Day 18. Concepts Plaintext: the original message Ciphertext: the transformed message Encryption:...
-
Upload
eugenia-norton -
Category
Documents
-
view
224 -
download
0
Transcript of Day 18. Concepts Plaintext: the original message Ciphertext: the transformed message Encryption:...
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
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.