Are there any encryption algorithms that require significantly more work to decrypt, versus the amount of work required to encrypt?
I'm looking for a method of fast encryption, but which will require much more CPU time to decrypt later, as a method of spam prevention (prevent people from decrypting too many files in a short amount of time).
One thing I should mention, is that the encryption key itself will be public knowledge.
I'm not trying to lock anyone out, I just want to force them to work before they can acquire data.