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I finally made an ubuntu live usb on a compromised ubuntu desktop laptop. Startup disk creator told me the usb was 7.2 gb, instead 7.8 gb and I believe the original hack involved my flash drive. Is there any way to make sure the usb isn't infected and is there any way of completely cleanly and securely reinstalling ubuntu to lock out the hacker?

Also my ubuntu 14.04 iso said it was made thursday but modified the monday before. Is it safe to trust it?

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You can run a checksum verification on an .iso image. However, you should not be burning the image to disk from an infected system. Create your bootable medium on a different machine or order a disk online. An additional concern is that a virus can also be written into a boot record, firmware compromised, etc. So once a machine is compromised, you can't really trust it to be able to clean itself.

It seemed worth adding that since you are interested in checking file integrity in an environment with LIKELY malicious tampering that the MD5 algorithm is no longer considered secure:

The underlying MD5 algorithm is no longer deemed secure, thus while md5sum is well-suited for identifying known files in situations that are not security related, it should not be relied on if there is a chance that files have been purposefully and maliciously tampered. In the latter case, the use of a newer hashing tool such as sha256sum is highly recommended. md5sum wiki article

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