glacier, S3: Fix bug that caused embedded creds to not be encypted using the remote's key.

encryptionSetup must be called before setRemoteCredPair. Otherwise,
the RemoteConfig doesn't have the cipher in it, and so no cipher is used to
encrypt the embedded creds.

This is a security fix for non-shared encryption methods!

For encryption=shared, there's no security problem, just an
inconsistentency in whether the embedded creds are encrypted.

This is very important to get right, so used some types to help ensure that
setRemoteCredPair is only run after encryptionSetup. Note that the external
special remote bypasses the type safety, since creds can be set after the
initial remote config, if the external special remote program requests it.
Also note that IA remotes never use encryption, so encryptionSetup is not
run for them at all, and again the type safety is bypassed.

This leaves two open questions:

1. What to do about S3 and glacier remotes that were set up
   using encryption=pubkey/hybrid with embedcreds?
   Such a git repo has a security hole embedded in it, and this needs to be
   communicated to the user. Is the changelog enough?

2. enableremote won't work in such a repo, because git-annex will
   try to decrypt the embedded creds, which are not encrypted, so fails.
   This needs to be dealt with, especially for ecryption=shared repos,
   which are not really broken, just inconsistently configured.

   Noticing that problem for encryption=shared is what led to commit
   fbdeeeed5f, which tried to
   fix the problem by not decrypting the embedded creds.

This commit was sponsored by Josh Taylor.

Creds.hs

@@ -23,7 +23,7 @@ import Annex.Perms
 import Utility.FileMode
 import Crypto
 import Types.Remote (RemoteConfig, RemoteConfigKey)
-import Remote.Helper.Encryptable (remoteCipher, embedCreds)
+import Remote.Helper.Encryptable (remoteCipher, embedCreds, EncryptionIsSetup)
 import Utility.Env (getEnv)
 
 import qualified Data.ByteString.Lazy.Char8 as L
@@ -40,12 +40,17 @@ data CredPairStorage = CredPairStorage
 
 {- Stores creds in a remote's configuration, if the remote allows
  - that. Otherwise, caches them locally.
- - The creds are found in storage if not provided. -}
-setRemoteCredPair :: RemoteConfig -> CredPairStorage -> Maybe CredPair -> Annex RemoteConfig
-setRemoteCredPair c storage Nothing = 
-	maybe (return c) (setRemoteCredPair c storage . Just)
+ - The creds are found in storage if not provided.
+ -
+ - The remote's configuration should have already had a cipher stored in it
+ - if that's going to be done, so that the creds can be encrypted using the
+ - cipher. The EncryptionIsSetup phantom type ensures that is the case.
+ -}
+setRemoteCredPair :: EncryptionIsSetup -> RemoteConfig -> CredPairStorage -> Maybe CredPair -> Annex RemoteConfig
+setRemoteCredPair encsetup c storage Nothing = 
+	maybe (return c) (setRemoteCredPair encsetup c storage . Just)
 		=<< getRemoteCredPair c storage
-setRemoteCredPair c storage (Just creds)
+setRemoteCredPair _ c storage (Just creds)
 	| embedCreds c = case credPairRemoteKey storage of
 		Nothing -> localcache
 		Just key -> storeconfig key =<< remoteCipher c