diff --git a/lib/twik.rb b/lib/twik.rb
index 85a88eb..b4bfcb9 100644
--- a/lib/twik.rb
+++ b/lib/twik.rb
@@ -1,5 +1,104 @@
-require "twik/version"
+require 'twik/version'
+require 'twik/exceptions'
+require 'openssl'
+require 'base64'
 
-module Twik
-  # Your code goes here...
+class Twik
+  TYPE = %i{alphanumeric_and_special_chars alphanumeric numeric}
+
+  attr_accessor :privatekey, :length, :type
+
+  def initialize(privatekey, length: 16, type: :alphanumeric_and_special_chars)
+    raise UnknownType.new("type should be one of (#{TYPE.join(', ')})")  unless TYPE.include?(type)
+    raise InvalidLength.new("length should be between 1 and 26") unless (1..26).include?(length)
+
+    self.privatekey = privatekey
+    self.length = length
+    self.type = type
+  end
+
+  def generate(tag, masterkey, length: self.length, type: self.type)
+    tag = hash(privatekey, tag, 24) unless privatekey.nil?
+
+    hash(tag, masterkey, length, type)
+  end
+
+  private
+
+  def hash(data, key, length, type = :alphanumeric_and_special_chars)
+    digest = OpenSSL::Digest.new('sha1')
+    data_hash = Base64.strict_encode64(
+      OpenSSL::HMAC.digest(digest, key, data)).sub(/=*\z/, '')
+
+    sum = data_hash.chars.map(&:ord).inject(:+)
+
+    if type == :numeric
+      data_hash = convert_to_digits(data_hash, sum, length)
+    else
+      data_hash = inject_character(data_hash, 0, 4, sum, length, '0', 10)
+
+      if type == :alphanumeric_and_special_chars
+        data_hash = inject_character(data_hash, 1, 4, sum, length, '!', 15)
+      end
+
+      data_hash = inject_character(data_hash, 2, 4, sum, length, 'A', 26)
+      data_hash = inject_character(data_hash, 3, 4, sum, length, 'a', 26)
+
+      if type == :alphanumeric
+        data_hash = remove_special_characters(data_hash, sum, length)
+      end
+    end
+
+    data_hash.slice(0, length)
+  end
+
+  def convert_to_digits(input, seed, length)
+    pivot = 0
+
+    input.chars.map.with_index do |char, i|
+      if char =~ /[[:digit:]]/
+        res = char
+      else
+        res = ((seed + input[pivot].ord) % 10 + '0'.ord).chr
+        pivot = i + 1
+      end
+
+      res
+    end.join
+  end
+
+  def inject_character(input, offset, reserved, seed, length, c_start, c_num)
+    pos0 = seed % length
+    pos = (pos0 + offset) % length
+
+    c_start = c_start.ord
+
+    (0..length - reserved).each do |i|
+      tmp = (pos0 + reserved + i) % length
+      char = input[tmp].ord
+
+      return input if char >= c_start && char < c_start + c_num
+    end
+
+    head = input[0..pos-1]
+    inject = ((seed + input[pos].ord) % c_num) + c_start
+    tail = input[pos+1..-1]
+
+    head + inject.chr + tail
+  end
+
+  def remove_special_characters(input, seed, length)
+    pivot = 0
+
+    input.chars.map.with_index do |char, i|
+      if char =~ /[[:alnum:]]/
+        res = char
+      else
+        res = ((seed + input[pivot].ord) % 26 + 'A'.ord).chr
+        pivot = i + 1
+      end
+
+      res
+    end.join
+  end
 end
diff --git a/lib/twik/exceptions.rb b/lib/twik/exceptions.rb
new file mode 100644
index 0000000..0e22789
--- /dev/null
+++ b/lib/twik/exceptions.rb
@@ -0,0 +1,4 @@
+class Twik
+  UnknownType = Class.new(StandardError)
+  InvalidLength = Class.new(StandardError)
+end
diff --git a/lib/twik/version.rb b/lib/twik/version.rb
index cbf329a..64b099a 100644
--- a/lib/twik/version.rb
+++ b/lib/twik/version.rb
@@ -1,3 +1,3 @@
-module Twik
+class Twik
   VERSION = "0.0.1"
 end