# All Questions

452 views

### Cracking secret key of a MD5 hash?

I have a MD5 hash which is obtained by hashing certain strings appended by a pipe character ( | ). (I know the values of these) A secret key is added as a suffix to this (I don't know the secret key). ...
90 views

### Is $(f(X), G(X))$ pseudorandom?

If $f:\{0,1\}^n \to \{0, 1\}^n$ is a one-way function, $G:\{0, 1\}^n \to \{0, 1\}^m$ is a pseudorandom generator, and $U_m$ is the uniform distribution over $m$-bit strings. Is the joint distribution ...
83 views

### Why do we need password in a server which uses TLS?

Suppose a server uses TLS to protect communication between us and the server. I observe that in the TLS handshake protocol, password is not mentioned anywhere. Then why do we need to have a ...
414 views

### rsa public and private keys

I have a question about RSA key generation. This is a simple RSA algorithm: Choose p = 3 and q = 11 Compute n = p * q = 3 * 11 = 33 Compute ?(n) = (p - 1) * (q - 1) = 2 * 10 = 20 Choose e such that ...
613 views

### Use case of RSA CRT

I discovered the CRT version of RSA cryptosystem which is used in many crypto libraries (openSSL, Java...). The use of the Chinese Remainder Theorem improves the speed of decryption so why it's not ...
125 views

### DES-X , computation load and storage

The passage said that the computational load to attack DES-X can be reduced to approximately $2^{(56+64)}=2^{120}$ steps,and the storage of data sets should be $2^{64}$. But I can't figure why ...
168 views

### What can be learned from the ciphertext of LibSodium's crypto_box_detached()?

LibSodium, (https://github.com/jedisct1/libsodium), has a function crypto_box_detached() which does authenticated encryption using the public key of the recipient ...
123 views

### Protection of Elliptic Curve Implementations against side-channel attacks [closed]

Recent fast elliptic curve implementations, for example a presentation at Eurocrypt 2014 (earlier presentation slides, the paper) talks about protection against only timing attacks. Why only timing ...
270 views

### Compare two approaches for cracking RSA key

I came across these questions while studying for a crypto course, does anyone have any ideas on how to answer these? (a) Random prime numbers of size 1536 bits are chosen to generate an RSA modulus ...
102 views

### Is it just as difficult to find a desired output in a hash digest, for MSB vs LSB?

This question comes to mind after seeing "custom" Bitcoin addresses with some special string at the beginning of the address. They are generated in the same way Bitcoins are "mined", continually ...
199 views

### Elliptic curve parameters

What's the meaning of 160 bit Curve in Elliptic curve ? or 192 or 224 or 256 and etc. And What is the standard for selecting this number of bit ? why they don't say 100 bit curve?
893 views

### Static DH/Static ECDH certificate using OpenSSL

I've been searching for some time and have found no way to create a working certificate and private key pair with a DH or ECDH public key using OpenSSL. Once I do this, I'd like to sign it with an RSA ...
72 views

### Hardness proof?

I am little curious about how do we calculate hardness proof of any cryptography algorithm? Also where can I find hardness proof for AES and AES-CBC ?
161 views

### INT_32_BE in PBKDF2

Trying to understand PBKDF2 in detail. It is clear that the password and the salt is given to the PRF (which is HMAC-SHA-1 in the specification). But what is the role of the INT_32_BE(i)? What does it ...
114 views

### Question about second preimage resistance of hash function combiner

Let $\Pi=(Gen_1,H_1)$ and $\Pi=(Gen_2,H_2)$ be two hash functions. Define $(Gen, H)$ so that $Gen$ runs $Gen_1$ and $Gen_2$ obtaining $s_1$ and $s_2$ respectively. Then let ...
341 views

### what is the difference between IBE and ABE schemes

Can somebody explain in simple words. real life example light help and limitations of these schemes etc.
309 views

### Modes of operation for Public Key Encryption schemes

Symmetric key encryption schemes have well defined modes of operations for achieving deterministic (ECB) or Randomized (CBC, OFB, CTR ) etc. and other applications of the same. But are they defined ...
180 views

### How hard to break a cipher if it has a different key for each word?

Recently I watched the movie 'The Prestige' again but this time the cipher diary impressed me very well. I like to create my own cipher for my diary because I have some time to kill everyday after ...
842 views

