# All Questions

1answer
68 views

### Unpredictability vs randomness

I'm experimenting with creating an entropy source for an environment where I have no hardware available and the language is very high level (executed in a virtual machine). I know it's not a good idea ...
2answers
88 views

### Why is “multiplying” $g^x$ and $g^y$ not possible?

The computational Diffie-Hellman problem states that for a cyclic group $G$ of order $p$ and a generator $g$, it is hard to find the value $g^{xy}$ given only $g^x$ and $g^y$ (but easy if either $x$ ...
2answers
32 views

### How to decrypt 3DES in ECB mode (using a wordlist)?

I have some encrypted texts (encrypted with 3DES in ECB mode without salt). My question: How can I decrypt them using a wordlist? (or without one) Example: Encrypted text: ...
0answers
181 views

### twin Prime vulnerability for RSA Cryptography? [on hold]

For every set of twin primes $y,z$ let $n=8z^{2}y$ and ${e}=z^{3}+2yz$. Then I claim: $$\cfrac{4e}{n}=\cfrac{1}{2}+\cfrac{1}{y}+\cfrac{1}{z},$$ That this is true can be seen simplifying the ...
0answers
86 views

As everyone is now aware, Mt Gox has declared bankruptcy. As what appears to be the very last thing they did, Mt Gox accepted my $35 and fulfilled my special Mt Gox Yubikey. Having just arrived in the ... 1answer 95 views ### Fast modular reduction I am looking at ways to speed up modular reduction for the polynomial $$2^{256}-2^{32}-2^9-2^8-2^7-2^6-2^4-1$$ I have read the paper "Generalized Mersenne numbers" by J.A. Solinas, but it does not ... 0answers 50 views ### Entropy of Androids motion-sensor data My app on Android uses the built-in motion sensors to collect seed-data for the PRNG (java.security.SecureRandom) that is later to be used for key-generation. The user is told to shake the device so ... 0answers 55 views ### encrypted file sharing protocol I am trying to design a file exchange protocol for learning purposes, inspired by PGP, but fixing some of its shortcomings. Assuming the machine where the creation and decryption of the file happens ... 2answers 151 views ### Is Dropbox's hashing method cryptographically secure? Dropbox uses CRC32 and md5 to create a checksum of each 4 mb "block" as part of the their compression/file system. I suspect that Dropbox uses an additional cryptographic hash to check if there's a ... 1answer 140 views ### Insecurity in Needham and Schroeder protocol? I've been studying specifically the authentication algorithm Needham and Shroeder, and understand the idea in general of the message exchange. But, researching on the subject, I found a question that ... 2answers 438 views ### Attacks of the MAC construction$\mathcal{H}(m||k)$for common hashes$\mathcal{H}$? Consider a common practically-collision-resistant hash function$\mathcal{H}$(e.g. SHA-1, SHA-256, SHA-512, RIPEMD-160), perhaps based on the Merkle–Damgård construction as are the first three. We ... 1answer 67 views ### Prime factors of non-random keys I have been working on a challenge i found on the internet. It is as follows: You've stumbled onto a significant vulnerability in a commonly used cryptographic library. It turns out that the ... 2answers 76 views ### Is it possible to calculate the 'skeleton key' for DUAL_EC_DRBG? What would it take? According to Bruce Schneier the constants used in the spec of DUAL_EC_DRBG may be related to a secret set of numbers, that could function as a master key for ... 1answer 53 views ### Convincing “someone” on the (in)validity of DH tuple without revealing$u,v$Is there a way of convincing someone on (in)validity of Diffie Hellman tuple$<g, g^u, g^v, g^{w}>$without giving away$u,v$. or at least solve the Decisional Diffie Hellman Problem without ... 0answers 23 views ### GnuTLS test 'chat' equivalent of OpenSSL s_client s_server [migrated] How can I set up a test encrypted "chat" with GnuTLS, equivalent to the OpenSSL s_client and s_server using GPG credentials? I would like to tell GnuTLS-serv and -cli to use keys generated with GPG ... 2answers 2k views ### Why is H(k||x) not a secure MAC construction? If H(m) is a secure hash function, can't we implement a MAC using H(k||m)? However, it seems the more widely used MACs, such as NMAC and HMAC (both originally defined in Keying hash functions for ... 