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Cryptography 1

Cryptography 1. Cyber Attacks Cryptography Terminology Secret-Key Encryption. Reading Assignment. Reading assignments for this lecture Required: Pfleeger: Ch 2 Recommended: C. Dupuis, A Short History of Cryptography, http://jproc.ca/crypto/crypto_hist.html

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Cryptography 1

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  1. Cryptography 1 Cyber Attacks Cryptography Terminology Secret-Key Encryption

  2. Reading Assignment • Reading assignments for this lecture Required: • Pfleeger: Ch 2 Recommended: • C. Dupuis, A Short History of Cryptography, http://jproc.ca/crypto/crypto_hist.html • Navajo Code Talkers: World War II Fact Sheet, http://www.historynet.com/world-war-ii-navajo-code-talkers.htm CSCE 522 - Farkas

  3. Snooper Insecure channel Recipient Sender Insecure communications Confidential CSCE 522 - Farkas

  4. Cryptographic Protocols • Messages should be transmitted to destination • Only the recipient should see it • Only the recipient should get it • Proof of the sender’s identity • Message shouldn’t be corrupted in transit • Message should be sent/received once only CSCE 522 - Farkas

  5. Terminology • Plaintext (cleartext): a message in its original form • Ciphertext (cyphertext): an encrypted message • Encryption: transformation of a message to hide its meaning • Cipher: cryptographic algorithm. A mathematical function used for encryption (encryption algorithm) and decryption (decryption algorithm). CSCE 522 - Farkas

  6. Terminology • Decryption: recovering meaning from ciphertext • Cryptography: art and science of keeping messages secure • Cryptanalysis: art and science of breaking ciphertext • Cryptology: study of both cryptography and cryptanalysis CSCE 522 - Farkas

  7. Encryption and Decryption Plaintext Plaintext Ciphertext Encryption Additional requirements: • Authentication • Between communicating parties • Third-party authentication • Non-repudiation • Integrity verification • Key distribution • Secret key (secure distribution) • Public key (reliable distribution) Decryption CSCE 522 - Farkas

  8. Conventional (Secret Key) Cryptosystem Plaintext Ciphertext Plaintext Encryption Decryption Sender Recipient K C=E(K,M) M=D(K,C) K needs secure channel CSCE 522 - Farkas

  9. Public Key Cryptosystem Recipient’s public Key (Kpub) Recipient’s private Key (Kpriv) Plaintext Ciphertext Plaintext Encryption Decryption Sender Recipient C=E(Kpub,M) M=D(Kpriv,C) Kpubneeds reliable channel CSCE 522 - Farkas

  10. Security Objectives Question 1: How can cryptography support these objectives? Confidentiality Integrity Availability Authenticity Non-repudiation CSCE 522 - Farkas

  11. Cryptography and Security Objectives CSCE 522 - Farkas

  12. Security Objectives • Confidentiality: Hiding message/file content • Secret key, public key encryption • Integrity: Detecting modification • Hash function • Availability: Not much – hiding existence of data • Secret key, public key encryption • Authenticity: Verify origin • Public key encryption • Non-repudiation: Verify activity • Public key encryption CSCE 522 - Farkas CSCE 522 - Farkas 12

  13. Cryptanalysis Cryptanalyst’s goal: • Break message • Break key • Break algorithm CSCE 522 - Farkas

  14. Taxonomy of Attacks • Ciphertext-only attack: attacker has ciphertext for messages encrypted with K. Deduce keys and/or plaintext messages. • Known plaintext attack: attacker additionally knows the plaintext of the messages. Deduce keys or a decryption algorithm. • Chosen plaintext attack: attacker can obtain the ciphertext for selected plaintext messages. Deduce as above. • Chosen ciphertext attack: attacker can obtain decrypted (plaintext) versions of selected ciphertext. Deduce as above. CSCE 522 - Farkas

  15. Breakable versus Practically breakable • Unconditionally secure: impossible to decrypt. No amount of ciphertext will enable a cryptanalyst to obtain the plaintext • Computationally secure: an algorithm that is not breakable in practice based on worst case scenario • Breakable: all algorithms (except one-time pad) are theoretically breakable CSCE 522 - Farkas

  16. What makes a good cryptosystem? • A good cryptosystem is one whose security does not depend upon the secrecy of the algorithm. • From Bruce Schneier: • “Good cryptographers rely on peer review to separate the good algorithms from the bad.'' CSCE 522 - Farkas

  17. Secret Key Cryptosystem Plaintext Ciphertext Plaintext Encryption Decryption Sender Recipient K C=E(K,M) M=D(K,C) K needs secure channel CSCE 522 - Farkas

