From 88e3091bb0a5aa5067a83348ca3c3cf8522bde98 Mon Sep 17 00:00:00 2001
From: mo khan <mo@mokhan.ca>
Date: Wed, 22 Jan 2025 11:36:48 -0700
Subject: Finish assignment 3

---
 3431709-assignment-3.pdf  | Bin 144185 -> 151482 bytes
 assignments/3-solution.md |  17 +++++++++++++++++
 assignments/3/caesar.rb   |  10 ++++++++++
 3 files changed, 27 insertions(+)
 create mode 100644 assignments/3/caesar.rb

diff --git a/3431709-assignment-3.pdf b/3431709-assignment-3.pdf
index 7fe9214..5112b1d 100644
Binary files a/3431709-assignment-3.pdf and b/3431709-assignment-3.pdf differ
diff --git a/assignments/3-solution.md b/assignments/3-solution.md
index 843ff0a..eb3e5c0 100644
--- a/assignments/3-solution.md
+++ b/assignments/3-solution.md
@@ -87,3 +87,20 @@ Chapter 7:
   ```
 
 Chapter 8:
+
+> 9. Using a Caesar cipher with s = 5, decode the received message RTAJ TZY FY IF
+
+```plaintext
+| 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 |
+
+Key: s = 5
+
+| F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | A | B | C | D | E |
+
+| Ciphertext | RTAJ TZY FY IFBS |
+| Plaintext  | RTAJ TZY FY IF   |
+```
+
+```ruby
+!include assignments/3/caesar.rb
+```
diff --git a/assignments/3/caesar.rb b/assignments/3/caesar.rb
new file mode 100644
index 0000000..bc70f75
--- /dev/null
+++ b/assignments/3/caesar.rb
@@ -0,0 +1,10 @@
+#!/usr/bin/env ruby
+
+key = 5
+alphabet = ('A'..'Z').to_a
+cipher = alphabet.rotate(key)
+map = Hash[cipher.zip(alphabet)]
+map[' '] = ' '
+
+ciphertext = "RTAJ TZY FY IFBS"
+puts ciphertext.chars.map { |x| map[x] }.join
-- 
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