Symmetric Cryptography

What is Symmetric Cryptography?

• both encryption and decryption operations use the same key

• secret-key systems have been around for many hundreds of years

What are the Types of Symmetric Cryptography?

What is a Stream Cipher?

a method that encrypts data one bit or byte at a time by combining it with a pseudorandom keystream, typically using the XOR operation

require sender and receiver synchronicity

What is the Method for a Stream Cipher?

take as input a (short) key

• key is often combined with an initialisation vector (IV)

key is converted into a continuous keystream

working with one bit at a time, the plaintext is mixed with the keystream

What are the Advantages of Stream Cipher?

• no error propagation

• on-the-fly-encryption - good choice for real-time services

• fast and easy to implement, especially in hardware

How does Stream Cipher Work?

• keystream generation - a small secret key and initialisation vector (IV) feed a PseudoRandom number generator (PRNG) to produce a long, seemingly random keystream

• encryption - each bit/byte of the plaintext is XORed with the corresponding bit/byte of the keystream to produce ciphertext

• decryption - the same keystream is XORed with the ciphertext to recover the plaintext

What is a Block Cipher?

a method to transform fixed-length blocks of data (e.g., 128 bits) into ciphertext using a secret key

What is the Method for a Block Cipher?

• take as input a key and a block of plaintext and outputs a block of ciphertext

• a good block cipher should be designed to provide confusion and diffusion

  • confusion hides the relationship between the plaintext and ciphertext

  • diffusion spreads the statistics of the plaintext through the ciphertext

• usually obtain protection from chosen plaintext attacks if the block cipher behaves like a pseudorandom permutation

• block length should be chosen to ensure a balance of efficiency and security

What are the Block Cipher Modes of Operation?

• Electronic Code Books (ECB)

• Counter Block Chaining (CBC)

• Cipher Feedback (CFB)

• Counter (CTR)

What is Electronic Code Books (ECB)?

the simplest mode of operation where each plaintext block is encrypted independently using the same key, resulting in identical ciphertext blocks for identical plaintext

What is Counter Block Chaining (CBC)?

a mode by XORing each plaintext block with the previous ciphertext block before encryption, breaking up patterns

requires an initialisation vector (IV) for the first block to ensure uniqueness

What is Cipher Feedback (CFB)?

works by encrypting the previous ciphertext block (or IV for the first block) using the block cipher algorithm, and XORing the result with the current plaintext to produce the ciphertext

What is Counter (CTR)?

works by encrypting successive values of a counter to generate a keystream, which is then XORed with plaintext to produce ciphertext

What is Padding?

extending a plaintext message to multiple of the block size, needed when data doesn’t fit the exact block size