// Package crypto provides AES-256-GCM encrypt/decrypt for exchange
// credentials stored in the database, keyed by the app_secret from
// data/settings.conf (see internal/config).
package crypto

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"fmt"
)

// Encrypt returns nonce||ciphertext, base64-encoded, for storage in a DB column.
func Encrypt(key []byte, plaintext string) (string, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return "", err
	}
	sealed := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(sealed), nil
}

// Decrypt reverses Encrypt.
func Decrypt(key []byte, encoded string) (string, error) {
	raw, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	if len(raw) < gcm.NonceSize() {
		return "", fmt.Errorf("ciphertext too short")
	}
	nonce, ciphertext := raw[:gcm.NonceSize()], raw[gcm.NonceSize():]
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plaintext), nil
}