encryptor.go

// Package crypto provides AES-256-GCM encryption for Vault secrets.
package crypto

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

// ErrInvalidKeySize is returned when the key is not exactly 32 bytes.
var ErrInvalidKeySize = errors.New("crypto: key must be exactly 32 bytes for AES-256")

// Encryptor provides AES-256-GCM encryption and decryption.
type Encryptor struct {
	aead cipher.AEAD
}

// NewEncryptor creates an Encryptor from a 32-byte AES-256 key.
func NewEncryptor(key []byte) (*Encryptor, error) {
	if len(key) != 32 {
		return nil, ErrInvalidKeySize
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("crypto: new cipher: %w", err)
	}

	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("crypto: new gcm: %w", err)
	}

	return &Encryptor{aead: aead}, nil
}

// Encrypt encrypts plaintext using AES-256-GCM.
// The returned ciphertext has the random nonce prepended: nonce || ciphertext.
func (e *Encryptor) Encrypt(plaintext []byte) ([]byte, error) {
	nonce := make([]byte, e.aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("crypto: generate nonce: %w", err)
	}

	// Seal appends encrypted data to nonce.
	return e.aead.Seal(nonce, nonce, plaintext, nil), nil
}

// Decrypt decrypts ciphertext produced by Encrypt.
// It expects the nonce prepended: nonce || ciphertext.
func (e *Encryptor) Decrypt(ciphertext []byte) ([]byte, error) {
	nonceSize := e.aead.NonceSize()
	if len(ciphertext) < nonceSize {
		return nil, errors.New("crypto: ciphertext too short")
	}

	nonce := ciphertext[:nonceSize]
	ct := ciphertext[nonceSize:]

	plaintext, err := e.aead.Open(nil, nonce, ct, nil)
	if err != nil {
		return nil, fmt.Errorf("crypto: decrypt: %w", err)
	}

	return plaintext, nil
}