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Information Security Basics

Using public-key cryptography, A wants to encrypt a message so that only B can read it. Which key is used for the encryption?

  1. AA's private key
  2. BB's public key
  3. CB's private key
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Answer: B. B's public key

In public-key cryptography each person has a pair of keys: a public key and a private key. The public key may be known to anyone, while only its owner keeps the private key, closely guarded. Data encrypted with a public key can be decrypted only with the matching private key. So A encrypts with B's public key and sends the message, and B decrypts it with B's own private key. Even if the ciphertext is intercepted on the way, anyone without B's private key cannot read it. Only B holds B's private key, so A cannot use it. Processing with A's private key is what happens in a digital signature, which shows that A created the data.

  • A. Anything processed with A's private key can be checked by anyone with A's public key, so it cannot keep a message secret. That key is used for digital signatures.
  • B. Correct. If it is encrypted with B's public key, only B's private key can decrypt it.
  • C. B's private key is held only by B, so A cannot use it. It is the key used for decryption.
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  1. 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)?
  2. When sending a large file encrypted to someone, which procedure describes the commonly used hybrid encryption scheme?
  3. Which statement correctly describes a property of hash functions used in cryptography (cryptographic hash functions)?
  4. 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)?
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