diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 4d0e188..a09f8e8 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -3,23 +3,36 @@ name: Test
 on: [push, pull_request]
 
 jobs:
+  rubocop:
+    runs-on: ubuntu-latest
+    env:
+      CI: true
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Ruby 3.2
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 3.2
+          bundler-cache: true
+      - name: Run RuboCop
+        run: bundle exec rubocop --parallel
+
   test:
-    name: Unit test
     strategy:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
         ruby-version: [2.6, 2.7, 3.0, 3.1, 3.2, jruby-9.4, truffleruby]
     runs-on: ${{ matrix.os }}
+    env:
+      CI: true
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Set up Ruby ${{ matrix.ruby-version }}
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby-version }}
-
       - name: Install dependencies
         run: bundle install
-
       - name: Run tests
         run: bundle exec rspec
diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 0000000..f48bd2a
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,37 @@
+inherit_from: .rubocop_todo.yml
+
+# inherit_from: .rubocop_todo.yml
+
+require:
+  - rubocop-performance
+  - rubocop-rake
+  - rubocop-rspec
+
+AllCops:
+  TargetRubyVersion: 2.6
+  NewCops: enable
+  Exclude:
+    - 'data/**/*'
+    - 'vendor/**/*'
+
+Metrics/CollectionLiteralLength:
+  Exclude:
+    - 'lib/paygate/aes.rb'
+
+Naming/FileName:
+  Exclude:
+    - 'lib/paygate-ruby.rb'
+
+Naming/MethodParameterName:
+  Exclude:
+    - 'lib/paygate/aes.rb'
+    - 'lib/paygate/aes_ctr.rb'
+
+RSpec/NotToNot:
+  EnforcedStyle: to_not
+
+Style/Documentation:
+  Enabled: false
+
+Style/ModuleFunction:
+  EnforcedStyle: extend_self
diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
new file mode 100644
index 0000000..53a7ed1
--- /dev/null
+++ b/.rubocop_todo.yml
@@ -0,0 +1,27 @@
+# This configuration was generated by
+# `rubocop --auto-gen-config`
+# on 2023-04-26 08:36:11 UTC using RuboCop version 1.50.0.
+# The point is for the user to remove these configuration records
+# one by one as the offenses are removed from the code base.
+# Note that changes in the inspected code, or installation of new
+# versions of RuboCop, may require this file to be generated again.
+
+# Offense count: 7
+# Configuration parameters: AllowedMethods, AllowedPatterns, CountRepeatedAttributes.
+Metrics/AbcSize:
+  Max: 93
+
+# Offense count: 1
+# Configuration parameters: CountComments, CountAsOne.
+Metrics/ClassLength:
+  Max: 113
+
+# Offense count: 8
+# Configuration parameters: CountComments, CountAsOne, AllowedMethods, AllowedPatterns.
+Metrics/MethodLength:
+  Max: 35
+
+# Offense count: 1
+# Configuration parameters: CountComments, CountAsOne.
+Metrics/ModuleLength:
+  Max: 123
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 05f61b5..7722067 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 - [PR#9](https://github.com/tablecheck/paygate-ruby/pull/9) Do not include Paygate::FormHelper in ActionView::Base. ([johnnyshields](https://github.com/johnnyshields))
 - [PR#9](https://github.com/tablecheck/paygate-ruby/pull/9) Update BIN list. Note BINs can now be a flexible number of digits. ([johnnyshields](https://github.com/johnnyshields))
 - [PR#9](https://github.com/tablecheck/paygate-ruby/pull/9) Rename data config YAML keys. ([johnnyshields](https://github.com/johnnyshields))
+- [PR#10](https://github.com/tablecheck/paygate-ruby/pull/10) Add Rubocop and fix compliance issues. ([johnnyshields](https://github.com/johnnyshields))
 
 ## 0.1.11 - 2022-05-13
 
diff --git a/Gemfile b/Gemfile
index e121039..a2f8224 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 group :development, :test do
@@ -6,6 +8,10 @@ end
 
 group :test do
   gem 'rspec'
+  gem 'rubocop', '~> 1.49.0'
+  gem 'rubocop-performance', '~> 1.16.0'
+  gem 'rubocop-rake', '~> 0.6.0'
+  gem 'rubocop-rspec', '~> 2.19.0'
   gem 'timecop'
 end
 
diff --git a/Rakefile b/Rakefile
index c702cfc..7398a90 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1 +1,3 @@
+# frozen_string_literal: true
+
 require 'bundler/gem_tasks'
diff --git a/lib/paygate-ruby.rb b/lib/paygate-ruby.rb
index 785acb2..29742c0 100644
--- a/lib/paygate-ruby.rb
+++ b/lib/paygate-ruby.rb
@@ -1,43 +1,3 @@
-require 'yaml'
-require 'paygate/version'
-require 'paygate/configuration'
-require 'paygate/aes'
-require 'paygate/aes_ctr'
-require 'paygate/member'
-require 'paygate/response'
-require 'paygate/transaction'
-require 'paygate/profile'
-require 'paygate/action_view/form_helper' if defined?(ActionView)
+# frozen_string_literal: true
 
