Characteristics of cryptographic systems

Question 1

What are the three characteristics of cryptographic systems?

Answer 1

1. Type of operations used to transform plaintext into ciphertext.
2. Number of keys used.
3. Way in which plaintext is processed.

Question 2

Explain characteristic 1 (Type of operations used to transform plaintext into ciphertext)

Answer 2

Two general principles for encryption algorithms:

1. Substitution - each element in the plaintext (bit, letter…) is mapped into another element.
2. Transposition - elements in the plaintext are rearranged.

All operations MUST BE REVERSIBLE and most systems (product systems) have multiple stages of substitution and transposition

Question 3

Explain characteristic 2 (Number of keys used)

Answer 3

1. Symmetric, single-key, secret-key, or conventional
   encryption: both sender and receiver use “same” key.
2. Asymmetric, two-key, or public-key encryption: sender and receiver use different keys.

Question 4

Explain characteristic 3 (way in which plaintext is processed)

Answer 4

1. Block cipher: processes input one block of elements at a time, producing an output block for each input block.
2. Stream cipher: processes input elements continuously, producing in output one element at a time, as it goes along.

Question 5

What is a block cipher? (DETAILED VERSION)

Answer 5

1. A block cipher is an encryption scheme that breaks up the plaintext message into strings (blocks) of a fixed length n and encrypts one block at a time.
2. It takes in input one block of n bits of plaintext and a key of k bits, producing in output one block of ciphertext of n bits.
3. For decryption, it takes in input a block of n bits of ciphertext and a key of k bits, producing in output a plaintext block of n bits.

Question 6

What is a stream cipher? (DETAILED VERSION)

Answer 6

1. A stream cipher is (typically) an XOR operation that encrypts and decrypts one bit or one byte at a time (input block and key) to obtain the ciphertext/ plaintext.
2. In other words, blocks of plaintext, key and ciphertext are one-bit long.

Question 7

What are codes?

Answer 7

1. They work on words of varying length.
2. The translation is given by a code-book.
3. Problems:
   a. if there’s no entry for a word in the code book, then you can’t say it or use it.
   b. Security is “pushed” to the code-book, which needs to be protected.