// Package cbk derives NaCl crypto_box keypairs (Curve25519) deterministically
// from BIP39 mnemonic phrases. That is all it does; the resulting keys are
// plain 32-byte crypto_box keys for use with golang.org/x/crypto/nacl/box.
//
// The same mnemonic and passphrase always produce the same keypair:
//
//  1. The mnemonic is validated and canonicalized with
//     bip39wordlist.NormalizeWordlist (lowercase, single spaces).
//  2. The standard 64-byte BIP39 seed is computed from the canonical
//     mnemonic and the optional passphrase with PBKDF2-HMAC-SHA512 (2048
//     rounds, salt "mnemonic" + passphrase), exactly as a BIP39 wallet
//     would.
//  3. The 32-byte crypto_box private key is derived from that seed with
//     HKDF-SHA256, using no salt and DerivationInfo as the info string.
//  4. The public key is the Curve25519 scalar multiplication of the
//     private key with the base point, via golang.org/x/crypto/nacl/box.
package cbk

import (
	"bytes"
	"crypto/hkdf"
	"crypto/sha256"
	"encoding/hex"
	"fmt"

	"github.com/tyler-smith/go-bip39"
	"golang.org/x/crypto/nacl/box"

	"sneak.berlin/go/x/pkg/bip39wordlist"
)

const (
	// KeySize is the length in bytes of a crypto_box public or private key.
	KeySize = 32

	// DerivationInfo is the HKDF info string that binds derived keys to
	// this package. Changing it changes every derived key.
	DerivationInfo = "sneak.berlin/go/x/pkg/cbk crypto_box v1"
)

// CryptoBoxKey is a 32-byte crypto_box key, either public or private.
type CryptoBoxKey [KeySize]byte

// Bytes returns a copy of the key as a byte slice.
func (k CryptoBoxKey) Bytes() []byte {
	return k[:]
}

// Hex returns the key as 64 lowercase hexadecimal characters.
func (k CryptoBoxKey) Hex() string {
	return hex.EncodeToString(k[:])
}

// Params configures New.
type Params struct {
	// Mnemonic is the BIP39 mnemonic phrase. Any letter case and any
	// whitespace between words is accepted; it is normalized before use.
	Mnemonic string

	// Passphrase is the optional BIP39 passphrase (sometimes called the
	// "25th word"). It is used verbatim: it is case sensitive and is not
	// trimmed. An empty passphrase is the common case.
	Passphrase string
}

// CryptoBoxKeypair is a crypto_box public and private key pair.
type CryptoBoxKeypair struct {
	PublicKey  CryptoBoxKey
	PrivateKey CryptoBoxKey
}

// New derives the keypair for the mnemonic and passphrase in p. If the
// mnemonic is not valid it returns an error wrapping one of the
// bip39wordlist sentinel errors.
func New(p Params) (CryptoBoxKeypair, error) {
	mnemonic, err := bip39wordlist.NormalizeWordlist(p.Mnemonic)
	if err != nil {
		return CryptoBoxKeypair{}, err
	}

	seed := bip39.NewSeed(mnemonic, p.Passphrase)

	privateKeyBytes, err := hkdf.Key(sha256.New, seed, nil, DerivationInfo, KeySize)
	if err != nil {
		return CryptoBoxKeypair{}, fmt.Errorf("cbk: deriving private key: %w", err)
	}

	publicKey, privateKey, err := box.GenerateKey(bytes.NewReader(privateKeyBytes))
	if err != nil {
		return CryptoBoxKeypair{}, fmt.Errorf("cbk: computing public key: %w", err)
	}

	return CryptoBoxKeypair{
		PublicKey:  CryptoBoxKey(*publicKey),
		PrivateKey: CryptoBoxKey(*privateKey),
	}, nil
}

// FromMnemonic derives the keypair for mnemonic with an empty passphrase.
// It is shorthand for New(Params{Mnemonic: mnemonic}).
func FromMnemonic(mnemonic string) (CryptoBoxKeypair, error) {
	return New(Params{Mnemonic: mnemonic, Passphrase: ""})
}

// PublicKeyBytes returns a copy of the public key as a byte slice.
func (kp CryptoBoxKeypair) PublicKeyBytes() []byte {
	return kp.PublicKey.Bytes()
}

// PrivateKeyBytes returns a copy of the private key as a byte slice.
func (kp CryptoBoxKeypair) PrivateKeyBytes() []byte {
	return kp.PrivateKey.Bytes()
}

// PublicKeyHex returns the public key as 64 lowercase hexadecimal
// characters.
func (kp CryptoBoxKeypair) PublicKeyHex() string {
	return kp.PublicKey.Hex()
}

// PrivateKeyHex returns the private key as 64 lowercase hexadecimal
// characters.
func (kp CryptoBoxKeypair) PrivateKeyHex() string {
	return kp.PrivateKey.Hex()
}