-module Paygate
-  CONFIG = YAML.load(File.read(File.expand_path('../data/config.yml', File.dirname(__FILE__)))).freeze
-  LOCALES_MAP = CONFIG[:locales].freeze
-  INTL_BRANDS_MAP = CONFIG[:intl_brands].freeze
-  KOREA_BIN_NUMBERS = CONFIG[:korea_bin_numbers].freeze
-  DEFAULT_CURRENCY = 'WON'.freeze
-  DEFAULT_LOCALE = 'US'.freeze
-
-  def mapped_currency(currency)
-    return DEFAULT_CURRENCY unless currency.present?
-
-    currency.to_s == 'KRW' ? 'WON' : currency.to_s
-  end
-  module_function :mapped_currency
-
-  def mapped_locale(locale)
-    locale.present? ? LOCALES_MAP[locale.to_s] : DEFAULT_LOCALE
-  end
-  module_function :mapped_locale
-
-  class << self
-    attr_writer :configuration
-  end
-
-  def self.configuration
-    @configuration ||= Configuration.new
-  end
-
-  def self.configure
-    yield configuration
-  end
-end
+require 'paygate'
diff --git a/lib/paygate.rb b/lib/paygate.rb
new file mode 100644
index 0000000..13a1038
--- /dev/null
+++ b/lib/paygate.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'yaml'
+require 'paygate/version'
+require 'paygate/configuration'
+require 'paygate/aes'
+require 'paygate/aes_ctr'
+require 'paygate/member'
+require 'paygate/response'
+require 'paygate/transaction'
+require 'paygate/profile'
+require 'paygate/action_view/form_helper' if defined?(ActionView)
+
+module Paygate
+  extend self
+
+  CONFIG = YAML.safe_load(File.read(File.expand_path('../data/config.yml', __dir__)),
+                          permitted_classes: [Symbol]).freeze
+  LOCALES_MAP = CONFIG[:locales].freeze
+  INTL_BRANDS_MAP = CONFIG[:intl_brands].freeze
+  KOREA_BIN_NUMBERS = CONFIG[:korea_bin_numbers].freeze
+  DEFAULT_CURRENCY = 'WON'
+  DEFAULT_LOCALE = 'US'
+
+  def mapped_currency(currency)
+    currency = currency&.to_s
+    return DEFAULT_CURRENCY if currency.nil?
+
+    currency.to_s == 'KRW' ? 'WON' : currency.to_s
+  end
+
+  def mapped_locale(locale)
+    LOCALES_MAP[locale&.to_s] || DEFAULT_LOCALE
+  end
+
+  def configuration
+    @configuration ||= Configuration.new
+  end
+
+  def configure
+    yield(configuration)
+  end
+end
diff --git a/lib/paygate/action_view/form_helper.rb b/lib/paygate/action_view/form_helper.rb
index 5cd434a..5e52081 100644
--- a/lib/paygate/action_view/form_helper.rb
+++ b/lib/paygate/action_view/form_helper.rb
@@ -1,155 +1,159 @@
-module Paygate
-module ActionView
-  module FormHelper
-    PAYGATE_FORM_TEXT_FIELDS = {
-      mid: {
-        placeholder: 'Member ID'
-      },
-
-      locale: {
-        name: 'langcode',
-        default: 'KR',
-        placeholder: 'Language'
-      },
-
-      charset: {
-        default: 'UTF-8',
-        placeholder: 'Charset'
-      },
-
-      title: {
-        name: 'goodname',
-        placeholder: 'Title'
-      },
-
-      currency: {
-        name: 'goodcurrency',
-        default: 'WON',
-        placeholder: 'Currency'
-      },
-
-      amount: {
-        name: 'unitprice',
-        placeholder: 'Amount'
-      },
-
-      meta1: {
-        name: 'goodoption1',
-        placeholder: 'Good Option 1'
-      },
-
-      meta2: {
-        name: 'goodoption2',
-        placeholder: 'Good Option 2'
-      },
-
-      meta3: {
-        name: 'goodoption3',
-        placeholder: 'Good Option 3'
-      },
-
-      meta4: {
-        name: 'goodoption4',
-        placeholder: 'Good Option 4'
-      },
-
-      meta5: {
-        name: 'goodoption5',
-        placeholder: 'Good Option 5'
-      },
-
-      pay_method: {
-        name: 'paymethod',
-        default: 'card',
-        placeholder: 'Pay Method'
-      },
-
-      customer_name: {
-        name: 'receipttoname',
-        placeholder: 'Customer Name'
-      },
-
-      customer_email: {
-        name: 'receipttoemail',
-        placeholder: 'Customer Email'
-      },
-
-      card_number: {
-        name: 'cardnumber',
-        placeholder: 'Card Number'
-      },
-
-      expiry_year: {
-        name: 'cardexpireyear',
-        placeholder: 'Expiry Year'
-      },
-
-      expiry_month: {
-        name: 'cardexpiremonth',
-        placeholder: 'Expiry Month'
-      },
-
-      cvv: {
-        name: 'cardsecretnumber',
-        placeholder: 'CVV'
-      },
-
-      card_auth_code: {
-        name: 'cardauthcode',
-        placeholder: 'Card Auth Code'
-      },
-
-      response_code: {
-        name: 'replycode',
-        placeholder: 'Response Code'
-      },
-
-      response_message: {
-        name: 'replyMsg',
-        placeholder: 'Response Message'
-      },
-
-      tid: {
-        placeholder: 'TID'
-      },
-
-      profile_no: {
-        placeholder: 'Profile No'
-      },
-
-      hash_result: {
-        name: 'hashresult',
-        placeholder: 'Hash Result'
-      }
-    }
-
-    def paygate_open_pay_api_js_url
-      (Paygate.configuration.mode == :live) ?
-        'https://api.paygate.net/ajax/common/OpenPayAPI.js'.freeze :
-        'https://stgapi.paygate.net/ajax/common/OpenPayAPI.js'.freeze
-    end
+# frozen_string_literal: true
 
