secure is the AES master key if Round Keys are found

If an attacker finds some round key of AES256, is it possible to find the master key? How safe is the master key if an attackers finds multiple round keys?
41 views

### Distributed authentication in low trust environment

I would like to inquire if there is any Method to achieve the following: There is a list $L$ of $n$ entities that are allowed to perform a certain action. $L$ is cryptographically signed by some ...
116 views

### Looking for a detailed example of proof by reduction

I'm looking for a very detailed example of proof by reduction. Say we have two or three protocols (that have been proven secure) and we construct a new protocol. We want to provide a proof of security ...
358 views

### Third-party security issues with Shamir's secret sharing scheme

In Shamir's secret sharing scheme, we are trusting a third party who generates the secret polynomial. How can we ensure security here when we are involving a third party?
340 views

### Construct an encryption algorithm using hash function [duplicate]

We know that Cryptographic hash function is generally a one-way function where we can't retrieve the original message. On the other hand, encryption is a two-way-function where we get the original ...
240 views

### Is a random oracle controled by the challenger?

When proving a Crypto scheme security under random oracle model, is the random oracle always controlled by the challenger? What if the Hash is only used by the adversary?
245 views

### Is it safe to reuse ECDH asymmetric keys for authentication?

Alice, Bob, and Carol each generate ECDH keypairs. Alice and Bob establish a communication channel and negotiate an AliceBob secret. The question is: Is it safe for Alice and/or Bob to reuse their ...
104 views

### Encryption mode with chained algorithms

Suppose that I want to encrypt some data using two different algorithm chained (e.g. AES and Serpent) and harnessing the benefits of CBC. I can do: ...
345 views

### Proof of storage scheme

I'm familiar with client puzzles, where the client performs some work and there is a way to prove to the server that the client has done so. Is there a way to do this, but where the client's resource ...
70 views

### How to invalidate decryption key in multi user model?

I want to develop an application that will allow a user to create a public/private key pair, and encrypt it symmetrically with a key such that once they enter their password, they will be able to ...
1k views

### Is it true that for RSA with no padding, the length of data must be equal to the length of key?

The question pertains not in terms of security but computing operational functionality, as it how the computation is done. Is it true that for RSA with no padding, the length of data must be equal to ...
56 views

### 1-1 trapdoor function mapping the set of $n$-bit numbers to themselves

I am looking for a trapdoor one-to-one function $f : \{0,1\}^n \rightarrow \{0,1\}^n$ (that is, mapping the set of $n$ bit numbers 1-1 onto itself [any $n$ is OK]). I was considering the use of ...
66 views

### Client puzzle using hash

When a server is under attack, it send the client the following data: Nonce - a byte array of Y length Hash - the hash digest of the puzzle solution The client has to find X such as ...
974 views

### Is PKCS #1 v1.5 RSA encryption padding secure under these conditions?

The PKCS #1 v1.5 padding scheme for RSA has been proven to have some weakness when used with TLS for example. My question is: is it still secure under the following conditions? Alice sends a ...
132 views

Let $H$ be a secure hash function (e.g. SHA256), is it possibile to find $H(M1)$ given $H(M1||M2)$ if the length of $M1$ and/or $M2$ is exact the block size of the hash (that is, 64 bytes for SHA ...
252 views

### Hash function and outputs of arbitrary length

I know that Skein can output a digest of arbitrary size, with a kind of counter mode. Blocks of the resulting digest can be computed in parallel. What about Keccak ? I know that a sponge function can ...
267 views

### Rock-paper-scissors over network, how to protect from cheating server?

I'm trying to design cryptographic protocol to play Rock-Paper-Scissors with two parties, neither trusting each other, nor trusting server they use for communication, so game is 'provably fair'. So ...
179 views

### No of keys used in Triple DES Algorithm

I am studying Triple DES. I have got two basic questions. One, why are we following a sequence like encrypt(decrypt(encrypt)); what is the problem using a sequence encrypt(encrypt(encrypt))? ...
149 views

### Explanations for the complexity values for second preimage attack on GOST?

I've been reading the article "A (second) preimage attack on the GOST hash function" by F. Mendel et al (link) and I'm having some difficulty to grasp some of the values of complexities/probabilities ...
1k views

### SHA-224 Purpose

One of the new features of Java 8 is the SHA-224 message digest. What is the use case for having a 224-bit-length hash?
140 views

### Why is DDH not hard over $\mathbb{Z}^*_p$?

Why is Diffie-Hellman key exchange not hard over $\mathbb{Z}^{*}_p$?
221 views

### What is difference among eavesdropper attack, multiple message attack and CPA attack?

What is/are difference(s) among eavesdropper attack, multiple message attack and CPA attack in context of symmetric key encryption? I only know that in eavesdropper attack there is an adversary ...
235 views

### Difference between lightweight, online and low memory Authenticated Encryption schemes

What is the basic difference between the following types of Authenticated Encryption (AE) schemes? Lightweight Authenticated encryption Online Authenticated Encryption Authenticated Encryption for ...
57 views

### Ensuring that an operation takes a relatively specific amount of time, but easily verify the result

I want an algorithm of some sort that can ensure that an operation takes a fairly specific amount of time, but proof that this operation was done can be completed relatively inexpensively. For ...
Alice and Bob want to agree on a bit $0$ or $1$. Both know it would be fair to pick that at random, but there's no way they could meet to throw a dice and no third party they could trust. Are there ...