// Package agekey turns derived bytes into an age identity and uses
// that identity to encrypt and decrypt.
package agekey

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"strings"

	"filippo.io/age"
	"filippo.io/age/armor"
	"github.com/btcsuite/btcd/btcutil/bech32"
)

const (
	// Application is the number this key type occupies in the
	// derivation path. It spells AGE the way BIP-85 spells RSA, as
	// the ASCII codes of the letters written out.
	Application = 657169

	// keySize is how long an X25519 secret key is.
	keySize = 32

	// humanPart is what age puts in front of a secret key when it
	// writes one down.
	humanPart = "age-secret-key-"
)

// ErrSize is returned when the derived bytes are not the length an
// X25519 secret key has to be.
var ErrSize = errors.New("an age identity needs 32 derived bytes")

// ErrNotRecipient is returned when the file was encrypted to someone
// else, so this key cannot open it.
var ErrNotRecipient = errors.New(
	"this mnemonic and index are not a recipient of the file",
)

// Key is one age identity.
type Key struct {
	identity *age.X25519Identity
}

// New makes the identity whose X25519 secret key is the derived bytes,
// clamped the way that curve requires.
func New(derived []byte) (*Key, error) {
	if len(derived) != keySize {
		return nil, fmt.Errorf("%w, got %d", ErrSize, len(derived))
	}

	scalar := make([]byte, keySize)
	copy(scalar, derived)
	clamp(scalar)

	// age offers no way to make an identity out of bytes, so the
	// scalar goes in the way age writes a secret key down.
	written, err := bech32.EncodeFromBase256(humanPart, scalar)
	if err != nil {
		return nil, fmt.Errorf("writing the secret key: %w", err)
	}

	identity, err := age.ParseX25519Identity(strings.ToUpper(written))
	if err != nil {
		return nil, fmt.Errorf("reading the secret key back: %w", err)
	}

	return &Key{identity: identity}, nil
}

// clamp makes the scalar one the curve accepts: the lowest three bits
// off, the highest bit off and the one below it on, as RFC 7748 says.
func clamp(scalar []byte) {
	const (
		lowestThreeOff  = 0b1111_1000
		highestOff      = 0b0111_1111
		secondHighestOn = 0b0100_0000
	)

	scalar[0] &= lowestThreeOff
	scalar[len(scalar)-1] &= highestOff
	scalar[len(scalar)-1] |= secondHighestOn
}

// Recipient returns the public key, the age1... line.
func (k *Key) Recipient() string {
	return k.identity.Recipient().String()
}

// Identity returns the secret key, the AGE-SECRET-KEY-1... line.
func (k *Key) Identity() string {
	return k.identity.String()
}

// Encrypt copies src to dst, encrypted to this key and to every extra
// recipient named, so the same mnemonic can always read it back again.
// Armored output is the text form age also reads.
func (k *Key) Encrypt(
	dst io.Writer, src io.Reader, to []string, armored bool,
) error {
	all, err := recipients(k.identity.Recipient(), to)
	if err != nil {
		return err
	}

	out := dst

	var text io.WriteCloser

	if armored {
		text = armor.NewWriter(dst)
		out = text
	}

	err = encrypt(out, src, all)
	if err != nil {
		return err
	}

	if text == nil {
		return nil
	}

	err = text.Close()
	if err != nil {
		return fmt.Errorf("finishing the text form: %w", err)
	}

	return nil
}

// recipients returns the key's own recipient followed by the ones
// named on the command line, so what the key encrypts it can read.
func recipients(mine age.Recipient, to []string) ([]age.Recipient, error) {
	list := []age.Recipient{mine}

	for _, name := range to {
		parsed, err := age.ParseX25519Recipient(name)
		if err != nil {
			return nil, fmt.Errorf("reading recipient %q: %w", name, err)
		}

		list = append(list, parsed)
	}

	return list, nil
}

// encrypt writes src into dst for the recipients.
func encrypt(dst io.Writer, src io.Reader, to []age.Recipient) error {
	sealed, err := age.Encrypt(dst, to...)
	if err != nil {
		return fmt.Errorf("starting the encryption: %w", err)
	}

	_, err = io.Copy(sealed, src)
	if err != nil {
		return fmt.Errorf("encrypting: %w", err)
	}

	err = sealed.Close()
	if err != nil {
		return fmt.Errorf("finishing the encryption: %w", err)
	}

	return nil
}

// Decrypt copies src to dst, decrypted with this key. The text form is
// recognised by the line it starts with, so it needs no flag.
func (k *Key) Decrypt(dst io.Writer, src io.Reader) error {
	plain, err := age.Decrypt(unarmored(src), k.identity)

	noMatch := &age.NoIdentityMatchError{}
	if errors.As(err, &noMatch) {
		return ErrNotRecipient
	}

	if err != nil {
		return fmt.Errorf("decrypting: %w", err)
	}

	_, err = io.Copy(dst, plain)
	if err != nil {
		return fmt.Errorf("reading the decrypted file: %w", err)
	}

	return nil
}

// unarmored strips the text form when the input begins with the line
// that starts one, and leaves binary input alone.
func unarmored(src io.Reader) io.Reader {
	buffered := bufio.NewReader(src)

	start, err := buffered.Peek(len(armor.Header))
	if err == nil && string(start) == armor.Header {
		return armor.NewReader(buffered)
	}

	return buffered
}