# All Questions

15k views

### RSA encryption with private key and decryption with a public key

When using the RSA cryptosystem, does it still work if you instead encrypt with the private key and decrypt with the public key? What about in the case of using RSA for sender authentication?
797 views

### How can I improve a password generation scheme based on a shared secret and URL?

I currently use the following method to generate a different password on every website I have to login: password = SHA1 ( mainPassword . domainName . number ) ...
1k views

### Can someone explain the ECB Penguin?

I've seen the ECB Penguin used to demonstrate why ECB is not a recommended method of encryption, but I do not understand how this translates to text or passwords. Aren't the people who create these ...
2k views

### Is there a way to make RC4 (ARCFOUR) secure, or is it completely broken?

I need a method to authenticate a process with another in order to establish interprocess communication between them, to prevent malicious processes from trying to hook onto the system. Currently I ...
296 views

### Using hashes as passwords

I have thought of a system for generating passwords which works as follows: Take the following items: A password, such as williamwallace. A secret 1000 digit ...
1k views

### Pre-image resistant but not 2nd pre-image resistant?

Are there any cryptographic hash functions for which there is a known pre-image attack, or a known second pre-image attack, but not both? The attack doesn't have to be practical - just anything that ...
495 views

### Can I determine if a user has the wrong symmetric encryption key?

We're using the Objectivity/DB object database with a custom encryption plugin that encrypts serialized objects on disk. Encryption uses AES with a shared secret key held by all users. I would like to ...
303 views

### Reduction from signatures to encryption?

Is it possible to construct an (asymmetric) encryption scheme from a signature scheme? If the signature scheme is deterministic and allows existential forgery (e.g. RSA), then the answer is yes ...
627 views

### Are there cryptographic hash functions that can be computed using only paper and pen without leaking any information about the plaintext?

I am looking for a cryptographic hash function that can be computed by a human using only paper and pen without ever leaking any information about the plaintext on the paper. The cryptographic hash ...
656 views

### Can one implement AES on 4-bit microcontroller?

It is possible to implement AES on 4-bit microcontroller? As far as I know, AES on 8-bit microcontroller are common especially for light weight application such as the RFID. I have yet to come across ...
518 views

### Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable?

A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden ...
610 views

### How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?

If cryptanalysts today were to crack the original Enigma machine, how fast/ how easily could they do it? What methods would they use? The original cracking was significantly helped by operator ...
348 views

### Should I delete cryptographic data from memory?

When dealing with cryptographic data, is there a chance that attacker recovers portion of data I used in my program? Should I try to delete every bit of keys and encrypted data that I used, overwrite ...
1k views

### How many RSA keys before a collision?

I was wondering how many possible private/public keys exist? If a million people – for whatever reason – would try to generate 5 keys each in the same minute (on the same date and time) is there a ...
1k views

### Modern integer factorization software

What are the modern software packages that can be used to factoring large numbers into primes. By modern I mean developed and made public within the last 5 years. I'm interested in things that are ...
949 views

### SHA-224 Purpose

One of the new features of Java 8 is the SHA-224 message digest. What is the use case for having a 224-bit-length hash?
2k views

### Unpredictability of X.509 serial numbers

About X.509 certificates serial numbers the RFC 5280 says: The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a ...
461 views

### Why initialize SHA1 with specific buffer?

SHA-1 is initialize with a specific buffer: h0 = 0x67452301 h1 = 0xEFCDAB89 h2 = 0x98BADCFE h3 = 0x10325476 h4 = 0xC3D2E1F0? Why?
2k views

### Why do block ciphers need a non-linear component (like an S-box)?

Why is there a requirement of "Non-Linear functions" as a component of many popular block ciphers (e.g. the S-box in DES or 3DES)? How does it make the cipher more secure? The only intuition I have ...
694 views

### Predicting PRNG given some of its previous output

I a have a question about PRNGs and this is my very first experience with them. I have the following generator that takes a 56-bit seed $p$ during initialization and then chooses both $X$ and $Y$ ...
505 views

### Why should I use Authenticated Encryption instead of just encryption?

There are various different modes of operation for block cipher use, some of which provide "encryption" and some of which provide authenticated encryption. Why should I use an authenticated ...
3k views

### Why is TLS susceptible to protocol downgrade attacks?

A recent blog post from Ivan Ristić (expert extraordinaire on all things SSL) says: all major browsers are susceptible to protocol downgrade attacks; an active MITM can simulate failure conditions ...
261 views

### AES CTR with similar IVs and same key

Lets say there is a piece of software that uses AES CTR to encrypt different messages using the same key but with slightly different IVs So for example, a 16 byte IV, the 2nd 8 bytes are always the ...
1k views

### Why are the constants so simple in Keccak?

Keccak, the construction selected for SHA-3 is very interesting. It seems unlike other primitives and has chosen very simple constants. (Keccak talk PDF) The initial values of the state in Keccak is ...
2k views

### What is the most secure hand cipher?

By "hand cipher", I mean a symmetric cipher for which encryption and decryption can can both be performed with a pencil on graph paper, consuming about 10-20 seconds per character by a proficient ...
848 views

