# All Questions

308 views

### Practical (and secure) PRGs

Does anyone know if there exists a PRG construction which takes as input an (RSA-sized) integer seed and outputs a fixed number of bits? There are number-theoretic PRGs such as the RSA, Micali-Schnorr,...
870 views

### What was the problem in the SSL version 2.0?

What is the main reason (attacks) the appearance of the protocol SSL version 2.0 to replace version 3.0?
253 views

### Attack against modular inversion operation using side-channels?

I'm building a device that performs a modular inversions using a secret modulus. I would like to know if it is possible to recover all or part of this modulus by side-channels (timing, power, EMR, etc....
150 views

### How does secret sharing solve the partial exposure problem?

I have been trying to understand how secret sharing methods like Shamir's secret sharing solve the problem of a share revealing information about the secret. I guess there are some random numbers ...
322 views

### Public-key cryptosystems without poly-time quantum attacks

It's well-known that Shor's algorithm can solve integer factorization, discrete logarithm and discrete log over elliptic curves in cubic time. This implies that cryptosystems like RSA, ElGamal, and ...
279 views

### Isn't the deterministic property of (EC)DSA a security problem?

From what I read in Wikipedia's article "Elliptic Curve DSA ~ Security", the "deterministic" generation of $k$ can help prevent certain type of attacks (such as the one happened to Sony). However, ...
406 views

### Elliptic Curve Cryptography Encryption Results

I'm playing around with a package that does Elliptic Curve Cryptography from http://jecc.sourceforge.net/ Every time I encrypt a value it produced a different result (same private key). However I'm ...
747 views

### Textbook-RSA meet-in-the-middle attack against other RSA based schemes?

A “meet-in-the-middle” (not “man-in-the-middle”!) attack on textbook-RSA was presented to me. The only requirements for it was that the attacker gets the output of RSA and the public key, and that the ...
130 views

### How difficult is it to find the “pre-image” of a block cipher?

Say that I have $$C_1 = AES_{k_1}(M_1)$$ How difficult would it be to find a key, K2 and plaintext M2 $$C_2 = AES_{k_2}(M_2)$$ such that $$C_1 == C_2$$ How would using a block cipher ...
103 views

### Can CBC-encrypted files be modified in-place?

When changing a (small) part of the plaintext of a CBC-encrypted file, how much of the file will have to be rewritten? My suspicion is that everything after the modified block will have to be ...
951 views

