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Unformatted text preview: Crypto Module Friday, February 26, 2010 2 Data Encryption Encryption is the process of encoding a message such that its meaning is not obvious. Decryption is the reverse process, ie, transforming an encrypted message to its original form. We denote plaintext by P and ciphertext by C. C = E(P), P = D(C) and P = D(E(P)), where E() is the encryption function (algorithm) and D() the decryption function. Encryption Decryption Plaintext Plaintext Ciphertext Friday, February 26, 2010 Kerckhoff’s Principle How do you prevent and eavesdropper from computing P, given C? Keep the encryption algorithm E() secret. BAD IDEA!! Choose E() (and corresponding D()) from a large collection, based on secret key. GOOD IDEA!! Kerckhoff’s principle . C = E(K, P) and P = D(K, C) Encryption Decryption Plaintext Plaintext Ciphertext Secret Key Friday, February 26, 2010 Symmetric and Asymmetric Cryptosystems Just by changing key we have different encryptions of one plaintext. If the encryption key and the decryption key are the same then we have a symmetric encryption scheme (also private key, onekey). If the encryption key and the decryption key are different then we have an asymmetric encryption scheme (also public key, twokey). Friday, February 26, 2010 5 Example – Caesar Cipher Let messages be all lower case from a through z (no spaces or punctuation). itsnotthathardtoread Represent letters by numbers from 0 to 25. Encryption function C i = E(P i ) = P i + K. where K is secret key and addition done modulo 26. Decryption is P i = D(C i ) = C i K. UNIX ROT13 uses K as 13. Friday, February 26, 2010 6 Cryptanalysis A cryptosystem had to be secure against the following kinds of attacks: Ciphertext only attack. Known plaintext attack. Chosen plaintext attack. Adaptive chosen plaintext attack. Chosen ciphertext attack. Chosen key attack. Of course there is one attack against which no cryptosystem can offer protection – rubber hose attack. Friday, February 26, 2010 7 Brute Force Attacks. Since the key space is finite, given a ciphertext a cryptanalyst can try and check all possible keys. For above to be not feasible, key space should be large!! How large? How about 2 56 ? Large enough to make it impractical for an adversary. But what is impractical today, may not be so tomorrow. In practice, for a “good” cryptosystem, the only possible attack should be the brute force attack, which should be impractical into the foreseeable future, as slong as message may have value. Friday, February 26, 2010 8 Substitution Ciphers Basic idea – substitute each block of plaintext by a different block....
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 Spring '08
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 Cryptography, Alice, decryption key, Bob exchange

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