package scrypt

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/binary"
	"encoding/hex"
	"errors"
	"fmt"
	"hash"
	"io"
	"math/bits"
	"os"
	"slices"
)



const (
	saltMinLength = 32
	desiredLength = 32
	maxInt                = int((^uint(0)) >> 1)
	MinimumPasswordLength = 16

	_N = 1 << 15
	r  = 8
	p  = 1
)


var (
	ErrBadN           = errors.New("scrypt: N must be > 1 and a power of 2")
	ErrParamsTooLarge = errors.New("scrypt: parameters are too large")
	ErrSaltTooSmall   = errors.New("scrypt: salt is too small")
)



// Package pbkdf2 implements the key derivation function PBKDF2 as defined in
// RFC 2898 / PKCS #5 v2.0.
//
// A key derivation function is useful when encrypting data based on a password
// or any other not-fully-random data. It uses a pseudorandom function to derive
// a secure encryption key based on the password.
//
// While v2.0 of the standard defines only one pseudorandom function to use,
// HMAC-SHA1, the drafted v2.1 specification allows use of all five FIPS
// Approved Hash Functions SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512 for
// HMAC. To choose, you can pass the `New` functions from the different SHA
// packages to pbkdf2.Key.
//
//
// Key derives a key from the password, salt and iteration count, returning a
// []byte of length keylen that can be used as cryptographic key. The key is
// derived based on the method described as PBKDF2 with the HMAC variant using
// the supplied hash function.
//
// For example, to use a HMAC-SHA-1 based PBKDF2 key derivation function, you
// can get a derived key for e.g. AES-256 (which needs a 32-byte key) by
// doing:
//
//	dk := pbkdf2.Key([]byte("some password"), salt, 4096, 32, sha1.New)
//
// Remember to get a good random salt. At least 8 bytes is recommended by the
// RFC.
//
// Using a higher iteration count will increase the cost of an exhaustive
// search but will also make derivation proportionally slower.
func _PBKDF2Key(
	password []byte,
	salt []byte,
	iter int,
	keyLen int,
	h func() hash.Hash,
) []byte {
	prf := hmac.New(h, password)
	hashLen := prf.Size()
	numBlocks := (keyLen + hashLen - 1) / hashLen

	var buffer [4]byte
	dk := make([]byte, 0, numBlocks*hashLen)
	U := make([]byte, hashLen)
	for block := 1; block <= numBlocks; block++ {
		// N.B.: || means concatenation, ^ means XOR
		// for each block T_i = U_1 ^ U_2 ^ ... ^ U_iter
		// U_1 = PRF(password, salt || uint(i))
		prf.Reset()
		prf.Write(salt)
		buffer[0] = byte(block >> 24)
		buffer[1] = byte(block >> 16)
		buffer[2] = byte(block >> 8)
		buffer[3] = byte(block >> 0)
		prf.Write(buffer[:4])
		dk = prf.Sum(dk)
		T := dk[len(dk) - hashLen:]
		copy(U, T)

		// U_n = PRF(password, U_(n - 1))
		for n := 2; n <= iter; n++ {
			prf.Reset()
			prf.Write(U)
			U = U[:0]
			U = prf.Sum(U)
			for x := range U {
				T[x] ^= U[x]
			}
		}
	}
	return dk[:keyLen]
}

// blockCopy copies n numbers from src into dst.
func blockCopy(dst []uint32, src []uint32, n int) {
	copy(dst, src[:n])
}

// blockXOR XORs numbers from dst with n numbers from src.
func blockXOR(dst []uint32, src []uint32, n int) {
	for i, v := range src[:n] {
		dst[i] ^= v
	}
}

// salsaXOR applies Salsa20/8 to the XOR of 16 numbers from tmp and in,
// and puts the result into both tmp and out.
func salsaXOR(tmp *[16]uint32, in []uint32, out []uint32) {
	w0  := tmp[0]  ^ in[0]
	w1  := tmp[1]  ^ in[1]
	w2  := tmp[2]  ^ in[2]
	w3  := tmp[3]  ^ in[3]
	w4  := tmp[4]  ^ in[4]
	w5  := tmp[5]  ^ in[5]
	w6  := tmp[6]  ^ in[6]
	w7  := tmp[7]  ^ in[7]
	w8  := tmp[8]  ^ in[8]
	w9  := tmp[9]  ^ in[9]
	w10 := tmp[10] ^ in[10]
	w11 := tmp[11] ^ in[11]
	w12 := tmp[12] ^ in[12]
	w13 := tmp[13] ^ in[13]
	w14 := tmp[14] ^ in[14]
	w15 := tmp[15] ^ in[15]

