How to Verify a Source File Before Patching

What is a checksum?

A checksum is a value calculated from every byte of a file. The same file always gives the same checksum, and even a one-byte difference gives a completely different one. Comparing two checksums is a reliable way to tell whether two files are identical, without comparing them byte by byte.

For checking that a file is the right version, any of these works. CRC32 is weaker against deliberate tampering, but it’s perfectly good at telling two versions of a file apart.

Step 1: Find the expected checksum

Look in these places, in order:

1. The patch’s README or included text file.
2. The Patch info tooltip Delta Patcher shows when you load the patch.
3. The patch’s download page or release notes.

Note the algorithm and the value, plus any file size, region or revision listed. No checksum published? Compare the exact file size in bytes instead. It won’t catch every difference, but it rules out many.

Step 2: Calculate your file’s checksum

Windows

Open PowerShell (right-click the Start button → Terminal), go to the folder with your file, and run:

Get-FileHash ".\Game (USA).iso" -Algorithm SHA1
Get-FileHash ".\Game (USA).iso" -Algorithm SHA256
Get-FileHash ".\Game (USA).iso" -Algorithm MD5

Tip: type Get-FileHash and then drag the file into the window to paste its full path. The older Command Prompt equivalent is certutil -hashfile "Game (USA).iso" SHA1.

CRC32 on Windows: Windows has no built-in CRC32 command. If you use 7-Zip (opens external site), right-click the file and choose 7-Zip → CRC SHA → CRC-32 (or * for all hashes at once).

macOS

Open Terminal, type the command followed by a space, drag the file onto the Terminal window, and press Return:

shasum -a 1 "Game (USA).iso"
shasum -a 256 "Game (USA).iso"
md5 "Game (USA).iso"

Linux

sha1sum "Game (USA).iso"
sha256sum "Game (USA).iso"
md5sum "Game (USA).iso"

CRC32 on macOS and Linux: the standard cksum command uses a different CRC and won’t match a CRC32 value. With 7-Zip installed (7z or 7zz depending on the package), run 7z h -scrcCRC32 "Game (USA).iso".

In a web browser

Rom Patcher JS (opens external site) shows a file’s CRC32, MD5 and SHA-1 before patching, and works on any system with a browser.

Step 3: Compare the values

• Compare the whole value, not just the first few characters.
• Case doesn’t matter: 1A2B3C4D and 1a2b3c4d are the same.
• Make sure you used the same algorithm as the published value. A SHA-1 will never equal an MD5.

Source file verification checklist

Use this before applying any patch. Tick items as you go.

• I found the file name, region and revision the patch needs.
• I extracted my file from any .zip, .7z or .rar archive.
• My file’s size in bytes matches the size listed (if given).
• I calculated the checksum with the same algorithm the author uses.
• The full checksum value matches exactly.
• The file hasn’t been patched, trimmed, converted or re-compressed since I checked.
• I’ve kept an untouched copy of this verified file somewhere safe.

Bonus: verify the patched result

Some authors also publish the checksum of the patched file. Calculating it after patching confirms the patch applied exactly as intended. If you created the patch yourself, compare the patched output with your modified file the same way. Testing a patch before sharing it.

Keep a record of your originals

If you patch files often, note the name, size and SHA-1 of each untouched original in a text file next to it. Next time a patch lists a checksum, you’ll know immediately whether you have the right file, without recalculating anything.