All Questions

0answers
43 views

Is there a way to analyse xxHash and tell, either by human analysis, or with an automated tool, that this hash function is or is not cryptographic? [duplicate]

After reading some excellent papers on SipHash, I understood that good non-cryptographic hashes such as MurmurHash and CityHash are not secure for Internet usage, due to a certain type of DDos attack ...
2answers
335 views

Is storing original file size in an encrypted file header a bad idea?

I'm encrypting a file using AES-256 in CBC mode. I'm padding to the 16-byte input multiple by using the PKCS#7 limit. The problem I'm currently having is that if I'm transferring or reading the ...
2answers
293 views

Implementing PKCS#7 Padding on a Stream of Unknown Length?

I have a fairly simple Python program using PyCrypto to use AES+CBC to encrypt a stream of input. In order to adhere to the 16-byte input size multiple, I've implemented PKCS#7 by hand. (While I know ...
1answer
66 views

RSA Certificate Owner and Issuer line meanings?

Apologies if this question is not in the right place. I've recently been looking through some RSA certificates ( specifically, Android apk .rsa files) and I'm having trouble deciphering the one and ...
1answer
129 views

Why is Blum-Goldwasser not IND-CCA-2 secure?

Consider the Blum-Goldwasser encryption scheme as described on Wikipedia. I was told that it was not IND-CCA-2 secure. I heard there was malleabilty, probably it has to do with XOR-ing. But I do not ...
1answer
60 views

Could use an explanation of the notation for an oracle adversary

In the definition below, what exactly does it mean for the adversary (not even sure that's the correct term?) to equal one? $$\underset{K}{Pr} \left [A^{F_k(\cdot)} = 1 \right ]$$ Source (Page ...
1answer
544 views

AES key/ciphertext space sizes

This is giving me a brain ache now... If I have AES-128, block is 128 bit, then every plaintext (128-bit) can be encrypted to some ciphertext that is also 128-bit. This is the block size. But: 128-bit ...
3answers
274 views

File encryption with one keypair?

I'm working on a program that uses an ECC keypair in a (password protected) PKCS12 file (.pfx) to encrypt files. I like this method because I think it will be higher security (using ECDH to negotiate ...
3answers
326 views

Master keys for decrypting?

I'm not a Cryptography expert, but i've seen this topic sometimes on the Web: Master decrypt keys. Most of the people said that "Master decrypt key" does not exist. This makes sense if you developed ...
0answers
47 views

How can I validate generated session keys in SRP-6a protocol

I have genereted two equal session keys on client and server successfully. But I couldn't understand how each other should validate that they have same session keys. On the client I generated a SHA1 ...
1answer
107 views

What are unified addition and differential addition in elliptic curve point arithmetic?

A lot of papers use these terms but I do not find a proper explanation of them. Can somebody tell the meaning / difference / intuition / application and if possible with an example.
2answers
666 views

Why is RSA encryption significantly faster than decryption?

I am designing an implementation of RSA . I recorded computation times in Java by using System.currentTimeMillis(). It returned an encryption time of 0.05 ms and ...
2answers
389 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 ...
1answer
243 views

Implementations of Ntru TLS

I posted this in a reply to What is the post-quantum cryptography alternative to Diffie-Hellman? but since it's actually another question, it was deleted. Has anyone come across any implementations ...
2answers
870 views

what kind of hash function can provide a short hash and be collision resistant?

When you try to connect via SSH, you see a signature which is short but I heard it is even stronger than sha256. It is perhaps stronger because it uses more rounds. Is there a hash function or a ...
1answer
366 views

How can an S-Box be reversed?

So, as an exercise to greater understand some things about cryptography, I decided to try and implement GOST 28147-89, which uses eight 4x4 S-Boxes. I can grasp the concept of putting 4 bits through ...
1answer
561 views

Is it possible to generate an RSA SHA1 signature only from a public key?

