Chapter 5 - Substitution Ciphers

Question 1

What are the goals of substitution ciphers?

Answer 1

1. unbreakable in practice if not in theory
2. the above is true even if the enemy learns the system
3. the key should be easy to remember(without notes) and easy to change
4. possible to transmit the enciphered messages digitally
5. apparatus and documents should be easily carried and operated by one person
6. the cipher should be easy to use; no complex rules or computations

Question 2

What does simple substitution require?

Answer 2

• a mixed alphabet

Question 3

What techniques are used for solving simple substitution ciphers?

Answer 3

• letter frequencies and letter contacts

Question 4

What is used to defeat an attack that analyzes letter frequencies and letter contacts?

Answer 4

• Disrupt the letter frequencies and letter contacts

Question 5

What is a letter contact table?

Answer 5

• A table showing the contact characteristics for letters of a specific language

Question 6

What are the characteristics of a symmetric cipher?

Answer 6

• when a letter X with key K is the same as letter K with key X