  18. Secret Key Cryptosystem Vulnerabilities (1 Passive Attacker (Eavesdropper) • Obtain and/or guess key and cryptosystem use these to decrypt messages • Capture text in transit and try a ciphertext-only attack to obtain plaintext. CSCE 522 - Farkas

  19. Secret Key Cryptosystem Vulnerabilities Active Attacker • Break communication channel (denial of service) • Obtain and/or guess key and cryptosystem and use these to send fake messages CSCE 522 - Farkas

  20. Inherent Weaknesses of Symmetric Cryptography • Key distribution must be done secretly (difficult when parties are geographically distant, or don't know each other) • Need a key for each pair of users • n users need n*(n-1)/2 keys • If the secret key (and cryptosystem) is compromised, the adversary will be able to decrypt all traffic and produce fake messages CSCE 522 - Farkas

  21. Basic Encryption Techniques • Substitution • Permutation • Combinations and iterations of these CSCE 522 - Farkas

  22. Simple Alphabetic Substitution • Assign a new symbol to each plain text symbol randomly or by key, e.g., C k, A h, B  l M=CAB C =k h l • Advantages: large key space 26! • Disadvantages: trivially broken for known plaintext attack, repeated pattern, letter frequency distributionsunchanged CSCE 522 - Farkas

  23. Question 2: Does multiple substitutions increase security? Yes, because the attacker must decrypt the cypher text twice No, because it is equivalent to a single substitution Maybe, depending on the complexity of each substitution CSCE 522 - Farkas

  24. Polyalphabetic Substitution • Frequency distribution: reflects the distribution of the underlying alphabet  cryptanalysts find substitutions • E.g., English: e – 14 %, t – 9.85%, a – 7.49%, o- 7.37%, … • Need: flatten the distribution • E.g., combine high and low distributions: t  a (odd position), b (even position) x  a (even position) , b (odd position) CSCE 522 - Farkas

  25. Cryptanalysis of Polyalphabetic Substitution • Determine the number of alphabets used • Solve each piece as monoalphabetic substitution. Kasiski Method: • Uses regularity of English: letters, letter groupings, full words • e.g., endings: -th, -ing, -ed, -ion, -ation, -tion,… beginnings: im-, in-, re-, un-, ... patterns: -eek-, -oot-, -our-, … words: of, end, to, with, are, is, … CSCE 522 - Farkas

  26. One-Time Pad • Perfect Secrecy! • Large, non-repeating set of keys • Key is larger than the message • Advantages: immune to most attacks • Disadvantages: • Need total synchronization • Need very long, non-repeating key • Key cannot be reused • Key management: printing, storing, accounting for CSCE 522 - Farkas

  27. Question 3: Recommend a practical approach for generating a large key … Discussion topic… CSCE 522 - Farkas

  28. Summary of Substitution • Advantages: • Simple • Easy to encrypt • Disadvantages: • Easy to break!!! CSCE 522 - Farkas CSCE 522 - Farkas 28

  29. Transposition • Letters of the message are rearranged • Break patterns, e.g., columnar transposition Plaintext: this is a test t h i s i s a t tiehssiatst! e s t ! • Advantages: easy to implement • Disadvantages: • Trivially broken for known plaintext attack • Easily broken for cipher only attack CSCE 522 - Farkas

  30. Cryptanalysis • Rearrange the letters • Digrams, Trigrams, Patterns • Frequent digrams: -re-, -th-, -en-, -ed-, … • Cryptanalysis: • Compute letter frequencies  subst. or perm. • Compare strings of ciphertext to find reasonable patterns (e.g., digrams) • Find digram frequencies CSCE 522 - Farkas

  31. Double Transposition • Two columnar transposition with different number of columns • First transposition: breaks up adjacent letters • Second transposition.: breaks up short patterns CSCE 522 - Farkas

  32. Product Ciphers One encryption applied to the result of the other En(En-1(…(E1(M)))), e.g., • Double transposition • Substitution followed by permutation, followed by substitution, followed by permutation… • Broken for • Chosen plaintext CSCE 522 - Farkas

  33. Shannon’s Characteristics of “Good” Ciphers The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption The set of keys and the enciphering algorithm should be free from complexity The implementation of the process should be simple and possible CSCE 522 - Farkas

  34. Shannon’s Characteristics of “Good” Ciphers (cont.) Errors in ciphering should not propagate and cause corruption of further information in the message The size of the enciphered text should be no larger than the original message CSCE 522 - Farkas

