Asymmetric vs Symmetric - secuguru/security-terms GitHub Wiki

Asymmetric and symmetric encryption are two distinct approaches to encryption, each with its own strengths and applications. Here’s an in-depth look at their differences, typical uses, and how they work together in protocols. We’ll also cover perfect forward secrecy (PFS) and its significance in secure communication.

1. Asymmetric Encryption

• Definition: Asymmetric encryption, also known as public-key encryption, uses a pair of keys: a public key (for encryption) and a private key (for decryption). Only the private key can decrypt data encrypted with the corresponding public key.
• Strengths:
  • Secure Key Distribution: Asymmetric encryption allows secure communication between parties without needing to exchange a shared key in advance.
  • Authentication: Because only the holder of the private key can decrypt data encrypted with the public key, asymmetric encryption also enables digital signatures, ensuring data authenticity.
• Weaknesses:
  • Slower Performance: Asymmetric encryption requires complex mathematical operations, making it slower and more computationally intensive compared to symmetric encryption.
• Use Case: Asymmetric encryption is commonly used for establishing a trusted connection, such as during the initial handshake in secure protocols like TLS, where it exchanges a symmetric session key for faster encryption.
• Example Algorithms: RSA, Elliptic Curve Cryptography (ECC).

2. Symmetric Encryption

• Definition: Symmetric encryption uses a single, shared key for both encryption and decryption. Both parties must have the same key to communicate securely.
• Strengths:
  • Faster Performance: Symmetric encryption is significantly faster than asymmetric encryption and is well-suited for encrypting large amounts of data.
  • Efficient for Data Transmission: Once a secure key exchange is established (often using asymmetric encryption), symmetric encryption becomes the preferred method for ongoing secure data transmission.
• Weaknesses:
  • Key Distribution Problem: Symmetric encryption requires a secure way to share the key beforehand, which can be challenging without secure channels.
• Use Case: Symmetric encryption is typically used after a secure channel is established (e.g., using asymmetric encryption), as in TLS, where a symmetric session key is used for encrypting data efficiently.
• Example Algorithms: AES, ChaCha20.

How They Work Together in Protocols

Protocols often combine asymmetric and symmetric encryption to balance security with performance:

• Handshake (Asymmetric): Initially, asymmetric encryption is used in the handshake phase to exchange a symmetric session key. This allows two parties to establish a shared key securely without pre-sharing it.
• Data Transfer (Symmetric): Once the session key is shared, the protocol switches to symmetric encryption to encrypt the actual data transferred, benefiting from its faster processing.

For example, in the TLS (Transport Layer Security) protocol:

• The initial handshake uses asymmetric encryption (e.g., RSA or ECC) to securely exchange a symmetric key.
• Once the key exchange is complete, the symmetric key is used to encrypt the rest of the communication.

Perfect Forward Secrecy (PFS)

• Definition: Perfect Forward Secrecy (PFS) is a security property that ensures even if the private key is compromised, past session keys remain secure. In PFS, each session generates a unique, temporary session key that is not linked to the long-term keys used in other sessions.
• How It Works:
  • PFS uses ephemeral keys, which are generated for each session and discarded afterward. Common algorithms used in PFS include Ephemeral Diffie-Hellman (DHE) and Elliptic Curve Diffie-Hellman Ephemeral (ECDHE).
  • Because each session key is unique, compromising one session key does not compromise previous or future sessions.
• Example: Signal, a widely used secure messaging app, uses PFS to ensure that each message session has its own encryption key, so intercepting one message does not compromise the entire conversation.
• Security Implications: PFS is crucial in protecting data from retrospective decryption, meaning that even if an attacker later obtains a device’s private key, they cannot decrypt previously recorded sessions.

Comparison Table

Property	Asymmetric Encryption	Symmetric Encryption
Key Usage	Uses a key pair (public and private)	Uses a single shared key
Speed	Slower, computationally intensive	Faster, efficient for large data
Key Distribution	No need to pre-share (secure exchange)	Must securely pre-share key
Common Use Cases	Initial handshake, key exchange, digital signatures	Bulk data encryption, ongoing communication
Examples	RSA, ECC	AES, ChaCha20
Perfect Forward Secrecy	Achieved with ephemeral keys (e.g., DHE, ECDHE)	Often supported in secure sessions after key exchange

Summary

• Asymmetric Encryption: Provides secure key exchange and authentication but is slower, making it suitable for establishing connections. Commonly used in handshakes to transfer symmetric keys.
• Symmetric Encryption: Fast and efficient, ideal for encrypting large amounts of data after a secure key has been exchanged via asymmetric encryption.
• Perfect Forward Secrecy: Ensures that each session has a unique key, preventing past communications from being compromised even if a private key is exposed.

Combining asymmetric and symmetric encryption with PFS in secure protocols provides a robust approach to protecting data in transit, ensuring both speed and security while safeguarding against retrospective decryption attacks.