-    def paygate_open_pay_api_form(options = {})
-      form_tag({}, name: 'PGIOForm') do
-        fields = []
-
-        PAYGATE_FORM_TEXT_FIELDS.each do |key, opts|
-          arg_opts = options[key] || {}
-          fields << text_field_tag(
-            key,
-            arg_opts[:value] || opts[:default],
-            name: opts[:name] || key.to_s,
-            placeholder: arg_opts[:placeholder] || opts[:placeholder]
-          ).html_safe
+module Paygate
+  module ActionView
+    module FormHelper
+      PAYGATE_FORM_TEXT_FIELDS = {
+        mid: {
+          placeholder: 'Member ID'
+        },
+
+        locale: {
+          name: 'langcode',
+          default: 'KR',
+          placeholder: 'Language'
+        },
+
+        charset: {
+          default: 'UTF-8',
+          placeholder: 'Charset'
+        },
+
+        title: {
+          name: 'goodname',
+          placeholder: 'Title'
+        },
+
+        currency: {
+          name: 'goodcurrency',
+          default: 'WON',
+          placeholder: 'Currency'
+        },
+
+        amount: {
+          name: 'unitprice',
+          placeholder: 'Amount'
+        },
+
+        meta1: {
+          name: 'goodoption1',
+          placeholder: 'Good Option 1'
+        },
+
+        meta2: {
+          name: 'goodoption2',
+          placeholder: 'Good Option 2'
+        },
+
+        meta3: {
+          name: 'goodoption3',
+          placeholder: 'Good Option 3'
+        },
+
+        meta4: {
+          name: 'goodoption4',
+          placeholder: 'Good Option 4'
+        },
+
+        meta5: {
+          name: 'goodoption5',
+          placeholder: 'Good Option 5'
+        },
+
+        pay_method: {
+          name: 'paymethod',
+          default: 'card',
+          placeholder: 'Pay Method'
+        },
+
+        customer_name: {
+          name: 'receipttoname',
+          placeholder: 'Customer Name'
+        },
+
+        customer_email: {
+          name: 'receipttoemail',
+          placeholder: 'Customer Email'
+        },
+
+        card_number: {
+          name: 'cardnumber',
+          placeholder: 'Card Number'
+        },
+
+        expiry_year: {
+          name: 'cardexpireyear',
+          placeholder: 'Expiry Year'
+        },
+
+        expiry_month: {
+          name: 'cardexpiremonth',
+          placeholder: 'Expiry Month'
+        },
+
+        cvv: {
+          name: 'cardsecretnumber',
+          placeholder: 'CVV'
+        },
+
+        card_auth_code: {
+          name: 'cardauthcode',
+          placeholder: 'Card Auth Code'
+        },
+
+        response_code: {
+          name: 'replycode',
+          placeholder: 'Response Code'
+        },
+
+        response_message: {
+          name: 'replyMsg',
+          placeholder: 'Response Message'
+        },
+
+        tid: {
+          placeholder: 'TID'
+        },
+
+        profile_no: {
+          placeholder: 'Profile No'
+        },
+
+        hash_result: {
+          name: 'hashresult',
+          placeholder: 'Hash Result'
+        }
+      }.freeze
+
+      def paygate_open_pay_api_js_url
+        if Paygate.configuration.mode == :live
+          'https://api.paygate.net/ajax/common/OpenPayAPI.js'
+        else
+          'https://stgapi.paygate.net/ajax/common/OpenPayAPI.js'
         end
-
-        fields.join.html_safe
-      end.html_safe
-    end
-
-    def paygate_open_pay_api_screen
-      content_tag(:div, nil, id: 'PGIOscreen')
+      end
+
+      def paygate_open_pay_api_form(options = {})
+        form_tag({}, name: 'PGIOForm') do
+          fields = []
+
+          PAYGATE_FORM_TEXT_FIELDS.each do |key, opts|
+            arg_opts = options[key] || {}
+            fields << text_field_tag(
+              key,
+              arg_opts[:value] || opts[:default],
+              name: opts[:name] || key.to_s,
+              placeholder: arg_opts[:placeholder] || opts[:placeholder]
+            ).html_safe
+          end
+
+          fields.join.html_safe
+        end.html_safe
+      end
+
+      def paygate_open_pay_api_screen
+        content_tag(:div, nil, id: 'PGIOscreen')
+      end
     end
   end
 end
-end
diff --git a/lib/paygate/aes.rb b/lib/paygate/aes.rb
index d6efb07..9ff432a 100644
--- a/lib/paygate/aes.rb
+++ b/lib/paygate/aes.rb
@@ -1,22 +1,24 @@
+# frozen_string_literal: true
+
 module Paygate
   class Aes
     # Pre-computed multiplicative inverse in GF(2^8)
-    S_BOX = [0x63,0x7c,0x77,0x7b,0xf2,0x6b,0x6f,0xc5,0x30,0x01,0x67,0x2b,0xfe,0xd7,0xab,0x76,
-             0xca,0x82,0xc9,0x7d,0xfa,0x59,0x47,0xf0,0xad,0xd4,0xa2,0xaf,0x9c,0xa4,0x72,0xc0,
-             0xb7,0xfd,0x93,0x26,0x36,0x3f,0xf7,0xcc,0x34,0xa5,0xe5,0xf1,0x71,0xd8,0x31,0x15,
-             0x04,0xc7,0x23,0xc3,0x18,0x96,0x05,0x9a,0x07,0x12,0x80,0xe2,0xeb,0x27,0xb2,0x75,
-             0x09,0x83,0x2c,0x1a,0x1b,0x6e,0x5a,0xa0,0x52,0x3b,0xd6,0xb3,0x29,0xe3,0x2f,0x84,
-             0x53,0xd1,0x00,0xed,0x20,0xfc,0xb1,0x5b,0x6a,0xcb,0xbe,0x39,0x4a,0x4c,0x58,0xcf,
-             0xd0,0xef,0xaa,0xfb,0x43,0x4d,0x33,0x85,0x45,0xf9,0x02,0x7f,0x50,0x3c,0x9f,0xa8,
-             0x51,0xa3,0x40,0x8f,0x92,0x9d,0x38,0xf5,0xbc,0xb6,0xda,0x21,0x10,0xff,0xf3,0xd2,
-             0xcd,0x0c,0x13,0xec,0x5f,0x97,0x44,0x17,0xc4,0xa7,0x7e,0x3d,0x64,0x5d,0x19,0x73,
-             0x60,0x81,0x4f,0xdc,0x22,0x2a,0x90,0x88,0x46,0xee,0xb8,0x14,0xde,0x5e,0x0b,0xdb,
-             0xe0,0x32,0x3a,0x0a,0x49,0x06,0x24,0x5c,0xc2,0xd3,0xac,0x62,0x91,0x95,0xe4,0x79,
-             0xe7,0xc8,0x37,0x6d,0x8d,0xd5,0x4e,0xa9,0x6c,0x56,0xf4,0xea,0x65,0x7a,0xae,0x08,
-             0xba,0x78,0x25,0x2e,0x1c,0xa6,0xb4,0xc6,0xe8,0xdd,0x74,0x1f,0x4b,0xbd,0x8b,0x8a,
-             0x70,0x3e,0xb5,0x66,0x48,0x03,0xf6,0x0e,0x61,0x35,0x57,0xb9,0x86,0xc1,0x1d,0x9e,
-             0xe1,0xf8,0x98,0x11,0x69,0xd9,0x8e,0x94,0x9b,0x1e,0x87,0xe9,0xce,0x55,0x28,0xdf,
-             0x8c,0xa1,0x89,0x0d,0xbf,0xe6,0x42,0x68,0x41,0x99,0x2d,0x0f,0xb0,0x54,0xbb,0x16].freeze
+    S_BOX = [0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe, 0xd7, 0xab, 0x76,
+             0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0,
+             0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,
+             0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75,
+             0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84,
+             0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,
+             0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8,
+             0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2,
+             0xcd, 0x0c, 0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,
+             0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14, 0xde, 0x5e, 0x0b, 0xdb,
+             0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79,
+             0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,
+             0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,
+             0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e,
+             0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,
+             0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16].freeze
 
