move cryptographicallySecure into Backend type

This is groundwork for external backends, but also makes sense to keep
this information with the rest of a Backend's implementation.

Also, removed isVerifiable. I noticed that the same information is
encoded by whether a Backend implements verifyKeyContent or not.

Types/Backend.hs

@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2010-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -18,7 +18,8 @@ import Utility.FileSystemEncoding
 data BackendA a = Backend
 	{ backendVariety :: KeyVariety
 	, getKey :: Maybe (KeySource -> MeterUpdate -> a Key)
-	-- Verifies the content of a key.
+	-- Verifies the content of a key using a hash. This does not need
+	-- to be cryptographically secure.
 	, verifyKeyContent :: Maybe (Key -> FilePath -> a Bool)
 	-- Checks if a key can be upgraded to a better form.
 	, canUpgradeKey :: Maybe (Key -> Bool)
@@ -28,6 +29,8 @@ data BackendA a = Backend
 	-- Checks if a key is known (or assumed) to always refer to the
 	-- same data.
 	, isStableKey :: Key -> Bool
+	-- Checks if a key is verified using a cryptographically secure hash.
+	, isCryptographicallySecure :: Key -> Bool
 	}
 
 instance Show (BackendA a) where

Types/Key.hs

@@ -23,8 +23,6 @@ module Types.Key (
 	HashSize(..),
 	hasExt,
 	sameExceptExt,
-	cryptographicallySecure,
-	isVerifiable,
 	formatKeyVariety,
 	parseKeyVariety,
 ) where
@@ -262,36 +260,6 @@ sameExceptExt (SHA1Key _) (SHA1Key _) = True
 sameExceptExt (MD5Key _) (MD5Key _) = True
 sameExceptExt _ _ = False
 
-{- Is the Key variety cryptographically secure, such that no two differing
- - file contents can be mapped to the same Key? -}
-cryptographicallySecure :: KeyVariety -> Bool
-cryptographicallySecure (SHA2Key _ _) = True
-cryptographicallySecure (SHA3Key _ _) = True
-cryptographicallySecure (SKEINKey _ _) = True
-cryptographicallySecure (Blake2bKey _ _) = True
-cryptographicallySecure (Blake2bpKey _ _) = True
-cryptographicallySecure (Blake2sKey _ _) = True
-cryptographicallySecure (Blake2spKey _ _) = True
-cryptographicallySecure _ = False
-
-{- Is the Key variety backed by a hash, which allows verifying content?
- - It does not have to be cryptographically secure against eg birthday
- - attacks.
- -}
-isVerifiable :: KeyVariety -> Bool
-isVerifiable (SHA2Key _ _) = True
-isVerifiable (SHA3Key _ _) = True
-isVerifiable (SKEINKey _ _) = True
-isVerifiable (Blake2bKey _ _) = True
-isVerifiable (Blake2bpKey _ _) = True
-isVerifiable (Blake2sKey _ _) = True
-isVerifiable (Blake2spKey _ _) = True
-isVerifiable (SHA1Key _) = True
-isVerifiable (MD5Key _) = True
-isVerifiable WORMKey = False
-isVerifiable URLKey = False
-isVerifiable (OtherKey  _) =  False
-
 formatKeyVariety :: KeyVariety -> S.ByteString
 formatKeyVariety v = case v of
 	SHA2Key sz e -> adde e (addsz sz "SHA")