module SKIPJACK where

/**
 * SKIPJACK encryption algorithm [SKIPJACK] B. SKIPJACK Specification.
 */

encrypt : [10][8] -> [4][16] -> [4][16]
encrypt cv w = last ws
  where
    cv' = cv # cv'
    ws  = [w]
        # [ AB Gk wi counter
            where Gk = G cvi
          | wi  <- ws
          | counter <- [1 .. 32]
          | cvi <- take`{32} (split cv')
          | AB <- [ A, A, A, A, A, A, A, A
                  , B, B, B, B, B, B, B, B
                  , A, A, A, A, A, A, A, A
                  , B, B, B, B, B, B, B, B
                  ]
          ]

/**
 * SKIPJACK decryption algorithm [SKIPJACK] B. SKIPJACK Specification.
 */

decrypt : [10][8] -> [4][16] -> [4][16]
decrypt cv w = last ws
  where
    cv' = cv # cv'
    ws  = [w]
        # [ AB Gk' wi counter
            where Gk' = G' cvi
          | wi  <- ws
          | counter <- [32, 31 .. 1]
          | cvi <- reverse (take`{32} (split cv'))
          | AB <- [ B', B', B', B', B', B', B', B'
                  , A', A', A', A', A', A', A', A'
                  , B', B', B', B', B', B', B', B'
                  , A', A', A', A', A', A', A', A'
                  ]
          ]

private

  /**
   * SKIPJACK stepping rule A [SKIPJACK] B. SKIPJACK Specification.
   */

  A : ([16] -> [16]) -> [4][16] -> [16] -> [4][16]
  A Gk [w1, w2, w3, w4] counter =
      [ Gk w1 ^ w4 ^ counter
      , Gk w1
      , w2
      , w3
      ]

  /**
   * SKIPJACK stepping rule B [SKIPJACK] B. SKIPJACK Specification.
   */
   
  B : ([16] -> [16]) -> [4][16] -> [16] -> [4][16]
  B Gk [w1, w2, w3, w4] counter =
      [ w4
      , Gk w1
      , w1 ^ w2 ^ counter
      , w3
      ]

  /**
   * Inverse SKIPJACK stepping rule A [SKIPJACK] B. SKIPJACK
   * Specification.
   */

  A' : ([16] -> [16]) -> [4][16] -> [16] -> [4][16]
  A' Gk' [w1, w2, w3, w4] counter =
      [ Gk' w2
      , w3
      , w4
      , w1 ^ w2 ^ counter
      ]

  /**
   * Inverse SKIPJACK stepping rule B [SKIPJACK] B. SKIPJACK
   * Specification.
   */

  B' : ([16] -> [16]) -> [4][16] -> [16] -> [4][16]
  B' Gk' [w1, w2, w3, w4] counter =
      [ Gk' w2
      , Gk' w2 ^ w3 ^ counter
      , w4
      , w1
      ]

  /**
   * Nonlinear SKIPJACK G function [SKIPJACK] B. SKIPJACK
   * Specification.
   */

  G : [4][8] -> [16] -> [16]
  G [cv0, cv1, cv2, cv3] (g1 # g2) = g5 # g6
    where
      g3 = F @ (g2 ^ cv0) ^ g1
      g4 = F @ (g3 ^ cv1) ^ g2
      g5 = F @ (g4 ^ cv2) ^ g3
      g6 = F @ (g5 ^ cv3) ^ g4

  /**
   * Inverse nonlinear SKIPJACK G function [SKIPJACK] B. SKIPJACK
   * Specification.
   */

  G' : [4][8] -> [16] -> [16]
  G' [cv0, cv1, cv2, cv3] (g5 # g6) = g1 # g2
    where
      g1 = F @ (g2 ^ cv0) ^ g3
      g2 = F @ (g3 ^ cv1) ^ g4
      g3 = F @ (g4 ^ cv2) ^ g5
      g4 = F @ (g5 ^ cv3) ^ g6

