Kryptering & Kodning

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• asymmetric cryptosystem ?
• binary symmetric channel ?
• chosen ciphertext attack ?
• cyclic code A # is a linear code such that any cyclic right shift of a codeword is again a codeword.
• Diffie-Hellman problem ?
• diffusion # is the idea that a small change of a plaintext should have a large effect on the corresponding ciphertext. With a high diffusion the chance for a ciphertext character to change given that one single character of the plaintext is modified should be the same no matter the position of the ciphertext character.
• discrete logarithm if h≡g^x (mod p) ⇒ x=log_g(h), Given a prime p; a primitive root  modulo p and any element  ∈ (Z_p)* the # of  (with respect to  modulo p) is the (unique) element x ∈ Z_{p-1} that fulfills ^x≡ (mod p)
• Electronic Code Book (ECB) ?
• Feistel system ?
• Hamming weight ?
• Kerckhoffs principle When designing new cryptosystems the security should only rely on keeping the keys secret and not on keeping the algorithms for encryption and decryption secret.
• length of a code ?
• linear code ?
• Meet-in-the-Middle Attack The # is a known-plaintext attack that can be used (for instance) in Double DES. Let E_K and D_K denote the DES encryption and decryption mapping respectively where K is a DES key. The pair of keys used for Double DES must then be found among all pair of keys (K';K'') that fulfils E_K'(m) = D_K''(c) where (m; c) denotes the known plaintext/ciphertext pair.
• minimal distance of a code ?
• minimal Hamming weight wt(C) of a code C The minimal weight of a code C is wt(C) = min{wt(c) | c ∈ C; c ≠ 0} i.e. it is equal to the least number of non-zero bits among all the non-zero words of the code.
• size of a code ?
• symmetric cryptosystem ?
• Vigenère encrypted ?

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