What is Stream Cipher and Block Cipher?

What is a Block Cipher?

A block cipher is a cryptographic algorithm that encrypts fixed-size data blocks, commonly 128 bits. It operates with a fixed key and encrypts data groups to ensure maximum confidentiality and security. Modern block ciphers are based on the iterated product cipher design, using a fixed key for encrypting data groups.

How does Block Cipher work?

In a block cipher, encryption occurs in fixed-length blocks, typically chunks of 128 bits. Block ciphers operate on larger data units than stream ciphers, which encrypt one byte at a time. A key, denoted as K, transforms each plaintext block into a corresponding ciphertext block.

Key features and benefits of the block cipher:

• Block Size

  Encryption is performed on fixed-size blocks, often consisting of 128 bits. Each block undergoes a series of transformations using the encryption key.

• Key Expansion

  The encryption key, K, is expanded to produce subkeys for each round of encryption, enhancing the security of the block cipher.

• Confusion and Diffusion

  Confusion involves complexing the relationship between the key and ciphertext. Diffusion ensures that a change in one part of the plaintext affects a large portion of the ciphertext.

• Rounds of Transformation

  Each block undergoes multiple rounds of transformation, each involving substitution, permutation, and mixing operations.

• Cryptographic Security

  Block ciphers are designed to resist various cryptographic attacks, including differential and linear cryptanalysis. The careful design of the cipher’s structure achieves robust security.

• Electronic Codebook (ECB) Mode

  Each block is independently encrypted in ECB mode. Identical plaintext blocks yield identical ciphertext blocks.

• Cipher Block Chaining (CBC) Mode

  CBC mode introduces an initialization vector (IV) to enhance security. Before encryption, each block is XORed with the previous ciphertext block.

Example Encryption:

• The encryption process involves multiple rounds of substitution and permutation.

• Each block of plaintext is transformed into a corresponding block of ciphertext.

Example Decryption:

• Decryption reverses the encryption process, applying inverse transformations.

• Each block of ciphertext is decrypted to reveal the original plaintext.

Block ciphers are foundational in various cryptographic protocols, ensuring secure communication and data protection across diverse applications. Their fixed block size and complex encryption mechanisms contribute to their resilience against attacks.

What is Stream Cipher?

Encryption is performed one byte at a time in a stream cipher, providing a continuous stream of pseudorandom bits for increased security. The process begins with initializing a key, denoted as k, which is fed into a pseudorandom bit generator. This generator produces an 8-bit keystream, serving as the foundation for encryption.

Key features and benefits of the stream cipher:

1. Keystream Generation

   • A key, k, is input into a pseudorandom bit generator.

   • The generator produces an 8-bit keystream, forming the basis for encryption.

2. Cryptanalysis Resistance

   • The sequential nature of stream ciphers enhances resistance against cryptanalysis.

   • Increasing the length of the keystream makes cryptanalysis more challenging.

3. Brute Force Protection

   • Longer keys contribute to resistance against brute force attacks.

   • Strengthening security is achieved by employing longer key lengths.

4. Efficient Keystream Design

   • Keystreams are designed for optimal efficiency, incorporating a balanced mix of 1s and 0s.

   • This design choice aims to heighten the complexity of cryptanalysis.

5. Stream Cipher Operation

   • The encryption process

     It involves XORing each plaintext bit with the corresponding bit in the keystream

     Example: Plain Text: 10011001, Keystream: 11000011, Cipher Text: 01011010

   • Decryption process

     Decryption mirrors the encryption process using the same keystream. XORing the ciphertext with the keystream yields the original plaintext.

     Example: Cipher Text: 01011010, Keystream: 11000011, Plain Text: 10011001

   The stream cipher’s simplicity, efficiency, and cryptographic strength make it suitable for various applications where real-time encryption and decryption are crucial.