# All Questions

5k views

### Is there a secure cryptosystem that can be performed mentally?

I, myself, do not plan on getting into a situation where I would be unable to use a computer in order to communicate securely. However, I can think of many practical situations in which mental ...
115k views

### How secure is AES-256?

The cipher AES-256 is used among other places in SSL/TLS across the Internet. It's considered among the top ciphers. In theory it's not crackable since the combinations of keys are massive. Although ...
6k views

### How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?

I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world. How can we reason about the symmetric ciphers cryptanalysis ...
2k views

### How is the MD2 hash function S-table constructed from Pi?

For fun, I'm learning more about cryptography and hashing. I'm implementing the MD2 hash function following RFC 1319 (http://tools.ietf.org/html/rfc1319). I'll preface by saying I know there are ...
21k views

### Basic explanation of Elliptic Curve Cryptography?

I have been studying Elliptic Curve Cryptography as part of a course based on the book Cryptography and Network Security. The text for provides an excellent theoretical definition of the algorithm but ...
4k views

### Examples of modern, widely used ciphers that suddenly fell?

RC4 and GOST are two major ciphers (defined as being widely used to encrypt large amounts of data) that fell to cryptanalysis (relatively) suddenly. The first becoming totally broken and the second ...
4k views

### Does the generator size matter in Diffie-Hellman?

For the Diffie-Hellman protocol I've heard that the generator 3 is as safe as any other generator. Yet, 32-bit or 256-bit exponents are sometimes used as generators. What is the benefit of using ...
23k views

### Best way to reduce chance of hash collisions: Multiple hashes, or larger hash?

I would like to maintain a list of unique data blocks (up to 1MiB in size), using the SHA-256 hash of the block as the key in the index. Obviously there is a chance of hash collisions, so what is the ...
7k views

### Why does nobody use (or break) the Camellia Cipher?

If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does ...
12k views

### Impacts of not using RSA exponent of 65537

This RFC says the RSA Exponent should be 65537. Why is that number recommended and what are the theoretical and practical impacts & risks of making that number higher or lower? What are the ...
2k views

### What security authorities and standards reject $e=3$ in RSA, when, and with what rationale?

In RSA, some security authorities and/or standards allow the public exponent $e=3$, others require or recommend $e>2^{16}$ (or perhaps some other minimum). I gathered the following: PKCS#1 allows ...
20k views

### What is the difference between CBC and GCM mode?

I am trying to learn more about GCM mode and how it differs between CBC. I already know that GCM provides a MAC which is used for message authentication. From what I have read, and seen code ...
2k views

### Prevent double-spending with decentralized digital currencies without all transactions being public?

A recent approach to creating a decentralized online currency, called Bitcoin, has been generating some interest. The goal is to have a way to transfer currency without a central authority and without ...
6k views

### How was the special DES key 0E329232EA6D0D73 found?

The DES key 0E329232EA6D0D73 has the unusual property that decrypting a ciphertext block consisting entirely of zeros gives a plaintext block consisting of eight repetitions of the same byte (0x87). ...
3k views

### In which cases should the IV be kept secret?

When must an IV be kept secret? There is an ambiguity in the way that the lack of need for secrecy of the IV is described in multiple places. Does this ambiguity suggest that there are, in fact, ...
32k 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 ...
6k views

### Is truncating a SHA512 hash to the first 160 bits as secure as using SHA1?

I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms. Consider a hash of the word '...
3k views

### Information leakage from the ecryptfs filesystem

I'm wondering what information might be leaked from the ecryptfs filesystem. This is what Ubuntu uses if you check the box for "encrypted home directory" when using the desktop installer, so is ...
47k views

### What is safer: ZipCrypto or AES-256?

Like in title: which one of these encryption methods (ZipCrypto, AES-256) is more secure and why? I am asking about it because I'd like to know which should be preferred when compressing files with ...
2k views

### Is this password migration strategy secure?

I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ...
3k views

### Are checksums essentially non-secure versions of cryptographic hashes?

Are checksums basically toned-down versions of cryptographic hashes? As in: they are supposed to detect errors that occur naturally/randomly as opposed to being designed to prevent a knowledgeable ...
9k views

### What is entropy?

We discuss a lot of topics and use measures of entropy to determine how difficult it is for an attacker to be successful. What does entropy mean in the context of cryptography? How is entropy ...
21k views

### What are the advantages of TOTP over HOTP?

HMAC-based One Time Password (HOTP) was published as an informational IETF RFC 4226 in December 2005. In May, 2011, Time-based One-time Password Algorithm (TOTP) officially became RFC 6238. What ...
2k views

### How do I apply differential cryptanalysis to a block cipher?

I have read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are – for example – vulnerable to differential cryptanalysis....
3k views

### Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
2k views

### Is it feasible to build an index of prime factors?

Would it be possible to break an RSA key, in for example 1 week of time, if the cracker have already spent X number of years building an index of primes by performing every permutation of existing ...
4k views

### How were the DES S-box values determined?

It seems like the S-boxes in DES have essentially random values. How were these chosen?
7k views

### understanding a length extension attack

I have been trying to understand exactly how a length extension attack works on SHA-1. I'll detail below what I've understood so far, so that I can convey my understanding of the same and hopefully ...
15k views

### Why is AES resistant to known-plaintext attacks?

At least it's my understanding that AES isn't affected by known-plaintext. Is it immune to such an attack, or just resistant? Does this vary for chosen-plaintext?
6k views

### Is it possible to derive the encryption method from encrypted text?

Is it possible to identify the encryption method, or at least rule out some of them, by looking at the encrypted text? For example, if you have 3 encrypted strings where the first 10 characters are ...
33k views

### How can I use SSL/TLS with Perfect Forward Secrecy?

I'm new to the field of cryptography, but I want to make the web a better web by setting up the sites that I host with Perfect Forward Secrecy. I have a list of questions regarding the setup of ...
8k views

### Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times?

I'm wondering why we use encrypt-decrypt-encrypt (EDE) sequence in 3DES (also known as DES-EDE, TDES or TDEA) with three keys instead of three times encryption (EEE) with three different keys?
6k views

### Why is OTP not vulnerable to brute-force attacks?

I saw this question on the book Understanding Cryptography. At first glance it seems as though an exhaustive key search is possible against an OTP system. Given is a short message, let’s say 5 ...
4k views

### Why can't I reverse a hash to a possible input?

I'm going to provide “proof” why a hash function can be reversed, and I hope you can tell my why I'm wrong So, a hash function can be implemented as a series of logic gates. All logic gates can be ...
2k views

### Hash function that allows to decide if A > B if you only have hash(A) and hash(B)?

Is it possible to construct a hash function with the following property? If you have hash(A) and hash(B) with A and B being integers, you can tell if A is greater than B -- without however knowing ...
42k 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?
2k views

### Why is PuTTYgen-created RSA public exponent(e) not in {3,17,65537}?

As far as I know, RSA public exponent(e) should be one of {3,17,65537}. However, I found PuTTYgen-created RSA public exponent(e) is 0x25(37) by default,as follows, (PuTTYgen version: 0.66) I am ...
14k views

### What is the lowest level of mathematics required in order to understand how encryption algorithms work?

What mathematical fields of knowledge would be required in order to get a good understanding of encryption algorithms? Is it basic algebra, or is there a "higher education" mathematical field ...
10k views

### Can you explain Bleichenbacher's CCA attack on PKCS#1 v1.5?

I've studied that the Bleichenbacher's CCA attack on PKCS#1 v1.5. is a base to many versions of attacks in the area. I'm trying to understand that attack, but every explanation I saw starts with the ...
11k views

### Definition of Textbook RSA

What is the definition of Textbook RSA? What are some of the properties of textbook RSA? How does it differ from other RSAs?
14k 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” or “how easily” could they do it? What methods would they use? The original cracking was significantly helped by operator ...
5k views

### For Diffie-Hellman, must g be a generator?

Due to a number of recently asked questions about Diffie-Hellman, I was thinking this morning: must $g$ in Diffie-Hellman be a generator? Recall the mathematics of Diffie-Hellman: Given public ...
3k views

### Why is plain-hash-then-encrypt not a secure MAC?

It seems that even in MAC-then-encrypt systems like SSL, something like HMAC is used rather than a plain hash. Why? Suppose we use some stream cipher; then why can't we use $Encrypt(m | H(m))$ as ...
686 views

### Necessity of Randomness of Salts?

Given the desire to have unique salts for each user of your system, is it actually necessary to create a cryptographically-random salt for each user? If your system already has some other unique user ...
2k views

### How to fairly select a random number for a game without trusting a third party?

Several people are playing a game with random events and require a way to produce a random number. (Such as dice rolls or a lottery.) Can this be done such that each player has the power to be ...
7k views

### How can SSL secure a two-way communication with only one key-pair?

As I understand it, SSL involved the use of a public-private key pair. How does this enable two-way communication? Suppose I have some server with which I wish to communicate securely. I connect to ...
1k views

### Proof for the SHA3 claim that 256 bit security is “post-quantum sufficient”?

On page 14 of "Keccak and the SHA-3 Standardization" (February 6, 2013) it says: Instantiation of a sponge function the permutation KECCAK-f 7 permutations: b → {25,50,100,200,400,800,...
5k views

### Why can't one implement bcrypt in Cuda?

I had heard that although it's easy to implement message digest functions like MD5, SHA-1, SHA-256 etc. in CUDA (or any other GPU platform), it is impossible to implement bcrypt there. bcrypt is ...