Fix Not a Valid IPSW Error while Creating Custom IPSW file for iPhone 3GS

Hello Simon, basically, Sn0wbreeze – as well as PwnageTool – is throwing Not a Valid IPSW error because you were trying to restore / jailbreak a corrupt or an unsupported IPSW file. Just to make sure, I would recommend you to re-download the stock iOS 4.3.3 from our website using this link. Additionally, it is highly recommended to use Internet Explorer (the latest version is much preferred) in downloading the IPSW file as there are tendencies that other web-browsers (i.e. Mozilla Firefox, Chrome, and even Safari) auto extract it or save it in different format. Once you have successfully downloaded the stock IPSW file, you can now start creating custom iOS 4.3.3 using Sn0wbreeze 2.7.3 by following this tutorial.

If Sn0wbreeze is still throwing Invalid IPSW error, then you could download Marxio File Checksum Verifier from here and check MD5 and SHA1 checksums of the downloaded IPSW. You could also use TheiPhoneWiki website to check the correct checksum values of your IPSW file. If you get different checksum values form the downloaded IPSW file into the firmware file listed on the website then re-download the fresh IPSW for your iPhone from this website or from iClarified. That’s it! Hope this helps.