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74 votes

Password cracking: What if attacker is lucky?

"lucky" is not a property of the attacker. There's no "lucky" attacker nor "normal" attacker. They both have the same probability (low, very low) to guess the key. You can decrease the probability at ...
Gianluca Ghettini's user avatar
42 votes

Password cracking: What if attacker is lucky?

Note: This answer assumes that by "lucky" OP meant "able to remove X% of valid answers", because I believe that was intent. Of course you can't measure luck ;) And if he is very lucky, say 90% ...
axapaxa's user avatar
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29 votes

Are longer passwords really safer against brute force attacks?

If we take two password strings of different length and attempt to bruteforce match them, it is obvious that the longer one will take longer to crack on average. Actually, that might be obvious to ...
poncho's user avatar
  • 149k
28 votes
Accepted

How is encryption broken today?

Modern encryption can be broken in practice even when the algorithms are theoretically secure. There are a variety of ways this can happen: Side channel analysis could have played a part. The ...
Ella Rose's user avatar
  • 19.7k
28 votes

How long would it take to brute force an AES-128 key?

I wrote a similar answer in the past, where the assumption was half the key is known. Since then, the bitcoin hashrate almost tripled (it's used in the estimation, as below). The estimation for half ...
tylo's user avatar
  • 12.7k
25 votes
Accepted

Password cracking: What if attacker is lucky?

So if at each bit he has a 50% chance, that means that 1 bit is actually only half bit. And if he is very lucky, say 90% chance, that means that 1 bit is actually only 0.1 bit.So in face of a very ...
Chris Dodd's user avatar
19 votes

How many KDF rounds for an SSH key?

Slower is better, as slow as you can tolerate. Timing for different -a values, each measured 20 times: ...
Luc's user avatar
  • 1,528
19 votes
Accepted

Why does Birthday attack work only with random messages and not with chosen messages?

Birthday attacks do indeed work also for messages chosen by the attacker. For example, consider the case that I want to get a collision between a letter of recommendation and a letter that I was fired....
Yehuda Lindell's user avatar
19 votes

Are longer passwords really safer against brute force attacks?

There's a 2013 article in Ars Technica that refutes the notion that long passwords are necessarily hard to crack. It details how security researchers Kevin Young and Josh Dustin turned to text from ...
Luis Casillas's user avatar
18 votes
Accepted

Security levels in NIST Post-quantum project: e.g. AES-128 vs SHA-256

This is due to the Brassard et al.'s method on hash functions. That has $\mathcal{O}(\sqrt[3]{n})$ attack time for n-bit hash function where as the Grover's method has $\mathcal{O}(\sqrt{n})$-time. ...
kelalaka's user avatar
  • 49.1k
17 votes

Does one time pad randomness help brute force attacks?

There is no such thing as a random key. There are only randomly generated keys. What I mean by that is that randomness is not a property of the key (or message, or number, or whatever), but of the ...
Ilmari Karonen's user avatar
17 votes
Accepted

How many KDF rounds for an SSH key?

I did also tried to find a good value for the -a flag, in a MacBook Pro Mid14 (i7), trying to login in to a Debian 8.5, I had this results: ...
zzantares's user avatar
  • 302
15 votes

Password cracking: What if attacker is lucky?

I'm not sure what you're trying to understand and if the other answers cover it, so I'm trying a different approach and interpret your question like this: What if an attacker guesses the right ...
David Foerster's user avatar
14 votes

Brute-forcing Symmetric Keys

No, it doesn't help. It doesn't hurt either; as long as you don't repeat keys, the probability of success is always the same. That is, if there are $2^n$ possible keys, and you test $\lambda$ of ...
poncho's user avatar
  • 149k
14 votes
Accepted

Why do look up tables speed things up compared to brute force?

Suppose you have an $n$-bit key. Suppose further you have some reliable predicate $P(k,m)$ which decides whether a key $k$ is the key you are looking for given the reference $m$. Furthermore, suppose ...
SEJPM's user avatar
  • 46.2k
14 votes

For Symmetric Cryptography, why is it considered more important to safeguard a key than the function/algorithm for encrypting/decrypting a message?

