ITM 820 Lecture Notes - Lecture 5: Yottabyte, Steganography, Keystream

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Single key encryption: the same key is used for encryption and decryption. Cryptography: change the plain text into unreadable without the correct. Cryptanalysis: read the plain text without having the correct key, Classic cryptography cracking the cipher: three letter shift, julius caesar was the first proof of usage, define a=0, b=1, c=2, d=3, , z=25, then ci = pi + 3 mod 26. Z = 25 +3 mod 26 = 2, therefore since c = 2, z = c. X = a, y = b, z = c, a = d. Ibn ad-duraidhim first evidence of cryptanalysis: two types of cipher systems emerged. Substitution: a monoalphabetic substitution is a cipher in which each occurrence of a plaintext symbol is replaced by a corresponding ciphertext symbol to generate ciphertext. Permutation (transposition: key (alphabet) is arbitrary (randomly) chosen, must be written down, we have to know the whole table. Reciprocal: if x -> y then y -> x.

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