Menu-based Caesar Cipher with encryption and decryption

I am about to start a very large project in Ruby. To get myself in the Ruby mindset I have been doing all of my homework and other side work in Ruby. I am hoping that some of you who know Ruby very well can look at this small program I have written and point out areas that aren't written in a Ruby-like fashion. Any and all tips and criticisms would be appreciated.

This program is a menu-based Caesar Cipher with encryption and decryption.

```
KEY = 3
$caesar_map = {}

#initializes the caesar_map and reads input from the user
#starting point of the program
def init()
#create empty hashes
plain_alphabet = {}
cipher_alphabet = {}

#fill in the alphabet hashes with the letter and its position
#in the english alphabet
"a".upto("z") {|x| plain_alphabet[x] = ((x[0] - "a"[0]) % 26) }
"A".upto("Z") {|x| cipher_alphabet[x] = ((x[0] - "A"[0]) % 26) }

#fill in the caesar_map, maps every letter KEY positions
#to the right in the alphabet
plain_alphabet.each do |k,v|
$caesar_map[k] = cipher_alphabet.index((v+KEY) % 26)
#puts "key: #{k} mapped value: #{cipher_alphabet.index((v+3) % 26)}"
end
#special case, I want spaces to be maintained in the messages
$caesar_map[" "] = " "

#print menu
choice = -1
while(choice != 3)
puts "What would you like to do?"
puts "\n1. Encrypt a message"
puts "\n2. Decrypt a message"
puts "\n3. Exit\n"

choice = Integer(gets.chomp)

if(choice == 1)
puts "Please enter a message you would like to Encrypt\n"
message = gets.chomp
encrypt(message)
elsif(choice == 2)
puts "Please enter a message you would like to Decrypt\n"
message = gets.chomp
decrypt(message)
elsif(choice == 3)
abort("Exiting...")
else
puts "You didn't enter a valid choice. Enter a number between 1..3\n"
end
end
end

#encrypts a plaintext message using the caesar cipher
def encrypt(plaintext)
#make sure the message is in lowercase
plaintext.downcase!
ci

I'd initialize with something closer to this (although I might inline the two arrays):

plain_alphabet = ('a'..'z').to_a
cipher_alphabet = ('A'..'Z').to_a.rotate KEY
$caesar_map = Hash[plain_alphabet.zip cipher_alphabet]

The en/decryption (which could likely be handled by the built-in tr method) might look more like:

plaintext.split("").inject("") {|s, c| s << $caesar_map[c]}

I'd put the whole thing in a class, too; skip the global.

StackExchange Code Review Q#7777, answer score: 5