Is it possible to generate signature only from Public key. See example below where i have valid signature but what if I change hash ? Is then any way to do this? Public key(160 bytes): ...
3answers
272 views

Is sharing the modulus for multiple RSA key pairs secure?

In the public-key system RSA scheme, each user holds beyond a public modulus $m$ a public exponent, $e$, and a private exponent, $d$. Suppose that Bob's private exponent is learned by other users. ...
2answers
186 views

AES encryption with shared IV

Sorry about the horrible title, didn't now what to say. I'm trying to avoid having to send an IV with every message purely to keep packet sizes down, so I thought of a method to establish a "IV ...
1answer
165 views

Scrypt as a KDF with one-time high-entropy input

I'm looking at using Scrypt as a KDF. Assume the following: the input will always be high-entropy random bytes generated by a CSPRNG the length of the input can vary from between 8 to 32 bytes the ...
4answers
289 views

Encrypting a key with the same key

I am new to crypto and trying to understand why it would be insecure to use AES to encrypt a key with the same key. Basically, something like this: encrypt(key, key) What happens when both key and ...
1answer
173 views

Low complexity implementation of a small blocksize cipher (< 64 bit)

Searching for "small blocksize cipher" finds a number of discussions on the topic, mostly refering to FPE. This one in particular suggests using AES as the round function of a Feistel network. The ...
1answer
455 views

Extract private RSA key from USB cryptographic token using Bardou et al. attack (variant of “million message attack”)

There is a side channel attack on tamper-resistant USB cryptographic tokens using padding-oracle, described by Bardou, Focardi, Kawamoto, Simionato, Steel and Tsay, titled "Efficient padding oracle ...
1answer
3k views

Why is triple-DES using three different keys vulnerable to a meet-in-the-middle-attack?

Among other sources, this wikipedia entry states that triple DES using three seperate keys (k1, k2, k3) is vulnerable to meet-in-the-middle-attacks, while triple DES using only two keys (k1, k2, k1) ...
1answer
457 views

How to break AES CBC with PKCS#5 padding?

If I have a string encoded with AES CBC with PKCS#5 padding, how can one possibly decrypt/crack it? What are the approaches that one can take to break its security?
0answers
88 views

SRP6a client server exchanged message order

In SRP6a, the public key of server is send after receiving the public key of client A. Its that okay that B send along together with s right after client send the username, and then later on client ...
1answer
489 views

Use of CBC-AES-256 to encrypt usernames

I came across some code in our project that encrypts usernames (limited to 50 chars, most are less than 10) using the following parameters: IV size is 16 bytes Key size is 32 bytes Algorithm is ...
1answer
696 views

Selective format-compliant JPEG encryption?

I am working towards building a format-compliant encryption system for pictures. The aim of it is to be able to obscure specific areas of a picture (i.e. faces, car license numbers...) while keeping ...
3answers
243 views

Is signing a hash instead of the full data considered secure?

I reference object in some of my code in hashes. The signature is itself is another object separate from the one I am signing (for obvious reason). I consider simpler to simply sign the reference to ...
1answer
126 views

ASCII Message in TV show? [closed]

So I was watching a popular series tonight (Person Of Interest) when suddenly during a frame change I noticed a very quick blue screen with writing. At the time I never thought anything of it as it ...
1answer
228 views

Measuring Shannon's diffusion

Shannon's idea of diffusion is fundamental to cryptography. Besides being a descriptive idea, is there any work on measuring or expressing it? Saying something like "System A has more diffusion than ...
0answers
121 views

Why does the SRP6 calculation of B included a multiplier k = 3?

In SRP6 $B$ is calculated as $B=kv+ g^b, k=3$. What is the purpose of $k$, and why was it fixed as $3$? (In SRP 6b, this value $3$ is replaced by $k = H(N,g)$, but this question is about SRP 6.)
3answers
688 views

PBKDF2 for key diversification

I am looking for a secure key diversification function to create individual AES keys for a local smart card deployment. The keys need to be derived from a secret master key and the smart card serial ...
1answer
141 views

MD5 implementation

As a personal project, I want to implement MD5 on an FPGA, but I have some doubts about the specifics of the implementation. My first source of how the algorithm is implemented was the RFC 1321, where ...
2answers
189 views

Is semantic security important in a hybrid cryptosystem?

