Lecture2

# Lecture2 - Lecture 2 Confidential Channels Encryption and...

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1 Lecture 2 Confidential Channels, Encryption and Ciphers CNT 5412 Network Security

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2 Cryptography Greek for “hidden writing” • “The art and science of keeping messages secure” • A field of study relating to the above • Note: Cryptology is often defined to include the fields of: –Cryptography –Cryptanalysis –Steganography
3 What is encryption? • Encryption is used to achieve confidentiality • Alice and Bob, wish to communicate secretly. • Curious Carl wants to listen into their private chat. 01001110… As root, try: tcpdump -a -s 0

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4 Ciphers • Plaintext • Ciphertext • Encipher: transform plaintext into ciphertext (encrypt) • Decipher: transform ciphertext into plaintext (decrypt) • Key: used in the transformation between plaintext and ciphertext. • Cipher: an encryption (and decryption) algorithm – Symmetric or secret key – Asymmetric or public/private key • Goal of good ciphers: make the ciphertext look like “random bits”
5 Ciphers • Ciphers operate to “garble” their input to make it unintelligible. The output of a cipher (ciphertext) does not bear any clear relation to the input (clear- text or plaintext). – The earliest recorded example of the use of a cipher is by Julius Caesar to his generals: He would shift each letter to the third letter following it in the alphabet. • Example: Attack now Dwwdfn qrz How can you cryptanalyze this cipher?

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6 Generalizing the Caesar Cipher (a substitution cipher) • Caesar: given plaintext p, use s, where s = E(p + 3) mod 26 • Can obviously use s = E(p + k) mod 26 for arbitrary k. How many keys are there? • Mono-alphabetic cipher: use an arbitrary permutation of the 26 letters. – N! permutations or keys (26! ≈ 4 × 10 26 ) Many more keys – how much harder to analyze?
7 Assumptions about cipher design • The adversary knows the cipher algorithm. – also the design process, the implementation, etc. • To achieve secrecy, ciphers use keys. • A key is an auxiliary input to the algorithm

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Lecture2 - Lecture 2 Confidential Channels Encryption and...

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