大きなデータを暗号化する方法
RSA 鍵サイズより大きなファイルやデータを、ハイブリッド(エンベロープ)暗号化で暗号化します。
问题
大きなファイルを暗号化する必要がありますが、RSA は数百バイトしか暗号化できず、公開鍵暗号の安全性も確保したい。
前提条件
- AES と RSA の理解
- ハイブリッド暗号に関する知識
ステップ
-
1. Generate a random symmetric key
Generate a fresh random AES key (256 bits). This key will encrypt the large data. It is called the session key or data encryption key (DEK). A new DEK should be generated for each file.
入力random AES-256 key输出32 random bytes -
2. Encrypt the data with AES
Encrypt the large file using AES-GCM with the session key and a fresh IV. AES can handle data of any size efficiently. Store the IV and authentication tag with the ciphertext.
入力large file + AES key输出AES ciphertext + IV + tag -
3. Encrypt the session key with RSA
Encrypt the small AES session key using the recipient's RSA public key with OAEP padding. Store the RSA-encrypted session key alongside the AES ciphertext. The recipient decrypts the session key with their private key, then decrypts the file with AES.
入力AES key + RSA public key输出RSA-encrypted session key
期待される結果
ハイブリッド暗号化パッケージ。大きなデータは AES で、AES 鍵は RSA で暗号化し、両者の長所を組み合わせます。
よくあるエラー
- Trying to encrypt the large file directly with RSA
- RSA can only encrypt data up to key_size - padding bytes. Use hybrid encryption for anything larger.
- Reusing the AES session key across files
- Generate a fresh session key for each encryption. Reusing it with the same IV breaks security.
トラブルシューティング指引
- If the RSA step fails with 'message too long', the session key exceeds the RSA limit. This should not happen with a 32-byte key and 2048+ bit RSA.
- Verify the GCM tag after AES decryption before using the data.
セキュリティ上の制限
- Hybrid encryption is the standard for TLS, PGP, and S/MIME. It provides public-key security with symmetric-key performance.
- All encryption is local; no keys or data are transmitted.