Best utility to securely wipe specific files from flash drives?

When sharing flash drives with others, I want to make sure I'm securely erasing old files—not just removing them from the directory, but completely wiping their actual contents.

What utility software would you recommend for this?

Ideally, I need a tool that:

• Securely deletes every single block of the targeted files.
• Does not erase any more than necessary, in order to conserve the flash media's write cycles.

I think you're obsessing way to much about the FLASH media life. Just do a wipe with any of the many utilities, problem solved. I can't imagine you'll use up the drive write cycles before you die of old age! I think almost any currently shipping FLASH drive should do at least 10,000 write cycles. Also, any FLASH drive you're giving away or sharing is doubtless not a super high capacity 1TB drive, so the possibility of a $20 drive failing is not exactly a financial catastrophe.

No off-the-shelf security tool exists, because of how SSD's handle wear leveling. An OS writes to a set of logical blocks, but the SSD's onboard controller decides the mapping of logical blocks to physical blocks (or cells).

Normally to securely erase a file on a non-SSD device, you overwrite the file's assigned data blocks with junk data. This works because the mapping of logical to physical blocks never changes on a HDD (with the exception of remapped failed disk blocks).

When you write junk data to a file on a SSD, the onboard controller writes that data to a different set of physical blocks and updates the logical mapping so the OS thinks the file is securely wiped. But the original data is untouched on the drive. To get around this behavior, you would need special firmware in the SSD to make sure wear leveling isn't obeyed. There's no industry standard for that on consumer drives.

Some SSD's have tools where you can trigger a secure wipe of the entire drive, but not for individual files or volumes.

You can share data in encrypted ZIP files. This way using a data recovery tool doesn't help someone recover your deleted files, because they're encrypted in the first place.