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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package des

import (
        "encoding/binary"
)

func cryptBlock(subkeys []uint64, dst, src []byte, decrypt bool) {
        b := binary.BigEndian.Uint64(src)
        b = permuteBlock(b, initialPermutation[:])
        left, right := uint32(b>>32), uint32(b)

        var subkey uint64
        for i := 0; i < 16; i++ {
                if decrypt {
                        subkey = subkeys[15-i]
                } else {
                        subkey = subkeys[i]
                }

                left, right = right, left^feistel(right, subkey)
        }
        // switch left & right and perform final permutation
        preOutput := (uint64(right) << 32) | uint64(left)
        binary.BigEndian.PutUint64(dst, permuteBlock(preOutput, finalPermutation[:]))
}

// Encrypt one block from src into dst, using the subkeys.
func encryptBlock(subkeys []uint64, dst, src []byte) {
        cryptBlock(subkeys, dst, src, false)
}

// Decrypt one block from src into dst, using the subkeys.
func decryptBlock(subkeys []uint64, dst, src []byte) {
        cryptBlock(subkeys, dst, src, true)
}

// DES Feistel function
func feistel(right uint32, key uint64) (result uint32) {
        sBoxLocations := key ^ permuteBlock(uint64(right), expansionFunction[:])
        var sBoxResult uint32
        for i := uint8(0); i < 8; i++ {
                sBoxLocation := uint8(sBoxLocations>>42) & 0x3f
                sBoxLocations <<= 6
                // row determined by 1st and 6th bit
                row := (sBoxLocation & 0x1) | ((sBoxLocation & 0x20) >> 4)
                // column is middle four bits
                column := (sBoxLocation >> 1) & 0xf
                sBoxResult |= uint32(sBoxes[i][row][column]) << (4 * (7 - i))
        }
        return uint32(permuteBlock(uint64(sBoxResult), permutationFunction[:]))
}

// general purpose function to perform DES block permutations
func permuteBlock(src uint64, permutation []uint8) (block uint64) {
        for position, n := range permutation {
                bit := (src >> n) & 1
                block |= bit << uint((len(permutation)-1)-position)
        }
        return
}

// creates 16 28-bit blocks rotated according
// to the rotation schedule
func ksRotate(in uint32) (out []uint32) {
        out = make([]uint32, 16)
        last := in
        for i := 0; i < 16; i++ {
                // 28-bit circular left shift
                left := (last << (4 + ksRotations[i])) >> 4
                right := (last << 4) >> (32 - ksRotations[i])
                out[i] = left | right
                last = out[i]
        }
        return
}

// creates 16 56-bit subkeys from the original key
func (c *Cipher) generateSubkeys(keyBytes []byte) {
        // apply PC1 permutation to key
        key := binary.BigEndian.Uint64(keyBytes)
        permutedKey := permuteBlock(key, permutedChoice1[:])

        // rotate halves of permuted key according to the rotation schedule
        leftRotations := ksRotate(uint32(permutedKey >> 28))
        rightRotations := ksRotate(uint32(permutedKey<<4) >> 4)

        // generate subkeys
        for i := 0; i < 16; i++ {
                // combine halves to form 56-bit input to PC2
                pc2Input := uint64(leftRotations[i])<<28 | uint64(rightRotations[i])
                // apply PC2 permutation to 7 byte input
                c.subkeys[i] = permuteBlock(pc2Input, permutedChoice2[:])
        }
}