{-# LANGUAGE CPP #-} module Hackage.Security.Trusted.TCB ( -- * Trusted values Trusted(DeclareTrusted) , trusted , trustStatic , trustVerified , trustApply , trustElems -- * Verification errors , VerificationError(..) , RootUpdated(..) , VerificationHistory -- * Role verification , SignaturesVerified -- opaque , signaturesVerified , verifyRole' , verifyFingerprints #if __GLASGOW_HASKELL__ >= 710 -- * Re-exports , StaticPtr #else -- * Fake static pointers , StaticPtr , static #endif ) where import MyPrelude import Control.Exception import Control.Monad (when, unless) import Control.Monad.Except (Except, runExcept, throwError) import Data.Typeable import Data.Time import Hackage.Security.TUF import Hackage.Security.JSON import Hackage.Security.Key import Hackage.Security.Util.Pretty import qualified Hackage.Security.Util.Lens as Lens #if __GLASGOW_HASKELL__ >= 710 import GHC.StaticPtr #else -- Fake static pointers for ghc < 7.10. This means Trusted offers no -- additional type safety, but that's okay: we can still verify the code -- with ghc 7.10 and get the additional checks. newtype StaticPtr a = StaticPtr { deRefStaticPtr :: a } static :: a -> StaticPtr a static = StaticPtr #endif -- | Trusted values -- -- Trusted values originate in only two ways: -- -- * Anything that is statically known is trusted ('trustStatic') -- * If we have "dynamic" data we can trust it once we have verified the -- the signatures (trustSigned). -- -- NOTE: Trusted is NOT a functor. If it was we could define -- -- > trustAnything :: a -> Trusted a -- > trustAnything a = fmap (const a) (trustStatic (static ())) -- -- Consequently, it is neither a monad nor a comonad. However, we _can_ apply -- trusted functions to trusted arguments ('trustApply'). -- -- The 'DeclareTrusted' constructor is exported, but any use of it should be -- verified. newtype Trusted a = DeclareTrusted { trusted :: a } deriving (Eq, Show) trustStatic :: StaticPtr a -> Trusted a trustStatic = DeclareTrusted . deRefStaticPtr trustVerified :: SignaturesVerified a -> Trusted a trustVerified = DeclareTrusted . signaturesVerified -- | Equivalent of '<*>' -- -- Trusted isn't quite applicative (no pure, not a functor), but it is -- somehow Applicative-like: we have the equivalent of '<*>' trustApply :: Trusted (a -> b) -> Trusted a -> Trusted b trustApply (DeclareTrusted f) (DeclareTrusted x) = DeclareTrusted (f x) -- | Trust all elements of some trusted (traversable) container -- -- If we have, say, a trusted list of values, we should be able to get a list -- of trusted values out of it. -- -- > trustElems :: Trusted [a] -> [Trusted a] -- -- NOTE. It might appear that the more natural primitive to offer is a -- 'sequenceA'-like operator such as -- -- > trustSeq :: Applicative f => Trusted (f a) -> f (Trusted a) -- -- However, this is unsound. To see this, consider that @((->) a)@ is -- 'Applicative' (it's the reader monad); hence, we can instantiate 'trustSeq' -- at -- -- > trustSeq :: Trusted (a -> a) -> a -> Trusted a -- -- and by passing @trustStatic (static id)@ make 'Trusted' a functor, which we -- certainly don't want to do (see comments for 'Trusted'). -- -- So why is it okay when we insist on 'Traversable' rather than 'Applicative'? -- To see this, it's instructive to consider how we might make a @((->) a)@ an -- instance of 'Traversable'. If we define the domain of enumerable types as -- -- > class Eq a => Enumerable a where -- > enumerate :: [a] -- -- then we can make @((->) r)@ traversable by -- -- > instance Enumerable r => Traversable ((->) r) where -- > sequenceA f = rebuild <$> sequenceA ((\r -> (r,) <$> f r) <$> enumerate) -- > where -- > rebuild :: [(r, a)] -> r -> a -- > rebuild fun arg = fromJust (lookup arg fun) -- -- The idea is that if the domain of a function is enumerable, we can apply the -- function to each possible input, collect the outputs, and construct a new -- function by pairing the inputs with the outputs. I.e., if we had something of -- type -- -- > a -> IO b -- -- and @a@ is enumerable, we just run the @IO@ action on each possible @a@ and -- collect all @b@s to get a pure function @a -> b@. Of course, you probably -- don't want to be doing that, but the point is that as far as the type system -- is concerned you could. -- -- In the context of 'Trusted', this means that we can derive -- -- > enumPure :: Enumerable a => a -> Trusted a -- -- but in a way this this makes sense anyway. If a domain is enumerable, it -- would not be unreasonable to change @Enumerable@ to -- -- > class Eq a => Enumerable a where -- > enumerate :: [StaticPtr a] -- -- so we could define @enumPure@ as -- -- > enumPure :: Enumerable a => a -> Trusted a -- > enumPure x = trustStatic -- > $ fromJust (find ((== x) . deRefStaticPtr) enumerate) -- -- In other words, we just enumerate the entire domain as trusted values -- (because we defined them locally) and then return the one that matched the -- untrusted value. -- -- The conclusion from all of this is that the types of untrusted input (like -- the types of the TUF files we download from the server) should probably not -- be considered enumerable. trustElems :: Traversable f => Trusted (f a) -> f (Trusted a) trustElems (DeclareTrusted fa) = DeclareTrusted `fmap` fa {------------------------------------------------------------------------------- Role verification -------------------------------------------------------------------------------} newtype SignaturesVerified a = SignaturesVerified { signaturesVerified :: a } -- | Errors thrown during role validation data VerificationError = -- | Not enough signatures signed with the appropriate keys VerificationErrorSignatures TargetPath -- | The file is expired | VerificationErrorExpired TargetPath -- | The file version is less than the previous version | VerificationErrorVersion TargetPath -- | File information mismatch | VerificationErrorFileInfo TargetPath -- | We tried to lookup file information about a particular target file, -- but the information wasn't in the corresponding @targets.json@ file. | VerificationErrorUnknownTarget TargetPath -- | The metadata for the specified target is missing a SHA256 | VerificationErrorMissingSHA256 TargetPath -- | Some verification errors materialize as deserialization errors -- -- For example: if we try to deserialize a timestamp file but the timestamp -- key has been rolled over, deserialization of the file will fail with -- 'DeserializationErrorUnknownKey'. | VerificationErrorDeserialization TargetPath DeserializationError -- | The spec stipulates that if a verification error occurs during -- the check for updates, we must download new root information and -- start over. However, we limit how often we attempt this. -- -- We record all verification errors that occurred before we gave up. | VerificationErrorLoop VerificationHistory deriving (Typeable) -- | Root metadata updated (as part of the normal update process) data RootUpdated = RootUpdated deriving (Typeable) type VerificationHistory = [Either RootUpdated VerificationError] #if MIN_VERSION_base(4,8,0) deriving instance Show VerificationError deriving instance Show RootUpdated instance Exception VerificationError where displayException = pretty instance Exception RootUpdated where displayException = pretty #else instance Exception VerificationError instance Show VerificationError where show = pretty instance Show RootUpdated where show = pretty instance Exception RootUpdated #endif instance Pretty VerificationError where pretty (VerificationErrorSignatures file) = pretty file ++ " does not have enough signatures signed with the appropriate keys" pretty (VerificationErrorExpired file) = pretty file ++ " is expired" pretty (VerificationErrorVersion file) = "Version of " ++ pretty file ++ " is less than the previous version" pretty (VerificationErrorFileInfo file) = "Invalid hash for " ++ pretty file pretty (VerificationErrorUnknownTarget file) = pretty file ++ " not found in corresponding target metadata" pretty (VerificationErrorMissingSHA256 file) = "Missing SHA256 hash for " ++ pretty file pretty (VerificationErrorDeserialization file err) = "Could not deserialize " ++ pretty file ++ ": " ++ pretty err pretty (VerificationErrorLoop es) = "Verification loop. Errors in order:\n" ++ unlines (map ((" " ++) . either pretty pretty) es) instance Pretty RootUpdated where pretty RootUpdated = "Root information updated" -- | Role verification -- -- NOTE: We throw an error when the version number _decreases_, but allow it -- to be the same. This is sufficient: the file number is there so that -- attackers cannot replay old files. It cannot protect against freeze attacks -- (that's what the expiry date is for), so "replaying" the same file is not -- a problem. If an attacker changes the contents of the file but not the -- version number we have an inconsistent situation, but this is not something -- we need to worry about: in this case the attacker will need to resign the -- file or otherwise the signature won't match, and if the attacker has -- compromised the key then he might just as well increase the version number -- and resign. -- -- NOTE 2: We are not actually verifying the signatures _themselves_ here -- (we did that when we parsed the JSON). We are merely verifying the provenance -- of the keys. verifyRole' :: forall a. HasHeader a => Trusted (RoleSpec a) -- ^ For signature validation -> TargetPath -- ^ File source (for error messages) -> Maybe FileVersion -- ^ Previous version (if available) -> Maybe UTCTime -- ^ Time now (if checking expiry) -> Signed a -> Either VerificationError (SignaturesVerified a) verifyRole' (trusted -> RoleSpec{roleSpecThreshold = KeyThreshold threshold, ..}) targetPath mPrev mNow Signed{signatures = Signatures sigs, ..} = runExcept go where go :: Except VerificationError (SignaturesVerified a) go = do -- Verify expiry date case mNow of Just now -> when (isExpired now (Lens.get fileExpires signed)) $ throwError $ VerificationErrorExpired targetPath _otherwise -> return () -- Verify timestamp case mPrev of Nothing -> return () Just prev -> when (Lens.get fileVersion signed < prev) $ throwError $ VerificationErrorVersion targetPath -- Verify signatures -- NOTE: We only need to verify the keys that were used; if the signature -- was invalid we would already have thrown an error constructing Signed. -- (Similarly, if two signatures were made by the same key, the FromJSON -- instance for Signatures would have thrown an error.) unless (length (filter isRoleSpecKey sigs) >= fromIntegral threshold) $ throwError $ VerificationErrorSignatures targetPath -- Everything is A-OK! return $ SignaturesVerified signed isRoleSpecKey :: Signature -> Bool isRoleSpecKey Signature{..} = signatureKey `elem` roleSpecKeys -- | Variation on 'verifyRole' that uses key IDs rather than keys -- -- This is used during the bootstrap process. -- -- See . verifyFingerprints :: [KeyId] -> KeyThreshold -> TargetPath -- ^ For error messages -> Signed Root -> Either VerificationError (SignaturesVerified Root) verifyFingerprints fingerprints (KeyThreshold threshold) targetPath Signed{signatures = Signatures sigs, ..} = if length (filter isTrustedKey sigs) >= fromIntegral threshold then Right $ SignaturesVerified signed else Left $ VerificationErrorSignatures targetPath where isTrustedKey :: Signature -> Bool isTrustedKey Signature{..} = someKeyId signatureKey `elem` fingerprints