     # Round Constant used for the Key Expansion [1st col is 2^(r-1) in GF(2^8)]
     R_CON = [[0x00, 0x00, 0x00, 0x00],
@@ -29,10 +31,10 @@ class Aes
              [0x40, 0x00, 0x00, 0x00],
              [0x80, 0x00, 0x00, 0x00],
              [0x1b, 0x00, 0x00, 0x00],
-             [0x36, 0x00, 0x00, 0x00] ].freeze
+             [0x36, 0x00, 0x00, 0x00]].freeze
 
     # Block size (in words): no of columns in state (fixed for AES)
-    CIPHER_BLOCK_SIZE = 4.freeze
+    CIPHER_BLOCK_SIZE = 4
 
     # AES Cipher function: encrypt 'input' state with Rijndael algorithm
     # applies Nr rounds (10/12/14) using key schedule w for 'add round key' stage
@@ -40,8 +42,8 @@ class Aes
     # @param int[][] w   Key schedule as 2D byte-array (Nr+1 x Nb bytes)
     # @returns int[]     Encrypted output state array
     def self.cipher(input, w)
-      nr = w.length / CIPHER_BLOCK_SIZE - 1      # no of rounds: 10/12/14 for 128/192/256-bit keys
-      state = [[],[],[],[]]                      # initialize 4x4 byte-array 'state' with input
+      nr = (w.length / CIPHER_BLOCK_SIZE) - 1 # no of rounds: 10/12/14 for 128/192/256-bit keys
+      state = [[], [], [], []] # initialize 4x4 byte-array 'state' with input
 
       (0...(4 * CIPHER_BLOCK_SIZE)).each do |i|
         state[i % 4][(i / 4.0).floor] = input[i]
@@ -74,17 +76,17 @@ def self.key_expansion(key)
 
       w = []
       temp = []
-      0.upto(nk - 1){ |i| w[i] = [key[4 * i], key[4 * i + 1], key[4 * i + 2], key[4 * i + 3]] }
+      0.upto(nk - 1) { |i| w[i] = [key[4 * i], key[(4 * i) + 1], key[(4 * i) + 2], key[(4 * i) + 3]] }
       (nk...(CIPHER_BLOCK_SIZE * (nr + 1))).each do |i|
         w[i] = []
-        0.upto(3){ |t| temp[t] = w[i - 1][t] }
-        if i % nk == 0
+        0.upto(3) { |t| temp[t] = w[i - 1][t] }
+        if (i % nk).zero?
           temp = sub_word(rotate_word(temp))
-          0.upto(3){ |t| temp[t] ^= R_CON[i / nk][t]}
+          0.upto(3) { |t| temp[t] ^= R_CON[i / nk][t] }
         elsif nk > 6 && i % nk == 4
           temp = sub_word(temp)
         end
-        0.upto(3){ |t| w[i][t] = w[i - nk][t] ^ temp[t] }
+        0.upto(3) { |t| w[i][t] = w[i - nk][t] ^ temp[t] }
       end
       w
     end
@@ -92,7 +94,7 @@ def self.key_expansion(key)
     # apply SBox to state
     def self.sub_bytes(state)
       0.upto(3) do |r|
-        0.upto(CIPHER_BLOCK_SIZE - 1){ |c| state[r][c] = S_BOX[state[r][c]] }
+        0.upto(CIPHER_BLOCK_SIZE - 1) { |c| state[r][c] = S_BOX[state[r][c]] }
       end
       state
     end
@@ -101,20 +103,20 @@ def self.sub_bytes(state)
     def self.shift_rows(state)
       t = []
       1.upto(3) do |r|
-        0.upto(3){ |c| t[c] = state[r][(c + r) % CIPHER_BLOCK_SIZE] }     # shift into temp copy
-        0.upto(3){ |c| state[r][c] = t[c] }                               # and copy back
-      end # note that this will work for nb = 4,5,6, but not 7,8 (always 4 for AES):
+        0.upto(3) { |c| t[c] = state[r][(c + r) % CIPHER_BLOCK_SIZE] }     # shift into temp copy
+        0.upto(3) { |c| state[r][c] = t[c] }                               # and copy back
+      end
       state # see asmaes.sourceforge.net/rijndael/rijndaelImplementation.pdf
     end
 
     # combine bytes of each col of state S
     def self.mix_columns(state)
       0.upto(3) do |c|
-        a=[]                  # 'a' is a copy of the current column from 'state'
-        b=[]                  # 'b' is a•{02} in GF(2^8)
+        a = []                  # 'a' is a copy of the current column from 'state'
+        b = []                  # 'b' is a•{02} in GF(2^8)
         0.upto(3) do |i|
           a[i] = state[i][c]
-          b[i] = (state[i][c] & 0x80 == 0) ? (state[i][c] << 1) ^ 0x011b : state[i][c] << 1
+          b[i] = (state[i][c] & 0x80).zero? ? (state[i][c] << 1) ^ 0x011b : state[i][c] << 1
         end
         # a[n] ^ b[n] is a•{03} in GF(2^8)
         state[0][c] = b[0] ^ a[1] ^ b[1] ^ a[2] ^ a[3]        # 2*a0 + 3*a1 + a2 + a3
@@ -128,21 +130,21 @@ def self.mix_columns(state)
     # xor Round Key into state S
     def self.add_round_key(state, w, rnd)
       0.upto(3) do |r|
-        0.upto(CIPHER_BLOCK_SIZE - 1){ |c| state[r][c] ^= w[rnd * 4 + c][r]}
+        0.upto(CIPHER_BLOCK_SIZE - 1) { |c| state[r][c] ^= w[(rnd * 4) + c][r] }
       end
       state
     end
 
