How SHA-256 Works Step-by-Step

#1

Post by kyleShropshire » Fri, Apr 22, 2022, 11:22 PM UTC

How SHA-256 Works Step-by-Step
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Re: How SHA-256 Works Step-by-Step

#2

Post by emerged » Sat, Apr 23, 2022, 12:29 AM UTC

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

Re: How SHA-256 Works Step-by-Step

#3

Post by RcouF1uZ4gsC » Sat, Apr 23, 2022, 12:48 AM UTC

emerged wrote: post #2

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

Exactly. Also SHA-256 is in the same family as MD-5. It would be nice to kind of go over how the family in general works, what weaknesses were discovered, and what changes were incorporated into SHA-256 that addresses these issues.

Re: How SHA-256 Works Step-by-Step

#4

Post by pvg » Sat, Apr 23, 2022, 12:51 AM UTC

emerged wrote: post #2

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

You're better off reading the Wikipedia page which covers things like that (for instance, the Merkle–Damgård construction) in some detail and without all the oversimplifications and almost unavoidable wrongness of slight write-ups like this.

Re: How SHA-256 Works Step-by-Step

#5

Post by abotsis » Sat, Apr 23, 2022, 1:51 AM UTC

RcouF1uZ4gsC wrote: post #3

emerged wrote: post #2

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

Exactly. Also SHA-256 is in the same family as MD-5. It would be nice to kind of go over how the family in general works, what weaknesses were discovered, and what changes were incorporated into SHA-256 that addresses these issues.

I’ve always been curious about whether it’s possible to have a collision when your input is 256 bits or less (in the case of sha256)… I even emailed Bruce Schneier. I got a polite response that he didn’t have time to look at it, which indicated to me he didn’t know “off the top of his head”, which I interpreted as “it’s not impossible”… but I still don’t know.

Re: How SHA-256 Works Step-by-Step

#6

Post by userbinator » Sat, Apr 23, 2022, 1:55 AM UTC

emerged wrote: post #2

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

There's some good discussion in the comments of another "one of these" a short while ago: https://news.ycombinator.com/item?id=30244534

Re: How SHA-256 Works Step-by-Step

#7

Post by upofadown » Sat, Apr 23, 2022, 1:59 AM UTC

abotsis wrote: post #5

RcouF1uZ4gsC wrote: post #3

Earlier quoted context omitted.

Exactly. Also SHA-256 is in the same family as MD-5. It would be nice to kind of go over how the family in general works, what weaknesses were discovered, and what changes were incorporated into SHA-256 that addresses these issues.

I’ve always been curious about whether it’s possible to have a collision when your input is 256 bits or less (in the case of sha256)… I even emailed Bruce Schneier. I got a polite response that he didn’t have time to look at it, which indicated to me he didn’t know “off the top of his head”, which I interpreted as “it’s not impossible”… but I still don’t know.

What sort of collision? Or asking in a different way, what would you consider a collision?

Re: How SHA-256 Works Step-by-Step

#8

Post by _vdpp » Sat, Apr 23, 2022, 2:09 AM UTC

upofadown wrote: post #7

abotsis wrote: post #5

Earlier quoted context omitted.

I’ve always been curious about whether it’s possible to have a collision when your input is 256 bits or less (in the case of sha256)… I even emailed Bruce Schneier. I got a polite response that he didn’t have time to look at it, which indicated to me he didn’t know “off the top of his head”, which I interpreted as “it’s not impossible”… but I still don’t know.

What sort of collision? Or asking in a different way, what would you consider a collision?

2 messages producing the same digest. That’s actually a really interesting question.

Re: How SHA-256 Works Step-by-Step

#9

Post by edflsafoiewq » Sat, Apr 23, 2022, 2:23 AM UTC

abotsis wrote: post #5

RcouF1uZ4gsC wrote: post #3

Earlier quoted context omitted.

Exactly. Also SHA-256 is in the same family as MD-5. It would be nice to kind of go over how the family in general works, what weaknesses were discovered, and what changes were incorporated into SHA-256 that addresses these issues.

I’ve always been curious about whether it’s possible to have a collision when your input is 256 bits or less (in the case of sha256)… I even emailed Bruce Schneier. I got a polite response that he didn’t have time to look at it, which indicated to me he didn’t know “off the top of his head”, which I interpreted as “it’s not impossible”… but I still don’t know.

There are 2^257 - 1 inputs of 256 bits or less and only 2^256 possible hashes, so there must be a collision.

Re: How SHA-256 Works Step-by-Step

#10

Post by tptacek » Sat, Apr 23, 2022, 2:35 AM UTC

emerged wrote: post #2

Every time one of these is posted, I’m expecting the steps to explain why they are being done. Like what makes this combination of operations have the particular properties we need?

You're not wrong. You're 100% right. This is like posting a story about how a game engine works, and having it be just blocks of assembly language with no explanation other than "here's some code multiplying a matrix, and then...".