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Questions tagged [sha-256]

SHA-256 is part of the SHA-2 family of hash functions with a 256-bit output and a 128-bit security level.

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15 votes
2 answers
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The effect of truncated hash on entropy

Suppose I have a 128-bit random binary string (128 bits of entropy), then I hash it using SHA-256, then I take the first 128 bits of the output hash. Does the taken bit string still have (almost) 128 ...
user40602's user avatar
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22 votes
4 answers
3k views

Cycles in SHA-256

Let's say I start with a particular 256 bit value. Call this $v$. I then hash that value, and get another 256 bit value. Call this $\text{SHA256}(v)$. I take this value and get another 256 bit value. ...
bnsh's user avatar
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9 votes
1 answer
3k views

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

In an article about NIST Post-quantum Standardization project I read about the security criteria of the proposed schemes and there was this table (Level I lowest security, level V highest): Level I: ...
gorte's user avatar
  • 167
26 votes
3 answers
10k views

"Weaknesses" in SHA-256d?

According to this answer, "SHA-256d" was proposed in one of the Ferguson/Schneier books like so: SHA-256d(x) = SHA-256(SHA-256(x)) Apparently, the motivation for ...
Nemo's user avatar
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18 votes
5 answers
11k views

Is it easy to crack a hashed phone number?

I want to SHA256 hash phone numbers in order to hide them. Is this a good idea? Are there any other ways I could make this safe?
Jack Resone's user avatar
15 votes
1 answer
20k views

Are hash functions strong against quantum cryptanalysis and/or independent enough of mathematics?

I work on ethereum and other blockchain technologies. And seeing that quantum pc's are someday going to see the light I have some questions / doubts. I was wondering if hash functions are strong ...
CPereez19's user avatar
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12 votes
1 answer
2k views

Why does SHA2-224 use different IV's than SHA2-256?

Given that it's otherwise just a truncation, I can guess that being able to compute the 224 value from the 256 value is an unwanted property, but that's just speculation.
hanshenrik's user avatar
10 votes
1 answer
4k views

How hard is it to generate a simultaneous MD5 and SHA1 collision?

I was recently reading that MD5 is "broken" because it's pretty easy to generate collisions (like 2^(L/2)). And the SHA1 (theoretically) fares no better. The ...
xyz's user avatar
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75 votes
6 answers
64k views

SHA-512 faster than SHA-256?

I'm getting this strange result that SHA-512 is around 50% faster than SHA-256. I'm using .net's SHA512Managed and SHA256Managed ...
ispiro's user avatar
  • 2,025
33 votes
1 answer
9k 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 ...
Peter Smit's user avatar
8 votes
3 answers
3k views

If a SHA256 hash with high entropy is then hashed with one made from low entropy, is the resulting hash higher/same/lower entropy?

If one were to create a SHA256 hash using 256 coin flips converted to hex (which, as I understand it, is as "entro-phized" as one can capture in SHA256) then take that hash as a string, combine that ...
travisjd's user avatar
7 votes
3 answers
3k views

A simple block cipher based on the SHA-256 hash function [duplicate]

I've come up with this little routine for doing encryption using the SHA-2 (in this case SHA-256) hash function. As such it is a block cipher with a 256 bit (32 byte) block size and an arbitrary key ...
user avatar
56 votes
11 answers
22k views

How do hashes really ensure uniqueness?

This might seem an impractical and unnecessary conversation, but I feel it's something I need to clarify. Especially, as I just got my first developer job in a blockchain startup. So hashes are said ...
James Kumar's user avatar
18 votes
1 answer
5k views

What are the consequences of removing a single byte from a sha256 hash?

I'm working on a system (Ethereum) where it is significantly cheaper to store 32 bytes than 33 bytes. I'd like to create a table where data is stored based on its hash. Sha256 would meet this ...
Akhil F's user avatar
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3 votes
1 answer
5k views

Hash functions and the Avalanche effect

Informally, the Avalanche effect says that two similar, but not identical, inputs should produce radically different outputs when fed through a hash function. I've seen, at least, two formal ...
Fredrik Savje's user avatar
11 votes
3 answers
13k views

SHA256 output to 0-99 number range?

Is it mathematically possible to take a SHA256 hash and turn it into a 0-99 number where each number in 0-99 range is equally likely to be picked? As a 256 bit hash means the highest value possible ...
John T's user avatar
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60 votes
3 answers
27k views

"SHA-256" vs "any 256 bits of SHA-512", which is more secure?

