File hashes

SFV used CRC for file checksums - which were fast to compute, but trivial to generate collisions for. We should probably move on to something like SHA256.

This is already in PowerShell:

Get-FileHash -Algorithm SHA256 

Nicer Outputs

Pipe a list of files, sort by name, hash them and format the output into a table.

$Table =
Get-ChildItem -Path $Path |     
Sort-Object -Property Name |
Get-FileHash -Algorithm SHA256 | 
Format-Table @{ Label = 'File'; Expression = { $_.Path | Split-Path -Leaf }}, Hash

Running this gives you something like:

File		Hash
----		----
Pack1.zip	E25F45F2B360642B8CEACFD7F00796BE355DC720D510343EE4F106BB8EA89D2B
Pack2.zip	FED441E7DC68C4E41F2928A9CD7B393FFCD7A602A28BFC942071109D447D0F1D
Pack4.zip	D4F7BDFC7227601F5677F1E8DBD9E5009FD2BE8A109EDD4F4C8DB175200C4C69
Pages.gz	548EE10D5D59ED535F96C08B2F9999B8866F85C40F0C6BB22DA5A07BD8C035F7
Pages.zip	E65277A391BE9AFCEF046650F23AB0ECB67DE4334C449F21271068239A413F18

We can now write this to a file and we have our list of hashes.

Comparing

Comparing is equally simple - read a saved file and use Compare-Object to roughly diff it with a calculated result.

Compare-Object ($Table) (Get-Content $Verify)

Here’s the final powershell module:

Usage

Drop it in your PowerShell modules directory in Verify\Verify.psm1.

In any folder, use

Create-Verification

to generate (current folder).sha2

The defaults are:

Create-Verification -Path . -Filter *.*

To verify, use:

Create-Verification -Verify (folder.sha2) 

-Path and -Filter options still apply.

Note that the function produces a object stream of { Side, File }, so you can still post-process the output with scripts that handle Compare-Object output.

Last modified on 2019-08-11