Simple Cipher Exercism Ruby Challenge - Nov 2019

The talk focused on Exercism.io challenge, "Implement a simple shift cipher like Caesar and a more secure substitution cipher." This 20 mins presentation was given at Ruby on Rails Oceania Sydney meet up on Thursday 19 November 2019.

https://exercism.io/tracks/ruby/exercises/simple-cipher

Su explored the following items from from a student's perspective:
- Simple ciper Caesar concepts
- Review ruby code
- Share 4 key lessons learnt by doing this coding challenge.

Transcript

1. Simple Cipher Exercism Challenge RoRoSydney Meetup Tuesday 19 November 2019 6:30pm Su Win 1
2. Aug 2019 Sep 2019 Oct 2019 Nov 2019 “Hello World” Self – Study Ruby Su’s Dev Career
3. Outline • Cryptography • Caeser Cipher • Substitution Cipher • Caeser Challenge • Step – 1 • Step – 2 • Step – 3 • Thought Process • Code • Lessons Learned 3
4. Cryptography Confidential important message! I will encrypt it! Thanks. I decrypt and can follow the instructions. Send via third party Julius Caesar Generals 4
5. Simple Cipher Algorithm Defend the east wall of the castle - Julius Caesar efgfoe uif fbtu xbmm pg uif dbtumf What the heck does this mean?? Decryption is easy. -1 letter. Yes, we will defend the east wall! 5
6. Simple Cipher Algorithm iamapandabear ldpdsdqgdehdu iamapandabear 6 Alice Bob Y Z A B C D E A B C D E Y Z A B C D E
7. Substitution Cipher Algorithm I am a panda bear. iawvawarwtqwamkwbx I am a panda bear. 7 Alice Bob PlainText Cipher A W B M D Q E K M V N T P R R B Empty space A Full Sop X
8. Caesar Cipher – Step 1 • Create a simple shift Caesar Cipher 8
9. Substitution Cipher – Step 2 • Amend the code to allow us to specify a key and use that for the shift distance 9
10. Ramdomness – Step 3.1 10 Hey, you. KEEP out. NO. I come from a source of randomness and have a key that contains only lowercase letters. I am VALID.
11. Ramdomness – Step 3.1 • Make the substitution cipher a little more fault tolerant 11
12. Ramdomness – Step 3.2 12 Oh Oh, I am in trouble! I cannot submit the key because I forget! What shall I do. SMILE What is your key?
13. Ramdomness – Step 3.2 • If someone doesn't submit a key at all, generate a truly random key of at least 100 characters in length. 13
14. Ramdomness – Step 3.3 • If the key submitted is not composed only of lowercase letters, your solution should handle the error in a language-appropriate way. 14
15. Thought Process 15 • How to convert letters to number? • s • Ruby Methods, Classes Constant Validate Encode Decode
16. DEMO 16 class Cipher ALPHABET = [*'a'..'z'] attr_reader :key
17. DEMO – Initialize Method 17 def initialize(key = nil) @key = key || 100.times.map { ALPHABET.sample }.join fail ArgumentError, 'invalid chars in key' unless valid?(@key) end
18. DEMO – Initialize Method 18 • .sample method – to randomize characters. • .map method – to transform the random characters • .time method – to iterate through 100 times • .join method – to join the 100element array into a string and assign it to @key
19. DEMO – Encode Method 19
20. DEMO – Encode Method 20 def encode(text) a = 'a'.ord text.chars.zip(@key.chars).map do |char, key| ALPHABET[(char.ord - a + key.ord -a) % ALPHABET.length] end.join end
21. DEMO – Encode Method 21 • .chars method - text string is separated into each individual character with .chars method. .chars more efficient than .split since it parses the underlying bytes to return the string's characters • .time method – to iterate through 100 times • .join method – to join the 100element array into a string and assign it to @key • .zip method – to create a new array and pair the original plain text characters and encryption characters • .ord method - convert a character to its ASCII value. a = 'a'.ord . • a = 97 • (Character position, not Unicode value. Therefore substract a.) • .map method - This method is called with a block and returns encrpyted text. • ALPHABET.Length => to ensure valid index of ALPHABET • .e.g ALPHABET[3]
22. DEMO – Decode Method 22 def decode(code) code.chars.zip(@key.chars).map do |char, key| ALPHABET[char.ord - key.ord] end.join end
23. DEMO – Error Handling Method 23 private def valid? (key) !key.empty? && key !~ /[^a-z]/ end
24. DEMO 24 class Cipher ALPHABET = [*'a'..'z'] attr_reader :key def initialize(key = nil) @key = key || 100.times.map { ALPHABET.sample }.join fail ArgumentError, 'invalid chars in key' unless valid?(@key) end def encode(text) a = 'a'.ord text.chars.zip(@key.chars).map do |char, key| ALPHABET[(char.ord - a + key.ord - a) % ALPHABET.length] end.join end def decode(code) code.chars.zip(@key.chars).map do |char, key| ALPHABET[char.ord - key.ord] end.join end private def valid? (key) !key.empty? && key !~ /[^a-z]/ end end
25. Key Lessons Learnt 25 Simple Cipher Substitution Cipher 1
26. Key Lessons Learnt 26 2 • You never know what other drivers and pedestrians will do!! • Expect incorrect input and handle it correctly • Think of both usual execution flow and unexpected flows Defensive Programming principle terminology
27. Key Lessons Learnt 27 3
28. Key Lessons Learnt 28 2 • Bruce Force Terminology • Recursion Terminology
29. Ruby 29 • Enumerable Modules o .zip – Glues together two enumerable objects o .map – Transforms every element of the enumerable object & returns the new version as an array o .join – Convert each element of they array to a string • Array Methods o .times o .sample – choose random element or n random elements from the array o .map – create a new array containing the values returned by the bock o A = [“a”, “b”, “c”, “d”] • String Methods o .char = str.each_char.to_a o e.g. meetup = “rorosyd”.chars => [“r”, “o”, “r”, “o”, “s”, “y”, “d”] • Ruby Access Control –Private Method • Tests 4
31. References • How to write your own Caesar Cipher Encoder by Jusus Castello https://www.rubyguides.com/2015/03/caesar-cipher-in-ruby/ • Ruby Documentation, https://ruby-doc.org/ • Stack Overflow • The Well-Grounded Rubyist, Third Edition, https://www.manning.com/books/the-well-grounded-rubyist-third- edition • Youtube 31