Small Message Encryption

TL;DR

Asymmetric encryption (like RSA) has overheads that make it inefficient for very small messages. This guide shows how to combine symmetric encryption with asymmetric encryption to securely encrypt and transmit short data, focusing on practical steps and avoiding unnecessary complexity.

Steps

1. Choose Symmetric Encryption: Select a strong symmetric cipher like AES (Advanced Encryption Standard). AES-256 is generally recommended for its security.
2. Generate a Session Key: Create a random key specifically for this message. This is your session key.

   openssl rand -base64 32

   This command generates a 32-byte (256-bit) random key in base64 format, suitable for AES-256.

3. Encrypt the Message: Use the session key to encrypt your small message with AES.

   openssl enc -aes-256-cbc -salt -in input.txt -out encrypted.enc -pass pass:"your_session_key"

   Replace input.txt with the file containing your message and encrypted.enc with the desired output filename. The -salt option adds a random salt for extra security.

4. Encrypt the Session Key: Use the recipient’s public key to encrypt the session key.

   openssl rsautl -encrypt -pubin -inkey recipient_public.pem -in session_key.txt -out encrypted_session_key.enc

   Replace recipient_public.pem with the path to the recipient’s public key file and session_key.txt with a file containing your generated session key.

5. Transmit Both Files: Send both encrypted.enc (the encrypted message) and encrypted_session_key.enc (the encrypted session key) to the recipient. Ensure secure transmission – use HTTPS, SFTP, or another secure channel.
6. Recipient Decrypts Session Key: The recipient uses their private key to decrypt the session key.

   openssl rsautl -decrypt -inkey recipient_private.pem -in encrypted_session_key.enc -out decrypted_session_key.txt

   Replace recipient_private.pem with the path to their private key file.

7. Recipient Decrypts Message: The recipient uses the now-decrypted session key to decrypt the message.

   openssl enc -aes-256-cbc -d -salt -in encrypted.enc -out decrypted_message.txt -pass pass:"the_decrypted_session_key"

   Replace encrypted.enc with the filename of the encrypted message and decrypted_message.txt with the desired output file for the decrypted message.

Important Considerations

• Key Management: Securely storing and managing private keys is crucial.
• Random Number Generation: Use a cryptographically secure random number generator (CSPRNG) to generate session keys. openssl rand provides this.
• Padding: Ensure proper padding schemes are used with AES (e.g., PKCS#7). The -salt option in openssl handles some of this automatically, but be aware of it when using other tools or libraries.
• Error Handling: Implement robust error handling throughout the process to detect and respond to potential issues.
• cyber security Best Practices: Always follow cyber security best practices for encryption and key management.