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Unformatted text preview: Class Notes September 18, 2008 Transposition: the letter frequencies are preserved. The first step when trying to determine if a transposition is used is to note if the letter frequencies in a cipher text are preserved. Once you are sure it is a transposition, then you t ry to break it using digraphs and t ri- graphs. Tri-Graphs- Ent- Ion- And- Ing- I re- Tio- For- Ono- Thi- One Making a good encryption algorithm-Defined in 1949 by Claude Shannon 1. The amount of secrecy needed should determine (or is proportional) to the amount of work needed for en/decryption. 2. Set of keys and the algorithms should be free from complexity (as much as possible) 3. The implementation should be as simple as possible 4. Errors in ciphering should not propagate 5. The cipher text should not be longer than the size of the plain text Properties of trustworthy encryption schemes 1. Based on sound mathematics (provable) 2. Tested and evaluated by experts 3. Withstand the test of time Symmetric Encryption Algorithms...
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- Spring '10