     # apply SBox to 4-byte word w
     def self.sub_word(word)
-      0.upto(3){ |i| word[i] = S_BOX[word[i]] }
+      0.upto(3) { |i| word[i] = S_BOX[word[i]] }
       word
     end
 
     # rotate 4-byte word w left by one byte
     def self.rotate_word(word)
       tmp = word[0]
-      0.upto(2){ |i| word[i] = word[i + 1] }
+      0.upto(2) { |i| word[i] = word[i + 1] }
       word[3] = tmp
       word
     end
diff --git a/lib/paygate/aes_ctr.rb b/lib/paygate/aes_ctr.rb
index 1071da0..968f38e 100644
--- a/lib/paygate/aes_ctr.rb
+++ b/lib/paygate/aes_ctr.rb
@@ -1,8 +1,9 @@
+# frozen_string_literal: true
+
 require 'base64'
 
 module Paygate
   class AesCtr
-
     #  Encrypt a text using AES encryption in Counter mode of operation
     #
     #  Unicode multi-byte character safe
@@ -12,16 +13,19 @@ class AesCtr
     #  @param int num_bits     Number of bits to be used in the key (128, 192, or 256)
     #  @returns string         Encrypted text
     def self.encrypt(plaintext, password, num_bits)
-      block_size = 16      # block size fixed at 16 bytes / 128 bits (Nb=4) for AES
+      block_size = 16 # block size fixed at 16 bytes / 128 bits (Nb=4) for AES
       return '' unless [128, 192, 256].include?(num_bits)
 
       # use AES itself to encrypt password to get cipher key (using plain password as source for key
       # expansion) - gives us well encrypted key (though hashed key might be preferred for prod'n use)
-      num_bytes = num_bits / 8        # no bytes in key (16/24/32)
+      num_bytes = num_bits / 8 # no bytes in key (16/24/32)
       pw_bytes = []
-      0.upto(num_bytes - 1){ |i| pw_bytes[i] = password.bytes.to_a[i] & 0xff || 0}      # use 1st 16/24/32 chars of password for key #warn
-      key = Aes.cipher(pw_bytes, Aes.key_expansion(pw_bytes))        # gives us 16-byte key
-      key = key + key[0, num_bytes - 16]                             # expand key to 16/24/32 bytes long
+      # use 1st 16/24/32 chars of password for key #warn
+      0.upto(num_bytes - 1) do |i|
+        pw_bytes[i] = (password.bytes.to_a[i] & 0xff) || 0
+      end
+      key = Aes.cipher(pw_bytes, Aes.key_expansion(pw_bytes)) # gives us 16-byte key
+      key += key[0, num_bytes - 16] # expand key to 16/24/32 bytes long
 
       # initialise 1st 8 bytes of counter block with nonce (NIST SP800-38A §B.2): [0-1] = millisec,
       # [2-3] = random, [4-7] = seconds, together giving full sub-millisec uniqueness up to Feb 2106
@@ -29,14 +33,14 @@ def self.encrypt(plaintext, password, num_bits)
       nonce = Time.now.to_i
       nonce_ms = nonce % 1000
       nonce_sec = (nonce / 1000.0).floor
-      nonce_rand = (rand() * 0xffff).floor
-      0.upto(1){ |i| counter_block[i] = urs(nonce_ms, i * 8) & 0xff }
-      0.upto(1){ |i| counter_block[i + 2] = urs(nonce_rand, i * 8) & 0xff }
-      0.upto(3){ |i| counter_block[i + 4] = urs(nonce_sec, i * 8) & 0xff }
+      nonce_rand = (rand * 0xffff).floor
+      0.upto(1) { |i| counter_block[i] = urs(nonce_ms, i * 8) & 0xff }
+      0.upto(1) { |i| counter_block[i + 2] = urs(nonce_rand, i * 8) & 0xff }
+      0.upto(3) { |i| counter_block[i + 4] = urs(nonce_sec, i * 8) & 0xff }
 
       # and convert it to a string to go on the front of the ciphertext
       ctr_text = ''
-      0.upto(7){ |i| ctr_text += counter_block[i].chr }
+      0.upto(7) { |i| ctr_text += counter_block[i].chr }
 
       # generate key schedule - an expansion of the key into distinct Key Rounds for each round
       key_schedule = Aes.key_expansion(key)
@@ -46,85 +50,84 @@ def self.encrypt(plaintext, password, num_bits)
       0.upto(block_count - 1) do |b|
         # set counter (block #) in last 8 bytes of counter block (leaving nonce in 1st 8 bytes)
         # done in two stages for 32-bit ops: using two words allows us to go past 2^32 blocks (68GB)
-        0.upto(3){ |c| counter_block[15 - c] = urs(b, c * 8) & 0xff }
-        0.upto(3){ |c| counter_block[15 - c - 4] = urs(b / 0x100000000, c * 8) }
+        0.upto(3) { |c| counter_block[15 - c] = urs(b, c * 8) & 0xff }
+        0.upto(3) { |c| counter_block[15 - c - 4] = urs(b / 0x100000000, c * 8) }
 
         cipher_cntr = Aes.cipher(counter_block, key_schedule) # -- encrypt counter block --
         # block size is reduced on final block
-        block_length =  b < block_count - 1 ? block_size : (plaintext.length - 1) % block_size + 1
+        block_length = b < block_count - 1 ? block_size : ((plaintext.length - 1) % block_size) + 1
         cipher_char = []
         0.upto(block_length - 1) do |i|
-          cipher_char[i] = (cipher_cntr[i] ^ plaintext.bytes.to_a[b * block_size + i]).chr
+          cipher_char[i] = (cipher_cntr[i] ^ plaintext.bytes.to_a[(b * block_size) + i]).chr
         end
         cipher_text[b] = cipher_char.join
       end
 
       cipher_text = ctr_text + cipher_text.join
-      Base64.encode64(cipher_text).gsub(/\n/, '') + "\n" # encode in base64
+      "#{Base64.encode64(cipher_text).delete("\n")}\n" # encode in base64
     end
 
