As #19274 is now fixed in Go 1.9, there is no longer any reason to have
a duplicate implementation to the standard library. We can now move
safely to crypto/rand.

This commit changes all the internal import paths from `fscrypt/foo` to
`github.com/google/fscrypt/foo` so that it can be built once we release
externaly. The documentation in README.md is updated accordingly.

Also, the README has a note noting that we do not make any guarantees
about project stability before 1.0 (when it ships with Ubuntu).

Change-Id: I6ba86e442c74057c8a06ba32a42e17f94833e280

This commit changes the error handing for the crypto, filesystem,
metadata, pam, and util packages to use the error handling library
github.com/pkg/errors. This means elimination of the FSError type, an
increased use of wrapping errors (as opposed to logging), switching
on the Cause() of an error (as opposed to its value), and improving our
integration tests involving TEST_FILESYSTEM_ROOT.

This commit also fixes a few bugs with the keyring code to ensure that
our {Find|Remove|Insert}PolicyKey functions are always operating on the
same keyring. The check for filesystem support has been moved from the
filesystem package to the metadata package. Finally, the API for the
filesystem package has been slightly modified:
	* filesystem.AllFilesystems() now returns all the filesystems in
	  sorted order
	* certain path methods are now public

O_SYNC is also removed for writing the metadata. We don't get that much
from syncing the metadata, as the actual file data could also be
corrupted by and IO error. The sync operation is also occasionally very
slow (~3 seconds) and can be unfriendly to battery life.

Change-Id: I392c2655141714b16dfdbc84ac09780072be2cf0

This commit adds in the FindPolicyKey and RemovePolicyKey functions to
complement the InsertPolicyKey function. The existing functions were
also refactored slightly.

Change-Id: Iabd275f2186a9e3023d5efd44c772966123e3657

This changes the crypto package so it now builds in light of the changes
to the util and metadata package. This commit also improves the error
handling, adds tests, and makes it so recovery keys now correspond to
Policy keys (as they are used to recover a directory in the absence of
any metadata).

The only feature addition here is the ability to compute descriptors.
For backwards compatibility, we keep the same descriptor algorithm used
before (double SHA512).

Change-Id: Ia2b53c6e85ce65c57595e6823d3c4c92219bc8dc

This commit adds in the PassphraseHash function which hashes the
provided passphrase (in key form) using Argon2id. This cost parameters
for Argon2id and that salt are both fed into the function. It also
includes tests and benchmarks for the passphrase hashing.

Change-Id: I060db3e71213c756d45ce5603a0a59d3d7a1e609

This commit adds in the ability to use the WrappedKeyData from the
metadata package to wrap and unwrap cryptographic keys of any length.
This makes use of several cryptographic primitives:
	- Unsalted, SHA256-based HKDF for key stretching
	- AES256 in CTR mode for encryption
	- SHA256-based HMAC for authentication

Note that the key wrapping/unwrapping uses an "Encrypt then MAC" scheme
for doing authenticated unwrapping. This means we can detect if bogus
metadata has been given. This package also standardizes the length for
fscrypt's internal keys.

This CL is the first to add benchmarks, which can be run with:
	go test -bench=. ./...

Change-Id: I2e5fc23a8a8cc36b17ccb3f26f03edcaccc517e1