EKRJGTU CTG CYGUQOG! Can you break the code?

Ciphers are awesome! Can you break the code?

5 Min Task Convert a 5 letter word into Ciphertext using an any key of shift cipher. Pass on to partner to decipher without telling them the key. Use the shift cipher list to help you https://goo.gl/3Ah3G8

Brute Force Attack For the name ALICE and a key of 3 the encoded word would be DOLFH. Any Ceaser cipher (regardless of length) can be brute forced in 25 versions of this decoding.

Relative Frequency (Standard)

Relative Frequency (Cypher)

Relative Frequency (transition)

Programmatically… A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 10 11 12 13 O P Q R S T U V W X Y Z 14 15 16 17 18 19 20 21 22 23 24 25

Programmatically… Letter H E L O Number 7 4 11 14 Shift +16 D E F G H I J K L M N 1 2 3 4 5 6 7 8 9 10 11 12 13 O P Q R S T U V W X Y Z 14 15 16 17 18 19 20 21 22 23 24 25 Letter H E L O Number 7 4 11 14 Shift +16 New Position 23 20 27 30 Quotient   27 = 1 ● 26 + 1 30 = 1 ● 26 + 4 Remainder 1 New Letter X U B

Weakness and Improvements Number of possible keys is too small. (25) Same shift is applied to each character making it easy for relative frequency analysis.   Same shift is likely to be used for each message.  Use a key word at the beginning of the shift cipher – BESWAX example. Moving the order of the letters around in a known pattern.

Keyword Shift Example Select a keyword to use with cipher (BEESWAX) G H I J K L M N S W X O P Q R S T U V W X Y Z L M N Select a keyword to use with cipher (BEESWAX) Remove any duplicate characters (BESWAX) Place this at the first in the shift, then fill in unused letters. More possibilities (different words = different order). Letters not shifted by the same number.

Rail Fence Cipher Splits the letters of words into new order, known to both parties (so can be decoded). This_is_scrambled Ti_ssrmldhsi_cabe Can also be done over more than two lines. Other methods (such as a standard shift cipher or keyword cipher can be used). T i _ s r m l d h c a b e

One Time Pad Example Key length is same as plain text length C E Numierical Value 1 11 8 2 4 Key Random* Shift 24 25 Cipher Numerical 5 22 12 26 = 1 ● 26 + 0 29 = 1 ● 26 + 3 Remainder 3 Cipher Text F W M Key length is same as plain text length Shift is random (if not chosen by computer)

One Time Pad Shift does not fall into repetitive pattern. More possibilities for a brute force attack. Shift Cipher 25 total One Time Pad 25^5 = 11,881,376 (where 5 is length of message) Ciphertext destitution is ‘flat’ or even. https://youtu.be/Q-XKOPNIDAg?t=3m16s Sharing the pad can be difficult Can only be used once

Vernam Cipher Plaintext ⊕ Key = Ciphertext Uses similar process as One Time Pad Converts characters to binary values Random key is converted into binary values Uses eXclusive-OR (XOR) to generate cipher text Plaintext ⊕ Key = Ciphertext Ciphertext ⊕ Key = Plaintext

Vernam Cipher

… the method or process used when hiding the meaning of a message. A Cipher is… … the method or process used when hiding the meaning of a message.