Cryptography - Week 4

Cryptography - Week 4

1. Slidedeck 1

   • Protecting Information
     • Data Storage
     • Link Encryption
     • Data Transmission
     • End-to-End Encryption
   • Stream based Ciphers
     • RC 4
       • Key Scheduling Algorithm
       • Psuedo random generator algorithm
       • Used in WEP and TLS
       • Fast and SImple
       • Not Secure
   • modulo math
     • Number 1 mod Number 2
       • Number 1 / Number 2
         • Round Down(Remainder)
           • Remainder - RoundDown (Remainder)
             • (Remainder - RoundDown (Remainder)) * Number 2
   • Block Based Cipher
     • Splits plaintext up into blocks of sizes that are multiples of ASCII characters
     • Stronger than stream based
       • But more computationally expensive to implement

2. Playfair Cipher

   • Encryption
     • 5 * 5 Matrix
       • Fill in Matrix Row Wise with Keyword
         • Fill in rest of cypher with letters from alphabet not in the key word
           • i/j Interchangable
             • Split word into 2 char blocks
               • If a block has the same letter replace it with a placeholder (X for example) [[BA], [LX], [LO],[ON]]
                 • If two letters are in the same row replace them vertically with the row below it [BA] -> [JB]
                   • Letters horizontally above each other replace the char in the same row using the column index
                     • Each plaintext letter in a pair is replaced by the letter that lies in its own row and the column occupied by the other plaintext letter
                       • Two plaintext letters that fall in the same row of the matrix are each replaced by the letter to the right, with the first element of the row circularly following the last

3. Hill Cipher

   • Step One
     • Encode Letters for Example : P = 15, A = 0, Y = 24
       • Go Row Wise along Matrices [Attached]
         • \begin{array}{ccc} 17 & 17 & 5 \\ 21 & 18 & 21 \\ 2 & 2 & 19 \end{array}
         • Go Column Wise along the encoded plaintext
           • \begin{array}{ccc} 15  \\ 0 \\ 24  \end{array}
           • Mod result againt 26
             • Re-encode value for Ciphertext

4. Transposition, changing the order of words

   • Plaintext = "mark"
     • Encryption n = 3421
       • "rkam"

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