RSA doesn't provide semantic security when used unmodified, and neither does the commonly used PKCS#1 v.1.5 padding scheme for encryption. Is this a problem for hybrid cryptosystems at all? My ...
1answer
333 views

Is AES reducible to an NP-complete problem?

Is breaking AES NP-hard? Can the security of AES be reduced to a NP-complete problem? If it is reducible, what does the reduction look like? If it is not reducible, why do we assume it is secure?
1answer
153 views

Any historical accounts of cryptanalysis of Jefferson's wheel cipher?

David Kahn in his book "The Codebreakers" wrote about Jefferson's wheel cipher, saying that To this day the Navy uses it… (the book was first published in 1967) Are there any historical accounts ...
1answer
1k views

Why is Diffie-Hellman considered in the context of public key cryptography?

In all textbooks I used the Diffie-Hellman key exchange is under "public key cryptography". As far as I can see it is a method to exchange a key to be used with a symmetric cryptographic algorithm, ...
2answers
477 views

Attack XOR encryption of binary data compressed by zlib with known key length (very short key)

I am trying to break a packet format. The packet format simply packs several files into one big file. The file contents are plain. But the index data which contain offsets, file sizes and filenames ...
2answers
153 views

Can the hash of one message be used to make it easier to find the hash of a very similar message?

Background: I am trying to get an understanding of using a hash of a passphrase as a secret. Example: ...
1answer
406 views

Can we replace the XOR operation in DES with some other operation?

Can we replace the XOR operation in the DES algorithm with some other operation? If so, does it work for both encryption and decryption?
1answer
145 views

ElGamal signature: Forging a signature of a specific form

I have a question I can't solve from one of the courses I'm currently taking: Show that given a legitimate ElGamal signature $(S,R)$ on a given message $m$, an attacker can compute a signature ...
1answer
177 views

brute force attack on KDF vs KEY

if we believe 256bit keys are secure in AES against any brute force attack, is that possible we use a KDF (convert user's weak entered password in truecrypt into a fixed AES key) with too much round ...
2answers
144 views

Name for identical operations for encryption and decryption

Is there a name for the property of a cryptographic algorithm that the operations for encryption and decryption are identical, i.e. $E_k(x) = D_k(x)$ and accordingly $E_k(E_k(x)) = x$? An example ...
1answer
163 views

How to bring little changes in DES algorithm

I thought of implementing DES modes of operation as a project...But my guide asked to change or to modify the DES algorithm in such a way to overcome drawbacks of DES...So is there any way to do ...
1answer
164 views

What are rounds in the context of ciphers?

Another recent question on security margin included in its answer the discussion of how many rounds are broken so far. What is the definition of a round?
1answer
163 views

Request for 1024-bit primes $p$ , subgroup $q$ and subgroup generator $g$

I need to find a prime $p$ of $1024$ bits with a $160$ bit sub group size $q$, such that $q|p-1$ , and $g$ is the generator of the sub group size $q$. I'm looking for the numeric values of $p$ , $q$ ...
1answer
109 views

Secure way to compare two secret numbers without disclosing them?

For example, two people want to know who is the older, without disclosing their age. Is there a secure way to do this without a third party involved?
1answer
172 views

AES implementation in java that allows key of 320-bit length [closed]

I need an implementation of AES(Rijndael) in java that allows use of a 320-bit key. Please suggest a reliable implementation
1answer
348 views

Hashing a password with SHA256 on the client then bcrypt on the server

I'm trying to implement a cryptographically secured storage site (not Mega, or anything similar) and am trying to prevent the user's password from ever touching the server. The password is used to ...

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