It is also not a good idea from a technical perspective on magnetic storage. Data overwrite operations should not be reading from the disk, except to verify that data was written correctly. It is also slow, as it requires reading data from the disk as well as running the hash function. If you were thinking about reading the data from the device in chunks, running it through a hash function of equal size, Luis' answer points out why that is a bad idea from a security perspective. Several reasons, the main being that there is no need to use one. But why allow it in the first place? Particularly when the alternative is simple enough: overwrite with random or pseudorandom data that has no correlation at all to the sensitive contents you're overwriting. You may think that this sounds like a very unlikely attack, and you could well be right about that. This is the same principle as password cracking just like password hashes allow you to quickly test password guesses, the overwritten disk sectors' contents could perhaps be used to test whether the disk contained or not some specific data before being overwritten.

In particular, it could be used to efficiently test hypotheses about the content of the overwritten data. The problem then is that this output, being a function of the data you're overwriting, therefore provides information about that input. I take it you mean that we would overwrite the storage with output produced by feeding its sensitive contents to a hash function. Specifically, why should hash functions not be used for data clearing?

Dban blancco