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CS457 – Introduction to Information Systems Security Cryptography 1a

CS457 – Introduction to Information Systems Security Cryptography 1a. Elias Athanasopoulos elathan@ics.forth.gr. Cryptography Elements. Symmetric Encryption Block Ciphers Stream Ciphers Asymmetric Encryption Cryptographic Hash Functions Applications.

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CS457 – Introduction to Information Systems Security Cryptography 1a

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

  2. Cryptography Elements • Symmetric Encryption • Block Ciphers • Stream Ciphers • Asymmetric Encryption • Cryptographic Hash Functions • Applications Elias Athanasopoulos

  3. 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). ” Elias Athanasopoulos

  4. CIA • Confidentiality • Dataconfidentiality: 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. • Integrity • 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. • Availability • Assures that systems work promptly and service is not denied to authorized users. Elias Athanasopoulos

  5. An Example Confidentiality: Bad guys cannot see messages Availability: The system is operational System/Service Communication Integrity: Bad guys cannot change messages Elias Athanasopoulos

  6. 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 Elias Athanasopoulos

  7. The need to remain secret Elias Athanasopoulos

  8. 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. Elias Athanasopoulos

  9. 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 ciphertextwithout knowing the key Elias Athanasopoulos

  10. Symmetric Encryption Elias Athanasopoulos

  11. Caesar Cipher Elias Athanasopoulos

  12. Key: 3 C = E(3, p) = (p + 3) mod 26 Elias Athanasopoulos

  13. Or… Plain: abcdefghijklmnopqrstuvwxyz Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC Elias Athanasopoulos

  14. Encrypting a message Plain: meet me after the toga party Cipher: PHHW PH DIWHU WKH WRJD SDUWB Elias Athanasopoulos

  15. Simple Brute Force attack: we need to try 25 different keys. Is this secure? Elias Athanasopoulos

  16. 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! Elias Athanasopoulos

  17. Frequency Analysis Is this secure? Elias Athanasopoulos


  19. Count Frequencies Elias Athanasopoulos

  20. Compare with English Elias Athanasopoulos

  21. And try… Elias Athanasopoulos

  22. Voila! Elias Athanasopoulos

  23. One-Time Pad Ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key:pxlmvmsydofuyrvzwctnlebnecvgdupahfzzlmnyih plaintext:mrmustard with the candlestick in the hall Ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key:pxlmvmsydofuyrvzwctnlebnecvgdupahfzzlmnyih plaintext:miss scarlet with the knife in the library Key = Size of message Elias Athanasopoulos

  24. Transposition Elias Athanasopoulos

  25. Modern Symmetric Ciphers • DES, 3DES, and AES • AES is the dominant one, today • Based on • Substitutions and transpositions • Very complex • Type • Block • Stream Elias Athanasopoulos

  26. 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. Elias Athanasopoulos

  27. Block Cipher Elias Athanasopoulos

  28. Stream Cipher Elias Athanasopoulos

  29. Block cipher • Plaintext ofn bits produces a ciphertext of n bits • Block size: n bits • Space of different plaintext blocks: 2^n • Each block must be unique Elias Athanasopoulos

  30. Encryption should be reversible Elias Athanasopoulos

  31. General n-bit-n-bit Block (n = 4) Elias Athanasopoulos

  32. Decryption/Encryption Elias Athanasopoulos

  33. 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 Elias Athanasopoulos

  34. 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 Elias Athanasopoulos

  35. Feistel Cipher Substitution: right part is transformed by F(Ki) and XORed with left part. Permutation: right part swapped with left part. Elias Athanasopoulos

  36. 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. Elias Athanasopoulos

  37. 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. Elias Athanasopoulos

  38. 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. Elias Athanasopoulos

  39. Block modes Elias Athanasopoulos

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