// Package secrets encrypts connection credentials (API tokens, passwords)
// at rest using AES-256-GCM, keyed from the application secret.
package secrets

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

type Box struct {
	gcm cipher.AEAD
}

// New derives an AES-256 key from the application secret via SHA-256.
func New(secret string) (*Box, error) {
	if secret == "" {
		return nil, errors.New("secrets: app secret must not be empty")
	}
	key := sha256.Sum256([]byte(secret))
	block, err := aes.NewCipher(key[:])
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &Box{gcm: gcm}, nil
}

func (b *Box) Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	nonce := make([]byte, b.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ciphertext := b.gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

func (b *Box) Decrypt(encoded string) (string, error) {
	if encoded == "" {
		return "", nil
	}
	data, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}
	nonceSize := b.gcm.NonceSize()
	if len(data) < nonceSize {
		return "", errors.New("secrets: ciphertext too short")
	}
	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
	plaintext, err := b.gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plaintext), nil
}