### Encryption Take a message Put it in a box Lock the box - That is a physical version of Encryption We have tried to do this virtually using **ciphers** Ciphers scramble and de-scramble messages #### Hash vs Encryption The fundamental difference between hash and encryption techniques is that **hash is irreversible while encryption is reversible**. Hash algorithms generate a digest of fixed length output cipher text for a given input plain text. The output text cannot be converted back to input text. #### Caesar Cipher - 58BC - Shifting letters by agreeing the shift -- A # number to move letters forward by - Add shift to encrypt - Subtract shift to decrypt ###### Breaking the Ceasar Cipher Lock breaking and code breaking is similar - A lock is only as strong as its weakest point - Look for mechanical flaws / extract information to narrow down the correct combination Frequency Analysis by Al Kindi was a blow to the Caesar Cipher, 800 years later - Used a clue based on an important property of the language the message is written in --> The frequency with which certain letters in a language are used is called the Fingerprint. ![[Pasted image 20210727143449.png]] - Counting the letter frequency of the encrypted text in the language it is written in Eg. English to check how far the fingerprint has shifted ![[Pasted image 20210727143558.png]] - 1 of 26 encryptions --> Shifting every letter by the same shift, Easy to check them all #### Polyalphabetic ciphers > Strong cipher disguises ones fingerprint, the goal to make a ligther fingerprint and flatten the distribution of letter frequency > Polyalphabetic Ciphers use Multiple shifts --> Flatter letter distribution, Simply multiple Caesar Ciphers in a string - Share a secret shift word --> Convert to numbers based on letter position - longer the shift word stronger the cipher - Repeat that secret number sequence across the message - Each letter is encoded by shifting as per the number below it > Code breakers look for information leak, the same as finding a partial fingerprint - Every time there is a partial differential in letter frequencies a leak of information occurs. This happens because of **repetition** ![[Pasted image 20210727144244.png]] > Avoid repetition in secret ciphers for shifts ### One time pad - Strongest Possible Method of Encryption - 19th century > Designing a cipher that hides your fingerprint to stop a leak of information ** Answer is Randomness - Shifts never fall into a repetitive pattern - Encrypted message will form a uniform frequency distribution --> no frequency differential and no leak No frequency differential and thus no leak ![[Pasted image 20210727145234.png]] Strength of the one time pad --> Combinatorial explosion ![[Pasted image 20210727150052.png]] Paper stacked for ALICE as a coded message shifted would be 1 km high with 26x26x26x26x26 possibilities -- Almost twelve million possible 5 letter sequences ---- #security