     # Decrypt a text encrypted by AES in counter mode of operation
     #
     # @param string ciphertext Source text to be encrypted
     # @param string password   The password to use to generate a key
-    # @param int nBits      Number of bits to be used in the key (128, 192, or 256)
+    # @param int n_bits      Number of bits to be used in the key (128, 192, or 256)
     # @returns string
     #           Decrypted text
-    def self.decrypt(ciphertext, password, nBits)
-      blockSize = 16  # block size fixed at 16 bytes / 128 bits (Nb=4) for AES
-      return '' unless(nBits==128 || nBits==192 || nBits==256)
+    def self.decrypt(ciphertext, password, n_bits)
+      block_size = 16 # block size fixed at 16 bytes / 128 bits (Nb=4) for AES
+      return '' unless [128, 192, 256].include?(n_bits)
+
       ciphertext = Base64.decode64(ciphertext)
 
-      nBytes = nBits/8  # no bytes in key (16/24/32)
-      pwBytes = []
-      0.upto(nBytes-1){|i| pwBytes[i] = password.bytes.to_a[i] & 0xff || 0}
-      key = Aes.cipher(pwBytes, Aes.key_expansion(pwBytes)) # gives us 16-byte key
-      key = key.concat(key.slice(0, nBytes-16)) # expand key to 16/24/32 bytes long
+      n_bytes = n_bits / 8 # no bytes in key (16/24/32)
+      pw_bytes = []
+      0.upto(n_bytes - 1) { |i| pw_bytes[i] = (password.bytes.to_a[i] & 0xff) || 0 }
+      key = Aes.cipher(pw_bytes, Aes.key_expansion(pw_bytes)) # gives us 16-byte key
+      key.concat(key.slice(0, n_bytes - 16)) # expand key to 16/24/32 bytes long
       # recover nonce from 1st 8 bytes of ciphertext
-      counterBlock = []
-      ctrTxt = ciphertext[0,8]
-      0.upto(7){|i| counterBlock[i] = ctrTxt.bytes.to_a[i]}
+      counter_block = []
+      ctr_txt = ciphertext[0, 8]
+      0.upto(7) { |i| counter_block[i] = ctr_txt.bytes.to_a[i] }
 
-      #generate key Schedule
-      keySchedule = Aes.key_expansion(key)
+      # generate key Schedule
+      key_schedule = Aes.key_expansion(key)
 
       # separate ciphertext into blocks (skipping past initial 8 bytes)
-      nBlocks = ((ciphertext.length-8)/blockSize.to_f).ceil
-      ct=[]
-      0.upto(nBlocks-1){|b|ct[b] = ciphertext[8+b*blockSize, 16]}
+      n_blocks = ((ciphertext.length - 8) / block_size.to_f).ceil
+      ct = []
+      0.upto(n_blocks - 1) { |b| ct[b] = ciphertext[8 + (b * block_size), 16] }
 
-      ciphertext = ct;  # ciphertext is now array of block-length strings
+      ciphertext = ct; # ciphertext is now array of block-length strings
 
       # plaintext will get generated block-by-block into array of block-length strings
       plaintxt = []
-      0.upto(nBlocks-1) do |b|
-        0.upto(3){|c| counterBlock[15-c] = urs(b,c*8) & 0xff}
-        0.upto(3){|c| counterBlock[15-c-4] = urs((b+1)/(0x100000000-1),c*8) & 0xff}
-        cipherCntr = Aes.cipher(counterBlock, keySchedule)  # encrypt counter block
-        plaintxtByte = []
+      0.upto(n_blocks - 1) do |b|
+        0.upto(3) { |c| counter_block[15 - c] = urs(b, c * 8) & 0xff }
+        0.upto(3) { |c| counter_block[15 - c - 4] = urs((b + 1) / (0x100000000 - 1), c * 8) & 0xff }
+        cipher_cntr = Aes.cipher(counter_block, key_schedule) # encrypt counter block
+        plaintxt_byte = []
         0.upto(ciphertext[b].length - 1) do |i|
           # -- xor plaintxt with ciphered counter byte-by-byte --
-          plaintxtByte[i] = (cipherCntr[i] ^ ciphertext[b].bytes.to_a[i]).chr;
+          plaintxt_byte[i] = (cipher_cntr[i] ^ ciphertext[b].bytes.to_a[i]).chr
         end
-        plaintxt[b] = plaintxtByte.join('')
+        plaintxt[b] = plaintxt_byte.join
       end
-      plaintxt.join('')
+      plaintxt.join
     end
 
-    private
-
-     # Unsigned right shift function, since Ruby has neither >>> operator nor unsigned ints
-     #
-     # @param a  number to be shifted (32-bit integer)
-     # @param b  number of bits to shift a to the right (0..31)
-     # @return   a right-shifted and zero-filled by b bits
+    # Unsigned right shift function, since Ruby has neither >>> operator nor unsigned ints
+    #
+    # @param a  number to be shifted (32-bit integer)
+    # @param b  number of bits to shift a to the right (0..31)
+    # @return   a right-shifted and zero-filled by b bits
     def self.urs(a, b)
       a &= 0xffffffff
       b &= 0x1f
-      if a & 0x80000000 && b > 0    # if left-most bit set
+      if (a & 0x80000000) && b.positive? # if left-most bit set
         a = ((a >> 1) & 0x7fffffff) # right-shift one bit & clear left-most bit
         a = a >> (b - 1)            # remaining right-shifts
-      else                     # otherwise
+      else # otherwise
         a = (a >> b);            # use normal right-shift
       end
       a
diff --git a/lib/paygate/configuration.rb b/lib/paygate/configuration.rb
index 8164772..c8e217a 100644
--- a/lib/paygate/configuration.rb
+++ b/lib/paygate/configuration.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Paygate
   class Configuration
     MODES = %i[live sandbox].freeze
@@ -10,7 +12,8 @@ def initialize
 
