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CS457 – Introduction to Information Systems Security Cryptography 1a Elias Athanasopoulos elathan@ics.forth.gr

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Cryptography Elements SSymmetric Encryption -B-Block Ciphers -S-Stream Ciphers AAsymmetric Encryption CCryptographic Hash Functions AApplications CS-457Elias Athanasopoulos2

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Computer Security “ The protection afforded to an automated information system in order to attain the applicable objectives of preserving the integrity, availability, and confidentiality of information system resources (includes hardware, software, firmware, information/ data, and telecommunications). ” CS-457Elias Athanasopoulos3

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CIA C onfidentiality - Data confidentiality: Assures that private or confidential information is not made available or disclosed to unauthorized individuals. - Privacy: Assures that individuals control or influence what information related to them may be collected and stored and by whom and to whom that information may be disclosed. I ntegrity - Data integrity: Assures that information and programs are changed only in a specified and authorized manner. - System integrity: Assures that a system performs its intended function in an unimpaired manner, free from deliberate or inadvertent unauthorized manipulation of the system. A vailability - Assures that systems work promptly and service is not denied to authorized users. CS-457Elias Athanasopoulos4

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An Example CS-457Elias Athanasopoulos5 Confidentiality: Bad guys cannot see messages Integrity: Bad guys cannot change messages Availability: The system is operational System/Service Communication

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How to achieve CIA? Crypto algorithms - “Hide” messages, sign messages, provide guarantees about sent/received messages, etc. Secure systems - Apply Crypto concepts in protocols, systems, etc. Software defenses - Ensure that software is not manipulated CS-457Elias Athanasopoulos6

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CS-457Elias Athanasopoulos7

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Cryptographic Jargon Plaintext - This is the original intelligible message or data that is fed into the algorithm as input. Encryption algorithm - The encryption algorithm performs various substitutions and transformations on the plaintext. Secret key - The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time. Ciphertext: - This is the scrambled message produced as output. It depends on the plaintext and the secret key. Decryption algorithm - This is essentially the encryption algorithm run in reverse. CS-457Elias Athanasopoulos8

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NOT Security via Obscurity The encryption/decryption algorithm is assumed to be known Security is based on - Secrecy of the key - Hard to infer the plaintext via the ciphertext by just knowing the key Cryptanalysis - Infer the plaintext from ciphertext without knowing the key CS-457Elias Athanasopoulos9

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Symmetric Encryption CS-457Elias Athanasopoulos10

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Caesar Cipher CS-457Elias Athanasopoulos11

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CS-457Elias Athanasopoulos12 Key: 3

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Or… Plain: abcdefghijklmnopqrstuvwxyz Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC CS-457Elias Athanasopoulos13

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Encrypting a message Plain: meet me after the toga party Cipher: PHHW PH DIWHU WKH WRJD SDUWB CS-457Elias Athanasopoulos14

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CS-457Elias Athanasopoulos15 Simple Brute Force attack: we need to try 25 different keys.

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Increase the key space Permutations: {a, b, c} - abc, acb, bac, bca, cab, cba Size: 6 (or 3! ~ 1*2*3) - In general the size is n! (n is the size of the set) For the alphabet: 26! CS-457Elias Athanasopoulos16

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CS-457Elias Athanasopoulos17 Frequency Analysis

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Example UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPES XUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ CS-457Elias Athanasopoulos18

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Count Frequencies CS-457Elias Athanasopoulos19

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Compare with English CS-457Elias Athanasopoulos20

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And try… CS-457Elias Athanasopoulos21

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Voila! CS-457Elias Athanasopoulos22

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One-Time Pad Ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih plaintext: mr mustard with the candlestick in the hall Ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih plaintext: miss scarlet with the knife in the library Key = Size of message CS-457Elias Athanasopoulos23

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Transposition CS-457Elias Athanasopoulos24

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Modern Symmetric Ciphers DES, 3DES, and AES - AES is the dominant one, today Based on - Substitutions and transpositions Very complex Type - Block - Stream CS-457Elias Athanasopoulos25

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Block vs Stream Block cipher - one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. Typically, a block size of 64 or 128 bits is used. Stream cipher - one that encrypts a digital data stream one bit or one byte at a time. CS-457Elias Athanasopoulos26

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Block Cipher CS-457Elias Athanasopoulos27

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Stream Cipher CS-457Elias Athanasopoulos28

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Block cipher Plaintext of n bits produces a ciphertext of n bits - Block size: n bits Space of different plaintext blocks: 2^n - Each block must be unique CS-457Elias Athanasopoulos29

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Encryption should be reversible CS-457Elias Athanasopoulos30

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General n-bit-n-bit Block (n = 4) CS-457Elias Athanasopoulos31

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Decryption/Encryption CS-457Elias Athanasopoulos32

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Problems Vulnerable to statistical attacks - Small blocks can take limited transformations - Increase n Key size: 4 bits * 16 rows - In general: n * 2^n - Approximate the ideal case CS-457Elias Athanasopoulos33

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Feistel Cipher Goal - Approximate the ideal cipher - Reduce statistical properties between plaintext, ciphertext, and key(s) Difussion - Each plaintext digit affect the value of many ciphertext digits Confusion - The statistics of the ciphertext and the value of the encryption key is as complex as possible CS-457Elias Athanasopoulos34

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Feistel Cipher CS-457Elias Athanasopoulos35 Substitution: right part is transformed by F(Ki) and XORed with left part. Permutation: right part swapped with left part.

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Properties Block size: - Larger block sizes mean greater security but reduced encryption/decryption speed for a given algorithm. A block size of 64 bits is reasonable tradeoff. AES uses a 128-bit block size. Key size: - Larger key size means greater security but may decrease encryption/ decryption speed. Key sizes of 64 bits or less are now widely considered to be inadequate, and 128 bits has become a common size. CS-457Elias Athanasopoulos36

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Properties Number of rounds: - The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security. A typical size is 16 rounds. Subkey generation algorithm: - Greater complexity in this algorithm should lead to greater difficulty of cryptanalysis. Round function F: - Again, greater complexity generally means greater resistance to cryptanalysis. CS-457Elias Athanasopoulos37

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Extra (desired) properties Fast software encryption/decryption: - In many cases, encryption is embedded in applications or utility functions in such a way as to preclude a hardware implementation. Ease of analysis: - There is great benefit in making the algorithm easy to analyze. It is easier to analyze that algorithm for cryptanalytic vulnerabilities and therefore develop a higher level of assurance as to its strength. DES, for example, does not have an easily analyzed functionality. CS-457Elias Athanasopoulos38

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Block modes CS-457Elias Athanasopoulos39 ModeDescriptionTypical Application Electronic Codebook (ECB)Each block of 64 plaintext bits is encoded independently using the same key. Secure transmission of single values (e.g., an encryption key) Cipher Block Chaining (CBC) The input to the encryption algorithm is the XOR of the next 64 bits of plaintext and the preceding 64 bits of ciphertext. General-purpose block- oriented transmission Authentication

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