I was reading upon Biham and Shamir's paper and a fact has been presented over there: if $P_1 = \bar P_2$ and I choose a key $K_1 = \bar K_2$ then in that case $$T_1 = DES(P_1, K_1)$$ $$T_2 = ... 3answers 190 views ### How is the following different than RSA Signing of a document? If the goal is to demonstrate that you are who you say you are, I think the following will work: Bob wants to know that you are Amy. Bob encrypts a number using Amy's public key and then asks her what ... 1answer 70 views ### where to keep the AES keystream? I am trying to implement an AES encryption on my messages in my app and I want to have a different keystream for each "message group". Therefore, currently I am holding the keystreams in the database ... 1answer 152 views ### Which algorithm do you recommend for practical use to generate unique passwords for each website? So, in my imagination there is this simple algorithm with only one master password, let's say a prime number 'p', which I have to remember. Using only the address of the website and p, the algorithm ... 2answers 2k views ### In public key cryptography, how many keys are required for secure communication between n parties? In public key cryptography, how many keys are required for secure communication between n parties? In my view the answer should be n^2. Reason: There are ‘n’ Parties. Every Party has One (1) ... 2answers 281 views ### Deriving 2 keys using HKDF I have an encryption scheme that uses a 256-bit master key, from which 2 separate keys (one for AES-256-CTR encryption and one for a HMAC-SHA256) are derived using HKDF. However, I'm not sure exactly ... 1answer 244 views ### Is 256 a reasonable length for a Diffie-Hellman safe prime? There's a program that uses the following OpenSSL function... DH_generate_parameters(256, 5, NULL, NULL) ...to create DH parameters which are later used for ... 1answer 280 views ### How can I find the multiplicative inverse in the first transformation of the SubBytes() transformation in AES? In FIPS-197 §5.1.1 , it says the first transformation in the SubBytes() transformation is: Take the multiplicative inverse in the finite field \text{GF}(2^8) described in Sec. 4.2; the ... 1answer 167 views ### Public and private key lengths I am generating RSA Public and Private keys with node.js instead of reading them from pem/crt files as I can hide them inside a private object using this module https://www.npmjs.org/package/private ... 1answer 120 views ### LFSR and Markov chain question [closed] This may seem an elementary question about LFSRs, and their link to Markov chains. LFSRs show Markov chain behaviour in that there can be a transition matrix defined over the LFSR, this follows from ... 1answer 105 views ### How can a digital signature algorithm incorporate a random data if it is to be authenticated on a separate system I am trying to understand how RSA digital signatures can incorporate any data from a PRNG into the final signature. Does this mean that separate implementations might fail to verify each others ... 1answer 135 views ### Are formal methods used for cryptographic protocol design? I decided to learn more about the application of formal methods to cryptographic protocols and found this interesting talk by Cathy Meadows at YouTube. After doing research about how people are using ... 1answer 199 views ### Number of different substitution alphabet ciphers possible with given conditions? If you consider a 26 letter alphabet, and a cipher where 24 of the letters are sent to themselves, and only 2 letters switch, how many different substitution alphabet ciphers are there, and what ... 1answer 195 views ### ElGamal digital signature. why mod p-1 in delta? Im trying to figure out why \delta in the signature part of ElGamal has mod p-1 when \gamma has mod p? \gamma = \alpha^k \mod p \delta = (x-a\gamma)k^{-1} \mod p-1 1answer 380 views ### About the Rijndael/AES sbox polynomial (subBytes) I've recently read a question about the irreducible polynomial behind the subBytes() operation in the Rijndael that has awakened and old curiosity I have: Why \,m(x) was chosen as ... 2answers 208 views ### reverse an identicon to its source string Identicons are used to create unique images based on IP address or strings. Lots of programs are available to generate identicons based on an input string. My ... 1answer 190 views ### Are the RFC3526 MODP groups Schnorr groups? I was wondering if a group like the 1536-bit MODP Group from RFC 3526 was a Schnorr group? A Schnorr group must apparently have: p and q being primes p = q\cdot r+1 1 < h < p ... 1answer 104 views ### Numeric combination of passwords Pls I need explanation on How i can design a Cryptography that use multiple passwords or passphrase to open a safe(Lock). For example, if i need five people to unlock a secured device whereby all the ... 1answer 275 views ### Generalize the Merkle–Damgård construction for any compression function I am trying to figure out this question: Generalize the Merkle–Damgård construction for any compression func­tion that compresses by at least one bit . You should refer to a general input ... 1answer 97 views ### Definition of one-way functions: randomly-chosen point vs every point Notation: \Sigma^k is the set of k-strings on the alphabet \Sigma; \Sigma^\ast is the set of all finite dimensional strings on the alphabet \Sigma. In the context of computer-science a ... 1answer 162 views ### XORing a header with SHA512-hash - secure? I am writing a application that encrypts text several times and stores the result. The result looks like complete random bytes, therefore I add some random bytes at the end. To let the program know ... 1answer 76 views ### MAC security and adversaries with memory The usual definition of MAC security is (roughly) that there is no "efficient" adversary that has non-negligible advantage in the "MAC game" where the MAC game is as follows: the challenger chooses a ... 1answer 86 views ### Exchanging Keyspace and Message space in PRF Suppose there is a secure PRF F: K \times X \to Y. Then is F': X \times K \to Y, defined by F'(x,k):=F(k,x), a secure PRF? In my opinion it should be because exchanging the keyspace with ... 1answer 351 views ### AES-256 Shift-Rows Offsets I am currently trying to implement a AES-256 cipher according to this sepcificaciton: http://techheap.packetizer.com/cryptography/encryption/spec.v36.pdf As well as on Wikipedia and in the document ... 1answer 2k views ### TLS/SSL's usage of Non-Ephemeral DH vs DHE These questions revolve around DH and ECDH vs DHE and ECDHE. Specifically within the context of TLS/SSL. There are three ... 1answer 47 views ### Alert protocol in TLS Assuming the alert protocol is encrypted AFTER a session has been established and the structure below, how does the client know whether to expect encrypted data or not? ... 1answer 110 views ### Is it possible to encrypt data in such a way that it cannot be decrypted with the key used to encrypt it but can be decrypted with another key I'm very new to cryptography and encryption so I apologize if the answer to this is well known, but I can't find anything in my research. The crux of the problem is the need to store data on an ... 1answer 73 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 ... 1answer 287 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 ... 2answers 98 views ### Subexponential algorithms for DLP in \mathbb{Z}_s \times \mathbb{Z}_t Consider the accepted answer to the question: Why are elliptic curves better than cyclic groups? It seems to suggest there are subexponential algorithms (i.e., algorithms with running time$$ ...
Can a monoalphabetic substitution cipher attain perfect secrecy? Definition of perfect secrecy: $${\rm Pr}[\,{\rm Enc}_k(m_1) = c\,] = {\rm Pr}[\,{\rm Enc}_k(m_2) = c\,]$$