Information Security Basics
When sending a large file encrypted to someone, which procedure describes the commonly used hybrid encryption scheme?
- AEncrypt the file directly with the recipient's public key, without using symmetric-key cryptography
- BEncrypt the file with a symmetric key and send that key unencrypted together with the file
- CEncrypt the file with a one-time symmetric key, encrypt that symmetric key with the recipient's public key, and send them together
Show answer and explanation
Answer: C. Encrypt the file with a one-time symmetric key, encrypt that symmetric key with the recipient's public key, and send them together
Public-key cryptography makes key distribution easy, but its computation is heavy and it is not suited to encrypting large data. Symmetric-key cryptography is fast, but getting the key safely to the other party is the challenge. Hybrid encryption combines their strengths: the data itself is encrypted quickly with a one-time symmetric key (session key) generated on the spot, and only that short symmetric key is encrypted with the recipient's public key and sent along. The recipient recovers the symmetric key with their own private key and uses it to decrypt the data. This is used, for example, in email encryption. In TLS the mainstream approach is for both sides to derive the same symmetric key through a key exchange instead of sending it encrypted with a public key, but the idea is the same: use public-key techniques to set up the symmetric key safely, and protect the data with symmetric-key cryptography.
- A. Public-key cryptography is computationally heavy, so it is not suited to encrypting large data directly.
- B. Sending the symmetric key as is would let anyone who intercepts it decrypt the file.
- C. Correct. The data is protected by fast symmetric-key cryptography, and the key handover by public-key cryptography.