How to Use RSA Encryption

Problem

You need to encrypt data with a public key so that only the holder of the private key can decrypt it.

Prerequisites

• Understanding of public-key cryptography
• Knowledge of key sizes and padding

Steps

1. 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.

   Input

   key size: 4096

   Output

   public key + private key

2. 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.

   Input

   small message + public key

   Output

   ciphertext

3. 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.

   Input

   ciphertext + private key

   Output

   original message

Expected result

Data encrypted with a public key that only the corresponding private key can decrypt.

Common errors

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.

Troubleshooting

• 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.

Security limits

• 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.