1. ITMS – 3153 Information Systems Security
Chapter 2 – Elementary Cryptography
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2. Coverage Areas Concepts of encryption Cryptanalysis
Symmetric (secret key) Encryption
Asymmetric (public key) Encryption
Key exchange protocols and certificates
Digital Signatures
Cryptographic hash functions
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4. Terminology & Background
Sender (S), Recipient (R), Transmission media (T)
Interceptor / intruder (O) (availability)
O might block message from reaching R
O might intercept message (confidentiality)
O might modify message (integrity)
O might fabricate an authentic-looking message (integrity)
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5. Terminology & Background
Encryption – process of encoding a message
Decryption – transforming encoded message back to normal
Encrypt – encode , encipher
Decrypt – decode, decipher
Cryptosystem – system for encryption and decryption
Plaintext – original form of message
Ciphertext – encoded form of message
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6. Terminology & Background
Algorithms – rules for encryption and decryption
Key – value used to encrypt message
C = E(K, P) where P=plaintext, K = key, E = encryption algorithms, and C = ciphertext
Symmetric encryption P = D(K, E(K,P))
Asymmetric encryption P = D(KD, E(KE,P))
Keyless cipher
Cryptography (hidden writing) – uses encryption to hide message
Cryptanalysis – attempts to find meanings in encrypted messages
Cryptology – study of encryption and decryption
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7. P = D (K, E (K, P)) Symmetric
Sometimes the encryption and decryption keys are the same
P = D (K, E (K, P))
where P=plaintext, K = key, E = encryption algorithms, and C = ciphertext
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8. P = D (KD, E (KE, P)) Asymmetric
At other times, encryption and decryption keys come in pairs
Decryption key, KD
Encryption key KE
P = D (KD, E (KE, P))
where P=plaintext, K = key, E = encryption algorithms, and C = ciphertext
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9. Cryptography
Cryptography (secret writing) is the strongest tool for controlling against many kinds of security threats.
Well-disguised data cannot be read, modified, or fabricated easily. Cryptography is rooted in higher mathematics: group and field theory, computational complexity, and even real analysis, not to mention probability and statistics. Fortunately, it is not necessary to understand the underlying mathematics to be able to use cryptography.
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10. Cryptanalysis Attempt to break a single message
Attempt to recognize patterns in encrypted messages
Attempt to infer some meaning without breaking the encryption
Attempt to realize the key
Attempt to find weaknesses in the implementation or environment of use of encryption
Attempt to find general weaknesses in an encryption algorithm
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11. Cryptographer v Cryptanalyst
A cryptographer works on behalf of a legitimate sender or receiver
Cryptanalyst
A cryptanalyst works on behalf of an unauthorized interceptor
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12. Encryption
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13. Encryption with Keys
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14. Breakable Encryption
An encryption algorithm is called breakable when, given enough time and data, an analyst can determine the algorithm
May be impractical
A 25-character message of just uppercase letters has (1035) possible decipherments. A computer performing operations/sec would take 1011 years
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15. Representing Characters
we use the convention;
plaintext is written in UPPERCASE letters, and ciphertext is in lowercase letters
LASANTHA
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16. Arithmetic on the characters
A + 3 = D
N - 1 = ?
C+10 = ?
S + 9 = ?
X + 4 = ?
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17. Types of Encryption
Substitution – one or more characters are replaced with another
Transpositions (permutations) – order of characters is rearranged
Hybrid – combinations of the two types
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18. Substitution Ciphers
This technique is called a monoalphabetic cipher or simple substitution
A substitution is an acceptable way of encrypting text
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19. Caesar Cipher
Each letter is translated a fixed number of positions in the alphabet
Ci = E(pi) = pi + 3 (Caesar used a shift of 3)
Easy to perform; easy to break
Look for double letters and then use common words with double letters
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20. Other Substitution Ciphers
Use a key to scramble the letters
A B C D E F G H I J K L M N O …
c i p h e r s a b d f g j k l …
Rearrange using a fixed distance between letters (e.g. every 3rd)
a d g j m p s v y b e h k n r …
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21. Complexity of Substitution Encryption and Decryption
Substitution encryption algorithms can be performed by direct lookup in tables.
An important issue in using any cryptosystem is the time it takes to turn plaintext into ciphertext, and vice versa.
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22. Cryptanalysis of Substitution Ciphers
The techniques described for breaking the Caesar cipher can also be used on other substitution ciphers
Look for short words, words with repeated patterns, common first and last letters
Can use our knowledge of language
Look at frequency distributions
Could reduce time to hours
Nature and context of the text being analyzed
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23. One-Time Pads
The pad consists of a large number of pages where each page contains a non- repeating key
The sender would write the keys above the message (e.g. a 300 character message would require 30 pages of 10 character keys)
The message is scrambled using a Vigenere tableau built from the message and key
Problem is synchronizing the receiver’s pad with the senders pad
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24. Vigenere tableau
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25. How to use Vigenere tableau
I am, I exist, that is certain.
uaopm kmkvt unhbl jmed
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26. Vernam Cipher
One-time pad consists of an arbitrary long non-repeating sequence of numbers that are combined with the plaintext
Each plaintext character is represented by its numeric equivalent and is added to one of the random numbers. The ciphertext character is computed from the sum mod 26
Repeated characters are typically represented by different ciphertext characters
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27. Vernum Cipher
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28. Book Ciphers
Uses a passage from a book to form the letters at the top of a Vigenere Tableau
Computes ciphertext character by taking the intersection of the plaintext character and corresponding character at that position from the book passage
Relatively easy to break using frequency distributions
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29. Transpositions (Permutations)
Columnar Transposition rearranging plaintext message into columns and then reading it row by row
Transposition algorithms require a constant amount of time per character and are (n) algorithms, but space required to store results and delay in waiting for all characters to be read are dependent on the size of the plaintext
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30. Transpositions….
THIS IS A MESSAGE TO SHOW HOW A COLUMNAR TRANSPOSITION WORKS
tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasns
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31. Infrequent letter
If the message length is not a multiple of the length of a row, the last columns will be one or more letters short. When this happens, we sometimes use an infrequent letter, such as X, to fill in any short columns
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32. Cryptoanalysis of Transposition Algorithms
Compute letter frequencies of ciphertext; if appear with normal frequency, then assume a transposition algorithm was used
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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 as simple as possible
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 text of the original message
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34. Stream Ciphers
Most of the ciphers we have presented so far are stream ciphers (exception is the columnar transposition cipher)
convert one symbol of plaintext immediately into a symbol of ciphertext
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35. Stream Ciphers System
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36. Errors of Stream Ciphers
Skipping a character in the key during encryption
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37. Block Cipher
A block cipher encrypts a group of plaintext symbols as one block
Eg: columnar transposition
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38. Block Cipher Systems
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39. Advantages and disadvantages of stream and block encryption algorithms
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40. Confusion and Diffusion
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41. Confusion
The interceptor should not be able to predict what will happen to the ciphertext by changing one character in the plaintext
The goal of substitution is confusion
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42. Diffusion
The cipher should also spread the information from the plaintext over the entire ciphertext so that changes in the plaintext affect many parts of the ciphertext
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43. References
Charles P. Pfleeger, (2005) "Security in Computing (Fourth Edition)", Prentic-Hall International, Inc.
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44. thank you
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