RSA 暗号を使う方法
RSA 公開鍵暗号でデータの暗号化・復号を行います。鍵の生成とパディングについても扱います。
问题
公開鍵でデータを暗号化し、秘密鍵の保持者だけが復号できるようにする必要があります。
前提条件
- 公開鍵暗号の理解
- 鍵長とパディングに関する知識
ステップ
-
1. Generate or obtain a key pair
Generate an RSA key pair of at least 2048 bits; 3072 or 4096 bits are recommended for long-term security. The public key can be shared freely; the private key must be kept secret. Alternatively, obtain the recipient's public key.
入力key size: 4096输出public key + private key -
2. Encrypt with OAEP padding
Use OAEP (Optimal Asymmetric Encryption Padding) with a hash like SHA-256. Never use PKCS#1 v1.5 padding for encryption, as it is vulnerable to padding oracle attacks. RSA can only encrypt data smaller than the key size minus padding overhead.
入力small message + public key输出ciphertext -
3. Decrypt with the private key
The private key holder decrypts the ciphertext using OAEP with the same hash. For large data, encrypt a symmetric key with RSA and encrypt the data with AES (hybrid encryption). This is how TLS and PGP work.
入力ciphertext + private key输出original message
期待される結果
公開鍵で暗号化され、対応する秘密鍵でのみ復号できるデータ。
よくあるエラー
- Message is too long for RSA
- RSA can only encrypt data up to key_size - padding_overhead bytes. Use hybrid encryption: encrypt an AES key with RSA and the data with AES.
- Using PKCS#1 v1.5 padding
- PKCS#1 v1.5 is vulnerable to Bleichenbacher's padding oracle attack. Always use OAEP.
トラブルシューティング指引
- If decryption fails, verify the padding scheme and hash match on both sides.
- For large files, use hybrid encryption rather than chunking with RSA directly.
セキュリティ上の制限
- RSA key sizes below 2048 bits are considered insecure. Use 3072+ bits for new applications.
- All operations are local. Private keys never leave your machine.