In terms of security strength, Is there any difference in using the SHA-256 algorithm vs using any random 256 bits of the output of the SHA-512 algorithm? Similarly, what is the security difference ...
Pacerier's user avatar
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36 votes
2 answers
51k views

HMAC-SHA1 vs HMAC-SHA256

I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough to ...
Mario's user avatar
  • 361
31 votes
2 answers
10k views

Fixed point of the SHA-256 compression function

SHA256 Free Start Self Collision (Full 64 rounds) IVec: 72BF9EF1 27B82DFB F298F3B7 22B6C32C 18A54860 4C032D91 ADD7B85B 7ED1A4AC Block: ...
Nathan.Mariels's user avatar
10 votes
3 answers
6k views

Is double SHA-256 the best choice for Bitcoin?

So I was just curious. I really look up to Blockchain technology and I have read that Bitcoin uses double SHA-256 for hashing. ( from what I understood, double sha256 is essentially $\operatorname{SHA-...
Pinkovai Krisztian's user avatar
8 votes
1 answer
2k views

How can uniformity of hash functions (e.g. SHA-256) be proved?

In reading about the Bitcoin protocol I noticed how much its proofs-of-work apparently depend on uniformity of the SHA-256 hash function. And so presumably do many other applications. How do ...
Drux's user avatar
  • 243
7 votes
1 answer
2k views

Is it problematic to use PBKDF2-HMAC-SHA256 to derive a 512-bit XTS key?

PBKDF2 should only be used to generate a larger output than the hash function it uses if the output is used in such a way that it has a flat keyspace. As far as I am aware, XTS does not have a flat ...
forest's user avatar
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4 votes
1 answer
14k views

How is input message for SHA-2 padded?

I read about how is an input message prepared to be hashed by MD4,MD5 or SHA-1: Step1 Append padding bits The input message is "padded" (extended) so that its length (in bits) equals to 448 ...
yak's user avatar
  • 233
111 votes
3 answers
132k views

Why haven't any SHA-256 collisions been found yet?

I've been thinking about this for a few days, a SHA-256 algorithm outputs 64 characters which can either be a lowercase letter or a number from 0-9. Which should mean that there are 64^36 distinct SHA-...
ninesalt's user avatar
  • 1,245
61 votes
3 answers
13k views

Hashing or encrypting twice to increase security?

Over on the bitcoin forums I asked why the bitcoin client computes SHA-256(SHA-256(x)) as its cryptographic hash for a variety of purposes. The leading theory--since the bitcoin author has disappeared-...
maaku's user avatar
  • 711
8 votes
1 answer
1k views

Why does SHA-2 call for doing 10* padding in addition to appending the message length?

Appending the length of the message when padding of a SHA-2 message is sufficient to satisfy the Merkle-Damgård construction. However the padding in SHA-2 also pads with 10* between the end of the ...
Russell O'Connor's user avatar
6 votes
1 answer
8k views

SHA256 HMAC brute force with chosen plaintext attacks

This is a follow up to Is It Possible To Reconstruct a Cryptographic Hash's Key I am using a SHA-256 HMAC function on a single-word input: sha256hmac(privatekey,word) = output. The private key length ...
user1373208's user avatar
1 vote
1 answer
139 views

Can I retrieve a re-hashed (hash) value, for instance by additional hashing?

I have a double-SHA-256 of some text $$ h_2 = \operatorname{SHA256}(\operatorname{SHA256}(m)).$$ I don't need the plain text ($m$), but somehow I need to get first $$h_1 = \operatorname{SHA256}(m).$$ ...
AmirHossein Zarei's user avatar
95 votes
4 answers
26k views

Has SHA256 been broken by Treadwell Stanton DuPont?

In a recent press release issued by Treadwell Stanton DuPont, the claim is made that their research laboratories have successfully broken all 64 rounds of the SHA256 hashing algorithm. They further ...
Gary's user avatar
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23 votes
2 answers
11k views

What makes SHA-256 secure?

For example, RSA relies on a mathematically hard problem, factoring, while ECDSA or similar rely on discrete logarithm problem. What makes SHA-256 and similar hash functions, of the same family, ...
rapadura's user avatar
  • 341
19 votes
2 answers
9k views

SHA-256: (Probabilistic?) partial preimage possible?

Currently busying myself with the Bitcoin "mining" algorithm, I am wondering if the process really cannot be simplified. For reference, the algorithm is basically SHA-256d: $$\mathit{success} := \...
JimmyB's user avatar
  • 312
16 votes
2 answers
2k views

Why is the salt used only once in PBKDF2, while the password is used often?

The purpose of PBKDF2 is to create a derived key (DK) from a master password (PW) and a salt, often using a function like HMAC-SHA256. I have read that the salt should be as random as possible. But ...
ericball's user avatar
  • 413
16 votes
2 answers
28k views

SHA-256 "almost unique"?

I have seen numerous references on the internet of people describing SHA-256 as generating an "almost unique" hash. Exhibit A. there are more. Is there some mathematical basis to the almost ...
El Ronnoco's user avatar
11 votes
1 answer
3k views

Using SHA-256 with different initial hash value

FIPS 180-3 defines the initial hash value for SHA-256 as the first 32 bits of the fractional parts of the square roots of the first 8 primes 2..19. What would be the risks of using a different value (...
ericball's user avatar
  • 413
5 votes
2 answers
4k views

What does "message schedule" mean in SHA-256?

