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EEC 688/788 Secure and Dependable Computing

Learn about the basics of cryptography, including encryption and decryption, symmetric and asymmetric encryption, and cryptanalysis. Explore different encryption methods such as substitution and transposition ciphers.

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EEC 688/788 Secure and Dependable Computing

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  1. EEC 688/788Secure and Dependable Computing Lecture 3 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org

  2. Outline Introduction to cryptography Terminology Basic encryption methods One time pad Symmetric-key algorithms DES, AES, etc EEC688: Secure & Dependable Computing

  3. Cryptography Terminology Encryption is the process of encoding a message so that its meaning is not obvious Equivalent terms: encode, encipher Decryption is the reverse process, transforming an encrypted message back into its normal, original form Equivalent terms: decode, decipher Plaintext: message to be encrypted Ciphertext: encrypted message EEC688: Secure & Dependable Computing

  4. Cryptography Terminology The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext Why encryption? It addresses the need for confidentiality of data, also helps to ensure integrity It forms the basis of protocols that enableus to provide security while accomplishing system or network tasks EEC688: Secure & Dependable Computing

  5. Cryptography Terminology The encryption and decryption rules are called encryption and decryptionalgorithms Encryption/decryptions algorithms often use a device called a key, denoted by K, so that the resulting ciphertext depends on the original plaintext message, the algorithm, and the key value An encryption scheme that does not require the use of a key is called a keyless cipher EEC688: Secure & Dependable Computing

  6. Symmetric Encryption The encryption and decryption keys are the same, so P = D(K, E(K,P)) D and E are closely related. They are mirror-image processes The symmetric systems provide a two-way channel to their users The symmetry of this situation is a major advantage of this type of encryption, but it also leads to a problem: key distribution EEC688: Secure & Dependable Computing

  7. Symmetric Encryption DK(EK(P)) = P EEC688: Secure & Dependable Computing

  8. Asymmetric Encryption Encryption and decryption keys come in pairs. The decryption key, KD, inverts the encryption of key KE, so that P = D(KD, E(KE,P)) Asymmetric encryption systems excel at key management EEC688: Secure & Dependable Computing

  9. Cryptology Cryptologyis the research into and study of encryption and decryption; it includes both cryptography and cryptanalysis Cryptography– art of devising ciphers Comes from Greek words for“secret writing”. It refers to the practice of using encryption to conceal text Cryptanalysis– art of breaking ciphers Study of encryption and encrypted messages, hoping to find the hidden meanings EEC688: Secure & Dependable Computing

  10. Cryptanalysis Attempt to break a single message Attempt to recognize patterns in encrypted messages, to be able to break subsequent ones Attempt to deduce the key, in order to break subsequent messages easily Attempt to find weaknesses in the implementation or environment of use of encryption Attempt to find general weaknesses in an encryption algorithm EEC688: Secure & Dependable Computing

  11. Cryptanalysis Traffic analysis: attempt to infer some meaning without even breaking the encryption, e.g., Noticing an unusual frequency of communication Determining something by whether the communication was short or long EEC688: Secure & Dependable Computing

  12. Cryptanalysis –Breaking Encryption Schemes Ciphertext-only: cryptanalyst has a quantity of ciphertext and no plaintext Known plaintext: cryptanalyst has some matched ciphertext and plaintext Chosen plaintext: cryptanalyst has the ability to encrypt pieces of plaintext of his own choosing 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  13. Basic Encryption Methods Substitution ciphers: one letter is exchanged for another Transposition ciphers: order of letters is rearranged EEC688: Secure & Dependable Computing

  14. Substitution Ciphers Idea: each letter or group of letters is replaced by another letter or group of letters Caesar cipher – circularly shift by 3 letters a -> D, b -> E, … z -> C More generally, shift by k letters, k is the key Monoalphabetic cipher – map each letter to some other letter A b c d e f … w x y z Q W E R T Y … V B N M <= the key EEC688: Secure & Dependable Computing

