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Knowledge/A Level/H446/01 Computer systems/Compression, Encryption and Hashing/Symmetric and asymmetric encryption

Symmetric and asymmetric encryption

1.3.1 Compression, Encryption and Hashing

Compression, Encryption and Hashing — Note 3 of 4

Candidates should understand the need for encryption. Candidates should understand how symmetric and asymmetric encryption work to encrypt and decrypt data.

What you need to know

Encryption scrambles data using a key so that it cannot be understood if it is intercepted. Symmetric encryption uses the same key to encrypt and decrypt; asymmetric encryption uses a pair of mathematically linked keys, one public and one private.

What to know

  • Symmetric drawback: the key must be shared securely. If it is intercepted during the key exchange, all messages can be decrypted.
  • Symmetric benefit: fast, so suitable for encrypting large amounts of data.
  • The public key can be shared openly; the private key is never shared, which solves the key exchange problem.
  • Asymmetric drawback: much slower than symmetric encryption.
  • In practice (e.g. HTTPS), asymmetric encryption is used to exchange a symmetric key securely, and the symmetric key then encrypts the rest of the communication.
  • Digital signatures reverse the process: the sender encrypts a hash of the message with their PRIVATE key, and anyone can verify it with the sender's PUBLIC key, proving who sent it and that it was not changed.

Key terms

Exam tipCommon mistake

Encryption

Converting plaintext into ciphertext using an algorithm and a key, so that only someone with the correct key can convert it back.

  • Common mistake: Saying encryption stops data being intercepted. It stops intercepted data being UNDERSTOOD.
Symmetric encryption

The SAME key is used to encrypt and decrypt the data. Both sender and receiver must have the key.

Asymmetric encryption

Uses a key pair. Data encrypted with the recipient's PUBLIC key can only be decrypted with their matching PRIVATE key.

Exam technique

  • Tip:State precisely which key is used for which step: 'encrypted with the receiver's public key, decrypted with the receiver's private key'.
← Previous noteRun length encoding and dictionary coding for lossless compression1.3.1 Compression, Encryption and HashingNext note →Different uses of hashing1.3.1 Compression, Encryption and Hashing

Back to Compression, Encryption and Hashing

Compression, Encryption and Hashing

Note 3 of 4

  1. 1Lossy vs lossless compression
  2. 2Run length encoding and dictionary coding for lossless compression
  3. 3Symmetric and asymmetric encryption
  4. 4Different uses of hashing
Topic overview