1answer 59 views ### Is script execution time a decent source of pseudorandom number generation? This is for a PHP script for password generation. Naturally there are other things being calculated before the password is generated. But I figure the time it takes for the script to initialize and ... 0answers 44 views ### Why don't we use H(m||k) as a MAC? [duplicate] HMAC seems a bit complicated. Why can't we use$H(m||k)$as a MAC? Unlike$H(k||m)$, length extension attacks won't work. Is there some other obvious attack? 0answers 10 views ### Cracking DES(Unix)/MD5 ARP [migrated] This is a basic exemple of Unix Pass Encryption. I have little Hash of MD5 ARP : ... 1answer 100 views ### GCM vs CTR+HMAC tradeoffs So these days I see everyone using AES-GCM. What are its advantages over simple CTR+HMAC modes? Is it speed? Or ciphertext length? And what are the security tradeoffs, both in terms of practical ... 2answers 197 views ### NTRU crypto from unseen.is; myth busting help I am always on the look out for privacy enhancing technologies. I stumbled upon the service from unseen.is. I had a look at their site (specifically here) and even though I lack knowledge there are a ... 1answer 66 views ### Can I use asymmetric encryption for more powerful write-protection? I have a game that I am developing which uses a locally-stored internal database. It does not matter to me whether a user can read the file (which, to me, means I don't need a symmetric cipher), but I ... 2answers 97 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 164 views ### Is it meaningful to consider the leakage of master key of KGC? In the Identity-based cryptography, the user's private key in the systems is delivered by the key generation center(KGC). My question is : Is it meaningful to consider the leakage of master key of ... 1answer 111 views ### Questions about OAEP for RSA I have two questions about OAEP for RSA. How are the number of bits to pad with 0 chosen? For example, if I'm sending a 255 byte message with RSA-2048 I have 8 unused bits (1 byte). Should I split ... 0answers 29 views ### Should layered use of crypto always have a cryptographic binding between the layers? Reading the latest TLS weakness, it seems that multiple weaknesses have been found over the years caused by not ensuring that messages are sent over the same channel, and in general a lack of ... 2answers 121 views ### OpenSSL PBKDF2 documentation I'm using OpenSSL 0.9.8 on OSX Maverick, and I want to use a PBKDF but I can't find any doc/man about it on OpenSSL's website. I'm working on QCA, an OpenSSL wrapper for Qt, and when I arrived there ... 0answers 99 views ### What RC4 key value will completely invert$S$after initial permutation? What RC4 key value will completely invert$S$after the initial permutation, so that$S[0] = 255, S[1] = 254, …, S[254] = 1, S[255] = 0$? 4answers 115 views ### How does TLS generate the shared secret? From many readings, I got to know that TLS generates master shared secrets using pre-master keys and uses the DH algorithm to generate the shared secret/Master secret. What values during the hello ... 0answers 128 views +50 ### How do multiple parties agree on a target block to hash for? Bitcoin style blockchain… I have party(node) A, B, C and algorithm X. Algorithm X generates the first half of a target hash input - the "solution" to the problem. I want A, B and C to sign their ... 0answers 60 views ### Machine Learning with Encryption [duplicate] Machine Learning is sometimes associated with cryptography. Many of such cross field studies have been majorly done in use of machine learning for cryptanalysis/decryption. I am interested in knowing ... 3answers 89 views ### Frequency of letters change by the length of the texts? In terms of the frequency of letters, how is it possible to have different frequent letters when the length of the text I'm analyzing is shorter? At the moment, I'm comparing the frequencies of a ... 1answer 60 views ### Check the validity of a rsa key pair with only the public key? [duplicate] I am currently developing a messaging application that uses rsa end to end encryption. The client generates permanent rsa keys, and sends the public key to my server for account management. As it ... 1answer 58 views ### If$G'(s)=G(s0^{|s|})$and$G$is a PRNG, is$G'$necessarily a PRNG? I argue it is because there should be no more correlation between the outputs$G(s0^{|s|})$than between the outputs$G(s)$. Thus we can say the$G'(s)$is a PRNG that satisfies$|G'(s)| \geq 4|s|$. ... 