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What is Symmetric-key cryptography?

A cryptographic method that uses the same key for encryption and decryption, also called secret-key cryptography. Its computation is light and it can encrypt large amounts of data quickly, but securely sharing the key with the other party (key distribution) is a challenge. AES is a typical example.

Japanese name: 共通鍵暗号

Symmetric-key cryptography appears in 3 practice questions across 1 topic, including Information Security Basics.

Field: Information Security Basics

Try it: questions about Symmetric-key cryptography

Which statement correctly describes symmetric-key cryptography?

  1. AIt uses different keys for encryption and decryption, and the encryption key may be made public
  2. BIt uses the same key for encryption and decryption and is fast, but securely sharing the key with the other party is a problem
  3. CIt transforms data by a fixed procedure without any key, so it is secure as long as the procedure is not known
Show answer and explanation

Answer: B. It uses the same key for encryption and decryption and is fast, but securely sharing the key with the other party is a problem

Symmetric-key cryptography uses the same key for encryption and decryption; AES is a typical example. Its computation is light and it can encrypt large amounts of data quickly, so it does the bulk of the work in encrypting communications and disks. On the other hand, the sender and receiver must share the same key in advance without letting anyone else learn it, and this key distribution problem is its weakness. Using different keys for encryption and decryption, with a public encryption key, is public-key cryptography. Modern ciphers are also designed to remain secure even when the procedure (algorithm) is public, as long as the key is secret (Kerckhoffs's principle).

Fifty members want every pair of them to be able to communicate with encryption without the other members learning the contents. Which combination gives the number of keys needed with symmetric-key cryptography and with public-key cryptography (counting public and private keys together)?

  1. ASymmetric-key: 1225 keys; public-key: 100 keys
  2. BSymmetric-key: 2450 keys; public-key: 100 keys
  3. CSymmetric-key: 100 keys; public-key: 1225 keys
Show answer and explanation

Answer: A. Symmetric-key: 1225 keys; public-key: 100 keys

With symmetric-key cryptography each pair needs its own key, because if three or more people shared a key, other members could read the contents too. The number of pairs that can be chosen from n people is n(n−1)/2, so 50 people need 50 × 49 / 2 = 1225 keys. With public-key cryptography, if each person has one public key and one private key, anyone can encrypt to anyone else with the recipient's public key, so 2n = 2 × 50 = 100 keys are enough. As the number of people grows, the number of symmetric keys grows roughly with the square of the number of people, whereas public-key cryptography grows only in proportion to it. This is one reason public-key cryptography makes key management easier.

Symmetric-key cryptography in the explanations

Questions that mention Symmetric-key cryptography (3)

  1. Which statement correctly describes symmetric-key cryptography?
  2. Fifty members want every pair of them to be able to communicate with encryption without the other members learning the contents. Which combination gives the number of keys needed with symmetric-key cryptography and with public-key cryptography (counting public and private keys together)?
  3. When sending a large file encrypted to someone, which procedure describes the commonly used hybrid encryption scheme?

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Related terms

Public-key cryptography TLS Cross-site scripting (XSS) Digital signature Hash function Message authentication code Multi-factor authentication Phishing Principle of least privilege Ransomware

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