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13 votes
4 answers
591 views

Tactics available to help prove security of a new system?

I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new ...
31 votes
1 answer
7k views

Should I use the first or last bits from a SHA-256 hash?

I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a SHA-256 hash. Should I take the first bits or the last bits? Which of them ...
11 votes
3 answers
436 views

Is it possible to create an asymmetric cryptosystem where the private keys are not easily verifiable as such?

Plaintext that consists of an RSA key is easily recognizable as such, because it satisfies certain mathematical properties, in particular (See the answer for Why can an encrypted private key be brute ...
  • 213
15 votes
2 answers
844 views

Why is there an enormous difference between SAT solvers?

SAT solvers are very important in algebraic attacks, for example walksat and minisat. However, when solving the benchmark problems available here there is an enormous performance difference between ...
  • 3,892
13 votes
5 answers
1k 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 (...
  • 1,659
10 votes
3 answers
22k views

Encrypted text length in AES

I have created an application that will be able to read any file and encrypt it using AES Encryption. For efficiency, I am reading a block of data, encrypting it and so on. So for decrypting, I just ...
  • 1,141
12 votes
1 answer
654 views

Can one efficiently iterate valid bcrypt hash output values?

bcrypt is an intentionally slow hash algorithm. In my last protocol idea, I wanted to use it to expand a password and then only transfer the bcrypt-hashed password. An efficient attack on this would ...
30 votes
3 answers
12k 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 ...
19 votes
6 answers
2k views

Why does PBKDF2 xor the iterations of the hash function together?

The definition of PBKDF2 states that I obtain a derived key (1) by calling a pseudorandom function a bunch of times recursively: $U_1 = PRF(password, salt)$ $U_2 = PRF(password, U_1)$ … $U_n = PRF(...
11 votes
4 answers
1k 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 ...
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16 votes
1 answer
1k views

Why did NIST remove The Lempel-Ziv Compression test from the Statistical Test Suite?

NIST removed "The Lempel-Ziv Compression" test from the Statistical Test Suite in revision 2008 and above and has not incorporated it since – see revision 2010. Why was it removed? Does it no longer ...
  • 3,892
5 votes
1 answer
615 views

Sites to find crypto intermediate values

Does anyone know of any sites where intermediate values of multiple crypto algorithms (not just the famous ones), including hashes, can be found? I don't just mean intermediate values of encrypting. I'...
7 votes
2 answers
335 views

Are there any tools for expressing the cipher operations as a system of equations?

The first step In Algebraic-attack is expressing the cipher operations as a system of equations. Is there an automated tool to do this?
  • 3,892
22 votes
3 answers
3k views

Is using slow password hashing on the client side easier attackable than on the server side?

As we know, one should use a slow password hashing algorithm instead of a fast one for storing passwords, to hinder brute force attacks when the database is compromised. The problem with this is that ...
28 votes
9 answers
32k views

RSA with small exponents?

Just to establish notation with respect to the RSA protocol, let $n = pq$ be the product of two large primes and let $e$ and $d$ be the public and private exponents, respectively ($e$ is the inverse ...
  • 1,551
16 votes
1 answer
2k views

Replacing the Rijndael S-Box?

The Rijndael S-Box design generates a permutation cycle of type $2+27+59+81+87$. What effect would replacing that permutation with a cycle of type $256$ have on the security of AES?
22 votes
6 answers
485 views

advances in usability for cryptography/authentication

I'm wondering if there have been any recent advances (say, the past 5-10 years) in human usability for cryptography and/or authentication? By that I mean something that makes it easier for an ...
  • 722
11 votes
4 answers
1k views

Should DES be avoided when developing new systems?

According to wikipedia… DES is now considered to be insecure for many applications However, since it can be broken relatively easy, should it not just be avoided for all applications?
95 votes
6 answers
92k views

How can I generate large prime numbers for RSA?

What is the currently industry-standard algorithm used to generate large prime numbers to be used in RSA encryption? I'm aware that I can find any number of articles on the Internet that explain how ...
  • 1,347
4 votes
2 answers
264 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 ...
10 votes
1 answer
8k views

What are the details of the DES weakness of reusing the same IV in CBC mode with the same key?

I think I once faced the recommendation, that the initialization vector should always be random and never be used twice with the same key. How serious is this weakness? Also, is AES less effected ...
111 votes
7 answers
109k views

Taking advantage of one-time pad key reuse?

Suppose Alice wants to send encryptions (under a one-time pad) of $m_1$ and $m_2$ to Bob over a public channel. Alice and Bob have a shared key $k$; however, both messages are the same length as the ...
  • 1,551
22 votes
3 answers
5k views

Known methods for constant time (table-free) AES implementation using 'standard' operations?

