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Information Security Basics

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
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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).

  • A. Using different keys with a public encryption key is a feature of public-key cryptography.
  • B. Correct. Using the same key makes it fast, but securely sharing the key is the challenge.
  • C. Modern ciphers are designed to be secure even with a public procedure, as long as the key is secret; their security rests on the key.
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More Information Security Basics questions

  1. Using public-key cryptography, A wants to encrypt a message so that only B can read it. Which key is used for the encryption?
  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?
  4. Which statement correctly describes a property of hash functions used in cryptography (cryptographic hash functions)?
  5. For an ideal hash function with a 256-bit output, which combination gives the approximate number of computations needed for a brute-force attack in two cases: finding an input that produces a given hash value (a preimage attack), and finding any two different inputs that produce the same hash value (a collision attack)?