Some facts for you to consider: Brutal-force a cryptographic key is much harder than brutal-force breaking into a house - the former can take as long as for a star to explode, while the latter take ...
DannyNiu's user avatar
  • 9,509
13 votes

For Symmetric Cryptography, why is it considered more important to safeguard a key than the function/algorithm for encrypting/decrypting a message?

At least two reasons. 1: security. You want your algorithm to be a good one. One of the best ways we know of ensuring cryptographic algorithms are good is to have as many experts as possible assess ...
ignis volens's user avatar
12 votes
Accepted

Why are asymmetric cryptography keys more vulnerable to brute force attack than symmetric ones?

For your first question: The main point here (at least that comes to mind) is that of how the key is made, used, and subsequently how it is attacked. Good symmetric ciphers are designed so that the ...
henrheid's user avatar
  • 198
12 votes

Is AES-256 a post-quantum secure cipher or not?

We know Grover's algorithm speedup brute-force attacks two times faster in block ciphers (e.g brute-forcing 128-bit keys take 264 operations, not $2^{128}$). This is the advertisement of the Lov K. ...
kelalaka's user avatar
  • 49.1k
11 votes
Accepted

In the RSA DES challenges, how did the contestants know they had found the right key considering they weren't given any plaintext?

One can still access the challenge rules from the archive.org Each contest is based on a specified cipher. A brief piece of printable ASCII text (containing byte values in hexadecimal notation from ...
kelalaka's user avatar
  • 49.1k
10 votes

Why are asymmetric cryptography keys more vulnerable to brute force attack than symmetric ones?

Putting this paper into context, it was "the culmination of the research efforts of nine dedicated undergraduate students in the Computing Research Topics course at Villanova University" and this ...
tylo's user avatar
  • 12.7k
9 votes

Password cracking: What if attacker is lucky?

So if at each bit he has a 50% chance, that means that 1 bit is actually only half bit. No, it is not. Because 'bit' is a quantity of information we should know to reduce entropy twice by definition. ...
Enr1g's user avatar
  • 198
9 votes
Accepted

Are ciphertext-only attacks on LFSRs possible?

For text known to be ASCII encoded as octets with high-order bit of octets at zero, that reveals one bit of the output of the LFSR out of 8. It allows finding the original state of the LFSR from (say) ...
fgrieu's user avatar
  • 143k
9 votes
Accepted

AES-256 password cracking time

It will happen immediately, because you just posted your password to the internet, where the adversary is watching. On the other hand, if you can describe the procedure you used to generate it, we ...
Squeamish Ossifrage's user avatar
8 votes

How is encryption broken today?

Orin Kerr & Bruce Schneier have a recent paper out titled Encryption Workarounds, where they group the techniques to indirectly attack theoretically-secure encryption. I think their break-down ...
Nick T's user avatar
  • 261
8 votes
Accepted

Why this brute force attack doesn't reduce all cryptographic hash functions' security bits against collision attacks to N/3?

Let $M$ be the number of queries to a uniform random function $F$ at distinct points $X_1, X_2, \dots, X_M$. The probability of a repeated value $F(X_i) = F(X_j)$ for $i \ne j$ is at most $M^2\!\big/...
Squeamish Ossifrage's user avatar
8 votes
Accepted

Does file size significantly affect brute-force time?

AES is a block cipher and typically operates on large plaintext data by dividing it into blocks of fixed size (128-bits in the case of AES) and processing these blocks individually. Similarly ...
Daniel S's user avatar
  • 24.1k
8 votes
Accepted

How weak is using AES with a 128 bit key but 64 bits of the secret key are public constants?

Using AES-128 with only 64-bit uniform random is not secure. There are many entities around that can break this easily by searching the $2^{64}$- space. Machines in a second in an hour in a day in a ...
kelalaka's user avatar
  • 49.1k
7 votes

Why are asymmetric cryptography keys more vulnerable to brute force attack than symmetric ones?

Asymmetric cryptography keys are NOT necessarily more vulnerable to brute force attack than their secret-key cryptography counterparts. Some asymmetric algorithms have short private keys (256-bit for ...
fgrieu's user avatar
  • 143k

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