package internal

import (
	"crypto/cipher"
	"crypto/rand"
	"fmt"

	"golang.org/x/crypto/argon2"
	"golang.org/x/crypto/chacha20poly1305"
)

const (
	SaltSize   = 32
	KeySize    = uint32(32) // KeySize is 32 bytes (256 bits).
	KeyTime    = uint32(5)
	KeyMemory  = uint32(1024 * 64) // KeyMemory in KiB. here, 64 MiB.
	KeyThreads = uint8(4)
)

// NewCipher creates a cipher using XChaCha20-Poly1305
// https://pkg.go.dev/golang.org/x/crypto/chacha20poly1305
// A salt is required to derive the key from a password using argon
func NewCipher(password string, salt []byte) (cipher.AEAD, error) {
	key := argon2.IDKey([]byte(password), salt, KeyTime, KeyMemory, KeyThreads, KeySize)
	return chacha20poly1305.NewX(key)
}

// Encrypt to encrypt a plaintext with a password
// Returns a byte slice with the generated salt, nonce and the ciphertext
func Encrypt(plaintext []byte, password string) (result []byte, err error) {
	salt := make([]byte, SaltSize)
	if n, err := rand.Read(salt); err != nil || n != SaltSize {
		return nil, err
	}

	aead, err := NewCipher(password, salt)
	if err != nil {
		return nil, err
	}

	result = append(result, salt...)

	nonce := make([]byte, aead.NonceSize())
	if m, err := rand.Read(nonce); err != nil || m != aead.NonceSize() {
		return nil, err
	}

	result = append(result, nonce...)

	ciphertext := aead.Seal(nil, nonce, plaintext, nil)
	result = append(result, ciphertext...)

	return result, nil
}

// Decrypt to decrypt a ciphertext with a password
// Returns the plaintext
func Decrypt(ciphertext []byte, password string) ([]byte, error) {
	if len(ciphertext) < SaltSize {
		return nil, fmt.Errorf("ciphertext is too short: cannot read salt")
	}
	salt := ciphertext[:SaltSize]

	aead, err := NewCipher(password, salt)
	if err != nil {
		return nil, err
	}

	if len(ciphertext) < SaltSize+aead.NonceSize() {
		return nil, fmt.Errorf("ciphertext is too short: cannot read nonce")
	}

	nonce := ciphertext[SaltSize : SaltSize+aead.NonceSize()]
	ciphertext = ciphertext[SaltSize+aead.NonceSize():]

	return aead.Open(nil, nonce, ciphertext, nil)
}