	x0  := w0
	x1  := w1
	x2  := w2
	x3  := w3
	x4  := w4
	x5  := w5
	x6  := w6
	x7  := w7
	x8  := w8
	x9  := w9
	x10 := w10
	x11 := w11
	x12 := w12
	x13 := w13
	x14 := w14
	x15 := w15

	for i := 0; i < 8; i += 2 {
		x4  ^= bits.RotateLeft32(x0  + x12, 7)
		x8  ^= bits.RotateLeft32(x4  + x0,  9)
		x12 ^= bits.RotateLeft32(x8  + x4,  13)
		x0  ^= bits.RotateLeft32(x12 + x8,  18)

		x9  ^= bits.RotateLeft32(x5  + x1,  7)
		x13 ^= bits.RotateLeft32(x9  + x5,  9)
		x1  ^= bits.RotateLeft32(x13 + x9,  13)
		x5  ^= bits.RotateLeft32(x1  + x13, 18)

		x14 ^= bits.RotateLeft32(x10 + x6,  7)
		x2  ^= bits.RotateLeft32(x14 + x10, 9)
		x6  ^= bits.RotateLeft32(x2  + x14, 13)
		x10 ^= bits.RotateLeft32(x6  + x2,  18)

		x3  ^= bits.RotateLeft32(x15 + x11, 7)
		x7  ^= bits.RotateLeft32(x3  + x15, 9)
		x11 ^= bits.RotateLeft32(x7  + x3,  13)
		x15 ^= bits.RotateLeft32(x11 + x7,  18)

		x1  ^= bits.RotateLeft32(x0  + x3,  7)
		x2  ^= bits.RotateLeft32(x1  + x0,  9)
		x3  ^= bits.RotateLeft32(x2  + x1,  13)
		x0  ^= bits.RotateLeft32(x3  + x2,  18)

		x6  ^= bits.RotateLeft32(x5  + x4,  7)
		x7  ^= bits.RotateLeft32(x6  + x5,  9)
		x4  ^= bits.RotateLeft32(x7  + x6,  13)
		x5  ^= bits.RotateLeft32(x4  + x7,  18)

		x11 ^= bits.RotateLeft32(x10 + x9,  7)
		x8  ^= bits.RotateLeft32(x11 + x10, 9)
		x9  ^= bits.RotateLeft32(x8  + x11, 13)
		x10 ^= bits.RotateLeft32(x9  + x8,  18)

		x12 ^= bits.RotateLeft32(x15 + x14, 7)
		x13 ^= bits.RotateLeft32(x12 + x15, 9)
		x14 ^= bits.RotateLeft32(x13 + x12, 13)
		x15 ^= bits.RotateLeft32(x14 + x13, 18)
	}

	x0  += w0
	x1  += w1
	x2  += w2
	x3  += w3
	x4  += w4
	x5  += w5
	x6  += w6
	x7  += w7
	x8  += w8
	x9  += w9
	x10 += w10
	x11 += w11
	x12 += w12
	x13 += w13
	x14 += w14
	x15 += w15

	out[0],  tmp[0]  = x0,  x0
	out[1],  tmp[1]  = x1,  x1
	out[2],  tmp[2]  = x2,  x2
	out[3],  tmp[3]  = x3,  x3
	out[4],  tmp[4]  = x4,  x4
	out[5],  tmp[5]  = x5,  x5
	out[6],  tmp[6]  = x6,  x6
	out[7],  tmp[7]  = x7,  x7
	out[8],  tmp[8]  = x8,  x8
	out[9],  tmp[9]  = x9,  x9
	out[10], tmp[10] = x10, x10
	out[11], tmp[11] = x11, x11
	out[12], tmp[12] = x12, x12
	out[13], tmp[13] = x13, x13
	out[14], tmp[14] = x14, x14
	out[15], tmp[15] = x15, x15
}

func blockMix(tmp *[16]uint32, in []uint32, out []uint32, r int) {
	blockCopy(tmp[:], in[(2 * r - 1) * 16:], 16)
	for i := 0; i < 2 * r; i += 2 {
		salsaXOR(tmp, in[i * 16:],      out[i * 8:])
		salsaXOR(tmp, in[i * 16 + 16:], out[i * 8 + r * 16:])
	}
}

func integer(b []uint32, r int) uint64 {
	j := (2 * r - 1) * 16
	return uint64(b[j]) | (uint64(b[j + 1]) << 32)
}

func smix(b []byte, r int, N int, v []uint32, xy []uint32) {
	var tmp [16]uint32
	R := 32 * r
	x := xy
	y := xy[R:]

