// GoGOST -- Pure Go GOST cryptographic functions library
// Copyright (C) 2015-2024 Sergey Matveev <stargrave@stargrave.org>
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, version 3 of the License.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

// GOST 28147-89 block cipher with ECB, CFB, CTR, MAC modes of operation.
// RFC 5830.
package gost28147

const (
	BlockSize = 8
	KeySize   = 32
)

// All 28147 operations are going with two 32-bit halves of the whole
// block. nv is representation of that one half.
type nv uint32

// Cyclic 11-bit shift.
func (n nv) shift11() nv {
	return ((n << 11) & (1<<32 - 1)) | ((n >> (32 - 11)) & (1<<32 - 1))
}

// Seq contains iteration numbers used in the encryption function
// itself. For example 28147 encryption and decryption process differs
// only with this sequence.
type Seq []uint8

var (
	SeqEncrypt = Seq([]uint8{
		0, 1, 2, 3, 4, 5, 6, 7,
		0, 1, 2, 3, 4, 5, 6, 7,
		0, 1, 2, 3, 4, 5, 6, 7,
		7, 6, 5, 4, 3, 2, 1, 0,
	})
	SeqDecrypt = Seq([]uint8{
		0, 1, 2, 3, 4, 5, 6, 7,
		7, 6, 5, 4, 3, 2, 1, 0,
		7, 6, 5, 4, 3, 2, 1, 0,
		7, 6, 5, 4, 3, 2, 1, 0,
	})
)

type Cipher struct {
	key  [KeySize]byte
	sbox *Sbox
	x    [8]nv
}

func NewCipher(key []byte, sbox *Sbox) *Cipher {
	if len(key) != KeySize {
		panic("invalid key size")
	}
	c := Cipher{sbox: sbox}
	copy(c.key[:], key)
	c.x = [8]nv{
		nv(key[0]) | nv(key[1])<<8 | nv(key[2])<<16 | nv(key[3])<<24,
		nv(key[4]) | nv(key[5])<<8 | nv(key[6])<<16 | nv(key[7])<<24,
		nv(key[8]) | nv(key[9])<<8 | nv(key[10])<<16 | nv(key[11])<<24,
		nv(key[12]) | nv(key[13])<<8 | nv(key[14])<<16 | nv(key[15])<<24,
		nv(key[16]) | nv(key[17])<<8 | nv(key[18])<<16 | nv(key[19])<<24,
		nv(key[20]) | nv(key[21])<<8 | nv(key[22])<<16 | nv(key[23])<<24,
		nv(key[24]) | nv(key[25])<<8 | nv(key[26])<<16 | nv(key[27])<<24,
		nv(key[28]) | nv(key[29])<<8 | nv(key[30])<<16 | nv(key[31])<<24,
	}
	return &c
}

func (c *Cipher) BlockSize() int {
	return BlockSize
}

// Convert binary byte block to two 32-bit internal integers.
func block2nvs(b []byte) (n1, n2 nv) {
	n1 = nv(b[0]) | nv(b[1])<<8 | nv(b[2])<<16 | nv(b[3])<<24
	n2 = nv(b[4]) | nv(b[5])<<8 | nv(b[6])<<16 | nv(b[7])<<24
	return
}

// Convert two 32-bit internal integers to binary byte block.
func nvs2block(n1, n2 nv, b []byte) {
	b[0] = byte((n2 >> 0) & 255)
	b[1] = byte((n2 >> 8) & 255)
	b[2] = byte((n2 >> 16) & 255)
	b[3] = byte((n2 >> 24) & 255)
	b[4] = byte((n1 >> 0) & 255)
	b[5] = byte((n1 >> 8) & 255)
	b[6] = byte((n1 >> 16) & 255)
	b[7] = byte((n1 >> 24) & 255)
}

func (c *Cipher) xcrypt(seq Seq, n1, n2 nv) (nv, nv) {
	for _, i := range seq {
		n1, n2 = c.sbox.k(n1+c.x[i]).shift11()^n2, n1
	}
	return n1, n2
}

// Encrypt single block.
// If provided slices are shorter than the block size, then it will panic.
func (c *Cipher) Encrypt(dst, src []byte) {
	n1, n2 := block2nvs(src)
	n1, n2 = c.xcrypt(SeqEncrypt, n1, n2)
	nvs2block(n1, n2, dst)
}

// Decrypt single block.
// If provided slices are shorter than the block size, then it will panic.
func (c *Cipher) Decrypt(dst, src []byte) {
	n1, n2 := block2nvs(src)
	n1, n2 = c.xcrypt(SeqDecrypt, n1, n2)
	nvs2block(n1, n2, dst)
}