All Questions

2answers
957 views

Finding a keylength in a repeating key XOR cipher

In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against: Discover the length of the key by counting ...
2answers
1k views

Why is asymmetric cryptography bad for huge data?

I've been told that asymmetric cryptography requires that the message to be encrypted be smaller than its key length. Why is this? I know about hybrid encryption, which uses symmetric encryption to ...
2answers
204 views

Are there any papers explaining how to port textbook RSA to realworld RSA?

Are there any papers, books, or link where it explains how the plain vanilla "textbook RSA" is actually implemented in practice with all the padding and stuff? Basically, I would like to know the ...
2answers
97 views

Diffie-Hellman insecure on addition modulo $n$

Assume that the group $G$ is the set $\mathbb{Z}_{n} = \{0,\ldots, n-1\}$ for a 1024 bit integer and $+$ is addition modulo $n$. Then why would Diffie-Hellman key exchange in this group be insecure?
2answers
214 views

Map Bytes to Number

Using node.js crypto library I can generate cryptographically strong bytes. How can I use this to generate a random number between 0 and 9999 (both inclusive)?
3answers
650 views

Export from US of crypto software with key-size > 56 bits still needs permission?

Elsewhere someone stated that he couldn't publish his crypto software on the Internet, because US Export Regulations require approval, if the key size is greater than 56 bits. I know that several ...
3answers
288 views

What is the difference between a HMAC and a hash of data?

On a recent question it became apparent that there's a significant difference between an HMAC of input data and a hash of input data. What exactly is the difference between an HMAC and a hash of a ...
3answers
140 views

Can the encryption exponent e be greater than ϕ(N)?

So I was just wondering in RSA, can the encryption exponent e be greater than ϕ(N)?? For an examples sake, lets just say N = 707, so p = 101 & q = 7. So, we have ϕ(707) = 600. Can I have e = ...
2answers
2k views

How to perform file encryption using 128-Bit AES?

I am confused, how can I encrypt a file using 128 Bit Advanced Encryption Standard? Do I need only to encrypt the file name and it's content or is there something that I need to do to encrypt it? Is ...
2answers
270 views

Feedback on rolling my own entropy gatherer

First of all, I don't recommend doing this. This was something I created when I didn't know better and didn't have a solution available to me. Long ago I created my own entropy gather for a ...
2answers
281 views

Why not use an algorithm's code rather than data itself for one time pads?

Instead of using actual data for one time pads, what would be different if the code for an algorithm was exchanged so that to acquire the pad one would execute the algorithm and use its output? Why ...
4answers
899 views

Webapp password storage: Salting a hash vs multiple hashes?

For security's sake, of course it's blasphemous to store passwords in plain-text; using a hash function and then doing a re-hash and comparison is considered much better. But, if bad guys steal your ...
2answers
385 views

Are there public slow-but-strong algorithms out there that resist brute-force attacks better?

I'm reading that AES uses 4x4 bytes (4*4*8 = 256 bits key) matrix for performances matters (since it's a requirement for common standard encryption algorithms), but are there implementations with ...
3answers
540 views

Is public-key cryptography the only option in this scenario?

Two parties: a client and a server are to a agree on a symmetric key. Both the client and the server are aware of a master password. The way this is currently done ...
3answers
140 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 ...
2answers
184 views

Avoiding overflow when encrypting with RSA

When encrypting with RSA one calculates $m^e \pmod n$ by doing the following: m^e % n Where $m$ is what we encrypt. Often $e$ is a very big number to make it ...
2answers
215 views

DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
3answers
293 views

Timing Attacks on ECDSA, ECDHE, AES and SHA2

Are there any known timing attacks (both practical and theoretical) on any implementations of the following? ECDSA (I'm aware of this one - are there any applicable to prime fields?), ECDHE (again, ...
3answers
186 views

How to attack a fixed LCG with partial output?

To show some colleague programmers exactly how broken C's rand() is (at least on Windows) I decided to break it. So everyone knows the exact parameters, MSVC's ...
2answers
993 views

Why is AES not a Feistel cipher?

I am studying for an exam right now. And I wanted to make sure I got this point correct. AES is not a Feistel cipher because the operations in AES are not invertible. Is the above statement ...
2answers
420 views

How is the key shared in symmetric key cryptography?

Symmetric key cryptography is an encryption system in which the sender and receiver of a message share a single, common key that is used to encrypt and decrypt the message. Is the key public or it is ...
2answers
141 views

Can you really insert the text you want in one-time pad?

