Notes

Outline

Announcements A review session is being scheduled for Saturday March 15

Encryption Encryption encodes information to hide it from everyone else … maintaining your privacy

Maintaining Privacy To keep information private it must be hidden from “prying” computers As children, most of us used “secret” codes Most often the code was a Caesar Cipher -- an alphabetic shift by a constant amount

Breaking Caesar Cipher Fixed substitutions don’t work, ‘cause letters have a known distribution In a large text, count the frequency of each letter, match the results to distribution The twelve most frequent letters account for 80% of English text ETAOINSHRDLU

Encryption Issue Traditionally, encryption technology has been “breakable” with effort Breakable codes let law enforcement and governments watch criminals and spies Codes are good enough for the honest

Encryption Issue Traditionally, encryption technology has been “breakable” with effort Breakable codes let law enforcement and governments watch criminals and spies Codes are good enough for the honest ÞNew computer encryption is unbreakableÜ It’s called “strong encryption” … should it be legal to be able to keep secrets absolutely?

General Encryption Setup Encryption is most important for when sending information

Problem: Key Exchange To communicate securely, users must meet before sending/receiving

Revise Encryption Setup Public Key Encryption is based on publishing the key Sender uses public key to encrypt

Public Key Encryption Using the public key, encrypt message Divide T, the clear text bytes, into blocks Treat each block as a number Cube number (raise to 3 pwr), divide by key Send the remainder for each block

Public Key Cryptography Does PKC work? Can’t it be cracked? Recall definition of divide: a=b×c + d For example, 50/6 implies 50=6×8 + 2 The encryption process is a division: T3=Kr×c + d so sending c&d determines clear text T

RSA Encryption Rivest, Adelman and Shamir invented a PKC scheme called RSA The secret is to pick the key, Kr, right Pick two prime numbers -- numbers divisible only by themselves and 1 -- that are 2 greater than a multiple of 3 … weird! Examples are 5, 11, 17, 23, 29, ... Kr = p×q  so that it is 129 digits

How To Recover Message Compute s=1/3(2(p-1)(q-1)+1) then compute Cs = Kr×c + T That is … The remainders (C) raised to s power equal Kr times some (quotient) c no one cares about plus the original clear text number! So, raise the remainders to s, divide by Kr and PRESTO! the new remainder is the answer

What Makes RSA Work? Though the numbers get huge, computer can handle them quickly These codes are strong because breaking them needs s, which needs p, q, which means factoring Kr Factoring is computationally tough -- best methods are only somewhat better than grammar school, “try all small primes” Picking 129 digit key, means no computer can factor it … so the code is unbreakable

RSA Challenge After inventing their scheme (1977), RSA challenged people to break it Their first key was broken in 1994 using 1000 computers over 8 months Their secret message: THE MAGIC WORDS ARE SQUEAMISH OSSIFRAGE Doomed? No. There are many other 129 digit keys, or if people get nervous make 200 digit keys or more … breaking gets harder very fast; encrypt/decrypt doesn’t

Is Strong Encryption Smart Should we allow people to use strong encryption? Or should only breakable codes be legal? It hampers law enforcement and security Most criminals reveal plans in other ways PKC exists and is known, so build in escape -- Trap door -- Key Escrow But are these schemes really secure?