  35. Trustworthy Encryption Systems Based on sound mathematics Has been analyzed by experts Has stood the test of time Examples: Data Encryption Standard (DES), Advanced Encryption Standard (AES), River-Shamir-Adelman (RSA) CSCE 522 - Farkas

  36. Stream Ciphers • Convert one symbol of plain text into a symbol of ciphertext based on the symbol (plain), key, and algorithm • Advantages: • Speed of transformation • Low error propagation • Disadvantages: • Low diffusion • Vulnerable to malicious insertion and modification CSCE 522 - Farkas

  37. Block Ciphers • Encrypt a group of plaintext as one block and produces a block of ciphertext • Advantages: • Diffusion • Immunity to insertions • Disadvantages: • Slowness of encryption • Error propagation CSCE 522 - Farkas

  38. Secret Key Cryptosystem Vulnerabilities (1) Passive Attacker (Eavesdropper) • Obtain and/or guess key and cryptosystem use these to decrypt messages • Capture text in transit and try a ciphertext-only attack to obtain plaintext. CSCE 522 - Farkas

  39. Secret Key Cryptosystem Vulnerabilities (2) Active Attacker • Break communication channel (denial of service) • Obtain and/or guess key and cryptosystem and use these to send fake messages • No third party authentication CSCE 522 - Farkas

  40. Inherent Weaknesses of Symmetric Cryptography • Key distribution must be done secretly (difficult when parties are geographically distant, or don't know each other) • Need a key for each pair of users • n users need n*(n-1)/2 keys • If the secret key (and cryptosystem) is compromised, the adversary will be able to decrypt all traffic and produce fake messages CSCE 522 - Farkas

  41. Data Encryption Standards DES CSCE 522 - Farkas

  42. Background and History • Developed by the U.S. government • Intended for general public • 1970s: NBS (National Bureau of Standards) — now named NIST (National Institute of Standards and Technology) — need for standard for encrypting unclassified, sensitive information • 1974: IBM’s candidate: Lucifer • November 1976 : DES was approved as a federal standard in CSCE 522 - Farkas

  43. DES Versions • Jan. 15, 1977: DES was published as FIPS PUB 46 (Federal Information Processing Standard), authorized for use on all unclassified data • 1988 (revised as FIPS-46-1) and 1993 (FIPS-46-2): DES is reaffirmed • Jan. 1999: DES key is broken in 22 hours and 15 minutes • 1999 (FIPS-46-3): DES, containing Triple DES, is reaffirmed • Nov. 26, 2001: The Advanced Encryption Standard (AES) is published in FIPS 197 • May 26, 2002: The AES standard becomes effective • May 19, 2005: FIPS 46-3 was officially withdrawn but Triple DES is approved by NIST until 2030 for sensitive government information CSCE 522 - Farkas

  44. Data Encryption Standard • Mathematics to design strong product ciphers is classified • Breakable by exhaustive search on 56-bit key size for known plaintext, chosen plaintext and chosen ciphertext attacks • Security: computational complexity of computing the key under the above scenarios (22 hours) CSCE 522 - Farkas

  45. Data Encryption Standard • DES is a product cipher • 56 bit key size • 64 bit block size for plaintext and cipher text • Developed by IBM and adopted by NIST with NSA approval • Encryption and decryption algorithms are public but the design principles are classified CSCE 522 - Farkas

  46. DES Controversies • Key size 56 bits – threshold of allowing exhaustive-search known plaintext attack • Built in trapdoor – allegations The US Senate Select Committee of Intelligence exonerated NSA from tampering with the design of DES in any way CSCE 522 - Farkas

  47. DES Multiple Encryption • 1992: proven that DES is not a group: multiple encryptions by DES are not equivalent to a single encryption CSCE 522 - Farkas

  48. DES Multiple Encryption Double DES P EK1(P) EK2[EK1(P)] Intermediate Ciphertext Ciphertext Plaintext Encryption Encryption K1 K2 Known-plaintext: meet-in-the-middle attack Effective key size: 57 bit -- Why not 112? CSCE 522 - Farkas

  49. DES Multiple Encryption Triple DES P EK1(P) DK2[EK1(P)] EK3[DK2[EK1(P)]] E E D K1 K2 K3 Tuchman: avoid meet-in-the-middle attack If K1=K2: single encryption CSCE 522 - Farkas

  50. Triple DES • Tuchman’s technique is part of NIST standard • Can be broken in 2^56 operations if one has 2^56 chosen plaintext blocks (Merkle, Hellman 1981) • Could use distinct K1,K2,K3 to avoid this attack -- 2^112 bit key CSCE 522 - Farkas

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