1answer 255 views ### How do institutions like banks do RSA with big primes? When encrypting with RSA it is often infeasible to decrypt by just doing c^d mod n, because for example when using the primes$(p,q)=(12553,1233)$, which are small ... 1answer 103 views ### Which data to share in CTR mode? Suppose I've got two clients$A$and$B$. Client$A$encrypts data in CTR (Counter) mode to get a ciphertext. What information will client$B$have to receive from$A$to decrypt the ciphertext? Just ... 0answers 54 views ### Question on factorization & polynomial time [closed] Factoring is the problem of computing, on input a positive integer n, a factorization of n in terms of integer powers of prime numbers. This problem can be "easy" (i.e., there exists a polynomial-time ... 0answers 53 views ### Question on BPP and NP (polynomial time) [on hold] Informally, BPP is the class of languages that can be decided by a probabilistic algorithm in polynomial time with an arrow probability of most 1/3 on any instance. More formally, a language L is in ... 2answers 80 views ### Need Help Reversing my Encryption Algorithm [closed] So I wrote a simple encryption algorithm (Not very secure, just a starting point. I don't want criticism on it.) and I have been having trouble finding a way to decrypt the text. The algorithm is here ... 0answers 60 views ### File Encryption (EFS method) I'm implementing (for learning purposes) a file encryptor, which uses the following method (from http://en.wikipedia.org/wiki/Encrypting_File_System). I have 3 kind of keys: RSA public/private, AES ... 1answer 102 views ### Derive both MAC and AES keys from same PBKDF2? When doing password-based encryption, is it OK to create one instance of PBKDF2 from the password, and then use it to create both the AES key and the MAC key? (Or should a new instance of PBKDF2 be ... 3answers 4k views ### Where i can find an AES implementation in python? Where i can find a "reviewed" version of AES implementation in python. Not an API like PyCrypto whereby you can call AES algorithms in a single line. 2answers 302 views ### Estimating random number entropy for input into 256 bit hash Assuming a random number generation process outputs lots of numbers between 0-9. First I gathered up a bunch of the numbers, converted them to binary and created a bitmap. Not so random as you can ... 1answer 133 views ### OpenPGP Brainwallet [closed] This question is an intersection of two pieces of technology. The first is on Hierarchical Deterministic Wallets from the Bitcoin Space. The second is on OpenPGP key generation. Has anyone created a ... 0answers 188 views ### Is there a “brainwallet” for GPG keys? [closed] Brainwallet is a program that takes a passphrase (hopefully a good one) and deterministically creates a Bitcoin wallet. This wallet can be used for transactions, then deleted from the computer. To get ... 0answers 70 views ### Using RSA key pair for file encryption I'm looking for a way to perform encrypted nightly backups of a server I administer, without storing the decryption key on the server. A form of PK encryption seems ideal for this, as I can store the ... 0answers 42 views ### IND-CPA Security Given a key$K$(the key is fixed) and a nonce$N$(changes from one message to another) Using a secure Key derivation function (KDF) such as the ANSI-X9.63-KDF with SHA-1 option to derive a key for ... 0answers 18 views ### Determine the iteration times using Pollard's rho Method for factoring Let's say, we have a large number$n=181937053$and$f(x)=x^2+1$. And also we know that$n=12391 \times 14683$. The problem is that ,using Pollard rho method, can we find the algorithm iteration ... 2answers 1k views ### How to calculate the time it'll take to crack RSA or DH? Sometimes the easiest way to describe security of a type of cryptography is to say that "the time it takes to solve for an x-bit key would be y years". How would one go about doing such a calculation ... 1answer 137 views ### Solving a discrete logarithm using GDlog I am trying to calculate an$x$, such that$t = g^x \pmod p\$ in order to crack a weak ElGamal encryption for university. I found GDlog, but I cant figure out how I can use the input to calculate my ...

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