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A Brief Introduction to Cryptography: Understanding Encryption and Message Security

This comprehensive introduction to cryptography explores the fundamental principles and historical evolution of message encryption. It covers key concepts such as plaintext vs. ciphertext, early techniques like the Caesar and Vigenère ciphers, and the advent of modern cryptography with the rise of digital technology. The importance of cryptography in various applications, from military communication to online security, is highlighted, demonstrating its critical role in protecting sensitive information in our digital age.

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A Brief Introduction to Cryptography: Understanding Encryption and Message Security

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  1. A brief introduction to cryptography by K. Kuypers

  2. Scenario TOP SECRET message

  3. Solution: Cryptography • Greek: • kryptos = “hidden, secret” • grapho = “I write” • Applications: • Sending messages during war • Sending messages between lovers • Security of ATM cards • Security of computer passwords • Security of e-commerce (internet) • Pay TV • …

  4. What do we need? • Aim: to convert the original message so that • If it is intercepted it makes no sense to the interceptor • The intended receiver can decipher it • Original message: “plaintext” • Transmitted message: “ciphertext” • The algorithm to convert between plaintext and ciphertext: “cipher” plaintext cipherciphertext

  5. Earlycryptography: Caesar cipher(Julius Caesar, 100 -44 B.C.) • Example: • Plaintext= e u r o l i n k s • Ciphertext = h x u r o l q n v • What is the cipher, what happened?

  6. Caesar cipher • Each letter in the plaintext is replaced by a letter 3 positions down the alphabet • The shift can be any number, but has to be known by the receiver therefore we need a “key” with the cipher.

  7. Is the Caesar cipher secure? • At that time: fairly • Reasons: • Most enemies were illiterate • They assumed that the message was written in a foreign language • Now: absolutely not • It can be easily broken by trying all shift possibilities (up to 25) when the Caesar cipher is suspected e.g. ciphertext= exxegoexsrgi

  8. It can be broken by frequency analyses (statistics) = comparison of • How many times a certain letter is used in the cyphertext • The expected frequency of those letters in the original language of the plaintext

  9. Earlycryptography: Vigenèrecipher(Blaise de Vigenère, 1523-1596) • The method was originally described by Giovan Batista Bellaso in 1467 • PublishedbyVigenère in 1553 • This cipher is • Easy to understand and implement • Using a series of Caesar ciphers with different shifts, based on the letters of a keyword

  10. Vigenèrecypher: example Plaintext= e u r o l i n k s Keyword= s t u d e n t s t Ciphertext = w n l r p v g c l

  11. Is the Vigenère cipher secure? • Yes • It is even theoretically proven to be unbreakable if: • The key is truly random • The key is only used once • The key is as long as the message, so no repetition occurs The cipher is then also referred to as“one time pad system”.

  12. A qualitycipher… • has to be very efficient (i.e., fast and requiring few resources) • makes cryptanalysis (breaking it) so inefficient and impractical as to be effectively impossible • stays secure even if the attacker knows the system, so secrecy of the key alone is sufficient (Kerckhoffs’ principle)

  13. Modern cryptography(from the mid 70s) • The development of digital computers and electronics after WWII ⇒ more complex ciphers • Encryption of any kind of data in binary format • Asymmetric key cryptography (public key and private key) • However, computers have also assisted cryptanalysis, which has compensated to some extent for increased cipher complexity

  14. Applications: Didyou ever… • Buy something on the Internet? • You must have submitted confidential user information, such as your credit card numbers through SSL (https://) • Send an e-mail? • electronic privacy is provided by PGP, S/MIME • Subscribe to pay TV? • You need a decoder for e.g. SECA2, DigiCipher2 (DCII) • UseMasterCardor VISA at an ATM? • They use 3DES to ensure the security of their networks

  15. Cryptography is everywhere And next time your computer seems a little bit slow… think about all the calculating and ciphering or deciphering that might be going on…

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