Cryptography a Presentation Prepared by Vytautas Kondratas

What is it? Kryptós + graphein -> secret writing Cryptography is the practice and study of techniques for secure communication in the presence of third parties called adversaries. In order to encrypt information, a cypher is used.

Cypher It’s an algorithm or technique that converts plain text into cypher text. Cypher text is gibberish unless you have the key, which lets you undo the cypher. ENCRYPTION – process of making text secret. DECRYPTION – reverse process. Transposition and Substitution cyphers. Permutation cyphers.

Permutation cypher

History Cyphers have been used long before computers ever showed up. Julius Caesar used a substitution cipher to encrypt private correspondence. Mary, Queen of Scots was executed after her plans to kill Queen Elizabeth were exposed by deciphering a secret message.

Enigma Automated encryption machine used by the Nazis to encrypt their wartime communications. It was the most complex encryption device of its time. Was eventually cracked by Alan Turing and his team.

How it worked

Software cyphers Data Encryption Standard – one of the earliest software cyphers to become widespread. At its inception used keys that were 56 bits long, which meant there were 256 different keys. With the increase in computing power this standard became easy to brute-force crack.

Software cyphers Advanced Encryption Standard – finalized version of DES, which used 128, 192 or 256bit long keys, making them near impossible to crack with brute force attacks. Principle: Chopping data into 16bit blocks Each block gets substitutions, permutations and some other operations based on key value Procedure is repeated 10+ times Used anywhere from iPhone encryption to HTTPS or WiFi Protection.

Key Exchange In the old days key were shared by voice or paper. This would never work in the information era. A way for a server to send a secret key over the public internet to a user, wishing to connect securely, was necessary. Key exchange – an algorithm that lets two computers agree on a key without ever sending one.

How it works? Diffie-Hellman Key exchange is achieved by one way functions (hard to reverse operations, e.g. xy mod z = a). Using these functions on both sender and receiver make a shared key which they use to encrypt their communications.

Asymmetric encryption Uses 2 keys: public and private. Anyone with a public key can encrypt a message but only the private key can be used to decrypt it. Reverse is possible too. Example use - signing. Only one person can encrypt a message, meaning if you can decrypt such a message, it is from a confirmed source. Most popular technique today: RSA.

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