Introduction to Cybersecurity: Cryptography

Introduction to Cybersecurity

• Cryptography involves converting data into a scrambled code, which is then decrypted for secure transmission across networks.
• Cryptography safeguards confidential information like emails, web transactions, and corporate data.

Objectives of Cryptography

• Confidentiality: Ensuring data is accessible only to authorized parties.
• Integrity: Maintaining the accuracy and completeness of data.
• Authentication: Verifying the identity of users and devices.
• Non-repudiation: Preventing senders from denying their actions.

Types of Cryptography

• Symmetric encryption uses the same key for both encryption and decryption.
• Asymmetric encryption employs different keys for encryption and decryption.

Symmetric Encryption

• Symmetric encryption, also known as secret-key, shared-key, or private-key encryption, uses an identical key for both encryption and decryption processes.

Asymmetric Encryption

• Asymmetric encryption, also known as public-key encryption, utilizes distinct keys for encryption and decryption, referred to as public and private keys.

Ciphers

• Ciphers are algorithms that encrypt or decrypt data.
• Classical ciphers involve substitution and transposition.
• Modern ciphers are categorized by the type of key used (private or public) and the type of input data (block or stream).

Classical Ciphers

• Classical ciphers include substitution ciphers and transposition ciphers.
• Substitution ciphers replace blocks of plaintext with ciphertext.
• Transposition ciphers shift plaintext letters to create a cryptogram.

Substitution Cipher

• In cryptography, a substitution cipher is a method of encrypting by which units of plaintext are replaced with ciphertext, according to a fixed system.
• The "units" in substitution ciphers can be single letters, pairs, triplets, or mixtures thereof.
• Decryption involves inverse substitution by the receiver.

Monoalphabetic Cipher (Caesar Cipher Improvement)

• Julius Caesar used a simple substitution system over 2,000 years ago for secure troop communication.
• This involved substituting letters by shifting them a predetermined number of positions.
• Caesar Cipher has 25 letter pairings.
• Monoalphabetic Cipher has 26! or 10^26 possible pairings.

Breaking Monoalphabetic Substitution Cipher

• Letter frequencies in ciphertext can be analyzed to decipher monoalphabetic substitution ciphers.
• Bigram and trigram frequency analysis can be used to break monoalphabetic substitution ciphers.

Ciphertext Only Scenario

• In ciphertext-only scenarios, the only available information is the ciphertext.
• Approaches to break include brute-force and statistical analysis.
• This scenario is difficult to break and easy to defend against, but weaker algorithms can be vulnerable.

Known Plaintext Scenario

• Analysts capture plaintext messages and their corresponding encryptions.
• Patterns, like standardized headers or placement of keywords, can help deduce the key.
• Sophisticated algorithms can withstand known plaintext attacks.

Chosen Plaintext Scenario

• An analyst inserts a chosen message into the system.
• Deliberate patterns are picked to reveal the structure of the key.
• Trudy might prompt Alice to send a specific message, like "The quick brown fox jumps over the lazy dog" to completely break the encryption scheme.

Polyalphabetic Cipher

• Employs 'n' substitution ciphers (M1, M2, ..., Mn) mapping letters variably based on their position.
• Cycling pattern use subsequent substitution patterns in a cyclic manner.
• For example, with n=4 pattern could be M1,M3,M4, M3,M2.

Transposition Cipher

• This method of encryption shifts the positions of plaintext units (characters or groups) based on a consistent system.
• The ciphertext is a reordered version of the plaintext.

Columnar Transposition Cipher

• The message is written in rows of fixed length and read out column by column in a scrambled order.
• With the keyword ZEBRAS (length 6), the order of columns is defined by alphabetical order in the keyword

What is Cryptography?

• Cryptography's core involves mathematical encryption and decryption techniques.
• Its purpose is secure communication, even when adversaries are present.
• It ensures confidentiality, authenticity, and integrity.

What is Cryptanalysis?

• In cryptanalysis, ciphertext, ciphers, and cryptosystems are analyzed to understand their functionality.
• Cryptanalysis aims to find ways to defeat or weaken these systems.

Simplified Model (Symmetric Cryptosystem)

• In a symmetric cryptosystem, plaintext is input and encrypted using a secret key shared by the sender and the receiver.
• The ciphertext is transmitted, then decrypted using the same secret key to produce the plaintext output.

Symmetric Encryption Types

• Block ciphers process plaintext in fixed-size blocks.

Block Ciphers

• Processes plaintext input in fixed-size blocks.
• Generates an equal-sized ciphertext block for each plaintext block.
• Have key size, use a number of rounds and algorithms for subkey generation and round function.
• Mapping is permutation of all possible inputs.