to understand RSA better I am doing a little calculation by hand, this is what I got: Choosing: $p = 3$ $q = 5$ $n = 15$ $\phi(pq) = 2 \cdot 4 = 8$ $e > \phi(n) => e = 13$ $e \cdot d = 1 \... 2answers 286 views ### Composition of block ciphers and 3DES There is some intriguing things about DES and 3DES. Now, I know that DES is weak and 3DES was an attempt to construct a more secure block-cipher from a deprecated one. Having this in mind, what ... 2answers 1k views ### Are block ciphers used in public key crypto? I was reading about block ciphers and most articles state they are being used in symmetric key cryptography. Are they also being used in public key cryptography? if not, what alternative pkc use? ... 2answers 2k views ### How to test if a number is a primitive root? How to test if a number is a primitive root, assuming the modulus is a prime? And if not? Is it not enough if the number is relatively prime to the modulus or prime? 1answer 705 views ### What are the potential security impacts of using CRAM-MD5 for Emails, when not using an SSL connection? Background: My current server-provider tells me it's no problem to store the passwords in plain-text in the database, saying he has to do so because they use CRAM-MD5 for email authentication. But ... 2answers 556 views ### Case insensitive verification of HMAC / Base64 signature I am signing a short message using the base64 of an HMAC, like this (python): ... 3answers 1k views ### RSA digital signature vs authenticated cipher I want to provide both confidentiality and integrity for data at rest (many large files stored on disk). I plan to encrypt the data using AES, which will cover the confidentiality requirement. So ... 2answers 847 views ### Simple RC4 key generation scheme I would like to ask you about your opinion on the following scheme of combining a master key and a nonce and stretching the result to the full 256-bytes RC4 key. Master key$K_m$is a pre-shared ... 2answers 1k views ### Berlekamp-Massey algorithm: case when sequence length is less than double the length of the LFSR Suppose that we have a sequence of$N$digits which is produced by a Linear Feedback Shift Register (LFSR) and the shortest such LFSR is of length$L$. A very important tool in cryptanalysis of stream ... 2answers 2k views ### Which categories of cipher are semantically secure under a chosen-plaintext attack? I am trying to get my head around the circumstances under which a cipher is (or is not) semantically secure under a chosen-plaintext attack. I can't seem to find a good reference explaining this. I ... 3answers 573 views ### A proof-of-work random number generation system for Pokémon [closed] (The original question that was here was considered too confusing, unclear, and rambling. You can still view it in the edit history, but the content is no longer useful by itself, and cannot be ... 1answer 214 views ### Does zero-padding the secret in Shamir's sharing scheme increase security? When performed in binary Galois fields$GF_{2^n}$, Shamir's threshold secret sharing scheme produces shares that are each the same bit-size as the secret. Though the scheme is "perfectly secure" in ... 2answers 697 views ### How can I add more rounds to AES? I'd like to understand a bit more about the AES key schedule if anyone would mind explaining that. For instance, why is the way the schedule generated (with recursive xors and occasional substitutions)... 3answers 261 views ### Message authentication codes construction I was reading the paper$[1]$and came across the scheme that I show below. While I understand the scheme well, I don't understand why they prepend a 0 to the block containing$r$and a 1 to all other ... 3answers 159 views ### The meaning of “scheme” This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography. Consider the following two sets of terms: Encryption ... 1answer 360 views ### Why pairing based crypto is suitable for some particular cryptographic primitives? Why pairing based crypto is being widely used in some special crypto primitives as ID based crypto and variations of standard signatures? I mean taking as deep as possible what makes it suitable for ... 2answers 1k views ### Difficulty of breaking RSA for a given key size Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus$N$. To factor it, you ... 3answers 441 views ### How much bigger does a precomputed lookup table get when salt is added? I am trying to wrap my head around the benefits of salt in cryptography. http://en.wikipedia.org/wiki/Salt_(cryptography) I understand that adding salt makes it harder to precompute a table. But ... 2answers 2k views ### What does SSL use? RSA? El-Gamal? Elliptic curves? I'm not sure what SSL uses to share the symmetric key to both end users, i.e. at the beginning of the communication. Is it RSA? Or El-Gamal? Or something else? 2answers 224 views ### Getting the encryption method and key from the encrypted data and the raw data I have some pairs of plaintext and ciphertext data, from which I need to be able to decrypt the other passwords stored in the database. The password field typically contains something like ... 2answers 184 views ### What is sent on the initial communication with Kerberos? When a user wants to communicate with another user (let's say Alice and Bob), what is sent over the network during the initial communication? Alice uses her Kerberos client and enters her password ... 