271
votes
Why can't we reverse hashes?
Take a simple mathematical operation like addition. Addition takes 2 inputs and produces 1 output (the sum of the two inputs). If you know the 2 inputs, the output is easy to calculate - and there's ...
- 2,314
153
votes
Accepted
Has SHA256 been broken by Treadwell Stanton DuPont?
[Update, 2019-09-12: Upon being exposed, and/or after taking advantage of market manipulation by their fraudulent announcements, the scam artists of Treadwell Stanton DuPont have retracted their claim,...
- 49.1k
133
votes
How do hashes really ensure uniqueness?
The simple answer is that hashes don't ensure uniqueness. Very broadly, hashes behave like "deterministic random numbers" – deterministic in the sense that hashing the same data always ...
- 1,221
112
votes
Accepted
What is the difference between a digest and a hash function?
The digest is the output of the hash function.
For example, sha256 has a digest of 256 bits, i.e. its digest has a length of 32 bytes.
That's it really.
- 1,592
85
votes
Accepted
What is the new attack on SHA-1 "SHAttered" and how does it work?
In order to get a collision on a $n$ bit Random Oracle using the birthday paradox, one needs $\sqrt{\pi / 2} \cdot 2^{n/2}$ calls. In other words, in the case of the 160 output bits of SHA-1 the limit ...
- 10k
70
votes
Accepted
Can a large file be hashed down to 32 bytes, and then reconstructed from the hash?
No, there is no way to compress (or hash or encrypt or whatever) a 5 MB file into a 32 byte hash and then reconstruct the original file just from the hash alone.
This is simply because there are many ...
- 46.4k
68
votes
Accepted
Other than password hashes, are there other uses for non-reversible crypto
Data integrity is another usage. For example, when you want to send/download data, you want to make sure that the data is not modified or transmitted/downloaded correctly. To achieve this the data ...
- 49.3k
67
votes
Cryptography algorithms that take longer to solve on a GPU than a CPU
This specific situation is a central part of the analysis of password hashing functions. Indeed, for hashing a password, we want a function which is:
slow in a tunable way;
such that the most cost-...
- 87.7k
60
votes
Accepted
How can hashes be unique if they are limited in number?
how can for example SHA-256 be unique if there is only a limited number of them?!
Where your issue occurs is that they're not unique. It's just very improbable that they'll reoccur. Unique in ...
- 15.7k
56
votes
Accepted
For a hashing function like MD5, how similar can two plaintext strings be and still generate the same hash?
This answer is based on the work by AleksanderRas, although my conclusion is different.
First, to lay out a definition, a hash is a function that takes an arbitrary length input to a fixed length ...
- 1,042
55
votes
Accepted
How secure is SHA1? What are the chances of a real exploit?
Actually SHA-1 has been "officially insecure" for a longer time, since an attack method was published in 2011. The 2017 collisions was just the first known case of actually running the attack. But ...
- 87.7k
53
votes
Accepted
Is it bad to expose the public key?
From a cryptographic standpoint it is OK to expose a public key in the sense of revealing its value. The most basic assumption in cryptography involving public/private key pairs is that the value of a ...
- 144k
53
votes
Accepted
Can I say "I have encrypted something" if I hash something?
Encryption algorithms and hash algorithms both belong to the realm of cryptography but are two different things: Encryption doesn't contain hash functions.
As stated on Wikipedia:
In cryptography, ...
- 6,502
52
votes
Accepted
Are cryptographic hash functions quantum secure?
It is a bit dubious to claim that hash functions "are not based on any hard problem": inverting a standard hash function, or finding a collision, is itself a very hard problem.
The point of ...
- 20.8k
45
votes
Accepted
Why do I need to store the salt for generated hashes?
The hash returned by bcrypt.hashSync is more than the hash itself, it contains all parameters needed by bcrypt. You do not need to store anything else yourself, ...
- 1,583
45
votes
Accepted
Does Terra Quantum AG break AES and Hash Algorithms?
