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# Unit-2 - Conventional Encryption Principles An encryption...

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1 Conventional Encryption Conventional Encryption Principles Principles An encryption scheme has five ingredients: Plaintext Encryption  algorithm Secret Key Ciphertext Decryption algorithm Security depends on the secrecy of the key, not the secrecy  of the algorithm

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2 Conventional Encryption Conventional Encryption Principles Principles
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6 Cryptography Cryptography Classified along three independent dimensions: The type of operations used for transforming plaintext to  ciphertext The number of keys used symmetric (single key) asymmetric (two-keys, or public-key encryption) The way in which the plaintext is processed
7 Average time required for Average time required for exhaustive exhaustive key search key search Key Size (bits) Number of Alternative Keys Time required at 10 6 Decryption/µs 32 2 32 = 4.3 x 10 9 2.15 milliseconds 56 2 56 = 7.2 x 10 16 10 hours 128 2 128 = 3.4 x 10 38 5.4 x 10 18 years 168 2 168 = 3.7 x 10 50 5.9 x 10 30 years

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9 Feistel Cipher Structure Feistel Cipher Structure Virtually all conventional block encryption  algorithms, including DES have a structure first  described by Horst Feistel of IBM in 1973 The realization of a Fesitel Network depends on  the choice of the following parameters and design  features (see next slide):

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10 Feistel Cipher Structure Feistel Cipher Structure Block size:  larger block sizes mean greater security Key Size:  larger key size means greater security Number of rounds:   multiple rounds offer  increasing security Subkey generation algorithm:  greater  complexity will lead to greater difficulty of cryptanalysis. Fast software encryption/decryption:  the  speed of execution of the algorithm becomes a concern
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13 Conventional Encryption Conventional Encryption Algorithms Algorithms Data Encryption Standard (DES) The most widely used encryption scheme The algorithm is reffered to the Data Encryption  Algorithm (DEA) DES is a block cipher The plaintext is processed in 64-bit blocks The key is 56-bits in length

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Henric Johnson 14
Henric Johnson 15

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Unit-2 - Conventional Encryption Principles An encryption...

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