King Fahd University of Petroleum Minerals College of

King Fahd University of Petroleum & Minerals College of Computer Science & Engineering SEC 511 Principles of Information Assurance and Security Lecture 3 Cryptography Overview

Cryptography Definition • Comes from Greek word meaning “secret writing” • It refers to the science of transforming information into a secure form while it is being transmitted or stored so that unauthorized users cannot access it 2

Goals of Cryptography Secure communication 3

Goals of Cryptography Secure storage 4

Services Provided by Cryptography Goal Description Confidentiality Keeping information secret and allow only authorized parties to access it [3]. Authentication Verifying an entity’s identity (e. g: user, computer) [3]. Integrity Ensuring that information is correct and has not been altered by unauthorized person or a malicious software [3]. Non-repudiation Proving that a user performed an action and preventing him/her from denying the performed action (e. g: sender/receiver sent/received a message) [3]. 5

Cryptography Components 6

Terminology • Plaintext: Original unencrypted information. • Ciphertext: The information after being encrypted by an encryption algorithm. • Encryption: Producing ciphertext from plaintext using cryptosystem (also called encipherment). • Decryption: Reverse process of encryption (also called decipherment). • Algorithm: Process of encrypting and decrypting information based on a mathematical procedure. 7

Terminology Key: Value used by an algorithm to encrypt or decrypt a message, and should only be known by sender/receiver. Cipher: Encryption or decryption algorithm tool used to create encrypted or decrypted a text. 8

Categories of Cryptography 9

Symmetric-Key Cryptography 10

Symmetric-Key Cryptography In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared. 11

Modes of operation* Figure 8. 1 Modes of operation 8. 12 * See: Forouzan, Chapter 8 12

Electronic Code Book (ECB) Figure 8. 2 Electronic codebook (ECB) mode 13

Cipher Block Chaining (CBC) Figure 8. 3 Cipher block chaining (CBC) mode 14

Cipher Feedback (CFB) Figure 8. 4 Encryption in cipher feedback (CFB) mode 15

CFB as a Stream Cipher Figure 8. 5 Cipher feedback (CFB) mode as a stream cipher 8. 16 16

Output Feedback (OFB) Each bit in the ciphertext is independent of the previous bit or bits. This avoids error propagation. Figure 8. 6 Encryption in output feedback (OFB) mode 17

OFB as a Stream Cipher Figure 8. 7 Output feedback (OFB) mode as a stream cipher 8. 18 18

Counter (CTR) In the counter (CTR) mode, there is no feedback. The pseudorandomness in the key stream is achieved using a counter. Figure 8. 8 Encryption in counter (CTR) mode 8. 19 19

CTR mode as a stream cipher Figure 8. 9 Counter (CTR) mode as a stream cipher 8. 20 20

Modern Symmetric-key Ciphers • Symmetric-key: A shared secret key is used for both encryption and decryption. • Block Ciphers: • • Each block of data is encrypted at once Examples: DES, AES • Stream Ciphers: • • Small unit of data (bit, byte, character) is encrypted individually. Examples: RC 4, A 5/1 21

Data Encryption Standard (DES) • • • 6. 22 DES: was published by NIST in 1975 Block size: 64 bits Key size: 56 bits Phase-out: NIST adopted 3 DES in 1999 Encryption and decryption with DES: 22

Advanced Encryption Standard (AES) • • AES: adopted by NIST in 2001 to replace 3 DES. Block size: 128 bits Key sizes: 128/192/256 bits AES Security: • More secure than DES and 3 DES due to larger blocks and keys. • Most of the known attacks on DES were already tested on AES. • AES Implementation: • Can be implemented in software, hardware, and firmware. • Implementation can use table lookup process or routines. • Simplicity and Cost: • AES is design to run efficiently on an 8 -bit processor. 6. 23 23

Symmetric Vs Asymmetric Cryptography 24

Asymmetric-Key Cryptography Asymmetric key cryptosystem: • Uses two separate keys: 1. 2. • Public: for encryption (receiver’s public key) Private: for decryption (receiver’s private key). Examples: • RSA • • El. Gamal Rabin 25

Asymmetric-Key Cryptography 26

Keys Used in Crcyptography 27

Kerckhoff’s principle • Based on Kerckhoff’s principle, one should always assume that the adversary knows the encryption/decryption algorithm. The resistance of the cipher to attack must be based only on the secrecy of the key. • Claude Shannon (father of Information Theory) reformulated the principle simply: The enemy knows the system • This to contrast to: Security through obscurity 28

Cryptanalysis As cryptography is the science and art of creating secret codes, cryptanalysis is the science and art of breaking those codes. 29

Cryptanalysis: Ciphertext-Only Attack The attacker has only a ciphertext 30

Cryptanalysis: Known-Plaintext Attack The attacker has access to a plaintext/ciphertext pair, in addition to the intercepted ciphertext. 31

Cryptanalysis: Chosen-Plaintext Attack The attacker chooses the pair plaintext/ciphertext 32

Cryptanalysis: Chosen-Ciphertext Attack The attacker chooses the ciphertext and decrypts it. 33

The Need for Integrity The cryptography systems that we have studied so far provide secrecy, or confidentiality, but not integrity. However, there are occasions where we may not even need secrecy but instead must have integrity. See: Forouzan Chapter 11 34

Document and Fingerprint One way to preserve the integrity of a document is through the use of a fingerprint. If Alice needs to be sure that the contents of her document will not be changed, she can put her fingerprint at the bottom of the document. 35

Message and Message Digest The electronic equivalent of the document and fingerprint pair is the message and digest pair. 36

Checking Integrity 37

Cryptographic Hash Function A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string. The data to be encoded is often called the "message, " and the hash value is sometimes called the message digest or simply digest. The ideal cryptographic hash function has four main or significant properties: 1. it is easy (but message 2. it is infeasible 3. it is infeasible 4. it is infeasible not necessarily quick) to compute the hash value for any given to generate a message that has a given hash to modify a message without changing the hash to find two different messages with the same hash 38

Cryptographic Hash Function 39

Summary How to guarantee each combination of the following: Confidentiality Authentication Integrity Non-Repudiation 40

The end Reading: Forouzan Chapters: 3, 6, 7, 8, 10, and 11