VFDecrypt ("VileFault Decrypt") is a program originally intended to decrypt FileVault-encrypted images. It is capable of decrypting the root filesystem image of iOS, with the appropriate key. The original program was written by Jacob Appelbaum (ioerror) and released at 23c3 2006.
To get the syntax for your build, run the program with no arguments. Different implementations of VFDecrypt have different rules about the flags. Some builds require the parameters to be in the exact order of
-iinput.dmg -kabcdef… -ooutput.dmg. Most builds require there be no space between the option and the parameter (i.e.