What is cryptography?• “hidden writing”

• Until recently: military tool

• Like any military technology: methods change over time

• Two sides: designing codes breaking codes (cryptanalysis)

• Computers have changed both

How do we encrypt?Protocol, or scheme: method of encryptionCryptovariable, or key: secret information

Symmetric encryption: decryption is the same

cryptovariableciphertext

plaintext protocol

Example: Caesar Shift• Protocol: shift each letter by the same

amount• Cryptovariable: amount to shift

-1

IBM HAL

Sam, Matt, Emmell

10• Decryption: shift back the same amount

Ckw, Wkdd, Owwovv

How could we break this?• Case I: we don’t know the protocolo Hard problem in cryptanalysiso “Clark Kent” effect

• Case II: we know the protocolo Need to guess the cryptovariableo Only 26 possibilities

Substitution Cipher• Allow any permutation of the alphabet• Key = permutation; 26! possibilities• 26! = 403,291,461,126,605,635,584,000,000• Roughly 288: checking 1 billion per second,

would take 12 billion years• Is there a better way?

• al-Kindi, ninth century: frequency analysis

H EKGGLHQNL KZEL AKGB PL ARHA

ARL CKSGB CHV XNGG KX UHB

VLENSTAF VFVALPV CSTAALZ UF

OLKOGL CRK SLHB HOOGTLB

ESFOAKQSHORF.

- USNEL VERZLTLS, VLESLAV HZB GTLV

L occurs 18 times, A occurs 10 times.

E E E T E T TH EKGGLHQNL KZEL AKGB PL ARHAT EARL CKSGB CHV XNGG KX UHB E T TE TTEVLENSTAF VFVALPV CSTAALZ UF E E E E OLKOGL CRK SLHB HOOGTLB TESFOAKQSHORF. E E E E ET E- USNEL VERZLTLS, VLESLAV HZB GTLV

E E E T E TH TH EKGGLHQNL KZEL AKGB PL ARHATHEARL CKSGB CHV XNGG KX UHB E T TE TTEVLENSTAF VFVALPV CSTAALZ UF E E H E E OLKOGL CRK SLHB HOOGTLB T HESFOAKQSHORF. E H E E E ET E- USNEL VERZLTLS, VLESLAV HZB GTLV

A EA E E T E THATH EKGGLHQNL KZEL AKGB PL ARHATHE A AARL CKSGB CHV XNGG KX UHB E T TE TTEVLENSTAF VFVALPV CSTAALZ UF E E H EA A E OLKOGL CRK SLHB HOOGTLB T A HESFOAKQSHORF. E H E E E ET A E- USNEL VERZLTLS, VLESLAV HZB GTLV

A OLLEA E O E TOL E THATH EKGGLHQNL KZEL AKGB PL ARHATHE O L A LL O AARL CKSGB CHV XNGG KX UHBSE T S STE S TTEVLENSTAF VFVALPV CSTAALZ UF PEOPLE HO EA APPL E OLKOGL CRK SLHB HOOGTLB PTO APHESFOAKQSHORF. E S H E E , SE ETS A L ES- USNEL VERZLTLS, VLESLAV HZB GTLV

A COLLEAGUE ONCE TOLD ME THATH EKGGLHQNL KZEL AKGB PL ARHATHE WORLD WAS FULL OF BADARL CKSGB CHV XNGG KX UHBSECURITY SYSTEMS WRITTEN BYVLENSTAF VFVALPV CSTAALZ UF PEOPLE WHO READ APPLIEDOLKOGL CRK SLHB HOOGTLBCRYPTOGRAPHY.ESFOAKQSHORF. BRUCE SCHNEIER, SECRETS AND LIES- USNEL VERZLTLS, VLESLAV HZB GTLV

But how do COMPUTERS do this?Let’s start by looking at how things are ‘encoded’…

Code Creation ExerciseIn groups of 3:you will be creating your own substitution cypher

● Prepare two separate pages● Page 1

- Write out the alphabet and your substitutions- Convert a common phrase into coded form using your code. Your phrase MUST be familiar, and must have a minimum of 10 words. For example, you could choose the phrase “The grass is always greener on the other side.” Try to select a message that you think will NOT be too easy for someone else to decipher, but that will not be too obscure also.

● Page 2 - Your cypher text You have 15 minutes!