Commit 76424a58cb enabled unique IV
for reverse mode by default, to get more testing and to increase
security of reverse mode.
The downside is that all IVs change when the inode numbers change,
which means that all of the ciphertext changes.
This may bite people who copied the plaintext to a new filesystem
- they will find out that they have to rsync everything again.
This commit disables unique IV for reverse mode by default. It can
still be enabled through expert mode.
Disabling the kernel cache make sure the encrypted view is always
up-to-date, however, it causes a factor 3 slowdown.
Please use --nocache manually if you want to disable caching.
Closes#60.
Also replaces the magic identifiers 513 and 514 by proper defines.