There are several known methods for implementing AES in constant time using SIMD operations, mostly based around fast byte shuffling (for instance Hamburg and Kasper/Schwabe). Are there any similar ...
  • 1,664
23 votes
4 answers
6k views

Is it feasible to build a stream cipher from a cryptographic hash function?

A few years ago I devised a symmetric-key system that worked like so: ...
  • 1,465
24 votes
5 answers
5k views

With sufficient randomness, is XOR an acceptable mechanism for encrypting?

I have heard criticism of various cryptosystems saying that "at their heart, they were just XOR." Is this just ignorance, or is there something inherently wrong with XOR based ciphers?
  • 1,465
12 votes
3 answers
4k views

Salting when encrypting?

I was attending a database encryption session at a developers conference. The presenter (who was a published author on the subject) said that MS SQL Server did not support salted hashes in the ...
20 votes
2 answers
13k views

What are the main weaknesses of a Playfair cipher, if any?

What are the main weaknesses of a Playfair cipher, if any? I know that they depend on none of the letters missing, but that is an easy fix if a letter gets dropped. Besides that, are there any other ...
23 votes
4 answers
1k 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 ...
  • 629
29 votes
7 answers
21k 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 ...
  • 629
22 votes
7 answers
10k views

Current mathematics theory used in cryptography/coding theory

What are the mainstream techniques borrowed from algebraic geometry (or some other branch of mathematics) which are currently used in cryptography/coding theory? I've only heard about a small subset ...
  • 421
30 votes
2 answers
5k 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....
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57 votes
2 answers
20k views

What makes a hash function good for password hashing?

Using a cryptographic hash to store e.g. passwords in a database is considered good practice (as opposed to storing them plaintext), but is subject to attacks on said cryptographic hash, assuming the ...
  • 673
87 votes
4 answers
99k views

What are the practical differences between 256-bit, 192-bit, and 128-bit AES encryption?

AES has several different variants: AES-128 AES-192 AES-256 But why would someone prefer use one over another?
  • 3,176
17 votes
3 answers
8k views

How can a random salt for a hash function work in practice?

I understand the theory behind the use salts in hash functions, but when I see it implemented, the implementations always generate the salt on the fly and the salt appears to be different for every ...
20 votes
1 answer
909 views

How to provide secure "vanity" bitcoin address service?

Bitcoin addresses are RIPEMD-160 hashes of the public portion of a public/private ECDSA keypair (along with an abbreviated hash of the hash to provide a check code, as @pulpspy notes in a comment). ...
  • 560
28 votes
2 answers
10k 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?
  • 3,176
70 votes
4 answers
39k views

How can I use asymmetric encryption, such as RSA, to encrypt an arbitrary length of plaintext?

RSA is not designed to be used on long blocks of plaintext like a block cipher, but I need to use it to send a large (encrypted) message. How can I do this?
  • 3,176
19 votes
2 answers
2k views

What is the general justification for the hardness of finding preimages for cryptographic hash functions?

Since most cryptographic hash functions are simple, compact constructions does this simplicity impose a limit on the complexity and the size of a function that can generate preimages? That is, given a ...
14 votes
2 answers
4k views

Why do we append the length of the message in SHA-1 pre-processing?

As we know, SHA-1 is irreversible, so why do we append the length of the message to the preimage?
  • 715
15 votes
2 answers
2k views

Why use a 1-2 Oblivious Transfer instead of a 1 out of n Oblivious Transfer?

When initiating an oblivious transfer, why would someone use a 1-2 oblivious transfer rather than going for an 1 out of n oblivious transfer? Perhaps a slight time overhead for the extra message ...
  • 253
47 votes
3 answers
18k views

What are the benefits of the two permutation tables in DES?

Why do we use a permutation table in the first step of DES algorithm and one at the end of the algorithm?
  • 715
183 votes
22 answers
27k views

Time Capsule cryptography?

Does there exist any cryptographic algorithm which encrypts data in such a way that it can only be decrypted after a certain period of time? The only idea that I can think of, is something like this: ...
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33 votes
1 answer
6k 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 ...
  • 572
16 votes
4 answers
3k views

Is Wiener's attack on RSA extendable to larger keys with low hamming weight?

Using small private exponents with RSA improves performance. However, it has been shown (Wiener, 1990) that if $\log d \leq \frac14 \log N$, the private exponent $d$ can be reconstructed from the ...
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7 votes
1 answer
317 views

Can a proof be constructed to show there is no distinguisher?

Let's assume a simple algorithm like the Skein hash function. Is it possible, given the algorithm, to construct a proof that it does not have a particular distinguisher, something like: $P(xyz)$ is ...
  • 179

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