Course 17 - Computer Network Security Protocols And Techniques | Episode 3: Modern Ciphers: Structure, Standards (DES/AES)

In this lesson, you’ll learn about:How modern cryptography differs from classical ciphersThe building blocks of bit-oriented encryptionHow DES, 3DES, and AES work at a high levelWhy block cipher modes of operation are necessaryIntroduction This lesson provides a structured overview of modern cryptographic techniques, focusing on how today’s encryption systems operate at the bit level, how complex standards like DES and AES are constructed, and how modes of operation securely apply block ciphers to real-world data. Foundational Concepts of Modern Ciphers Modern cryptography is bit-oriented, meaning it works directly on bits rather than characters. This allows encryption of all digital data types, including text, audio, images, and video. Basic Cipher Components Complex modern ciphers are built by combining several simple operations:XOR (Exclusive OR) CipherPerforms a bitwise XOR between data and a keySimple but essential for mixing key material with dataRotation CipherRotates bits left or right with wraparoundHelps spread bit influence across the dataSubstitution Ciphers (S-Boxes)Replace input bits with output bits using lookup tablesVariants include:Equal size substitution (n = m)Expansion (n < m)Compression (n > m)Transposition / Permutation Ciphers (P-Boxes or T-Boxes)Reorder bits based on fixed permutation patternsCan preserve size or perform expansion/reductionIncrease diffusion by spreading bit changesRound Cipher Structure Most modern block ciphers use a round-based design:Encryption is performed over multiple roundsEach round applies substitution, permutation, and XOREach round uses a different subkey derived from a master keySecurity increases with the number and complexity of roundsKey Encryption Standards Data Encryption Standard (DES)Early U.S. encryption standardOperates on 64-bit blocksUses a 56-bit key (stored as 64 bits)Consists of 16 roundsDES Round Function Each round includes:Splitting input into two 32-bit halvesExpansion P-box: 32 → 48 bitsXOR with a 48-bit round keyS-boxes: 48 → 32 bitsStraight permutationFeistel structure swaps halves each roundTriple DES (3DES)Designed to improve DES securityApplies DES three times in an Encrypt–Decrypt–Encrypt sequenceKey options:Two-key version: 112-bit securityThree-key version: 168-bit securityMore secure than DES, but slower and largely deprecatedAdvanced Encryption Standard (AES)Current global encryption standardReplaced DES and 3DESOperates on 128-bit blocksSupports three key sizes:128-bit192-bit256-bitMore rounds are used as key size increasesDesigned for high security and high performanceModes of Operation for Block Ciphers Block ciphers encrypt fixed-size blocks, but real data streams require modes of operation to handle multiple blocks securely. 1. Electronic Code Book (ECB)Each block encrypted independentlyIdentical plaintext blocks → identical ciphertext blocksLeaks patterns and is insecureNot recommended for real-world use2. Cipher Block Chaining (CBC)Each plaintext block is XORed with the previous ciphertextEliminates repeated ciphertext patternsRequires an Initialization Vector (IV)Suffers from error propagation across blocks3. Cipher Feedback (CFB)Converts block cipher into a stream-like cipherSupports encrypting smaller data units (R bits)Uses a shift register with feedback from ciphertextError propagation affects subsequent blocks4. Output Feedback (OFB)Similar to CFB but feeds back encrypted output instead of ciphertextEncryption stream is independent of ciphertextNo error propagationRequires careful IV synchronizationInitialization Vector (IV)Required for CBC, CFB, and OFB modesEnsures uniqueness of the first encryption blockMust be agreed upon by sender and receiverPrevents pattern reuse across messagesKey TakeawaysModern encryption operates at the bit levelStrong ciphers are built from simple operations combined over many roundsDES introduced round-based block encryption but is no longer secure3DES improved security but is inefficientAES is the modern standard due to strength and performanceModes of operation are essential for securely encrypting large or streaming dataECB is insecure, while CBC, CFB, and OFB address pattern leakage in different waysYou can listen and download our episodes for free on more than 10 different platforms:https://linktr.ee/cybercode_academy