2-Classical Encryption

2-Classical Encryption - Classical Encryption Techniques...

Info iconThis preview shows pages 1–13. Sign up to view the full content.

View Full Document Right Arrow Icon
Classical Encryption Techniques CSE 651: Introduction to Network Security
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Classical encryption techniques As opposed to modern cryptography Goals: to introduce basic concepts & terminology of encryption to prepare us for studying modern cryptography 2
Background image of page 2
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
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Symmetric Cipher Model 4
Background image of page 4
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
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 6
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
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 8
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
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 10
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
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 12
Image of page 13
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 01/22/2011 for the course CSE 651 taught by Professor Tenh.lai during the Spring '10 term at Ohio State.

Page1 / 54

2-Classical Encryption - Classical Encryption Techniques...

This preview shows document pages 1 - 13. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online