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,...
- 49.5k
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
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-...
- 87.8k
61
votes
Accepted
What is the difference between SHA-3 and SHA-256?
The main differences between the older SHA-256 of the SHA-2 family of FIPS 180, and the newer SHA3-256 of the SHA-3 family of FIPS 202, are:
Resistance to length extension attacks.
With SHA-256, ...
- 49.5k
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
52
votes
Accepted
How does hashing twice protect against birthday attacks?
A collision in any hash function gives a collision in a "squared" variant of the hash function. This is easy to see. If hash(x)==hash(y), then hashing the outputs ...
- 38.9k
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
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 ...
- 94.5k
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 ...
- 49.5k
34
votes
Fixed point of the SHA-256 compression function
SHA-256 is based on a Davies–Meyer compression function. Easy to find fixed-points are a known property of this construction.
A notable property of the Davies–Meyer construction is that even if the ...
- 25.1k
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 ...
- 7,082
32
votes
How can hashes be unique if they are limited in number?
You are right, hashes won't be all unique as you already have shown. The important part are practical collisions - how many SHA-512 hashes can the whole earth generate in its lifetime? Definitely much ...
- 3,900
32
votes
How do hashes really ensure uniqueness?
Firstly, some definitions;
Pre-image resistant: given a hash value $h$ find a message $m$ such that $h=Hash(m)$. Consider storing the hashes of passwords on the server. Eg. an attacker will try to ...
- 49.5k
31
votes
Accepted
Why hashing twice?
A common rationale for hashing twice is to guard against the length-extension property of the hash (if it has that property, as many hashes before SHA-3 did). For SHA-256, this property allows to ...
- 145k
25
votes
Accepted
Are hash functions strong against quantum cryptanalysis and/or independent enough of mathematics?
This depends on what kind of hash function you mean and what kind of security you want.
Poly1305 is an almost-universal hash family, which, when used with a uniform random key for a single message, ...
- 49.5k
24
votes
Accepted
What are the consequences of removing a single byte from a sha256 hash?
Does this bias the hash in any way?
We want the avalanche criteria on the output bits, that is a change in the any of input bit must randomly affect half of the output bits. Each bit of the hash ...
- 49.5k
23
votes
Accepted
How many hex digits do I need to compare when manually checking hash functions?
How many hex digits do I need to compare when manually checking hash
functions?
If you actually want the full security guarantees of the hash function to apply: all of them.
I usually just look ...
- 46.4k
23
votes
Accepted
What makes SHA-256 secure?
It's worth pointing out that in the case of SHA2 and most other hashes the compression function has a block cipher (keyed permutation) as its core.
Basically what you are asking is identical to ...
- 246
23
votes
Has SHA256 been broken by Treadwell Stanton DuPont?
As of today (12th September 2019) Tradwell Stanton DuPont have retracted their claim:
Sorry for the image link, but the text on their site doesn't seem to be permalinkable, and I wanted a more ...
- 331
22
votes
Accepted
Are there any well-known examples of SHA-256 collisions?
No, there is not any known SHA-256 collision. Publication of one, or of a remotely feasible method to obtain one, would be considered major.
It is next to impossible that two distinct strings with ...
- 145k
22
votes
What makes SHA-256 secure?
The design and security of SHA-256 rely on two cryptographic structures; one-way compression function which is based on Davies–Meyer structure which uses SHACAL-2 block cipher and on the top the ...
- 49.5k
21
votes
Can one use a reversible hash algorithm as a compression function?
Strictly speaking, all hash functions are compressing since the output can be smaller than the input, but I imagine you're asking about compressing data that can later be losslessly decompressed.
This ...
- 15.5k
21
votes
Accepted
Is it possible that a SHA256 hash has the same hex character over and over again?
First of all, the output of SHA-256 is binary and consists of 32 bytes (256 denotes the output size in bits). What you are talking about is apparently the hexadecimal encoding of these bytes.
The ...
- 94.5k
20
votes
Accepted
Why does SHA2-224 use different IV's than SHA2-256?
The $\operatorname{SHA-224}$ is defined in the exact same manner as $\operatorname{SHA-256}$ with different initial values and the digest is obtained truncating the hash value, FIPS PUB 180-4 Page 23.
...
- 49.5k
19
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.
...
- 49.5k
19
votes
Accepted
Is double SHA-256 the best choice for Bitcoin?
Since the initial release of Bitcoin is 9 January 2009, the designer had these NIST hash functions (NIST-FIPS 180-4) as available options: SHA-1( 1995), SHA-256 (2001), SHA-512 (2001), and some more....
- 49.5k
16
votes
Accepted
Which hash algorithms support binary input of arbitrary bit length?
What established hash algorithms are compatible with input binary data of arbitrary bit length?
The specifications of many hash algorithms include provisions for input of binary data of length not a ...
- 145k
15
votes
SHA-512 faster than SHA-256?
Benchmarks
I would like to see also some real-life measurements here, so I hope you'll like it ;)
Intel Core i7-7700HQ (7th gen = Kaby Lake); RAM (DDR4)
HW / OS configuration:
System: Linux Mint 20....
- 250
15
votes
Accepted
Argon2 with SHA-256 instead of Blake2
If you take a look at the Password Hashing Competiton, you can see, that most of the schemes use Blake2b, some of them uses SHA-512, none of them uses SHA-256.
Blake2b is optimized for 64-bit ...
- 995
Only top scored, non community-wiki answers of a minimum length are eligible