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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6k 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 ...
26
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1answer
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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 ...
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4answers
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Cycles in SHA256

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. ...
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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 ...
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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 ...
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4answers
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SHA512 faster than SHA256?

I'm getting this strange result that SHA512 is around 50% faster than SHA256. I'm using .net's SHA512Managed and SHA256Managed ...
8
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1answer
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Are hash functions strong against quantum cryptography 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 ...
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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 ...
4
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1answer
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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 ...
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1answer
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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 ...
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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 ...
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2answers
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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 ...
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2answers
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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: ...
5
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1answer
429 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 ...
5
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1answer
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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 ...
8
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1answer
600 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 ...
4
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1answer
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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 ...
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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-...
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“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 ...
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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 ...
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1answer
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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 ...
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2answers
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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 $N$: { ...
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3answers
581 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 $\operatorname{SHA256}(n)$ ...
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1answer
561 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 ...
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2answers
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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} := \...
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2answers
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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 ...
17
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1answer
3k 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 ...
8
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2answers
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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 ...
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2answers
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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 ...
11
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1answer
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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 (...
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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 ...
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2answers
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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 ...
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1answer
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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-...
9
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1answer
412 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 ...
6
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1answer
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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, ...
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1answer
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How does ROTL work?

what does ROTL stand for? I know it does left shifting but what about the acronym? When we do a left shift, do we take the leftmost bit and add it at the end, by making the second bit the first, and ...
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1answer
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SHA256: Padding a 512 bits length message

I am currently implementing the SHA256 hash-algorithm for a "custom-built" embeded-device. Obviously I have a problem with message padding. The routine I wrote does not work with message whose size ...
39
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1answer
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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-...
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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 ...
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7answers
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Is calculating a hash code for a large file in parallel less secure than doing it sequentially?

I would like to improve the performance of hashing large files, say for example in the tens of gigabytes in size. Normally, you sequentially hash the bytes of the files using a hash function (say, ...
9
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1answer
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What does Maj and Ch mean in SHA-256 algorithm?

I'm guessing they're some kind of standard function but what do they do and what do the names mean? A little explaination or link me to an article would be great.
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1answer
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Replay attack prevention in connectionless UDP encrypted communication

What are the options for replay attack prevention when two parties exchange UDP messages. No connection or session is estabilished. Communicating parties have pre shared key that is used for ...
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3answers
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Is there a way to use bcrypt with passwords longer than 72 bytes securely?

From my understanding BCrypt truncates the password to 72 bytes. If a password is longer than 72 bytes, what is a way to store that password using bcrypt securely without compromising it? Or is this ...
9
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2answers
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128 bit hash with least chance of collision?

I'm building a storage system for JSON documents where they are looked up on a 128 bit key. These JSON documents have a timestamp within them, but apart from that are user-entered data. These JSON ...
5
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2answers
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AES+CTR+HMAC Encryption and Authentication on an Arduino

In my project we would like to encrypt and authenticate the the communication channel between our server and our Arduino nodes, which relies on an underlying TCP channel. We have chosen AES in CTR ...
2
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1answer
2k views

Hashing a password with SHA256 on the client then bcrypt on the server

I'm trying to implement a cryptographically secured storage site (not Mega, or anything similar) and am trying to prevent the user's password from ever touching the server. The password is used to ...
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3answers
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At the current time, is SHA256 the de facto standard for strong cryptographic hashes?

At the current time, is SHA256 the de facto standard for strong cryptographic hashes? From what I am seeing of more sites utilizing it, I would suppose the answer to this is yes, but would like to ...
9
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2answers
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Is there a guarantee that for each possible hash y there exists a number x such that with hash function H, H(x) = y?

Specifically talking about SHA-256 here and its involvement in Bitcoin, this was a question someone asked me which I didn't know the answer to. Yes, I am aware that SHA-256 can only have a hash from $...
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2answers
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AES + CBC encryption for a stream of UDP packets

I am developing an application that is based on UDP, and I need to send a stream of packets. As you can imagine, packets can get lost or corrupted. I need to make sure that the content of those ...
2
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1answer
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Uniform distribution/randomness of characters in the sha256 hash?

I'm using the sha256 algorithm to generate a number in one of my programming projects. A sha256 hash is calculated from three strings: one the user can edit, one the user can view but not edit and one ...