     def mode=(value)
       value = value.to_sym
-      fail 'Invalid mode. Value must be one of the following: :live, :sandbox' unless value && MODES.include?(value)
+      raise 'Invalid mode. Value must be one of the following: :live, :sandbox' unless value && MODES.include?(value)
+
       @mode = value
     end
   end
diff --git a/lib/paygate/member.rb b/lib/paygate/member.rb
index 6023782..8fed3b4 100644
--- a/lib/paygate/member.rb
+++ b/lib/paygate/member.rb
@@ -1,9 +1,12 @@
+# frozen_string_literal: true
+
 module Paygate
   class Member
     attr_reader :mid, :secret
 
     def initialize(mid, secret)
-      @mid, @secret = mid, secret
+      @mid = mid
+      @secret = secret
     end
 
     def refund_transaction(txn_id, options = {})
diff --git a/lib/paygate/profile.rb b/lib/paygate/profile.rb
index 0bf0315..303238c 100644
--- a/lib/paygate/profile.rb
+++ b/lib/paygate/profile.rb
@@ -1,9 +1,11 @@
+# frozen_string_literal: true
+
 require 'uri'
 require 'net/http'
 
 module Paygate
   class Profile
-    PURCHASE_URL = 'https://service.paygate.net/INTL/pgtlProcess3.jsp'.freeze
+    PURCHASE_URL = 'https://service.paygate.net/INTL/pgtlProcess3.jsp'
 
     attr_reader :profile_no
     attr_accessor :member
diff --git a/lib/paygate/response.rb b/lib/paygate/response.rb
index 6038660..cbb96d7 100644
--- a/lib/paygate/response.rb
+++ b/lib/paygate/response.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Paygate
   class Response
     attr_accessor :transaction_type, :http_code, :message, :body, :raw_info, :json
diff --git a/lib/paygate/transaction.rb b/lib/paygate/transaction.rb
index a10af75..0e9fee8 100644
--- a/lib/paygate/transaction.rb
+++ b/lib/paygate/transaction.rb
@@ -1,10 +1,12 @@
+# frozen_string_literal: true
+
 require 'digest'
 require 'uri'
 require 'net/http'
 
 module Paygate
   class Transaction
-    FULL_AMOUNT_IDENTIFIER = 'F'.freeze
+    FULL_AMOUNT_IDENTIFIER = 'F'
 
     attr_reader :tid
     attr_accessor :member
@@ -15,8 +17,8 @@ def initialize(tid)
 
     def refund(options = {})
       # Encrypt data
-      api_key_256 = ::Digest::SHA256.hexdigest(member.secret)
-      aes_ctr = AesCtr.encrypt(tid, api_key_256, 256)
+      api_key256 = ::Digest::SHA256.hexdigest(member.secret)
+      aes_ctr = AesCtr.encrypt(tid, api_key256, 256)
       tid_enc = "AES256#{aes_ctr}"
 
       # Prepare params
@@ -32,7 +34,7 @@ def refund(options = {})
       response = ::Net::HTTP.get_response(uri)
 
       r = Response.build_from_net_http_response(:refund, response)
-      r.raw_info = OpenStruct.new(tid: tid, tid_enc: tid_enc, request_url: uri.to_s)
+      r.raw_info = OpenStruct.new(tid: tid, tid_enc: tid_enc, request_url: uri.to_s) # rubocop:disable Style/OpenStructUse
       r
     end
 
@@ -47,12 +49,12 @@ def verify
       Response.build_from_net_http_response(:verify, response)
     end
 
-    private
-
     def self.refund_api_url
-      (Paygate.configuration.mode == :live) ?
-        'https://service.paygate.net/service/cancelAPI.json' :
+      if Paygate.configuration.mode == :live
+        'https://service.paygate.net/service/cancelAPI.json'
+      else
         'https://stgsvc.paygate.net/service/cancelAPI.json'
+      end
     end
   end
 end
diff --git a/lib/paygate/version.rb b/lib/paygate/version.rb
index 6e3da7d..814b10d 100644
--- a/lib/paygate/version.rb
+++ b/lib/paygate/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Paygate
   VERSION = '0.2.0'
 end
diff --git a/paygate-ruby.gemspec b/paygate-ruby.gemspec
index 0777471..7200142 100644
--- a/paygate-ruby.gemspec
+++ b/paygate-ruby.gemspec
@@ -1,5 +1,6 @@
-# coding: utf-8
-lib = File.expand_path('../lib', __FILE__)
+# frozen_string_literal: true
+
+lib = File.expand_path('lib', __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 
 require 'paygate/version'
@@ -12,7 +13,8 @@ Gem::Specification.new do |spec|
   spec.summary       = 'Ruby wrapper for PayGate Korea payment gateway'
   spec.license       = 'MIT'
 
+  spec.required_ruby_version = '>= 2.6.0'
   spec.files         = Dir.glob('{data,lib,vendor}/**/*') + %w[CHANGELOG.md LICENSE.txt README.md Rakefile]
-  spec.test_files    = Dir.glob('spec/**/*')
   spec.require_paths = ['lib']
+  spec.metadata['rubygems_mfa_required'] = 'true'
 end
diff --git a/spec/integration/paygate_spec.rb b/spec/integration/paygate_spec.rb
index 3fd3deb..230e340 100644
--- a/spec/integration/paygate_spec.rb
+++ b/spec/integration/paygate_spec.rb
@@ -2,8 +2,8 @@
 
 require_relative '../spec_helper'
 
