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Add support for the key derivation method field in the NigoriSpecifics
proto in SyncEncryptionHandlerImpl. Persist the key derivation method
chosen for new passphrases in NigoriSpecifics. If updating from a remote
NigoriSpecifics, respect the key derivation method stored there. The
change is limited to cases when the passphrase type is
CUSTOM_PASSPHRASE.

A rationale for the change follows.

Prerequisite: if the custom passphrase key derivation method is set to
PBKDF2 (or UNSPECIFIED), the behavior of all decryption is exactly the
same as in previous versions.

Let "happy paths" be the encryption and decryption code paths which a
keystore-enabled client (referring to a client released after the
support for keystore migrations was introduced) will be able to hit
starting from a fresh account, if only other keystore-enabled clients
are syncing to that account.  Since legacy encryption methods are out of
scope for the change, we need to change the behavior only in these
"happy paths", and keep the behavior identical in others.

In ApplyNigoriUpdateImpl, we will read the key derivation method from
Nigori and save it in the class member.

In "happy paths", we will always know what key derivation method will be
used, and we will always know when we'll end up in a CUSTOM_PASSPHRASE
state. We will remember the key derivation method in the class as a
member when applicable. In particular, there are two such paths. Both
are short-circuits that are much easier to follow than the alternative,
legacy code paths:

1. |IsNigoriMigratedToKeystore() == true| case in
SetEncryptionPassphrase(), which calls SetCustomPassphrase() (only call
site). Here, it is us who are making the decision, so this is obvious.

2. |IsNigoriMigratedToKeystore() == true && ...| case in
SetDecryptionPassphrase(), which calls
DecryptPendingKeysWithExplicitPassphrase() (only call site). Here, we
will use the key derivation method from the member. Since we can't reach
this path in SetDecryptionPassphrase() without a valid Nigori node, we
assume that ApplyNigoriUpdateImpl() was called and has saved the key
derivation method from Nigori in the member.

Since we write the passphrase type to Nigori *only* in
AttemptToMigrateNigoriToKeystore, we will also write the custom
passphrase key derivation method *only* there. If the key derivation
method member is not set at that point (but the passphrase type we are
writing is CUSTOM_PASSPHRASE), that means we have reached the
CUSTOM_PASSPHRASE state through some other, non-"happy" path. In this
case, we will write PBKDF2 as the key derivation method. This will
ensure that decryption for these cases works the same way as in older
versions, no matter what.

The only thing that remains is converting an UNSPECIFIED key derivation
method in Nigori to PBKDF2. We will also do this in
ApplyNigoriUpdateImpl(), which is called when we receive a remote
Nigori. Because we expect to set the key derivation method explicitly
whenever we decide what it is, this will ensure that we use every chance
we can to set an explicit (rather than UNSPECIFIED) key derivation
method in Nigori.

Bug: 876408
Change-Id: Ia793f53f8777ed61af5edda9d8fa21090ad6da43
Reviewed-on: https://chromium-review.googlesource.com/1187148
Commit-Queue: David Davidović <davidovic@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587611}