I am trying to understand the sha-256 algorithm from FIPS 180-2. I understood the padding and parsing of the message string. However after that it states (page 15): For $i = 1$ to $N$: { ...
Developer Android's user avatar
3 votes
3 answers
1k views

Can the SHA256 hashes of consecutive integers be attacked?

Suppose that the attacker knows the SHA256 values of integers $n, n+1, n+2 ... n+k$. $n$ is sufficiently big, so we do not expect to be able to brute force $n$ just by $\operatorname{SHA256}(n)$ ...
sqd's user avatar
  • 133
1 vote
1 answer
997 views

How exactly is the input message of SHA-256 (pre-)processed?

I’m very interested how SHA-256 handles messages, but I’ve got 3 questions. (As I have already read in some answers before, SHA-256 is not directly performed on the message but on an array, thanks for ...
Lizz4rd merge me's user avatar
1 vote
2 answers
3k views

What is the difference between "Elliptic Curve Function" and "Hash Functions" like SHA256?

I am reading about bitcoin and I am a little confused about "elliptic curve function" and "SHA256". Do they have the same properties? Can both be used to generate private and ...
Narayan Choudhary's user avatar
44 votes
4 answers
16k views

Cryptography algorithms that take longer to solve on a GPU than a CPU

I know that Graphics cards are faster at solving algorithms like SHA-256 because of the many builtin processors, but are there Algorithms that take actually longer on a Graphics card than on a modern ...
user51749's user avatar
  • 449
25 votes
3 answers
22k views

What size should the HMAC key be with SHA-256?

I'm trying to generate a secret key to be used for HMAC SHA-256 signature processing. I've seen many sample of keys with variable length from 32 characters to 96 characters. What is the ironclad rule ...
user30041's user avatar
  • 251
22 votes
1 answer
21k views

Are there any well-known examples of SHA-256 collisions?

The popularity of SHA-256 as a hashing algorithm, along with the fact that it has 2256 buckets to choose from leads me to believe that collisions do exist but are quite rare. Are there any well-...
Ari Sweedler's user avatar
17 votes
2 answers
66k views

Do identical strings always have the same SHA-256 value?

If you hash a string using SHA-256 on your computer, and I hash the same string using SHA-256 on my computer, will we generate the same value? Does the algorithm depend on a seed (so we'd both need ...
conor's user avatar
  • 289
11 votes
1 answer
894 views

Minimizing exchanges for ZK proof of a message with given SHA-256

Consider the problem of proving knowledge of a message $m$ which has a certain public SHA-256 hash $h$, without disclosing $m$ or usable information about it, while minimizing the information exchange ...
fgrieu's user avatar
  • 143k
9 votes
1 answer
678 views

Are there any security issues when replacing the SHA-256 initialisation values?

As RFC 4634 describes in section 6.1, SHA-256 is initialized using eight 32-bit words. These were obtained by taking the first 32 bits of the fractional parts of the square roots of the first eight ...
e-sushi's user avatar
  • 18k
8 votes
1 answer
2k views

Will rehashing an SHA256 hash continually, eventually produce every possible value? [duplicate]

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
Earlz's user avatar
  • 253
7 votes
2 answers
829 views

Entropy preservation through cryptographic hash function

Background First, while studying MinEntropy a bit, I came across an NIST paper, "DRAFT SP 800-90B (second draft)," which suggests "twice" the entropy of the underlying block of a cryptographic hash ...
Gratis's user avatar
  • 187
6 votes
2 answers
20k views

What happens if a SHA-256 input is too long (longer than 512 bits)?

What I understand is: When we parse a message into 512 bit message blocks. Then we extend the first message block to 64 entry array and start with the compression function. What happens if the ...
Lizz4rd merge me's user avatar
4 votes
1 answer
5k views

SHA-256 Partial Collision of initial 36 bits and more

I was lucky enough to, by brute force, have found two different messages, whose SHA-256 hashes collide in the first 9 hexadecimal characters, which are 36 bits, let'...
nvbach91's user avatar
3 votes
2 answers
3k views

Would SHA-256(SHA-256(x)) produce collisions?

Was reviewing some Bitcoin public-key hash literature and the use of RIPEMD-160 and the SHA-256 as below: RIPEMD160(SHA256(ECDSA_publicKey)) The Proof-of-work ...
Gopalakrishna Palem's user avatar
3 votes
1 answer
1k views

Has a SHA256 output ever been found consisting entirely of the same character?

Is it possible to have an output consisting purely of one character and does this change the entropy of that output? (If that's even possible to ascertain.) As an example, the Bitcoin cryptocurrency ...
Wingsuit's user avatar
  • 149