# All Questions

801 views

### How good is using AES CTR mode with initial counter as 0?

Is it advisable to use AES CTR mode with the counter initialized to 0? How good is it in security terms?
397 views

### LFSR using words

If I've got an LFSR, let's say a 16-bit Fibonacci LFSR as shown in the corresponding wikipedia article, which generates maximum length sequences, could I use it to create word sequences instead of bit ...
106 views

### How do these figures represent a boolean function?

In the Wikipedia article "Bent functions", there are some figures representing those Bent functions: How do these figures represent a boolean function ?
1k views

### How much is slower GCM AES-128 than one of the stream chipher eSTREAM?

Evaluate the speed of the algorithm GCM AES-128 and compare it whith speed of one of the stream chipher in eSTREAM. Can someone give such an assessment?
204 views

### Decryption possible only within a period of time

Time-locked encryption and time-released encryption seem to be security systems focused on making sure that the decryption of certain message cannot take place before a certain time has passed. ...
189 views

### How should I interpret this note on diffusion of the internal state of a PRNG?

This question is about Tyche, a non-cryptographic PRNG. While the PRNG is not cryptographic the analysis and this question surely is heavily involved with cryptography, so I think it's on-topic here. ...
850 views

### Why does second pre-image resistance imply pre-image resistance

I am studying hash functions. I can understand why collision resistance implies second preimage resistance, but I don't get why second preimage resistance should imply first preimage resistance. ...
265 views

### Could completely public passphrase hashes ever be reliably secure?

This is a hypothetical question and I only have a basic understanding of Cryptography. If one were to follow the very best cryptographic practices for storing passphrases, could it ever be possible ...
350 views

### Can I dynamically calculate an appropriate number of iterations for PBKDF2 based on the system time, rather than using a fixed value?

Could you choose the number of PBKDF2 iterations based upon the system time? I've heard a few people recommend that the number of iterations for PBKDF2 should be doubled every two years (starting ...
104 views

### Recasting randomly generated numbers to other widths

Suppose I have a list of randomly generated uint_8's, uniform over the whole range of uint_8. I write them into a file as raw ...
92 views

### Is it possible to generate a message using the mac and the key?

Say that you have the Key and the MAC value that was generated from the Key using CBC. Is it possible to generate a message with just the mac and the key? If so, how would you do it?
136 views

### Can stream ciphers (usually) be “run backwards”?

Is it possible to reconstruct the previous output bits of a stream cipher, e.g. RC4, when only the current state is known, or is that computationally hard, or even impossible (due to ambiguous ...
164 views

### Security of cloud computational protocols in UC Framework?

The universal composability allows one to the analyze security of cryptographic protocols . But it does have some gaps when it comes to analyzing few protocols especially two party cases when there is ...
143 views

### Camellia cipher - is it a symmetric or asymmetric cipher?

I read a wiki page about Camellia cipher and know that it is a block cipher that can use 128-bit, 192-bit or 256-bit keys. But I cant find any information about if its a symmetric cipher (uses one key ...
167 views

### Public Keys on Social Media

Is there any problem with using social media (i.e. Facebook) as a directory of public keys? For example, couldn't Alice put a note containing $(g^a \bmod p, g, p)$ on her Facebook page so she could ...
135 views

### Can OTP still be broken if there are several messages being sent with only a slight variation in a password?

If an attacker were able to get a hold of 2 encrypted messages that were both encrypted in a one time pad with a password that were each similar but had one change in the first character, could it ...
190 views

### Why must we hash lot of times in rainbow tables?

I don't get the utility to chain and reduce in generating rainbow tables. Hash functions are made to not have collisions, or at least chain only 2 times so we don't return to the concept of the hash ...
2k views

### How is input message for SHA-2 padded?

I read about how is an input message prepared to be hashed by MD4,MD5 or SHA-1: Step1 Append padding bits The input message is "padded" (extended) so that its length (in bits) equals to 448 ...
130 views

### Can you explain what the AES paper means by “sharing active S-boxes”?

I am reading the "Biclique cryptanalysis of the full AES" paper. What do they mean by "sharing active S-boxes"? How can this concept can be advantageous to make a bicycle? If there is someone who ...
1k views

### How does SafeNet MobilePASS generate passwords?

We use MobilePASS at work but the latest version of the android client seems to be buggy so I wanted to have a go at implementing the algorithm myself. You can download the client to play with here: ...
185 views

### Pseudorandom functions

The usual case to distinguish a pseudorandom function from a random function is to assume that the adversary can choose the plaintext blocks. Is there another case (game) in which the adversary can ...
220 views

### Are SSL modes with forward secrecy vulnerable to the BEAST attack?

From the original BEAST paper, it seems to me that you need the same session key when encrypting data, so you can check if the original plaintext block is the same as your chosen plaintext (which is ...
507 views

I'm performing ElGamal encryption algorithm and using the additive homomorphic property so the product of two ciphertexts is the encryption of the sum of the plaintexts. The problem is that I need to ...
166 views

