// Package crypto provides AES-256-GCM field encryption keyed from EMCLI_KEY.
package crypto

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

// keyFromEnv reads and validates a base64-encoded 32-byte AES key from the
// named environment variable. Errors name the variable so callers get a
// role-appropriate message.
func keyFromEnv(varName string) ([]byte, error) {
	raw := os.Getenv(varName)
	if raw == "" {
		return nil, fmt.Errorf("%s is not set", varName)
	}
	key, err := base64.StdEncoding.DecodeString(raw)
	if err != nil || len(key) != 32 {
		return nil, fmt.Errorf("%s must be base64 of exactly 32 bytes", varName)
	}
	return key, nil
}

// AgentKeyFromEnv reads the agent KEK from EMCLI_KEY (agent commands only).
func AgentKeyFromEnv() ([]byte, error) { return keyFromEnv("EMCLI_KEY") }

// AdminKeyFromEnv reads the admin KEK from EMCLI_ADMIN_KEY (all commands).
func AdminKeyFromEnv() ([]byte, error) { return keyFromEnv("EMCLI_ADMIN_KEY") }

// NewDEK returns a fresh random 32-byte data-encryption key.
func NewDEK() ([]byte, error) {
	dek := make([]byte, 32)
	if _, err := io.ReadFull(rand.Reader, dek); err != nil {
		return nil, err
	}
	return dek, nil
}

func newGCM(key []byte) (cipher.AEAD, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}
	return cipher.NewGCM(block)
}

// Seal encrypts plaintext, returning nonce||ciphertext.
func Seal(key, plaintext []byte) ([]byte, error) {
	gcm, err := newGCM(key)
	if err != nil {
		return nil, err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, err
	}
	return gcm.Seal(nonce, nonce, plaintext, nil), nil
}

// Open reverses Seal. A wrong key or tampered blob returns an error.
func Open(key, blob []byte) ([]byte, error) {
	gcm, err := newGCM(key)
	if err != nil {
		return nil, err
	}
	ns := gcm.NonceSize()
	if len(blob) < ns {
		return nil, errors.New("ciphertext too short")
	}
	nonce, ct := blob[:ns], blob[ns:]
	return gcm.Open(nil, nonce, ct, nil)
}