Day 18

Concepts• Plaintext: the original message• Ciphertext: the transformed message• Encryption: transformation of plaintext

into ciphertext• Decryption: transformation of plaintext

into ciphertext• Key: some critical information used for

encryption and decryption, only known to the sender and/or receiver

Caesar Cipher• Each letter of the alphabet is

rotated 3 places.– A -> D– B -> E– C -> F– X -> A– Y -> B

• The key here is 3• The algorithm is to swap each

letter with the letter KEY letters away.

Bkzovmqflk Oribp!

A B C D E F G H I J K L MN O P Q R S T U V WX Y Z

X Y Z A B C D E F G H I J K L MN O P Q R S T U V W

Standard English Frequency

Special Rules• All Q’s in the English language are

followed by U.• T’s often are followed by h’s• Etc.

Breaking a Caesar Cipher• Figure out the frequency of each

letter.• Compare it to standard English• Figure out the mapping• Translate.

Polyalphabetic substitution• You can use multiple different

Caesar Ciphers on the same text.– First letter has a key of 5– Second letter has key of 7– Third letter has key of 11– Forth letter has key of 4– Fifth letter has key of 5 again.

Substitutions• The letters can be a random

mapping:– A -> X– B -> C– C -> P

• Slightly more difficult than Caesar but still has the same problems.

Enigma• World War 2 saw the creation of a

machine to perform substitutions one after another based on 3 wheels.– Each wheel had a substitution– After each letter, the wheels rotated.– The wheel choice, and starting

position was determined ahead of time by a code book and the day.

Transposition Cipher• Instead of changing letters, just

rearrange them.– Doesn’t suffer from same problems as

substitutions.– Quite difficult to decode on large

column counts.– Can be based on a key:

• Computer -> 1 4 3 5 8 7 2 6

Transpose and Substitute• Nothing says you can’t do both.• DES (Govt. standard for encryption until

Oct 2000 – replaced by AES (keys up to 256bits – blocks 128bit)– 56 bit key

• Broken into smaller bits for encryption

– 64 bit blocks of data.– 16 rounds of substitutions and

transformations– Both sides must know the key ahead of time.

• Involves Permutation• Series of substitutions• Swapping of ½ bits• More substitutions• Another permutation

Key difficulty• How do you get the key to the

other side?– If you can do that securely, why don’t

you just send the data?

• What if they key gets compromised?– You need to exchange new keys

Public Key Cryptography• Different keys used to encrypt and

decrypt the traffic.– Very complex polynomial factoring

used to create 2 keys.– The same key cannot be used to

encrypt AND decrypt. You MUST use the other key.

– Given one key it is impossible (as far as we know) to calculate the other key.

Encrypting with public key• I generate a public and private

key pair.• I publish the public key to anyone

who wants it• If someone wants to send me data

that only I can read, they encrypt it with my public key.– Only my private key will decrypt it.

SSL• Symmetric key is faster to use,

but has the problem of how to exchange keys.

• SSL uses public key to exchange a symmetric key, then DES or AES is used to encrypt traffic.

• SSL understands how to decide on best algorithm both sides understand.