A simple implementation of Caesar's Cipher.

ceasar_cipher.rb

puts "Welcome!"
puts "Please enter the phrase you wish to encrypt: "
phrase = gets.chomp
puts "Enter factor: "
factor = gets.chomp.to_i

ALPHABETS = {
    'lower' => [nil, ('a'..'z').to_a].flatten,
    'upper' => [nil, ('A'..'Z').to_a].flatten
}

def ceaser_cipher2(phrase, factor = 1)
    return if phrase.strip.empty?

    limit = ALPHABETS['lower'].size - 1
    phrase.split("").map do |char|
        if !char.strip.empty? && (ALPHABETS['lower'].include?(char) || ALPHABETS['upper'].include?(char))
            ref = /[[:upper:]]/.match(char) ? 'upper' : 'lower'
            index = ALPHABETS[ref].index(char)
            factored = index + factor
            if (factored > limit)
                factored = factored - limit
            end

            char = ALPHABETS[ref][factored]
        end

        char     
    end.join
end

encrypted = ceaser_cipher2(phrase, factor)
puts "Encrypted phrase: '#{encrypted}'"