Cryptanalysis is the study and practice of analyzing and breaking cryptographic systems. Its main goal is to find vulnerabilities or weaknesses in cryptographic algorithms and protocols to understand how they can be exploited.

1. Types of Attacks:
   • Ciphertext-only attack: The cryptanalyst has access only to the ciphertext.
   • Known-plaintext attack: The cryptanalyst has access to both the plaintext and its corresponding ciphertext.
   • Chosen-plaintext attack: The cryptanalyst can choose arbitrary plaintexts to be encrypted and obtain the corresponding ciphertexts.
   • Chosen-ciphertext attack: The cryptanalyst can choose arbitrary ciphertexts to be decrypted and obtain the corresponding plaintexts.
2. Common Techniques:
   • Frequency Analysis: Analyzing the frequency of letters or groups of letters in ciphertext. Useful against substitution ciphers.
   • Pattern Analysis: Identifying patterns in the ciphertext that could correspond to patterns in the plaintext.
   • Mathematical Attacks: Exploiting mathematical properties or flaws in the encryption algorithm. Common against modern cryptographic algorithms like RSA or ECC.

Steganography

Steganography is the practice of hiding a message within another medium to avoid detection. Unlike cryptography, which hides the content of a message, steganography hides the existence of the message.

1. Techniques:
   • Image Steganography: Embedding secret data into digital images by modifying the pixel values. The Least Significant Bit (LSB) method is commonly used.
   • Audio Steganography: Hiding data within audio files by altering the audio signal in ways that are inaudible to human ears.
   • Text Steganography: Concealing messages within text using methods like changing the formatting of existing text or using invisible characters.
   • Video Steganography: Embedding data in video files by altering frames or inserting data into motion vectors.
2. Applications:
   • Covert communication
   • Digital watermarking for copyright protection
   • Secure transmission of information in sensitive environments

Stream and Block Ciphers

Stream Ciphers

Stream ciphers encrypt plaintext digits one at a time, often combining it with a pseudorandom cipher digit stream (keystream).

1. Characteristics:
   • Symmetric key ciphers: Both the sender and receiver use the same key.
   • Bit-by-bit encryption: Each bit of plaintext is XORed with a bit from the keystream.
   • Keystream Generation: The keystream is generated from the key using an algorithm. It must be as long as the plaintext and appear random.
2. Examples:
   • RC4: A widely used stream cipher, though now considered insecure for some applications.
   • Salsa20 and ChaCha: Modern stream ciphers known for their security and performance.
3. Advantages:
   • High speed and low complexity
   • Suitable for real-time applications like streaming audio and video

Block Ciphers

Block ciphers encrypt data in fixed-size blocks (e.g., 64-bit or 128-bit blocks) using a symmetric key.

1. Characteristics:
   • Block size: Determines the size of the data chunks that are encrypted at a time.
   • Modes of Operation: How blocks are encrypted. Common modes include ECB, CBC, CFB, OFB, and CTR.
   • Key size: Determines the strength of encryption. Common key sizes are 128, 192, and 256 bits.
2. Examples:
   • DES (Data Encryption Standard): A historic block cipher with a 56-bit key, now considered insecure.
   • AES (Advanced Encryption Standard): The current standard, with key sizes of 128, 192, and 256 bits. Known for its security and efficiency.
3. Modes of Operation:
   • ECB (Electronic Codebook): Encrypts each block independently, but identical plaintext blocks yield identical ciphertext blocks.
   • CBC (Cipher Block Chaining): Each block of plaintext is XORed with the previous ciphertext block before encryption. Requires an initialization vector (IV).
   • CFB (Cipher Feedback): Turns a block cipher into a stream cipher. It encrypts segments of plaintext into ciphertext.
   • OFB (Output Feedback): Similar to CFB but generates keystream blocks, which are XORed with plaintext blocks.
   • CTR (Counter): Converts a block cipher into a stream cipher. Each block is encrypted with a counter value that changes for each block.

Understanding cryptanalysis, steganography, stream ciphers, and block ciphers is essential for anyone involved in cybersecurity. Cryptanalysis reveals the importance of robust cryptographic algorithms. Steganography offers methods for covert communication. Stream and block ciphers provide the foundational techniques for encrypting data, each with its own advantages and use cases. These concepts collectively form the backbone of modern secure communication systems.