The Wikipedia article "One-time pad ~ Authentication" says : For example, an attacker who knows that the message contains "meet jane and me tomorrow at three thirty pm" at a particular point can ...
1answer
418 views

CBC-MAC , fixed length, all blocks returned

CBC-MAC, with fixed length message. Is it safe to return all ciphered blocks instead of the last? My intuition says it is less secure, since is gives an attacker more information. But how could one ...
2answers
763 views

AES-CTR vulnerability to cryptographic oracle

I am new to cryptographic issues and from what I googled so far I could not retrieve the information I need. Consider the use of AES-128 in CTR mode. Let M be the set of possible plaintexts, for ...
2answers
347 views

Theoretical pi-based stream cipher

Let's pretend that all digits of pi are known and arbitrarily long sequences of digits are trivial to get. Further, some mathematician proves that there are no patterns in pi. We could create a stream ...
1answer
355 views

What's the main difference between Pohlig-Hellman and RSA?

Both Pohlig-Hellman and RSA perform encryption and decryption by exponentiation modulo some integer ($p$ prime for PH, $n$ composite for RSA). They both use a key $e$ as the exponent to encrypt a ...
2answers
2k views

Is it possible to obtain AES-128 key from a known ciphertext-plaintext pair?

I have a file, which was encrypted with AES-128 in ECB mode. I know the format of the original file and know that all files in this format have the same headers. So, I have an encrypted block and the ...
3answers
272 views

How to encrypt data and know it will be secure for at least a few decades?

I want to encrypt a small amount (few kilobytes) of data and be fairly confident it will be secure against brute force decryption and advances in cryptanalysis for at least a a few decades (or more). ...
1answer
222 views

What is the name of this kind of protocol

There is a communication protocol that I believe creates the equivalent of a one time pad, with the downside that the secret message must be transferred multiple times. The protocol is so simple that ...
2answers
223 views

Can a “pattern” in a series of passwords be detected from their hashes (and maybe a single raw password)?

Let's say I'm a lazy user of a system with annoyingly frequent password change policies. I may have a "good" password I used initially that is only used for this system but since I have to change it ...
2answers
268 views

Is there any recent cryptographic algorithm especially designed for low-level processors?

Most modern algorithms require relatively large amount of resources. Is there any recent (and freely usable) encryption/decryption algorithm which is specially designed for low-level microcontrollers ...
3answers
466 views

Getting started [closed]

I'm looking for a good place to start in cryptography and places to go to get free books etc on the topic. I have been looking online but I always get stuck at some point or another. I need something ...
2answers
327 views

Key space size when either of two public keys are valid for authentication?

If for authentication a user can own either A OR B public key instead of just one specific key is that equivalent to halving the key space. i.e. it it theoretically twice as easy to brute force and ...
2answers
1k views

Decrypting DES with decrypted and encrypted data

I got two 8-byte strings. One which is decrypted is: FF FF FF FF FF FF FF FF and one which is encrypted is: ...
2answers
175 views

Protocol to generate Client Certificates at the start of a SSL session automatically?

A more secure form of 'cookie' could be created for SSL communications through the following method. The client generates and requests the server to sign a certificate. Then the client authenticates ...
2answers
134 views

Do deterministic secret sharing schemes exist?

Are there algorithms for secure secret sharing such that the algorithm depends solely on the value being secured, relying on no randomness in its calculations. If there aren't, is such an algorithm ...
1answer
190 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, ...
2answers
112 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?
2answers
377 views

In RSA, why is it important choosing e so that it is coprime to φ(n)?

When choosing the public exponent e, it is stressed that $e$ must be coprime to $\phi(n)$, i.e. $\gcd(\phi(n), e) = 1$. I know that a common choice is to have $e = 3$ (which requires a good padding ...
1answer
153 views

Can two people with different one time pads securely exchange a message like this?

Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ...
2answers
320 views

Why is sharing the seed and using SecureRandom “deterministically” so bad?

I've recently been reading an article about "using cryptography to store credentials", and I would love it someone explained this in more detail… Up until Android 4.1, Android used a SecureRandom ...
2answers
138 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, ...
1answer
113 views

Is this really the correct way to do an IND-CPA test?

In a description of IND-CPA (indistinguishability under the chosen plaintext attack), I have been reading the following, simple test: The adversary can generate as many messages as he wants. Then, ...
1answer
101 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 ...
2answers
81 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 ...
2answers
256 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? ...
4answers
350 views

Tamper-proofing log files

Problem Overview I want to securely store log files so the contents are secret, and they can't be modified without detection. The files will be encrypted using authenticated encryption (AES in GCM ...
2answers
339 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
429 views

Why does the recommended key size between symmetric and assymetric encryption differ greatly?

In various articles it is mentioned that for secure communications, the recommended key sizes are 128-bit key size for symmetric encryption (which makes it $2^{128}$ possible keys?) and 2048-bit key ...
3answers
492 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 ...

15 30 50 per page