Symmetric Encryption: Substitution

Question 1

What are the 4 types of substitution ciphers?

Answer 1

Mono-alphabetic, Poly-alphabetic, Transposition, Composite.

Question 2

Define Mono-alphabetic substitution…

Answer 2

A message is encrypted via a character shift across all characters in the message. The character shift is the key. The result is a permutation of the original message.

Question 3

Why is mono-alphabetic substitution vulnerable?

Answer 3

Frequency analysis

Question 4

Define Poly-alphabetic substitution…

Answer 4

A block cipher in which every block of the plain text has a unique key (random character shift applied to it). For example, block 1 may have a key of 3, block 2 may have a key of 7 etc. This ensures that characters in the cipher text don’t all map to the same characters in the plain text. Thus, reducing vulnerability to frequency analysis. In essence, this substitution consists of many different caesars ciphers working on the different plain text blocks.

Question 5

Give an example of a famous poly-alphabetic substitution and explain how it works…

Answer 5

Vingere cipher - In this cipher, a key is chosen and repeated to be the same length as the plain text. This means each key value maps to a plain text value. Then, for each mapping, the numeric values are added and then modulo the length of the key.

Question 6

What is the benefit of Vingere cipher?

Answer 6

It gives uniform uniqueness across the cipher text.

Question 7

What is a disadvantage of Vingere cipher?

Answer 7

The use of a repeating key still leaves some vulnerability to frequency analysis.

Question 8

What is the One Time Pad?

Answer 8

Aka, Vernam Cipher. A cipher in which each character of plain text is encrypted by a unique key. In more detail, the key and plain text are converted to binary values, and each mapping is XORd. The key is discarded once the encryption / decryption process is complete.

Question 9

Is the OTP considered unbreakable? If so, why?

Answer 9

Yes, it’s considered mathematically unbreakable due to the notion of it being truly random (if this is actually possible).

Question 10

What are the core aspects of the Vernam cipher?

Answer 10

1. The key is only used once for encryption and decryption, then it’s discarded.
2. Key must be truly random (if that’s possible)
3. Binary values of the key must be the same length as the binary values of the plain text

Question 11

What are the pros and cons of OTP?

Answer 11

Pros:
- Mathematically unbreakable due to randomness.
- XOR is very efficient on modern operating systems, therefore, encryptions and decryption process is very fast.
- Keys are discarded, therefore, no replay attacks.
Cons:
- Key generation is required for every communication run.
- Key distribution - It’s not practical to frequently exchange large keys.

Question 12

Define Transposition Cipher

Answer 12

Ciphers that use only permutations to hide the message.

Question 13

What was the earliest transposition cipher?

Answer 13

Scytale

Question 14

Name a modern Transposition Cipher. Explain how it works…

Answer 14

Row Column cipher.
A number of columns are chosen for a table, and the message is written in the table. The columns are then uses as cipher text.

Question 15

What is the key in the Row Column transposition cipher?

Answer 15

The column count

Question 16

What is a Composite Cipher

Answer 16

These are a combination of substitution and transposition ciphers.

Question 17

What is a product cipher?

Answer 17

A chain of composite ciphers.

Question 18

Define a homophonic substitution cipher…

Answer 18

A substitution cipher in which each each plain text symbol maps to multipe cipher text symbols. This reduces vulnerability to frequency analysis.