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2-Classical Encryption

2-Classical Encryption - Classical Encryption Techniques...

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Classical Encryption Techniques CSE 651: Introduction to Network Security
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Classical encryption techniques As opposed to modern cryptography Goals: to introduce basic concepts & terminology of encryption to prepare us for studying modern cryptography 2
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Basic terminology Plaintext: original message to be encrypted Ciphertext: the encrypted message Enciphering or encryption: the process of converting plaintext into ciphertext Encryption algorithm: performs encryption Two inputs: a plaintext and a secret key 3
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Symmetric Cipher Model 4
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Deciphering or decryption: recovering plaintext from ciphertext Decryption algorithm: performs decryption Two inputs : ciphertext and secret key Secret key: same key used for encryption and decryption Also referred to as a symmetric key 5
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Cipher or cryptographic system : a scheme for encryption and decryption Cryptography: science of studying ciphers Cryptanalysis: science of studying attacks against cryptographic systems Cryptology: cryptography + cryptanalysis 6
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Ciphers Symmetric cipher: same key used for encryption and decryption Block cipher: encrypts a block of plaintext at a time (typically 64 or 128 bits) Stream cipher: encrypts data one bit or one byte at a time Asymmetric cipher: different keys used for encryption and decryption 7
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Symmetric Encryption or conventional / secret-key / single-key sender and recipient share a common key all classical encryption algorithms are symmetric The only type of ciphers prior to the invention of asymmetric-key ciphers in 1970’s by far most widely used 8
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Symmetric Encryption Mathematically: Y = E K ( X ) or Y = E( K , X ) X = D K ( Y ) or X = D( K , Y ) X = plaintext Y = ciphertext K = secret key E = encryption algorithm D = decryption algorithm Both E and D are known to public 9
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Cryptanalysis Objective: to recover the plaintext of a ciphertext or, more typically, to recover the secret key. Kerkhoff’s principle: the adversary knows all details about a cryptosystem except the secret key. Two general approaches: brute-force attack non-brute-force attack (cryptanalytic attack) 10
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Brute-Force Attack Try every key to decipher the ciphertext. On average, need to try half of all possible keys Time needed proportional to size of key space Key Size (bits) Number of Alternative Keys Time required at 1 decryption/μs Time required at 10 6 decryptions/μs 32 2 32 = 4.3 × 10 9 2 31 μs = 35.8 minutes 2.15 milliseconds 56 2 56 = 7.2 × 10 16 2 55 μs = 1142 years 10.01 hours 128 2 128 = 3.4 × 10 38 2 127 μs = 5.4 × 10 24 years 5.4 × 10 18 years 168 2 168 = 3.7 × 10 50 2 167 μs = 5.9 × 10 36 years 5.9 × 10 30 years 26 characters (permutation) 26! = 4 × 10 26 2 × 10 26 μs = 6.4 × 10 12 years 6.4 × 10 6 years 11
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12 Cryptanalytic Attacks May be classified by how much information needed by the attacker: Ciphertext-only attack Known-plaintext attack Chosen-plaintext attack Chosen-ciphertext attack
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13 Ciphertext-only attack Given: a ciphertext c Q: what is the plaintext m?
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