add notes

src/Data/ByteString/Base64/URL.hs

@@ -31,12 +31,14 @@ import Data.Text (Text)
 encodeBase64 :: ByteString -> ByteString
 encodeBase64 = encodeBase64_ True base64UrlTable
 
--- | Decode a padded base64-url encoded 'ByteString'
+-- | Decode a padded base64-url encoded 'ByteString'. If its length is not a multiple
+-- of 4, then padding chars will be added to fill out the input to a multiple of
+-- 4 for safe decoding.
 --
 -- See: <https://tools.ietf.org/html/rfc4648#section-4 RFC-4648 section 4>
 --
 decodeBase64 :: ByteString -> Either Text ByteString
-decodeBase64 = decodeBase64_ False decodeB64UrlTable
+decodeBase64 = decodeBase64_ True decodeB64UrlTable
 
 -- | Encode a 'ByteString' in base64-url without padding. Note that for Base64url,
 -- padding is optional. If you call this function, you will simply be encoding
@@ -51,10 +53,12 @@ encodeBase64Unpadded :: ByteString -> ByteString
 encodeBase64Unpadded = BS.takeWhile ((/=) 0x3d) . encodeBase64_ True base64UrlTable
 
 -- | Decode an unpadded base64-url encoded 'ByteString'. If its length is not a multiple
--- of 4, then padding chars will be added to fill out the input to a multiple of
--- 4 for safe decoding.
+-- of 4, then padding chars will __not__ be added to fill out the bytestring.
+--
+-- Only call this function if you are sure your bytestring length is a multiple of 4.
+-- Otherwise, use the standard decoding function.
 --
 -- See: <https://tools.ietf.org/html/rfc4648#section-4 RFC-4648 section 4>
 --
 decodeBase64Unpadded :: ByteString -> Either Text ByteString
-decodeBase64Unpadded = decodeBase64_ True decodeB64UrlTable
+decodeBase64Unpadded = decodeBase64_ False decodeB64UrlTable