Содержание
- 2. Introduction Symmetric encryption, also referred to as conventional encryption or single-key encryption, was the only type
- 3. Content Symmetric cipher Secure Use of Conventional Encryption 1. – – 2. 3. – – Symmetric
- 4. Symmetric cipher A form of cryptosystem in which encryption and decryption are performed using the same
- 5. Secure Use of Conventional Encryption Two requirements for secure use of conventional encryption: 1. Strong algorithm
- 6. Symmetric Encryption Scheme We do not need to keep the algorithm secret; we need to keep
- 7. Cryptographic systems Characterized 1. The type plaintext to along three independent dimensions: of operations used for
- 8. 2. – The number of keys used. Symmetric: both sender and receiver use the same key
- 9. Cryptanalysis Two general approaches to attacking a conventional encryption scheme: Cryptanalysis: exploits the characteristics of the
- 10. Cryptanalytic Attacks Ciphertext only: ● Encryption algorithm Ciphertext – – Known plaintext: ● Encryption algorithm Ciphertext
- 11. Chosen plaintext: ● Encryption algorithm Ciphertext Plaintext message chosen by cryptanalyst, – – – together with
- 12. Chosen text: ● Encryption algorithm Ciphertext Plaintext message chosen by cryptanalyst, – – – together with
- 13. A Brute-Force Attack Trying every possible key until an intelligible ● translation of the ciphertext into
- 14. Brute-Force Cryptanalysis of Caesar Cipher Three important characteristics of this problem enabled us to use a
- 15. Example: Let's encipher some text using Caesar cipher with the secret key is 3. toga WRJD
- 16. Monoalphabetic Ciphers With only 25 possible keys, the Caesar cipher is far ● from secure. A
- 17. Breaking Monoalphabetic Cipher The ciphertext to be solved is UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXU DBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSV PQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPF UPZHMDJUDTMOHMQ The relative frequency
- 18. Relative Frequency of Letters in English Text
- 19. Comparing two previously shown letters P and Z are the equivalents and t, but it is
- 20. A powerful tool is to look at the frequency of two-letter combinations, known as digrams. The
- 21. So far, then, we have UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ t a e e te a that e e a
- 22. Continued analysis of frequencies plus trial and error should easily yield a solution from this point.
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