hackage-security-0.5.2.2: Hackage security library

Hackage.Security.Trusted

Synopsis

# Trusted values

newtype Trusted a #

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.

Constructors

 DeclareTrusted Fieldstrusted :: a

Instances

 Eq a => Eq (Trusted a) # Methods(==) :: Trusted a -> Trusted a -> Bool #(/=) :: Trusted a -> Trusted a -> Bool # Show a => Show (Trusted a) # MethodsshowsPrec :: Int -> Trusted a -> ShowS #show :: Trusted a -> String #showList :: [Trusted a] -> ShowS #

trustApply :: Trusted (a -> b) -> Trusted a -> Trusted b #

Equivalent of <*>

Trusted isn't quite applicative (no pure, not a functor), but it is somehow Applicative-like: we have the equivalent of <*>

trustElems :: Traversable f => Trusted (f a) -> f (Trusted a) #

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) <$> enumerate) where rebuild :: [(r, a)] -> r -> a rebuild fun arg = fromJust (lookup arg fun) The idea is that if the domain of a function is enumerable, we can apply the function to each possible input, collect the outputs, and construct a new function by pairing the inputs with the outputs. I.e., if we had something of type a -> IO b and a is enumerable, we just run the IO action on each possible a and collect all bs to get a pure function a -> b. Of course, you probably don't want to be doing that, but the point is that as far as the type system is concerned you could. In the context of Trusted, this means that we can derive enumPure :: Enumerable a => a -> Trusted a but in a way this this makes sense anyway. If a domain is enumerable, it would not be unreasonable to change Enumerable to class Eq a => Enumerable a where enumerate :: [StaticPtr a] so we could define enumPure as enumPure :: Enumerable a => a -> Trusted a enumPure x = trustStatic$ 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.

# Verification errors

Errors thrown during role validation

Constructors

 VerificationErrorSignatures TargetPath Not enough signatures signed with the appropriate keys VerificationErrorExpired TargetPath The file is expired VerificationErrorVersion TargetPath The file version is less than the previous version VerificationErrorFileInfo TargetPath File information mismatch VerificationErrorUnknownTarget TargetPath We tried to lookup file information about a particular target file, but the information wasn't in the corresponding targets.json file. VerificationErrorMissingSHA256 TargetPath The metadata for the specified target is missing a SHA256 VerificationErrorDeserialization TargetPath DeserializationError Some verification errors materialize as deserialization errorsFor 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. VerificationErrorLoop VerificationHistory 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.

Instances

 # MethodsshowList :: [VerificationError] -> ShowS # # Methods # Methods

data RootUpdated #

Root metadata updated (as part of the normal update process)

Constructors

 RootUpdated

Instances

 # MethodsshowList :: [RootUpdated] -> ShowS # # Methods # Methods

# Role verification

Arguments

 :: 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)

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.

Arguments

 :: [KeyId] -> KeyThreshold -> TargetPath For error messages -> Signed Root -> Either VerificationError (SignaturesVerified Root)

Variation on verifyRole that uses key IDs rather than keys

This is used during the bootstrap process.

# Re-exports

data StaticPtr a :: * -> * #

A reference to a value of type a.

Instances

 Methods

# Derived functions

(<>) :: StaticPtr (a -> b) -> Trusted a -> Trusted b #

Apply a static function to a trusted argument

## Role verification

class VerifyRole a where #

Minimal complete definition

verifyRole

Methods

Instances

 # Methods # Methods # Methods # Methods

## File info verification

Variation on knownFileInfoEqual for Trusted FileInfo