checksums and unicode data

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checksums and unicode data

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Lowell SSC Guru Points: 323518 More actions · Answer 1

just looking for confirmation...i used my google-fu and found info and examples for using hashbytes with SHA1, ie

select HASHBYTES('SHA1','My Plain Text')

--results

0x6D99DDF6FE7A32547B6766E0BF88B1F50835F0FF

everything i read says that this is a one way operation, so you can use it to generate a unique value like checksum, but you cannot unhash this back to it's original value, right?

so this is not encryption, but rather a tool to generate a unique identifier, correct?

Lowell

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Jeff Moden SSC Guru Points: 1004407 More actions · Answer 2

Paul White (8/10/2009)
RBarryYoung (8/10/2009)
To clarify: HashBytes w/ SHA1 is nigh-unbreakable.
Hey Barry,
On the other hand, it is relatively slow, only works on strings, and returns varbinary(8000).
If the task is to detect changes in a row of data, would you use HashBytes (SHA1) alone?
Paul

In Lowell's example, it also turned a 13 byte string into a 20 byte Varbinary so it's expensive, too.

But, I believe that Barry's point was simply that it is unbreakable... not like CHECKSUM at all. 🙂

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Paul White SSC Guru Points: 150468 More actions · Answer 3

Jeff Moden (8/10/2009)
In Lowell's example, it also turned a 13 byte string into a 20 byte Varbinary so it's expensive, too. But, I believe that Barry's point was simply that it is unbreakable... not like CHECKSUM at all. 🙂

So...SHA1 always returns 160 bits (20 bytes) regardless of the input. HashBytes also appears to be limited to 8000 bytes of input - 8000 ANSI characters or 4000 Unicode. Finally, it doesn't appear unbreakable either: Wikipedia Link.

I mentioned HashBytes way back - now I'm just pointing out some of the reasons that I have yet to use it in a real production system.

Paul

Paul White
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RBarryYoung SSC Guru Points: 143329 More actions · Answer 4

Paul White (8/10/2009)
RBarryYoung (8/10/2009)
To clarify: HashBytes w/ SHA1 is nigh-unbreakable.
Hey Barry,
On the other hand, it is relatively slow, only works on strings, and returns varbinary(8000).
If the task is to detect changes in a row of data, would you use HashBytes (SHA1) alone?
Paul

Despite the datatype, I think that it actually only returns 19 bytes. And since VARBINARY is a string and *everything* converts to varbinary very easily, thats not that big a problem (nulls are a bigger problem). So yeah, I *might* use it in situations were I was doing remote comparisons through linked servers and just recording the hash every day for later comparison. The problem with the field by field comparison is that you have to have the entire previous record around to do it.

Though I think that I usually used MD5 in the past.

[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung [font="Arial Black"]
Proactive Performance Solutions, Inc. [/font][font="Verdana"] "Performance is our middle name."[/font]

RBarryYoung SSC Guru Points: 143329 More actions · Answer 5

Paul White (8/10/2009)
Jeff Moden (8/10/2009)
In Lowell's example, it also turned a 13 byte string into a 20 byte Varbinary so it's expensive, too. But, I believe that Barry's point was simply that it is unbreakable... not like CHECKSUM at all. 🙂
So...SHA1 always returns 160 bits (20 bytes) regardless of the input. HashBytes also appears to be limited to 8000 bytes of input - 8000 ANSI characters or 4000 Unicode. Finally, it doesn't appear unbreakable either: Wikipedia Link.
I mentioned HashBytes way back - now I'm just pointing out some of the reasons that I have yet to use it in a real production system.
Paul

Hmm, I though that I counted 19 bytes... Anyway, qualifying myself again :-), "nigh-unbreakable" was referring to its use as a signature in detecting random data changes, not as a security device.

[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung [font="Arial Black"]
Proactive Performance Solutions, Inc. [/font][font="Verdana"] "Performance is our middle name."[/font]