The onus is on the company to prove their claims, especially when they are extreme. There is also no financial motivation to not prove their claims. I can understand if they say that they want to keep ...
- 28.1k
42
votes
Why can't we reverse hashes?
Cryptographically secure hashes were specifically build to (among other things) make what you're asking hard!
Now, you could try to create an appropriate dictionary of all hashes, hoping to find ...
- 18k
39
votes
Accepted
Why use argon2i or argon2d if argon2id exists?
The current Argon2 draft RFC, I think, provides a good, reasonably brief answers to this question. TL;DR: most people will indeed want to use Argon2id and not the "pure" variants.
The ...
- 14.6k
38
votes
Accepted
What are the odds of collisions for a hash function with 256-bit output?
Birthday problem for cryptographic hashing, 101.
Let $p_n$ be the probability of collision for a number $n$ of random distinct inputs hashed to $k$ possible values (that is, probability that at least ...
- 144k
38
votes
Accepted
If a hash function $H$ is collision resistant, is it true that $H(x)\neq H(x')$ for all messages $x, x'$ with $x \neq x'$?
As you correctly observed, for any function $H\colon \{0,1\}^\ast\to\{0,1\}^n$ collisions must exist, simply because $\{0,1\}^\ast$ is an infinite set and $\{0,1\}^n$ is finite.
One could define "hash ...
- 12.2k
37
votes
Has SHA256 been broken by Treadwell Stanton DuPont?
This is firmly in "put up or shut up" territory.
It's easy to prove this claim. I will provide them with a string and its hash. They must provide a collision.
They could also mine bitcoin and earn a ...
- 906
37
votes
Accepted
Is it safe to create a public ID by hashing a private key?
Would it be safe to generate a public identifier by hashing this private key?
It depends on the hash.
If the hash is SHA-512 applied on the private key formatted as a 256-bit (32-byte) string, that's ...
- 144k
35
votes
What is the maximum size of the plaintext message for RSA OAEP?
Important note: usually OAEP is only used to encrypt a random symmetric key, which is then used to encrypt a plaintext message; this is called a hybrid cryptosystem. Repeated use of OAEP would require ...
- 93.9k
35
votes
Is it easy to crack a hashed phone number?
No, it is not a good idea to hash phone numbers. There are only a limited number of phone numbers, so it is pretty easy for an adversary to try and hash all of them. Then you can simply compare the ...
- 93.9k
35
votes
Accepted
Boss insists on storing SHA2(p) || SHA3(p), claiming it "doubles security"
"nobody has a rainbow table for [this scheme]"
Well, this is a huge claim so needs justification. Building a rainbow table is not hard. Once you are a target, the table will be ready. For ...
- 49.3k
34
votes
Accepted
Is there really no use for MD5 anymore?
I know that MD5 should not be used for password hashing, and that it also should not be used for integrity checking of documents. There are way too many sources citing MD5 preimaging attacks and MD5s ...
- 49.1k
34
votes
Accepted
Can we assume that a hash function with high collision resistance also means a highly uniform distribution?
Define $H(x) = \operatorname{SHA-256}(x) \mathbin\| 1$; that is, append a single 1 bit to SHA-256. Can you find a collision under $H$? Does $H$ have anything resembling uniform distribution?
This ...
- 49.1k
33
votes
Accepted
Looking at hash output – is Base64 encoding in any way better than HEX encoding?
The algorithms themselves just output binary (i.e. bytes) if you read their specifications. It's the implementation in API's and applications that output the hexadecimals and/or base64.
Sometimes ...
- 93.9k
33
votes
Accepted
Do identical strings always have the same SHA-256 value?
Yes, if you hash the same input with the same function, you will always get the same result.
This follows from the fact that it is a hash-function. By definition a function is a relation between a ...
- 7,038
32
votes
Accepted
What is a ratchet?
A ratchet is simply a device that only moves forward, one step at a time, much like fastening a tie rib (which is a ratchet device). So if you've got a state value in the past you can calculate all ...
- 93.9k
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