1 files changed, 54 insertions, 16 deletions

diff --git a/TLSA.hs b/TLSA.hs
index 4c4a3d4..16cc568 100644
--- a/TLSA.hs
+++ b/TLSA.hs
@@ -30,11 +30,35 @@ fromWord8 = toEnum . fromEnum
 toWord8 :: Enum a => a -> Word8
 toWord8 = toEnum . fromEnum
 
+-- | The Certificate Usage Field as described in RFC 6698, section 2.1.1.
 data CertUsage
+
+    -- | This usage limits which CA can be used to issue certificates for a
+    -- given service on a host.  PKIX-validated TLS connections for the domain
+    -- should be considered invalid if the certification path does not include
+    -- at least one certificate that 'match'es the 'TLSA' record.
     = CAConstraint
+
+    -- | This usage limits which end entity certificate can be used by a given
+    -- service on a host.  The TLS connection for the domain should be
+    -- considered invalid if the end entity certificate does not 'match' the
+    -- 'TLSA' record.
     | ServiceCertificateConstraint
+
+    -- | This usage allows a domain name administrator to specify a new trust
+    -- anchor.  This is useful if the domain issues its own certificates under
+    -- its own CA that is not expected to be in the end users' collection of
+    -- trust anchors.  When conducting PKIX validation for the domain, any
+    -- certificate matching the 'TLSA' record can be treated as a trust anchor
+    -- that does not require further validation.
     | TrustAnchorAssertion
+
+    -- | This usage allows for a domain name administrator to issue
+    -- certificates for a domain without involving a third-party CA.  The end
+    -- entity certificate MUST 'match' the 'TLSA' record.  Unlike for a
+    -- 'ServiceCertificateConstraint', PKIX validation should not be performed.
     | DomainIssued
+
     | CertUsage Word8
  deriving (Eq, Ord, Show, Read)
 
@@ -50,10 +74,11 @@ instance Enum CertUsage where
     toEnum 3 = DomainIssued
     toEnum n = CertUsage (toEnum n)
 
+-- | Indicates what sort of object should be compared with 'associationData'.
 data Selector
-    = FullCertificate
-    | SubjectPublicKeyInfo
-    | Selector Word8  -- Selector 255 reserved for private use
+    = FullCertificate      -- ^ x.509 certificate
+    | SubjectPublicKeyInfo -- ^ PKCS #8 formatted public key
+    | Selector Word8       -- ^ value 255 reserved for private use
  deriving (Eq,Ord,Show)
 
 instance Enum Selector where
@@ -64,11 +89,13 @@ instance Enum Selector where
     toEnum 1   = SubjectPublicKeyInfo
     toEnum n = Selector (toEnum n)
 
+-- | Is 'associationData' the an object of the form specified by 'Selector' or
+-- is it only a hash of it?
 data MatchingType
     = Match_Exact
     | Match_SHA256
     | Match_SHA512
-    | Match Word8 -- Match 255 reserved for private use
+    | Match Word8 -- ^ value 255 is reserved for private use
  deriving (Eq,Ord,Show)
 
 instance Enum MatchingType where
@@ -82,21 +109,28 @@ instance Enum MatchingType where
     toEnum n = Match (toEnum n)
 
 
+-- | The parsed RDATA field of a TLSA DNS resource record (type 52) as
+-- described in RFC 6698.
+--
+-- Use the 'match' function uses 'selector', 'matchingType' and
+-- 'associationData' to implement a predicate on certificates obtained via the
+-- TLS protocol.  The 'certUsage' field indicates what that predicate means.
 data TLSA = TLSA
-    { tlsaCertUsage       :: CertUsage
-    , tlsaSelector        :: Selector
-    , tlsaMatchingType    :: MatchingType
-    , tlsaAssociationData :: BS.ByteString
+    { certUsage       :: CertUsage
+    , selector        :: Selector
+    , matchingType    :: MatchingType
+    , associationData :: BS.ByteString
     }
  deriving (Eq, Ord, Show)
 
+-- | Parse RDATA for a TLSA resource record.
 fromByteString :: BS.ByteString -> TLSA
 fromByteString bs = TLSA (fromWord8 cu)
                          (fromWord8 sel)
                          (fromWord8 mat)
-                         associationData
+                         dta
  where
-    (csm,associationData) = BS.splitAt 3 bs
+    (csm,dta) = BS.splitAt 3 bs
     (cu,sel,mat) =
         case BS.unpack csm of
             [cu,sel,mat] -> (cu,sel,mat)
@@ -104,28 +138,32 @@ fromByteString bs = TLSA (fromWord8 cu)
             [cu]         -> (cu,0,0)
             []           -> (0,0,0)
 
+-- | Encode a valid RDATA field for a TLSA DNS record.
 toByteString :: TLSA -> BS.ByteString
-toByteString (TLSA cu sel mat associationData) = csm <> associationData
+toByteString (TLSA cu sel mat dta) = csm <> dta
  where
     csm = BS.pack [ toWord8 cu
                   , toWord8 sel
                   , toWord8 mat ]
 
-match :: Certificate -> TLSA -> Bool
-match cert tlsa = fromMaybe False $
-    (== tlsaAssociationData tlsa) <$> (hash <*> material)
+-- | Returns 'True' if the given certificate matches the given 'TLSA' object.
+-- The algorithm for matching depends on the values of 'selector' and
+-- 'matchingType' as described in RFC 6698.
+match :: TLSA -> Certificate -> Bool
+match tlsa cert = fromMaybe False $
+    (== associationData tlsa) <$> (hash <*> material)
 
  where
     encode obj = encodeASN1 DER (toASN1 obj [])
 
     material =
-        case tlsaSelector tlsa of
+        case selector tlsa of
             FullCertificate      -> Just $ encode cert
             SubjectPublicKeyInfo -> Just $ encode $ certPubKey cert
             _                    -> Nothing
 
     hash =
-        case tlsaMatchingType tlsa of
+        case matchingType tlsa of
             Match_Exact  -> Just L.toStrict
             Match_SHA256 -> Just SHA256.hashlazy
             Match_SHA512 -> Just SHA512.hashlazy