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Making “Good” Encryption Algorithms

Making “Good” Encryption Algorithms. Substitution algorithms “hide” the plaintext and dissipate high letter frequencies Transposition algorithms scramble text Many “good” algorithms combine both techniques. Shannon’s Characteristics of “Good” Ciphers.

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Making “Good” Encryption Algorithms

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  1. Making “Good” Encryption Algorithms • Substitution algorithms “hide” the plaintext and dissipate high letter frequencies • Transposition algorithms scramble text • Many “good” algorithms combine both techniques

  2. Shannon’s Characteristics of “Good” Ciphers • Amount of secrecy needed should determine the amount of labour appropriate for encryption/decryption. • Set of keys and enciphering algorithm should be free from complexity. • Implementation should be simple • Errors in ciphering should not propagate. • Size of ciphertext should be no larger than the size of the plaintext

  3. Properties of “Trustworthy” Encryption Systems • Based on sound mathematics • Been analyzed by competent experts and found to be sound • Stood the “test of time” • Three Examples: • DES (data encryption standard) • RSA (Rivest-Shamir-Adelman) • AES (Advanced Encryption Standard)

  4. Symmetric and Asymmetric Encryption Systems • Symmetric requires one “secret” key that is used for encryption AND decryption (e.g. Caesar cipher might use a “key” of 3 to indicate shift by 3) • As long as key remains secret, authentication is provided • Problem is key distribution; if there are n users, we need n * (n-1)/2 unique keys

  5. Symmetric and Asymmetric Encryption Systems • Asymmetric requires two keys one of which is a “public key” • The public key is used for encryption and the “private” key is used for decryption • If there are n users, there are n public keys that everyone knows and n private keys known only to the user

  6. Stream and Block Ciphers • Stream ciphers – convert one symbol of plaintext immediately into a symbol of ciphertext • Transformation depends on the plaintext symbol, the key, and the algorithm • Error can affect all text after the error

  7. Stream and Block Ciphers • Block cipher encrypts a group of plaintext symbols as one block (e.g. columnar transposition)

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