What kind of attacks is this code vulnerable to?

Question

module SimpleCrypt

    class Map
        attr_accessor :letters

        def initialize
            @letters = ('a'..'z').to_a
            @letters << "_"
            @letters.shuffle!
        end

    end

    class Cipher

        def initialize(map)
            @letters = map.letters
        end

        def mapping(type, character)
            matches(@letters, type, character)
        end

        def matches(letters, type, character)
            letters_copy = letters.dup
            encrypt = Proc.new{|letter| "#{letter}#{character}#{letters_copy.shift}" }
            decrypt = Proc.new{|letter| "#{letters_copy.shift}#{character}#{letter}" }
            if type == "encrypt"
                encrypt_map = ('a'..'z').to_a.map{ |letter|  encrypt[letter] }
                encrypt_map << "_#{character}#{letters_copy.shift}"
            else
                encrypt_map = ('a'..'z').to_a.map{ |letter|  decrypt[letter] }.sort
                encrypt_map << "#{letters_copy.shift}#{character}_"
            end
        end
    end

    class Encrypter < Cipher

        def mapping
            super('encrypt', @character)
        end

        def initialize(map, character = "=>")
            super(map)
            @character = character
        end

    end

    class Decrypter < Cipher

        def mapping
            super('decrypt', @character)
        end

        def initialize(map, character = "=>")
            super(map)
            @character = character
        end

    end

end

map = SimpleCrypt::Map.new
encrypter = SimpleCrypt::Encrypter.new(map)
puts encrypter.mapping
puts ""
decrypter = SimpleCrypt::Decrypter.new(map)
puts decrypter.mapping

Answer 1

This appears to be a simple substitution cipher. Substitution ciphers are vulnerable to frequency analysis, and would never be used in practice.

It would be very easy to guess what character is being used to encrypt underscores (spaces), and from there an attacker could guess common one-, two-, and three-letter words and begin unraveling your messages from there.

See the relevant section of the Wikipedia article for more information.