  /**
   * SKIPJACK F table [SKIPJACK] B. SKIPJACK Specification.
   */
   
  F : [256][8]
  F = [ 0xa3, 0xd7, 0x09, 0x83, 0xf8, 0x48, 0xf6, 0xf4, 0xb3, 0x21, 0x15, 0x78, 0x99, 0xb1, 0xaf, 0xf9
      , 0xe7, 0x2d, 0x4d, 0x8a, 0xce, 0x4c, 0xca, 0x2e, 0x52, 0x95, 0xd9, 0x1e, 0x4e, 0x38, 0x44, 0x28
      , 0x0a, 0xdf, 0x02, 0xa0, 0x17, 0xf1, 0x60, 0x68, 0x12, 0xb7, 0x7a, 0xc3, 0xe9, 0xfa, 0x3d, 0x53
      , 0x96, 0x84, 0x6b, 0xba, 0xf2, 0x63, 0x9a, 0x19, 0x7c, 0xae, 0xe5, 0xf5, 0xf7, 0x16, 0x6a, 0xa2
      , 0x39, 0xb6, 0x7b, 0x0f, 0xc1, 0x93, 0x81, 0x1b, 0xee, 0xb4, 0x1a, 0xea, 0xd0, 0x91, 0x2f, 0xb8
      , 0x55, 0xb9, 0xda, 0x85, 0x3f, 0x41, 0xbf, 0xe0, 0x5a, 0x58, 0x80, 0x5f, 0x66, 0x0b, 0xd8, 0x90
      , 0x35, 0xd5, 0xc0, 0xa7, 0x33, 0x06, 0x65, 0x69, 0x45, 0x00, 0x94, 0x56, 0x6d, 0x98, 0x9b, 0x76
      , 0x97, 0xfc, 0xb2, 0xc2, 0xb0, 0xfe, 0xdb, 0x20, 0xe1, 0xeb, 0xd6, 0xe4, 0xdd, 0x47, 0x4a, 0x1d
      , 0x42, 0xed, 0x9e, 0x6e, 0x49, 0x3c, 0xcd, 0x43, 0x27, 0xd2, 0x07, 0xd4, 0xde, 0xc7, 0x67, 0x18
      , 0x89, 0xcb, 0x30, 0x1f, 0x8d, 0xc6, 0x8f, 0xaa, 0xc8, 0x74, 0xdc, 0xc9, 0x5d, 0x5c, 0x31, 0xa4
      , 0x70, 0x88, 0x61, 0x2c, 0x9f, 0x0d, 0x2b, 0x87, 0x50, 0x82, 0x54, 0x64, 0x26, 0x7d, 0x03, 0x40
      , 0x34, 0x4b, 0x1c, 0x73, 0xd1, 0xc4, 0xfd, 0x3b, 0xcc, 0xfb, 0x7f, 0xab, 0xe6, 0x3e, 0x5b, 0xa5
      , 0xad, 0x04, 0x23, 0x9c, 0x14, 0x51, 0x22, 0xf0, 0x29, 0x79, 0x71, 0x7e, 0xff, 0x8c, 0x0e, 0xe2
      , 0x0c, 0xef, 0xbc, 0x72, 0x75, 0x6f, 0x37, 0xa1, 0xec, 0xd3, 0x8e, 0x62, 0x8b, 0x86, 0x10, 0xe8
      , 0x08, 0x77, 0x11, 0xbe, 0x92, 0x4f, 0x24, 0xc5, 0x32, 0x36, 0x9d, 0xcf, 0xf3, 0xa6, 0xbb, 0xac
      , 0x5e, 0x6c, 0xa9, 0x13, 0x57, 0x25, 0xb5, 0xe3, 0xbd, 0xa8, 0x3a, 0x01, 0x05, 0x59, 0x2a, 0x46 ]

  /**
   * Test from [SKIPJACK] Annex III
   */

  property test1 =
      encrypt k pt == ct
   /\ decrypt k ct == pt
   where
     k  = split 0x00998877665544332211
     pt = split 0x33221100ddccbbaa
     ct = split 0x2587cae27a12d300

/*
===== References =====

[SKIPJACK] (U) SKIPJACK and KEA Algorithm Specifications. Version 2.0,
29 May 1998.

======================
*/