  15. Cryptanalysis of Substitution Ciphers Brute force cryptanalysis would have to try 26! permutations of a particular ciphertext message Smarter way: use frequencies of letters, pairs of letter etc., or by guessing a probable word or phrase. Most frequently occurred Letters: e, t, o, a, n, … Digrams: th, in, er, re, an, … Trigrams: the, ing, and, ion, ent Words: the, of, and, to, a, in, that, … When messages are long enough, the frequency distribution analysis quickly betrays many of the letters of the plaintext EEC688: Secure & Dependable Computing

  16. Transposition Ciphers Substitution cipher – preserves order of plaintext symbols but disguises them Transposition cipher – reorders (rearrange) symbols but does not disguise them. It is also called permutation With transposition, the cryptography aims for Widely spreading the information from the message or the key across the ciphertext Transpositions try to break established patterns EEC688: Secure & Dependable Computing

  17. Columnar Transposition Plaintext written in rows, number of columns = key length Key is used to number the columns Ciphertext read out by columns, starting with column whose key letter is lowest EEC688: Secure & Dependable Computing

  18. Columnar Transposition A transposition cipher example EEC688: Secure & Dependable Computing

  19. One-Time Pads One-time pad: construct an unbreakable cipher Choose a random bit string as the key Convert the plaintext into a bit string Compute the XOR of these two strings, bit by bit The resulting ciphertext cannot be broken, because in a sufficiently large sample of ciphertext, each letter will occur equally often, as will every digram, every trigram, and so on => There is simply no information in the message because all possible plaintexts of the given length are equally likely 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  20. The Vernam Cipher The Vernam Cipher is a type of one-time pad devised by Gilbert Vernam for AT&T 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  21. The Vernam Cipher The encryption involves an arbitrarily long nonrepeating sequence of numbers that are combined with the plaintext Assume that the alphabetic letters correspond to their counterparts in arithmetic notation mod 26 That is, the letters are represented with numbers 0 through 25 To use the Vernam cipher, we sum this numerical representation with a stream of random two-digit numbers 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  22. The Vernam Cipher - Example 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  23. The Vernam Cipher - Observations The repeated letter t comes from different plaintext letters Duplicate ciphertext letters are generally unrelated when this encryption algorithm is used =>there is no information in the message to be exploited 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  24. The Vernam Cipher - Decryption To decrypt: (Ci – Ki) mod 26 Note on rules of mod on negative number: “The mod function is defined as the amount by which a number exceeds the largest integer multiple of the divisor that is not greater than that number” (http://mathforum.org/library/drmath/view/52343.html) Modula op always return non-negative number E.g., (19-76) mod 26 = (-57) mod 26 = (-78+21) mod 26 = 21 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  25. The Vernam Cipher - Decryption 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  26. One-Time Pads Disadvantages The key cannot be memorized, both sender and receiver must carry a written copy with them Total amount of data can be transmitted is limited by the amount of key available Sensitive to lost or inserted characters 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  27. Symmetric-Key Algorithms DES – The Data Encryption Standard AES – The Advanced Encryption Standard Other Ciphers Cipher Modes 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  28. Data Encryption Standard Developed by IBM. US standard for unclassified info(1977) Same key for encryption as for decryption Encrypts in 64-bit blocks Uses 56-bit key Has 19 stages, 16 parameterized by different functions of the key 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  29. Triple DES Triple DES – effectively increases the key length. It uses two keys and three stages In first stage, the plaintext is encrypted using DES in the usual way with K1 In second stage, DES is run in decryption mode, using K2 as the key In third stage, another DES encryption is done with K1 Triple DES encryption Triple DES decryption 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  30. AES – The Advanced Encryption Standard AES is a result of a cryptographic contest Organized by NIST in 1997 Rules for AES proposals The algorithm must be a symmetric block cipher The full design must be public Key lengths of 128, 192, and 256 bits supported Both software and hardware implementations required The algorithm must be public or licensed on nondiscriminatory terms Winner: Rijndael (from two Belgian cryptographers: Joan Daemen and Vincent Rijmen) 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

  31. Other Symmetric-Key Ciphers 1/1/2020 EEC688: Secure & Dependable Computing Wenbing Zhao

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