MD5 Hash question

PowerShell Help
1

I’m creating a file and to protect the integrity of it I would want to attach the MD5 checksum in the header.

Using the below script from Josep - Get-Hash, a powershell hash function I can get the MD5 checksum…But I’m just thinking that as soon as i generate it and then apply it to the header, the MD5 checksum has changed again…Is there a method to adding the checksum in the header without changing it by adding it to the header?

It would in theory look like…

Performed by: $username
Date: $date
MD5: $md5hash

everything else in the report below

  Get-Hash "hello_world.txt"
    Gets the SHA1 from myFile.txt file. When there's no explicit parameter, it uses -File

.EXAMPLE    
    Get-Hash -File "C:\temp\hello_world.txt"
    Gets the SHA1 from myFile.txt file
    
.EXAMPLE
    C:\PS> Get-Hash -Algorithm "MD5" -Text "Hello Wold!" 
    Gets the MD5 from a string
       
.EXAMPLE
    C:\PS> "Hello Wold!" | Get-Hash
    We can pass a string throught the pipeline  
    
.EXAMPLE
    Get-Content "c:\temp\hello_world.txt" | Get-Hash
    It gets the string from Get-Content

.EXAMPLE
    Get-ChildItem "C:\temp\*.txt" | %{ Write-Output "File: $($_)   has this hash: $(Get-Hash $_)" }
    This is a more complex example gets the hash of all "*.tmp" files
    
.NOTES
    DBA daily stuff (http://dbadailystuff.com) by Josep Martínez Vilà
    Licensed under a Creative Commons Attribution 3.0 Unported License
    
.LINK
    Original post: http://dbadailystuff.com/2013/03/11/get-hash-a-powershell-hash-function/
#>
function Get-Hash
{
    Param
    (
        [parameter(Mandatory=$true, ValueFromPipeline=$true, ParameterSetName="set1")]
        [String]
        $text,
        [parameter(Position=0, Mandatory=$true, ValueFromPipeline=$false, ParameterSetName="set2")]
        [String]
        $file = "",
        [parameter(Mandatory=$false, ValueFromPipeline=$false)]
        [ValidateSet("MD5", "SHA", "SHA1", "SHA-256", "SHA-384", "SHA-512")]
        [String]
        $algorithm = "SHA1"
    )

    Begin
    {
        $hashAlgorithm = [System.Security.Cryptography.HashAlgorithm]::Create($algorithm)
    }
	Process
	{
        $md5StringBuilder = New-Object System.Text.StringBuilder 50
        $ue = New-Object System.Text.UTF8Encoding 

        if ($file){
            try {
                if (!(Test-Path -literalpath $file)){
                    throw "Test-Path returned false."
                }
            }
            catch {
                throw "Get-Hash - File not found or without permisions: [$file]. $_"
            } 

            try {        
                [System.IO.FileStream]$fileStream = [System.IO.File]::Open($file, [System.IO.FileMode]::Open);
                $hashAlgorithm.ComputeHash($fileStream) | % { [void] $md5StringBuilder.Append($_.ToString("x2")) }
            }
            catch {
                throw "Get-Hash - Error reading or hashing the file: [$file]"
            } 
            finally {
                $fileStream.Close()
                $fileStream.Dispose()
            }
        }
        else {
            $hashAlgorithm.ComputeHash($ue.GetBytes($text)) | % { [void] $md5StringBuilder.Append($_.ToString("x2")) }
        }
        
        return $md5StringBuilder.ToString()
    }
}
2

Nope. As you say, adding any content to the file immediately changes the hash. Hashes are typically downloaded or sent out in a separate file.

3

Ok, that’s what I figured but just wanted to ask, just incase. Thanks!