A brief introduction to cryptography

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…