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PowerShell script to compute partial MD4 checksums

3 min read 22-10-2024
the ifix
PowerShell script to compute partial MD4 checksums

MD4 is a cryptographic hash function that produces a 128-bit hash value, typically expressed as a 32-digit hexadecimal number. While MD4 is not as commonly used today due to security vulnerabilities, there are still scenarios where it might be necessary to compute partial MD4 checksums for data verification or other purposes. In this article, we will discuss how to create a PowerShell script to compute partial MD4 checksums, understand the code, and explore some practical applications.

Problem Scenario

Imagine you have a large dataset and you only want to compute the MD4 checksum for a specific portion of each file. This scenario requires a PowerShell script that can read the designated portion of files and return their partial MD4 checksums.

Here is an example of the original code for computing a full MD4 checksum:

function Compute-MD4Hash {
    param (
        [string]$filePath
    )

    # Load the MD4 .NET assembly
    Add-Type -AssemblyName System.Security.Cryptography

    # Create an MD4 hash object
    $md4 = New-Object System.Security.Cryptography.HashAlgorithm -ArgumentList "MD4"
    
    # Read the file and compute the hash
    $fileStream = [System.IO.File]::OpenRead($filePath)
    $hashBytes = $md4.ComputeHash($fileStream)
    $fileStream.Close()
    
    # Convert hash to hexadecimal string
    return [BitConverter]::ToString($hashBytes) -replace '-', ''
}

# Example usage
$checksum = Compute-MD4Hash -filePath "C:\path\to\your\file.txt"
Write-Output $checksum

Understanding the Code

The above PowerShell function, Compute-MD4Hash, takes a file path as input and computes the MD4 hash for that entire file. Here's how it works:

  1. Loading the MD4 Assembly: The Add-Type cmdlet loads the required .NET assembly that provides the cryptographic functions.
  2. Creating an MD4 Hash Object: The function initializes a new MD4 hash object.
  3. Reading the File: The script opens the file in read mode and computes the hash.
  4. Returning the Hash: Finally, the hash bytes are converted into a human-readable hexadecimal string format.

Computing Partial MD4 Checksums

To modify the original script for partial MD4 checksum computation, we can adjust the function to accept additional parameters that specify the starting position and the number of bytes to read.

Modified Code Example

Here’s a revised version of the PowerShell script to compute partial MD4 checksums:

function Compute-PartialMD4Hash {
    param (
        [string]$filePath,
        [int]$offset,
        [int]$length
    )

    # Load the MD4 .NET assembly
    Add-Type -AssemblyName System.Security.Cryptography

    # Create an MD4 hash object
    $md4 = New-Object System.Security.Cryptography.HashAlgorithm -ArgumentList "MD4"
    
    # Open file stream
    $fileStream = [System.IO.File]::OpenRead($filePath)
    
    # Seek to the specified offset
    $fileStream.Seek($offset, [System.IO.SeekOrigin]::Begin)
    
    # Create a buffer and read the specified length
    $buffer = New-Object byte[] $length
    $bytesRead = $fileStream.Read($buffer, 0, $length)
    $fileStream.Close()
    
    # Compute the hash for the read portion
    $hashBytes = $md4.ComputeHash($buffer, 0, $bytesRead)
    
    # Convert hash to hexadecimal string
    return [BitConverter]::ToString($hashBytes) -replace '-', ''
}

# Example usage
$partialChecksum = Compute-PartialMD4Hash -filePath "C:\path\to\your\file.txt" -offset 10 -length 20
Write-Output $partialChecksum

Analysis and Explanation

In the modified function Compute-PartialMD4Hash, we introduced two new parameters:

  • offset: The position in the file from where the reading should begin.
  • length: The number of bytes to read from the file.

The function reads the specified number of bytes starting from the designated offset and computes the MD4 checksum only for that portion. This is especially useful for large files where you might be interested only in a specific segment rather than the entire content.

Practical Applications

  1. Data Integrity Verification: If you only want to check specific sections of a file for integrity, such as a header or footer, computing a partial hash could save time and resources.
  2. File Comparisons: For applications where you compare files, partial checksums can help in quickly validating the segments of files that have changed.
  3. Log File Analysis: When dealing with log files, often we are only interested in the most recent entries. A partial checksum could be used to verify that these entries have not been altered.

Conclusion

Computing partial MD4 checksums using PowerShell is a useful technique for scenarios where file integrity needs to be verified in segments rather than as a whole. The modified function provided allows for flexibility in reading and hashing file portions, helping save time and computational resources.

Additional Resources

By implementing the revised PowerShell script and understanding how it operates, readers can efficiently handle checksum calculations for various use cases in their workflows.

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