Web.ClientSession

Stores session data in a client cookie. In order to do so, we:

• Encrypt the cookie data using AES in CTR mode. This allows you to store sensitive information on the client side without worrying about eavesdropping.
• Authenticate the encrypted cookie data using Skein-MAC-512-256. Besides detecting potential errors in storage or transmission of the cookies (integrity), the MAC also avoids malicious modifications of the cookie data by assuring you that the cookie data really was generated by this server (authenticity).
• Encode everything using Base64. Thus we avoid problems with non-printable characters by giving the browser a simple string.

Simple usage of the library involves just calling getDefaultKey on the startup of your server, encryptIO when serializing cookies and decrypt when parsing then back.

Automatic key generation

data Key

The keys used to store the cookies. We have an AES key used to encrypt the cookie and a Skein-MAC-512-256 key used verify the authencity and integrity of the cookie. The AES key must have exactly 32 bytes (256 bits) while Skein-MAC-512-256 must have 64 bytes (512 bits).

See also getDefaultKey and initKey.

Instances

Eq Key

Show Key

Dummy Show instance.

Serialize Key

data IV

The initialization vector used by AES. Must be exactly 16 bytes long.

Instances

Eq IV

Ord IV

Show IV

Serialize IV

randomIV :: IO IV

Randomly construct a fresh initialization vector. You MUST NOT reuse initialization vectors.

mkIV :: ByteString -> Maybe IV

Construct an initialization vector from a ByteString. Fails if there isn't exactly 16 bytes.

getKey

Arguments

:: FilePath

File name where key is stored.

-> IO Key

The actual key.

Get a key from the given text file.

If the file does not exist or is corrupted a random key will be generated and stored in that file.

getKeyEnv

Arguments

:: String

Name of the environment variable

-> IO Key

The actual key.

Get the key from the named environment variable

Assumes the value is a Base64-encoded string. If the variable is not set, a random key will be generated, set in the environment, and the Base64-encoded version printed on devstdout.

defaultKeyFile :: FilePath

The default key file.

getDefaultKey :: IO Key

Simply calls getKey defaultKeyFile.

initKey :: ByteString -> Either String Key

Initializes a Key from a random ByteString. Fails if there isn't exactly 96 bytes (256 bits for AES and 512 bits for Skein-MAC-512-512).

Note that the input string is assumed to be uniformly chosen from the set of all 96-byte strings. In other words, each byte should be chosen from the set of all byte values (0-255) with the same probability.

In particular, this function does not do any kind of key stretching. You should never feed it a password, for example.

It's highly recommended to feed initKey only with values generated by randomKey, unless you really know what you're doing.

randomKey :: IO (ByteString, Key)

Generate a random Key. Besides the Key, the ByteString passed to initKey is returned so that it can be saved for later use.

randomKeyEnv :: String -> IO Key

Generate a random Key, set a Base64-encoded version of it in the given environment variable, then return it. Also prints the generated string to devstdout.

Actual encryption/decryption

encrypt

Arguments

:: Key

Key of the server.

-> IV

New, random initialization vector (see randomIV).

-> ByteString

Serialized cookie data.

-> ByteString

Encoded cookie data to be given to the client browser.

Encrypt (AES-CTR), authenticate (Skein-MAC-512-256) and encode (Base64) the given cookie data. The returned byte string is ready to be used in a response header.

encryptIO :: Key -> ByteString -> IO ByteString

Same as encrypt, however randomly generates the initialization vector for you.

decrypt

Arguments

:: Key

Key of the server.

-> ByteString

Encoded cookie data given by the browser.

-> Maybe ByteString

Serialized cookie data.

Decode (Base64), verify the integrity and authenticity (Skein-MAC-512-256) and decrypt (AES-CTR) the given encoded cookie data. Returns the original serialized cookie data. Fails if the data is corrupted.