How to Encrypt Large Data

Problem

You need to encrypt a large file but RSA can only encrypt a few hundred bytes, and you want the security of public-key encryption.

Prerequisites

• Understanding of AES and RSA
• Knowledge of hybrid encryption

Steps

1. 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.

   Input

   random AES-256 key

   Output

   32 random bytes

2. 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.

   Input

   large file + AES key

   Output

   AES ciphertext + IV + tag

3. 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.

   Input

   AES key + RSA public key

   Output

   RSA-encrypted session key

Expected result

A hybrid-encrypted package: large data encrypted with AES, and the AES key encrypted with RSA, combining the strengths of both.

Common errors

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.

Troubleshooting

• 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.

Security limits

• 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.