Documentation.SBV.Examples.Crypto.RC4

An implementation of RC4 (AKA Rivest Cipher 4 or Alleged RC4/ARC4), using SBV. For information on RC4, see: http://en.wikipedia.org/wiki/RC4.

We make no effort to optimize the code, and instead focus on a clear implementation. In fact, the RC4 algorithm relies on in-place update of its state heavily for efficiency, and is therefore unsuitable for a purely functional implementation.

Types

type S = STree Word8 Word8

RC4 State contains 256 8-bit values. We use the symbolically accessible full-binary type STree to represent the state, since RC4 needs access to the array via a symbolic index and it's important to minimize access time.

initS :: S

Construct the fully balanced initial tree, where the leaves are simply the numbers 0 through 255.

type Key = [SWord8]

The key is a stream of Word8 values.

type RC4 = (S, SWord8, SWord8)

Represents the current state of the RC4 stream: it is the S array along with the i and j index values used by the PRGA.

The PRGA

swap :: SWord8 -> SWord8 -> S -> S

Swaps two elements in the RC4 array.

prga :: RC4 -> (SWord8, RC4)

Implements the PRGA used in RC4. We return the new state and the next key value generated.

Key schedule

initRC4 :: Key -> S

Constructs the state to be used by the PRGA using the given key.

keySchedule :: Key -> [SWord8]

The key-schedule. Note that this function returns an infinite list.

keyScheduleString :: String -> [SWord8]

Generate a key-schedule from a given key-string.

Encryption and Decryption

encrypt :: String -> String -> [SWord8]

RC4 encryption. We generate key-words and xor it with the input. The following test-vectors are from Wikipedia http://en.wikipedia.org/wiki/RC4:

>>> concatMap hex2 $ encrypt "Key" "Plaintext"
"bbf316e8d940af0ad3"

>>> concatMap hex2 $ encrypt "Wiki" "pedia"
"1021bf0420"

>>> concatMap hex2 $ encrypt "Secret" "Attack at dawn"
"45a01f645fc35b383552544b9bf5"

decrypt :: String -> [SWord8] -> String

RC4 decryption. Essentially the same as decryption. For the above test vectors we have:

>>> decrypt "Key" [0xbb, 0xf3, 0x16, 0xe8, 0xd9, 0x40, 0xaf, 0x0a, 0xd3]
"Plaintext"

>>> decrypt "Wiki" [0x10, 0x21, 0xbf, 0x04, 0x20]
"pedia"

>>> decrypt "Secret" [0x45, 0xa0, 0x1f, 0x64, 0x5f, 0xc3, 0x5b, 0x38, 0x35, 0x52, 0x54, 0x4b, 0x9b, 0xf5]
"Attack at dawn"

Verification

rc4IsCorrect :: IO ThmResult

Prove that round-trip encryption/decryption leaves the plain-text unchanged. The theorem is stated parametrically over key and plain-text sizes. The expression performs the proof for a 40-bit key (5 bytes) and 40-bit plaintext (again 5 bytes).

Note that this theorem is trivial to prove, since it is essentially establishing xor'in the same value twice leaves a word unchanged (i.e., x xor y xor y = x). However, the proof takes quite a while to complete, as it gives rise to a fairly large symbolic trace.

hex2 :: (SymVal a, Show a, Integral a) => SBV a -> String

For doctest purposes only