package crypto

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

// Encrypt verschlüsselt plaintext mit AES-256-GCM (Key = SHA256(secret))
// und liefert base64(nonce + ciphertext).
func Encrypt(secret string, plaintext []byte) (string, error) {
	k := sha256.Sum256([]byte(secret))
	block, err := aes.NewCipher(k[:])
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	sealed := gcm.Seal(nonce, nonce, plaintext, nil)
	return base64.StdEncoding.EncodeToString(sealed), nil
}

// Decrypt macht Encrypt rückgängig.
func Decrypt(secret, s string) ([]byte, error) {
	data, err := base64.StdEncoding.DecodeString(s)
	if err != nil {
		return nil, err
	}
	k := sha256.Sum256([]byte(secret))
	block, err := aes.NewCipher(k[:])
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	if len(data) < gcm.NonceSize() {
		return nil, errors.New("ciphertext zu kurz")
	}
	return gcm.Open(nil, data[:gcm.NonceSize()], data[gcm.NonceSize():], nil)
}