AES で復号する方法
正しい鍵、IV、モードを使って AES 暗号文を復号し、認証付きモードでは整合性を検証します。
问题
AES で暗号化された暗号文と鍵があり、元の平文を復元する必要があります。
前提条件
- AES 暗号の理解
- 暗号化に使うモードと IV に関する知識
ステップ
-
1. Gather all parameters
You need the key, the IV (or nonce), the ciphertext, and the mode used during encryption. For GCM, you also need the authentication tag. All of these must match what was used to encrypt.
入力key + IV + ciphertext + mode输出ready to decrypt -
2. Verify the authentication tag (GCM)
If the data was encrypted with GCM, verify the tag before decrypting. If the tag does not match, the ciphertext has been tampered with and must be rejected. Do not attempt to use tampered data.
入力tag mismatch输出reject: data corrupted -
3. Decrypt and interpret the result
Decrypt the ciphertext using the key and IV. If the result is text, decode it with the correct character set. If it is binary, save it as a file. A wrong key produces garbage without error in non-authenticated modes.
入力ciphertext + correct key输出original plaintext
期待される結果
元の平文。認証付きモードを使用した場合は整合性も検証済みです。
よくあるエラー
- Decryption succeeds but output is garbage
- In CBC or CTR mode, a wrong key produces random-looking output without error. Use GCM to detect wrong keys via tag verification.
- Wrong IV produces partially correct output
- The IV affects only the first block in CBC. A wrong IV corrupts the first block but leaves the rest intact. Verify the IV matches.
トラブルシューティング指引
- If only the first block is corrupted, the IV is wrong but the key is correct.
- If all blocks are garbage, the key or mode is wrong.
セキュリティ上の制限
- Never use decrypted data without verifying the authentication tag in GCM mode.
- All decryption happens locally. No data is sent to any server.