# All Questions

2k 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 ...
338 views

### Number of states in a LFSR

Do all $2^{\ell}$ (where $\ell$ is the bit length of the shift register) states always occur in a LFSR or can I choose my taps badly so some states are skipped and the period is shortened? If so is ...
5k views

### SHA-256 and AES-128

Does it make an encrypted string more secure if I use SHA256(x) instead of x as the secret key for AES-128 encryption? I do know that SHA-256 produces 64 characters of hashed string regardless of what ...
215 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 ...
133 views

### Does a stream cipher require a KDF when it's key is variable length?

RC4 has a variable length key. Is it still necessary to use a key derivation function, such as PBKDF2, to initialize it? Would using a single character password weaken it's output when compared to a ...
240 views

### Regarding Key Strength with DES and Blowfish

I had a small discussion with my friend on combining two encryption algorithms based on 64-bit keys. Say I have a plain text of 64 bits which I want to encrypt. My Key size is 128 bits. I will use ...
245 views

### Can all asymmetric key pairs be reversed?

After reading about public/private key cryptography, there are still a few points I don't really understand. Based on the way (I understand) challenges work, the public key is used to encrypt a ...
796 views

### Theoretical attack on RSA

The high security of RSA is granted, because it is very hard to factorize $$N = p * q$$ Nevertheless, there is actually no need of factorizing $N$, in order to generate the $Private$ $Key$, but the ...
604 views

### Difference between a nonce and IV

I know the generic difference between a nonce and an IV. I am specifically looking for a clarification on these terms as used in the "Evaluation of Some Blockcipher Modes of Operation" by Phil ...
248 views

### Is RSA encryption of a cryptographic hash with a private key the same as signature generation?

It is often said that RSA encryption with a private key is the same as signing (signature generation). Will RSA encryption with a private key over a cryptographic hash give the same result as ...
122 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?
454 views

### Is there a cryptographic method to add noise to a plaintext instead of actually encrypting it?

I'm looking for a cryptographic technique that instead of permuting a cleartext, would add noise to the cleartext to obfuscate the true text among a bunch of garbage. For example, a list of 10 ...
428 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, ...
2k 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 ...
267 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)?
507 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 ...
163 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 = ...
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 ...
910 views

### How do public key rings work in cryptographic applications?

I am wondering if there are any links to articles or resources available online, or explanations that you can give, that would help me to understand the concept of a public key ring, and why I might ...
542 views

### Does RSA padding have to be unpredictable if the payload is?

I'm trying to understand the precise requirements on padding when using RSA for encryption. Suppose Alice uses RSA to encrypt a payload $M$ that cannot be guessed (say, a random nonce): Alice send ...
435 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 ...
647 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 ...
111 views

### CSPRNG that cannot be used as random extractor

Are there any specific CSPRNG's that cannot be used as randomness extractor within a TRNG? In other words, can you simply provide enough entropy gathered from a good entropy source as seed in any ...
111 views

### Associative standard cryptographic hash function

I am looking for a standard hash function which satisfies the following property: A hash function $H(a,b) = F(h(a),h(b))$ with $h$ (within $F$) any standard cryptographic hash function and $F$ an ...
384 views

### Shamir's secret sharing scheme - type of channels

For information theoretic security in Shamir's [m,m] secret sharing scheme, do i need both authentic and confidential channels? Another related query is ;is it true that if the channels are only ...
307 views

### Leaving authentication data blank less secure for AES GCM?

The AES GCM example for Ruby / OpenSSL shown at: http://ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html has a GCM example of the following: ...
238 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 ...
343 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?
336 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 ...
1k 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 ...
2k views

### Initialization vector in symmetric-key encryption

Can we use symmetric-key algorithms without an initialization vector? I am making an app where both the sender and receiver share a key and there is no way to create an initialization vector for each ...
174 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 ...
309 views

### Finite fields and ECC

I understand modular arithmetic(or at least I think I do!) and I've tried to read and learn about how the Math in RSA works(and I think it went pretty well). I've been reading up on ECC and it looks ...
639 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 ...