Feistel Structure

• Subkeys 1<K; < n is different from and derived from original key.
• Round function the same structure for each round.
• Used by all block ciphers.

Data Encryption Standard (DES)

• DES used a 64-bit plaintext block and a 56-bit key.
• DES is minor variation of Feistel network.
• Concerns exist regarding algorithm strength and the use of a 56-bit key.
• In July 1998, the Electronic Frontier Foundation (EFF) announced it had broken DES encryption.

Triple DES (3DES)

• Repeats the DES algorithm three times using either two or three unique keys.
• A 168-bit key length overcomes DES's vulnerability to brute-force attacks.
• The underlying encryption algorithm is the same as DES.
• Slower in software and uses a 64-bit block size.

Advanced Encryption Standard (AES)

• Was needed as a replacement for 3DES.
• NIST called for proposals for a new AES in 1997 that.
• Rijndael in 1997 should have a security strength equal to or better than 3DES.
• Must increase significantly improved efficiency and use Symmetric block cipher.
• Symmetric block cipher with 128 bit data and 128/192/256 bit keys.

AES Steps

• AES steps include Add round key, Substitute bytes, Shift rows and Mix columns.

AES

• Substitute bytes transformation uses constructed properties of finite-fields.

Key Size and Brute Force

• Time required to break algorithms, DES 56 bits, 1 hour compared to AES with 256 is orders of magnitudes greater.

AES Other Finalists

• Other AES finalists include, Serpent, Twofish, RC6 and MARS.

Stream Ciphers

• Processes input elements continuously.
• Stream ciphers produce output one element at a time.
• Typically, encrypts plaintext 1 byte at a time (but may be designed to operate on one bit)
• Pseudorandom stream that is unpredictable without knowledge of the input key and appears random.

Encryption decription stream

• Plaintext and key stream creates ciphertext throught encryption.
• Ciphertext and key stream creates plaintext through encryption.

Stream Cipher

• Stream ciphers can be as secure as block ciphers with comparable key length (128-bit).
• Faster and use less code, so can be used in browser over the web.
• But is vulnerable to cryptanalysis if the same key is used.
• Better alternative for encrypting a stream of data over communication channels, browser/Web-link.

RC4

Speed comparisons

• The fastest cipers on Pentiums are RC4, and then AES.
• The slowest are 3DES and DES.

Electronic Codebook (ECB)

• ECB is the simplest mode, handling plaintext 'b' bits at a time and encrypting each block with the same key.
• Have unique ciphertext value for each plaintext block, creating a "codebook".
• Not secure for long messages since repeated plainttext can be seen.
• Can be used for the secure transmission of a single values.
• It's necessary to repeatedly produce different ciphertext blocks from the same plaintext block.

Cipher Block Chaining (CBC)

• The main purpose of IV is to ensure that even if two plaintexts are identical, their ciphertexts are still different, because different IVs will be used.
• IV known to sender and receiver in plaintext.
• IV known be well protected as with the key.
• Decryption can be parallized.
• Encryption cannont be parallized
• Is Useful for general purpose block-oriented transmission and authentication.

Cipher Feedback (CFB)

• Input processed s-bits (s=8) at time.
• Any block cipher can be turned inco stream cipher.
• Ideal for encryping real-time data.
• Padding not required for the last block.
• Decryption can be parallized, encryption will be sequential.

Padding

• Block cipher encryption modes divide plaintext, and are used to make the last part conform.
• Last block of the plaintext needs padding i.e. before encryption, to ensure the original cypher can be decrypted.
• The padding schemes need to clearly mark where the padding starts so that the decryption works.

Output Feedback (OFB)

• Similar to CFB, but operates on blocks rather than s-bit subset.
• Stream oriented transmisison over a noisy chanel.
• IV must be nonce i.e. unique to each execution of encryption operation
• IV should not repeat.
• IV should nor be predictable.

Counter Mode (CTR)

• A counter is used to generate key streams.
• No key stream can be reused, so the counter value for each is preprended with a nonce.
• Nonce serves that same role as IV's.
• Used in protocols such as, Asynchronous Transfer Mode and IPSec and Secure Real-Time Transport Protocol(VoIP).
• Both encryption and decryption can be parallized.

Symmetric Encryption Devices Location of encryption

• Link encryption.
• End-to-end encryption.

Computationally secure requirements

• Strong encryption algorithm.
• Sender and receiver must have copies of key.

Key Distribution

• Key distribution can work by:
  • Key selected by one party and transferred to another party
  • Third party delivers key to 2 parties
  • Keys uses for communication or transmission of newly encryped key
  • Third party communication between parties is use to send a secured key

Key design

• Automated keys can be created and distributed between hosts to facilitate key generation and management.