I’m pretty sure that everyone has something similar to a hash calculator that has been written in PowerShell or some other language and I am no different. This is my take on a simple file hash calculator to determine the hash of a given file or files based off of a given algorithm.

For those of you who may not know what I am talking about, calculating a file hash is useful to determine if a file has been modified from its current state. Usually when you download a file from the internet, there will be an associated checksum that has the expected hash of the file you are downloading. If the hashes match up, then there is nothing to worry about. But if the hash of the downloaded file differs from the expected hash, then that may raise concern as to the integrity of the file and if it has been compromised in any way.

To calculate a hash for a file, I will use the Security.Cryptography.HashAlgorithm class and its static method Create() which will create an instance of a given algorithm to use for the calculation. So what are my possible algorithm choices? Good question, they are:

• MD5 
• SHA1
• SHA256
• SHA384
• SHA512
• RIPEM160

For this example, I will just stick with MD5 and continue by creating the instance.

$algorithm = [Security.Cryptography.HashAlgorithm]::Create("MD5")

From this instance, I will then use the ComputeHash() method which will allow me to compute the hash of a given file based on whether I supply a file stream or the bytes of a file. First I will use the bytes method to show how easy it is to get the MD5 hash from a file.

$fileBytes = [io.File]::ReadAllBytes("C:\users\Administrator\desktop\NewFile.txt")
$bytes = $algorithm.ComputeHash($fileBytes)
-Join ($bytes | ForEach {"{0:x2}" -f $_})

First, I get the read all of the bytes of the file into memory and then use the ComputeHash() method of the MD5 instance and then translate that into a hex output using the {0:x2} formatter and make it all on one line by specifying –Join at the beginning line of code. While this method does work great, the gotcha is that this will start to fall apart when working with larger files. If you try this on a 3GB file  for instance and you don’t have the memory to support it, then you will the operation fail due to insufficient memory.

The Factory.wim file is 4Gb in size and is more than double what the ReadAllBytes() method allows.

The way to get around this issue is by using the other possible parameter that ComputeHash accepts which is a stream.

$hash = [Security.Cryptography.HashAlgorithm]::Create( "MD5" )
$stream = ([IO.StreamReader]"C:\users\Administrator\desktop\newfile.txt").BaseStream
-join ($hash.ComputeHash($stream) | ForEach { "{0:x2}" -f $_ })
$stream.Close()

Same amount of code and the same results. But the difference here is that if I ran this against a large file, it wouldn’t freak out with an error about memory running out. Lets try that factory.wim file again and see what happens.

It took a little bit of time, but I had no issues with getting the hash of the 4Gb file using the stream. So with that, your best option is to use a stream to handle any file, regardless of size. This will save you from a hassle if you happen to try to calculate a hash for a larger file.

Of course, this is always better as a function that can be re-used with little to no effort and wouldn’t be nicer to allow the option to pick multiple algorithms or be able to run Get-ChildItem and then pipe the output into a function to get the hashes? Of course it would be! That is why I have a function called Get-FileHash that will do all of this for you. It is a simple function that performs a simple hash calculation of all of the possible hashes if you choose.

Lets take a look at some examples of using it.

Get-FileHash -Path .\NewFile.txt -Algorithm MD5

Get-FileHash -Path .\newfile.txt -Algorithm MD5,SHA1,SHA512,SHA256,RIPEMD160 | Format-List

Get-ChildItem -Filter *.txt | 
    Get-FileHash -Algorithm MD5

This even works against UNC paths as well!

And yes, it works against files of any size, such as the 4GB image.

:\dell\Image\Factory.wim -Algorithm SHA256

The download for this function is available at the link below. As always, thank you for stopping by and I am always interested in hearing what you think about this function.

Download Get-FileHash

Script Repository