	j := 0
	for i := 0; i < R; i++ {
		x[i] = binary.LittleEndian.Uint32(b[j:])
		j += 4
	}
	for i := 0; i < N; i += 2 {
		blockCopy(v[i * R:], x, R)
		blockMix(&tmp, x, y, r)

		blockCopy(v[(i + 1) * R:], y, R)
		blockMix(&tmp, y, x, r)
	}
	for i := 0; i < N; i += 2 {
		j := int(integer(x, r) & uint64(N - 1))
		blockXOR(x, v[j * R:], R)
		blockMix(&tmp, x, y, r)

		j = int(integer(y, r) & uint64(N - 1))
		blockXOR(y, v[j * R:], R)
		blockMix(&tmp, y, x, r)
	}
	j = 0
	for _, v := range x[:R] {
		binary.LittleEndian.PutUint32(b[j:], v)
		j += 4
	}
}

func validateParams(N int, r int, p int) error {
	if N <= 1 || N & (N - 1) != 0 {
		return ErrBadN
	}

	if ((uint64(r) * uint64(p)) >= (1 << 30)) ||
			r > maxInt / 128 / p ||
			r > maxInt / 256 ||
			N > maxInt / 128 / r {
		return ErrParamsTooLarge
	}

	return nil
}

// Package scrypt implements the scrypt key derivation function as defined in
// Colin Percival's paper "Stronger Key Derivation via Sequential Memory-Hard
// Functions" (https://www.tarsnap.com/scrypt/scrypt.pdf).
//
//
// Key derives a key from the password, salt, and cost parameters, returning
// a byte slice of length keyLen that can be used as cryptographic key.
//
// N is a CPU/memory cost parameter, which must be a power of 2 greater than 1.
// r and p must satisfy r * p < 2³⁰. If the parameters do not satisfy the
// limits, the function returns a nil byte slice and an error.
//
// For example, you can get a derived key for e.g. AES-256 (which needs a
// 32-byte key) by doing:
//
//	dk, err := scrypt.Key([]byte("some password"), salt, 32768, 8, 1, 32)
//
// The recommended parameters for interactive logins as of 2017 are N=32768, r=8
// and p=1. The parameters N, r, and p should be increased as memory latency and
// CPU parallelism increases; consider setting N to the highest power of 2 you
// can derive within 100 milliseconds. Remember to get a good random salt.
func scrypt(
	password []byte,
	salt []byte,
	N int,
	r int,
	p int,
	keyLen int,
) ([]byte, error) {
	err := validateParams(N, r, p)
	if err != nil {
		return nil, err
	}

	xy := make([]uint32, 64 * r)
	v  := make([]uint32, 32 * r * N)
	b  := _PBKDF2Key(password, salt, 1, p * 128 * r, sha256.New)

	for i := 0; i < p; i++ {
		smix(b[i * 128 * r:], r, N, v, xy)
	}

	return _PBKDF2Key(password, b, 1, keyLen, sha256.New), nil
}

func SaltFrom(r io.Reader) ([]byte, error) {
	buffer := make([]byte, saltMinLength)
	_, err := io.ReadFull(r, buffer)
	if err != nil {
		return nil, err
	}
	return buffer, nil
}

func HashFrom(password []byte, salt []byte) ([]byte, error) {
	if len(salt) < saltMinLength {
		return nil, ErrSaltTooSmall
	}

	hash, err := scrypt(
		password,
		salt,
		_N,
		r,
		p,
		desiredLength,
	)
	if err != nil {
		return nil, err
	}

	return hash, nil
}

func CheckFrom(password []byte, salt []byte, hash []byte) (bool, error) {
	candidate, err := HashFrom(password, salt)
	if err != nil {
		return false, err
	}

	return slices.Equal(candidate, hash), nil
}

func Salt() []byte {
	salt, err := SaltFrom(rand.Reader)
	if err != nil {
		panic(err)
	}
	return salt
}

func Hash(password []byte, salt []byte) []byte {
	hash, err := HashFrom(password, salt)
	if err != nil {
		panic(err)
	}
	return hash
}

func Check(password []byte, salt []byte, hash []byte) bool {
	check, err := CheckFrom(password, salt, hash)
	if err != nil {
		panic(err)
	}
	return check
}


func Main() {
	if len(os.Args) != 3 {
		fmt.Fprintf(os.Stderr, "Usage: scrypt PASSWORD SALT\n")
		os.Exit(2)
	}

	fmt.Println(hex.EncodeToString(
		Hash([]byte(os.Args[1]), []byte(os.Args[2]))),
	)
}