// Package secretbox encrypts small secrets (tokens, passwords, private keys)
// before they are written to the database, using AES-256-GCM.
//
// Sealed values look like "enc1:<base64>". Opening a value that isn't sealed
// returns it unchanged, so data written before encryption was turned on is
// still readable and gets sealed the next time it is saved.
package secretbox

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"strings"
)

const prefix = "enc1:"

type Box struct{ aead cipher.AEAD }

// GenerateKey returns a new random key in the form New accepts.
func GenerateKey() string {
	b := make([]byte, 32)
	rand.Read(b)
	return base64.StdEncoding.EncodeToString(b)
}

// New builds a Box from a key given as base64 or hex of 32 bytes. Any other
// text is treated as a passphrase and stretched with SHA-256.
func New(key string) (*Box, error) {
	key = strings.TrimSpace(key)
	if key == "" {
		return nil, errors.New("empty key")
	}
	raw := decodeKey(key)
	block, err := aes.NewCipher(raw)
	if err != nil {
		return nil, err
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &Box{aead: aead}, nil
}

func decodeKey(key string) []byte {
	if b, err := base64.StdEncoding.DecodeString(key); err == nil && len(b) == 32 {
		return b
	}
	if b, err := hex.DecodeString(key); err == nil && len(b) == 32 {
		return b
	}
	sum := sha256.Sum256([]byte(key))
	return sum[:]
}

// Seal encrypts s. The empty string stays empty so unset secrets stay visibly unset.
func (b *Box) Seal(s string) string {
	if s == "" || strings.HasPrefix(s, prefix) {
		return s
	}
	nonce := make([]byte, b.aead.NonceSize())
	rand.Read(nonce)
	return prefix + base64.StdEncoding.EncodeToString(b.aead.Seal(nonce, nonce, []byte(s), nil))
}

// Open decrypts a sealed value. A wrong key or damaged data is an error, so a
// bad key can never silently turn secrets into garbage.
func (b *Box) Open(s string) (string, error) {
	if !strings.HasPrefix(s, prefix) {
		return s, nil
	}
	raw, err := base64.StdEncoding.DecodeString(s[len(prefix):])
	if err != nil || len(raw) < b.aead.NonceSize() {
		return "", errors.New("damaged secret")
	}
	n := b.aead.NonceSize()
	plain, err := b.aead.Open(nil, raw[:n], raw[n:], nil)
	if err != nil {
		return "", errors.New("can't decrypt a stored secret: is HAMMERHEAD_MASTER_KEY the key it was saved with?")
	}
	return string(plain), nil
}