### Is it possible to distinguish a securely-encrypted ciphertext from random noise?

Say I have a bunch of data encrypted with a secure block cipher (such as AES). An attacker has unlimited access to this encrypted data. The attacker doesn't know whether the data is encrypted or if ...
6k views

### How does a chosen ciphertext attack work, with a simple example?

Can someone please explain - using a simple example - how a chosen ciphertext attack works?
345 views

### Choose a random number that is different from a bunch of other secret numbers

I'm looking for an algorithm where n participants each have a different secret number between $[0..x]$ (and where $x$ is known) and where the participants then select randomly another, non-secret, ...
361 views

### Building a hard to factor number without knowing its factorization

It is possible to find an efficient algorithm for constructing a provably hard to factor number $N$, together with a witness that shows that it is indeed hard to factor. EDIT, since it was not clear: ...
3k views

### Why exactly is Blowfish faster than AES?

I've not been able to understand exactly the reason behind Blowfish being faster than AES. Is it dependent on the block size? Or is it processor dependent? (if Yes, then lets assume that AES ...
533 views

### creating a small number from a cryptographically secure random string

i'm trying to figure out the best way to generate a cryptographically secure random number between 0 and 200 from a cryptographically random string of bytes (ie. read from /dev/urandom or some such) ...
2k views

### Use of salt to hash a password

In a few implementations of hashed passwords, I have seen that the length of the random salt is chosen to be, say, 10 or "some constant". Is there any specific reason why the salt is chosen to have a ...
722 views

### What is a hard-core predicate?

I read this article on Wikipedia: Hard-core predicate. Still I don't understand what exactly is a hard-core predicate. Is it possible to put this in simple English terminology, and perhaps with a ...
345 views

### Mapping between subgroups and the integers

This question is a companion to the equivalent question on elliptic curves. Preliminaries Diffie-Hellman, Elgamal, DSA, etc. are examples of protocols that work in the integers modulus a large prime ...
244 views

### Trial divisions before Miller-Rabin checks?

I'm trying to understand prime number generation (more correctly, the primality checking) as described in Handbook of Applied Cryptography. The context is circa pages 145 - 150, and specifically ...
6k views

### How does RSA signature verification work?

I understand how the RSA algorithm works for encryption and decryption purposes but I don't get how signing is done. Here's what I (think) I know and is common practice: If I have a message that I ...
754 views

### Why does DES use exactly 16 rounds?

Why does DES use exactly 16 rounds and not more or less than 16? What is the significance of using 16 rounds?
4k views

### How strong is the ECDSA algorithm?

Some cryptographic algorithms are as strong as the size of their key is, while other have some weaknesses that limit their strength (such as SHA-1). How strong is the ECDSA algorithm, and does that ...
318 views

### Encrypting files with known headers

If I encrypt a file of a known format that has a lengthy header (e.g. an XML Excel file), does that render the encrypted file susceptible to a "known plain text" attack? In other words, if the first ...
653 views

### Is a small size block cipher usable?

Consider a block cipher with a key of size 128 bits but a small block size, say 32 bits. Is this kind of block cipher secure? I would like to know to what extent I can use a small block cipher, like ...
452 views

### RSA leak bits to factor N

Suppose you randomly generate large primes p and q as in RSA, and then tell me N=pq but not p or q. Then, you would like to actually let me factor N, except you should tell me as few bits of ...
2k views

### Is Truecrypt's multiple/cascading encryption safe?

Is Truecrypt use of cascading encryption safe? Is it useful? Truecrypt is arguably one of the most popular and widely used encryption applications in use today, yet it seems to use a very ...
2k views

### Is it possible to validate a Public Key in RSA?

If I have a 1024-bit number, and someone is telling me that it is in fact a valid RSA public key, is there any way I can quickly validate that it is indeed so (without cracking RSA)? (I suppose I am ...
1k views

### Desirable S-box properties

What desirable properties should an S-box have? My current standard selection process is to just pick them at random and verify that they fit the following criteria: The probability that any random ...
1k views

### Is it okay to use a hash of a timestamp as the IV for AES?

The message format includes a datetime field in the clear. Is it okay to also use this field (or some hash thereof) as the initialization vector? In this case, CBC is the mode being used.
747 views

### How is SHA1 different from MD5?

On the surface, SHA1 and MD5 look pretty similar. Their diagrams include chunks of bits, bit rotation, xor and special functions. Their implementations are roughly the same length (at least the ones ...
1k views

### Does Microsoft use Dual_EC_DRBG by default?

So, as we all know, Dual_EC_DRBG contains an NSA back door. At this point, there is no reason to call it a "potential" or even an "alleged" back door; the presence is obvious even to the NY Times. As ...
206 views

### Is the inverse of a secure PRP, also a secure PRP?

If a block cipher is a secure PRP, is it's inverse a secure PRP as well? My intuition says yes but I'm not exactly sure. On a related note, if a block cipher is a secure sPRP, is it's inverse a ...