3answers 362 views ### Where can I find useful data for cryptography/coding theory? When implementing cryptographic/coding theory algorithms one need to use data like big prime numbers, numbers in$Z_n$and their inverses, irreducible polynomials in$Z_n[x]$and so on... While ... 1answer 127 views ### What do they mean when saying that a certain value should be “super-logarithmic”? What do they mean when saying that a certain value should be “super-logarithmic”? I've found the Wikipedia definition of a “super-logarithm”, but I'm having trouble understanding how a given value ... 2answers 44 views ### Are there some well-known crypto APIs that leak information? Are there any well-known crypto APIs or libraries that leak or used to leak information (it can be the key or anything else)? 2answers 107 views ### Importance of round complexity in determining the efficiency of an MPC protocol Literature has different ways of specifying complexity of an MPC protocol: computation complexity that measures the number of (assumed primitive) operations performed by all the parties; ... 1answer 140 views ### Is the use of AES in LoRaWAN correct? I'm reading about LoRaWAN (specification). This network protocol has a particular way to encrypt data, detailed in the part 4.3.3.1. Here is a recap of how the encryption work, as I understood it. ... 2answers 151 views ### Should I add randomness at the end before AES? Seen AES, changing a character drastically affects the encrypted text. I want to make it harder to decrypt AES I send. So, it seems this can be done by adding some special code and some random bytes <... 1answer 277 views ### AES 128 bits with 12 bytes IV vector composed of a 4 bytes counter : safe? I have a 128 bits AES cypher (GCM, Galois/Counter Mode) using a 12-byte initialization vector (IV) composed of: 8 constant bytes: 6 bytes (device ID) 2 bytes (zero) 4 bytes (incrementing frame ... 3answers 132 views ### Shamir's Secret Sharing vs. Asmuth-Bloom scheme I need to make use of a secret sharing scheme and I don't really know how to decide which one to use, Shamir's or Asmuth-Bloom (using CRT). The complexity for recovering the secret seems to be linear ... 1answer 152 views ### How to reverse engineer a cryptography algorithm if some input-output pair is known. I am trying to reverse engineer an unknown encryption algorithm. By this, I mean I dont know what kind of algorithm is used to encrypt the words. But I know some input-output pairs. From this point ... 1answer 294 views ### What's the safest TrueCrypt algorithm and hash? I have an encrypted file container inside my NTFS pendrive. The root folder Write permission is blocked for everyone. A folder inside it has all permissions unblocked. The dummy file of TrueCrypt is ... 2answers 291 views ### Finding Private key in RSA with public key, cipher text and plain text Is there a known 'non-brute force' method of determining a private key in an RSA system when all other parameters are know? I found the values of a ciphertext ($C$), its corresponding plaintext ($P$) ... 1answer 131 views ### How do I attack an RSA setup where e is even? As a challenge, we were shown an RSA setup where we have a big$n$(617 decimal digits),$e=4$, and asked to recover three messages. (However, the messages are padded to the same length as the key ... 1answer 304 views ### Periodic One Way Function Is there any notion of a periodic OWF? I.e. an OWF$f$that on every input$x$in its domain it holds that$f(f(f\ldots f(x))\ldots )=x$when$f$applied many times, each time on the previous output.... 1answer 90 views ### What is the correct name for bytes which are to be signed? I have some bytes, and I sign them with my private key using ECDSA. The resultant (71) bytes are the signature. But what is the name of the bytes I have signed? (If this were encryption, they would ... 1answer 113 views ### Is md5(x)&md5(y&x) secure? I was wondering if the following hash function f(x) := md5(x) & md5('abc'&x) (with & as concatenation operator) is secure. This schema can be even ... 1answer 303 views ### ECDH or RSA more secure for symmetric key wrapping? Suppose a message is encrypted with a symmetric block cipher with a random key. RSA is often used to wrap the symmetric key using the recipient's public key. In this case, the size of the message is ... 2answers 69 views ### Hiding identifying data while allowing matching? I do research with people about their illegal drug use, and for one project I'm doing surveys with them when they visit a needle exchange. So over time the same people get repeatedly surveyed as they ... 1answer 226 views ### Encrypt with one cipher and decrypt with a different cipher? I recently installed an android app called Secret Space Encryptor and think I found a serious flaw in it. Let me explain: This app provides a variety of different ciphers to use and I used Threefish ... 2answers 149 views ### What is the difference between the standard representants of$\mathbb Z/q\mathbb Z$? The symbol$\mathbb Z/q\mathbb Z$(given that$q$is prime) represents the prime field$\mathbb Z_q$. Basically, the elements of this field are represented by$\{0, 1, \dots, q-1\}\$, let's call this ...

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