### No IV for one off symmetric file encryption

My question is (hopefully, for somebody) a simple one; but my project is holding as I'm not sure. I read on SO that generating encryption keys without an IV is a bad idea, so is using a constant IV ...
245 views

### HMAC and assumptions on the cryptographic hash

According to Wikipedia, a cryptographic hash function has the following properties: Pre-image resistance: Given $h$, it's difficult to find any message $m$ such that $h = H(m)$. Second pre-image ...
285 views

### How insecure in practice?

I am in attempt to understand relative insecurity of certain encryption schemes. Particularly of interest is DES and RC2. I know AES is better and should be used to encrypt. But practically, if ...
810 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?
623 views

### Could quantum computers “break” symmetric crypto-systems (e.g. AES)?

These days I'm reading about quantum computing and quantum cryptography which I've found extremely interesting. Well, I also read some blog posts of Bruce Schneier talking about how quantum computers ...
148 views

### Is this algorithm secure?

I recently found this site, proposing a hashing algorithm for passwords. They describe the following: pad the password on both sides with SHA1(email) to ...
967 views

### Is Base64(SHA1(GUID)) still unique like the original GUID?

Basically what the title is; GUIDs are unique by design. If you run the GUID through SHA1 and then Base64 the hash, will the resulting string have the same guaranteed uniqueness as the GUID, or not?
483 views

### How does OAEP improve the security of RSA?

The heart of OAEP algorithm used for RSA encryption are the cryptographic hash functions $H$ and $G$. Does everybody (so also an adversary) know these functions? If YES: How does it help the ...
253 views

### Many consecutive hashes to slow down brute force attack?

I've heard that hash algorithms like bcrypt are more secure because they take longer to complete, and therefore take much longer to prute force, without a noticable ...
685 views

### risk of attacker decrypting RSA ciphertext without public or private key

As I describe in my previous question I am trying to decide if it's worth it for me to use the Offline Private Key Protocol in creating some long term private archives, instead of just going with a ...
320 views

### Why is $h(H, m) = E(m, H) \oplus m$ insecure?

I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies­-Meyer or Miyaguchi-Preneel are ...
471 views

### Attack on DSA with signatures made with k, k+1, k+2

For homework, I'm asked to find the private key, $x$, in a DSA digital signature scheme. In the particular instance, we are given the parameters $p$, $q$ and $g$, the public key $g^x$, 3 messages ...
821 views

### Brute forcing an HMAC

Given current technology such as GPUs and GPU cracking software I was wondering if anyone has an idea on how long it would take to brute force the key used to derive an HMAC?
253 views

### Proof of the standard pseudorandom generator + XOR encryption scheme in Goldreich

Reading Goldreich's Foundations of Cryptography II, I found this proof for the security of the common pseudorandom generator + XOR encryption scheme (Proposition 5.2.12 in the book): Assume you ...
125 views

### Padding always the same, problem or not?

I need to transmit long encrypted messages to a smartcard over a limited capacity link so I need to fragment the messages somewhere before sending thzm. My problem is that I split the message into ...
513 views

### What are the differences between proofs based on simulation and proofs based on games?

what are the main pros and cons of proving the "security" of a crypto scheme under simulation proofs instead of game based proofs?
387 views

### Is there a length-preserving encryption scheme?

Is there a length-preserving encryption scheme, that preserves the lengths of input sizes such that the length of the input plain text is same as length of the output cipher text ?
768 views

### Do known-plaintext attacks exist for public key encryption?

In asymmetric ciphers we publish the public key for anyone, which means an attacker can encrypt any message they want and compare the ciphertext and plaintext without communicating with the owner of ...
813 views

### Can a shift cipher attain perfect secrecy?

On a practice question for my intro cryptography exam, it asks the following: Assuming that keys are chosen with equal likelihood, the shift cipher provides:    A) computational security ...
498 views

### Diffie-Hellman is Post-quantum secured?

If Alice and bob have a secure channel for key-exchange and mallory don't man-in-the-middle attack them but in the future eavesdrop connection and see the key exchanged, can mallory break it like RSA ...
2k views

### Is it safer to encrypt twice with RSA?

I wonder if it's safer to encrypt a plain text with RSA twice than it is to encrypt it just once. It should make a big difference if you assume that the two private keys are different, and that the ...
1k views

### How can I use eulers totient and the chinese remainder theorem for modular exponentiation?

I'm trying to implement modular exponentiation in Java using Lagrange and the Chinese remainder theorem. The example we've been given is: Let $N = 55 = 5 · 11$ and suppose we want to compute ...
302 views

### Increase number of rounds for SPN and Feistel ciphers

Read a post on Schneiers blog (and again 2011) about increasing the number of rounds for AES from to "AES-128 at 16 rounds, AES-192 at 20 rounds, and AES-256 at 28 rounds" to raise the security. ...
587 views

### One time pad key exchange

If two parties want to communicate with securely using OTP , how is the key ( that is lengthy as plain text) shared with other party for decryption ?
113 views

### When is each key used when encrypting an email using OpenPGP?

When you send an email using PGP to encrypt emails, is the recipients public key used to encrypt the email, or is your private key used? Are they both used? At what points do each of the four keys ...