Short Messages & Public Key Encryption: Is it Safe?

TL;DR

Very short messages can be encrypted with public key encryption, but it’s generally not recommended on its own. The main problem is that short ciphertexts don’t hide much information about the original message, making them vulnerable to attacks. You need to combine public key encryption with other techniques like symmetric encryption and padding schemes for real security.

Why Short Messages Are a Problem

Public key encryption (like RSA or ECC) works by mathematically transforming your message into an unreadable format using the recipient’s public key. Only their private key can turn it back. However, these systems have weaknesses when dealing with very short messages:

• Frequency Analysis: If the ciphertext is too short, attackers might be able to guess the original message by looking at patterns in the encrypted text (especially if they know something about what you typically send).
• Known-Plaintext Attacks: If an attacker knows even a small part of your original message, it can help them break the encryption. Short messages make this easier.
• Mathematical Weaknesses: Some public key algorithms have inherent weaknesses that are more exposed with short inputs.

How to Secure Short Messages

Here’s a step-by-step guide to making short message encryption much safer:

1. Use Hybrid Encryption

1. Generate a Symmetric Key: Create a random, strong key for symmetric encryption (e.g., AES). A 256-bit key is good.

   openssl rand -base64 32

2. Encrypt the Message with Symmetric Encryption: Use the symmetric key to encrypt your short message. This is fast and secure for the actual data.

   openssl enc -aes-256-cbc -salt -in message.txt -out encrypted_message.enc -k <your_symmetric_key>

3. Encrypt the Symmetric Key with Public Key Encryption: Now, encrypt only the symmetric key using the recipient’s public key.

   openssl rsautl -encrypt -inkey <recipient_public_key.pem> -pubin -in <symmetric_key> -out encrypted_key.enc

4. Send Both: Send the recipient both the encrypted_message.enc file and the encrypted_key.enc file.

The recipient uses their private key to decrypt the symmetric key, then uses that key to decrypt the message.

2. Padding Schemes

Padding adds random data to your message before encryption. This makes it harder for attackers to analyse patterns and exploit weaknesses in the algorithm. Most modern libraries handle padding automatically (e.g., PKCS#7).

• Ensure Padding is Enabled: When using public key encryption directly, make sure your library or tool uses a proper padding scheme.

3. Message Authentication Codes (MACs)

1. Generate a MAC: Create a MAC of the original message using a secret key known only to you and the recipient.

   openssl dgst -sha256 -hmac <secret_key> message.txt

2. Send with Encryption: Send the encrypted message and the MAC.
3. Recipient Verification: The recipient decrypts the message and recalculates the MAC. If the calculated MAC matches the received MAC, it confirms the message hasn’t been tampered with.

4. Key Length

Use sufficiently long keys for your public key encryption algorithm. For RSA, 2048 bits is a minimum; 3072 or 4096 bits are better. For ECC, use at least 256-bit keys.

Important Considerations

• Algorithm Choice: Use well-vetted and widely used algorithms like RSA (with OAEP padding) or Elliptic Curve Cryptography (ECC).
• Library Security: Use reputable cryptography libraries that are regularly updated to address security vulnerabilities.
• Key Management: Securely store and manage your private keys! Losing them compromises all your encrypted messages.