Blown to Bits Chapter 5 pages 8-16 Answers

Questions corresponding to pages 8-16

1. What is encryption? (look on page 4 for this definition)

"Encryption is the art of encoding messages so they can't be understood by eavesdroppers or adversaries into whose hands the messages might fall."

2. What is cryptography?

"secret writing"

3. What is a cipher?

"A method of transforming a message into an obscured form, together with a way of undoing the transformation to recover the message"

4. Was is a Caesar cipher?

"If Caesar had anything confidential to say, he wrote it in a cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out."

5. What is a substitution cipher?

"Caesar's method is a representation of a larger class of ciphers, called substitution ciphers, in which one symbol is substituted for another according to a uniform rule (the same letter is always translated the same way)."

6. What is frequency analysis?

"the technique used to crack the code"

• What is a one-time pad?

A combination of ciphers where each cipher (or table row) is only used once, and there is a key that is as long as the plain-text.

• a. Who uses them?

"used during the Second World War and the Cold War in the form of booklets filled with digits. Governments still use one-time pads today for sensitive communications, with large amounts of keying material carefully generated and distributed on CDs or DVDs."

• b. Why are they impractical?

"Good one-time pads are hard to produce. If the pad contains repetitions or other patterns, Shannon's proof that one-time pads are uncrackable no longer holds. More seriously, transmitting a pad between the parties without loss or interception is likely to be just as difficult as communicating the plaintext of the message itself without detection."

• What are today's ciphers like?

"Rather than substituting message texts letter for letter, computers divide the ASCII-encoded plaintext message into blocks. They then transform the bits in the block according to some method that depends on a key. The key itself is a sequence of bits on which Alice and Bob must agree and keep secret from Eve. … there are no known shortcuts for breaking these ciphers (or at least none known publicly). … The amount of computation required to break a cipher by exhaustive search grows exponentially in the size of the key. Increasing the key length by one bit doubles the amount of work required to break the cipher, but only slight increases the work required to encrypt or decrypt."