lemma/server/internal/secrets/secrets.go

// Package secrets provides an Encryptor interface for encrypting and decrypting strings using AES-256-GCM.
package secrets

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

	"novamd/internal/logging"
)

// Service is an interface for encrypting and decrypting strings
type Service interface {
	Encrypt(plaintext string) (string, error)
	Decrypt(ciphertext string) (string, error)
}

type encryptor struct {
	gcm cipher.AEAD
}

var logger logging.Logger

func getLogger() logging.Logger {
	if logger == nil {
		logger = logging.WithGroup("secrets")
	}
	return logger
}

// ValidateKey checks if the provided base64-encoded key is suitable for AES-256
func ValidateKey(key string) error {
	log := getLogger()
	log.Debug("validating encryption key")
	_, err := decodeAndValidateKey(key)
	return err
}

// decodeAndValidateKey validates and decodes the base64-encoded key
// Returns the decoded key bytes if valid
func decodeAndValidateKey(key string) ([]byte, error) {
	if key == "" {
		return nil, fmt.Errorf("encryption key is required")
	}

	keyBytes, err := base64.StdEncoding.DecodeString(key)
	if err != nil {
		return nil, fmt.Errorf("invalid base64 encoding: %w", err)
	}

	if len(keyBytes) != 32 {
		return nil, fmt.Errorf("encryption key must be 32 bytes (256 bits): got %d bytes", len(keyBytes))
	}

	// Verify the key can be used for AES
	_, err = aes.NewCipher(keyBytes)
	if err != nil {
		return nil, fmt.Errorf("invalid encryption key: %w", err)
	}

	return keyBytes, nil
}

// NewService creates a new Encryptor instance with the provided base64-encoded key
func NewService(key string) (Service, error) {
	log := getLogger()
	log.Debug("creating new encryption service")

	keyBytes, err := decodeAndValidateKey(key)
	if err != nil {
		return nil, err
	}

	block, err := aes.NewCipher(keyBytes)
	if err != nil {
		return nil, fmt.Errorf("failed to create cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("failed to create GCM: %w", err)
	}

	log.Info("encryption service created")
	return &encryptor{gcm: gcm}, nil
}

// Encrypt encrypts the plaintext using AES-256-GCM
func (e *encryptor) Encrypt(plaintext string) (string, error) {
	log := getLogger()

	if plaintext == "" {
		log.Debug("empty plaintext provided, returning empty string")
		return "", nil
	}

	nonce := make([]byte, e.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}

	ciphertext := e.gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	encoded := base64.StdEncoding.EncodeToString(ciphertext)

	log.Debug("data encrypted", "inputLength", len(plaintext), "outputLength", len(encoded))
	return encoded, nil
}

// Decrypt decrypts the ciphertext using AES-256-GCM
func (e *encryptor) Decrypt(ciphertext string) (string, error) {
	log := getLogger()

	if ciphertext == "" {
		log.Debug("empty ciphertext provided, returning empty string")
		return "", nil
	}

	data, err := base64.StdEncoding.DecodeString(ciphertext)
	if err != nil {
		return "", fmt.Errorf("invalid base64 encoding: %w", err)
	}

	nonceSize := e.gcm.NonceSize()
	if len(data) < nonceSize {
		return "", fmt.Errorf("invalid ciphertext: too short")
	}

	nonce, ciphertextBytes := data[:nonceSize], data[nonceSize:]
	plaintext, err := e.gcm.Open(nil, nonce, ciphertextBytes, nil)
	if err != nil {
		return "", err
	}

	log.Debug("data decrypted", "inputLength", len(ciphertext), "outputLength", len(plaintext))
	return string(plaintext), nil
}