How to quickly hash growing files

[Sherlock 663]

I was tasked with finding a way to know if files have been altered. The possible alterations can only be caused by either system failure or a semi malicious person as the computer is not connected to a network and the gain for said person is...minimal at best. In both cases the altered files would have to be recreated instead of reused which would slow down a different process a bit. In short these are cache files. And of course the best way to know if a file has been altered without permission is a hash, since Windows permissions are sketchy at best. One of these files grows by roughly 2600 Bytes every 5 seconds, and therefore it would be necessary to rehash after each such interval. The system would be generating up to 20 of these files "simultaneously" with each of these files being about 6 hours long on average and may peak at 24 hours. So in Bytes we are talking on average 11MB peaking to 44MB.

Here's the question: What would be the best way to do this without to much of an impact on performance. I could use Delphis built-in hashing system.Hash.TSHA1. It is pretty straight forward, but before I dive in on the shallow end of this pool and break my neck...perhaps one of you has a better suggestion.

 

[Lajos Juhász 293]

Why not keep the file locked while writing into it? In that case you have to do the hashing only when the file is done.

[Sherlock 663]

OK, but my application might crash and then the file might not have been hashed or even written. But I guess I could express the need to perform a "complete reload" after a crash. But it's not nice...

[Kas Ob. 121]

As Lajos said, if they are only cache and will not be needed after a restart or to be shared with different process, then put them in system temp folder and don't share writing, as for hashing 44Mb is nothing to worry about, but you can build a table to hash parts like every 64kb this way you will check that part when you read (need) it.

[Stefan Glienke 2009]

SHA1 is dead. I suggest using SHA2 or SHA3 even if it is "just" for a file checksum. If you want performance-optimized implementations I would suggest using mormot2.

 

uses
  mormot.core.buffers,
  mormot.crypt.secure;

...

  HashFile(myFileName, THashAlgo.hfSHA256);  

Fun fact: mormot2 SHA256 is faster than RTL SHA1.

[Kas Ob. 121]

59 minutes ago, Stefan Glienke said:

Fun fact: mormot2 SHA256 is faster than RTL SHA1.

Also SHA3, SHA512 are faster by at least %50 than SHA256.

[Brian Evans 108]

For basic error / tamper detection a CRC would be easier and a lot faster since you can feed additional bytes into the calculation as the file grows. Can also keep a few length, CRC pairs around to re-check parts of the file as desired. The CRC value of the first 16MB could be used to either check the first 16MB of the file or to check from 16MB to another CRC at 20MB for example.