-RSpec.describe 'basic' do
+RSpec.describe Paygate do
   it 'can load gem' do
-    expect(true).to eq true
+    expect(described_class).to_not be_nil
   end
 end
diff --git a/spec/paygate/aes_ctr_spec.rb b/spec/paygate/aes_ctr_spec.rb
new file mode 100644
index 0000000..b6b2c31
--- /dev/null
+++ b/spec/paygate/aes_ctr_spec.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require_relative '../spec_helper'
+
+RSpec.describe Paygate::AesCtr do
+  let(:plaintext) { 'This is a test message' }
+  let(:password) { 'password' }
+  let(:num_bits) { 128 }
+  let(:encrypted_text) { "yACaHyMKGQCJtHL8KR0loRQOGV5zIjbGHltR8QL4\n" }
+  let(:invalid_num_bits) { 2560 }
+
+  describe '.encrypt' do
+    before do
+      allow(Object).to receive(:rand).and_return(0.123456789)
+    end
+
+    around do |example|
+      Timecop.freeze(Time.utc(2022, 1, 1)) { example.run }
+    end
+
+    it 'returns an encrypted text string' do
+      expect(described_class.encrypt(plaintext, password, num_bits)).to eq(encrypted_text)
+    end
+
+    it 'returns an empty string if num_bits is invalid' do
+      expect(described_class.encrypt(plaintext, password, invalid_num_bits)).to eq('')
+    end
+  end
+
+  describe '.decrypt' do
+    it 'returns a decrypted text string' do
+      decrypted_text = described_class.decrypt(encrypted_text, password, num_bits)
+      expect(decrypted_text).to eq(plaintext)
+    end
+
+    it 'returns an empty string if num_bits is invalid' do
+      expect(described_class.decrypt(encrypted_text, password, invalid_num_bits)).to eq('')
+    end
+  end
+end
diff --git a/spec/unit/aes_spec.rb b/spec/paygate/aes_spec.rb
similarity index 90%
rename from spec/unit/aes_spec.rb
rename to spec/paygate/aes_spec.rb
index 31565fe..188391f 100644
--- a/spec/unit/aes_spec.rb
+++ b/spec/paygate/aes_spec.rb
@@ -1,19 +1,21 @@
+# frozen_string_literal: true
+
 require_relative '../spec_helper'
 
 RSpec.describe Paygate::Aes do
-  let(:key_128) { '0123456789abcdef' }
-  let(:key_192) { '0123456789abcdef012345' }
-  let(:key_256) { '0123456789abcdef0123456789abcdef' }
+  let(:key128) { '0123456789abcdef' }
+  let(:key192) { '0123456789abcdef012345' }
+  let(:key256) { '0123456789abcdef0123456789abcdef' }
 
   describe '.cipher' do
     context 'when encrypting with 128-bit key' do
       it 'encrypts the input correctly' do
         input = '0123456789abcdef'
         expected_output = '72727e881edcfd0100a718687909b565'
-        key_schedule = described_class.key_expansion(key_128.bytes)
+        key_schedule = described_class.key_expansion(key128.bytes)
         output = described_class.cipher(input.bytes, key_schedule)
 
-        expect(output.pack('c*').unpack('H*').first).to eq(expected_output)
+        expect(output.pack('c*').unpack1('H*')).to eq(expected_output)
       end
     end
 
@@ -21,10 +23,10 @@
       it 'encrypts the input correctly' do
         input = '0123456789abcdef01234567'
         expected_output = '943cb7b4f5ec4afcbd2973b72ba25e4a'
-        key_schedule = described_class.key_expansion(key_192.bytes)
+        key_schedule = described_class.key_expansion(key192.bytes)
         output = described_class.cipher(input.bytes, key_schedule)
 
-        expect(output.pack('c*').unpack('H*').first).to eq(expected_output)
+        expect(output.pack('c*').unpack1('H*')).to eq(expected_output)
       end
     end
 
@@ -32,10 +34,10 @@
       it 'encrypts the input correctly' do
         input = '0123456789abcdef0123456789abcdef'
         expected_output = 'f83c9a60dc0cdb98219f79d6d5db1635'
-        key_schedule = described_class.key_expansion(key_256.bytes)
+        key_schedule = described_class.key_expansion(key256.bytes)
         output = described_class.cipher(input.bytes, key_schedule)
 
-        expect(output.pack('c*').unpack('H*').first).to eq(expected_output)
+        expect(output.pack('c*').unpack1('H*')).to eq(expected_output)
       end
     end
   end
@@ -92,7 +94,7 @@
       end
 
       it 'returns the expected key schedule' do
-        key = key_128.bytes
+        key = key128.bytes
         schedule = described_class.key_expansion(key)
         expect(schedule).to eq(expected_schedule)
       end
@@ -153,7 +155,7 @@
       end
 
       it 'returns the expected key schedule' do
-        key = key_192.bytes
+        key = key192.bytes
         schedule = described_class.key_expansion(key)
         expect(schedule).to eq(expected_schedule)
       end
@@ -226,7 +228,7 @@
       end
 
       it 'returns the expected key schedule' do
-        key = key_256.bytes
+        key = key256.bytes
         schedule = described_class.key_expansion(key)
         expect(schedule).to eq(expected_schedule)
       end
diff --git a/spec/unit/aes_ctr_spec.rb b/spec/unit/aes_ctr_spec.rb
deleted file mode 100644
index 75d4d5a..0000000
--- a/spec/unit/aes_ctr_spec.rb
+++ /dev/null
@@ -1,38 +0,0 @@
-require_relative '../spec_helper'
-
-RSpec.describe Paygate::AesCtr do
-  let(:plaintext) { "This is a test message" }
-  let(:password) { "password" }
-  let(:num_bits) { 128 }
-  let(:encrypted_text) { "yACaHyMKGQCJtHL8KR0loRQOGV5zIjbGHltR8QL4\n" }
-  let(:invalid_num_bits) { 2560 }
-
-  describe ".encrypt" do
-    before do
-      allow(Object).to receive(:rand).and_return(0.123456789)
-    end
-
-    around do |example|
-      Timecop.freeze(Time.utc(2022, 1, 1)) { example.run }
-    end
-
-    it "returns an encrypted text string" do
-      expect(Paygate::AesCtr.encrypt(plaintext, password, num_bits)).to eq(encrypted_text)
-    end
-
-    it "returns an empty string if num_bits is invalid" do
-      expect(Paygate::AesCtr.encrypt(plaintext, password, invalid_num_bits)).to eq('')
-    end
-  end
-
-  describe ".decrypt" do
-    it "returns a decrypted text string" do
-      decrypted_text = Paygate::AesCtr.decrypt(encrypted_text, password, num_bits)
-      expect(decrypted_text).to eq(plaintext)
-    end
-
-    it "returns an empty string if num_bits is invalid" do
-      expect(Paygate::AesCtr.decrypt(encrypted_text, password, invalid_num_bits)).to eq('')
-    end
-  end
-end