diff --git a/app/controllers/staffs/x509_certificates/private_keys_controller.rb b/app/controllers/staffs/x509_certificates/private_keys_controller.rb
index ef47221..76db01d 100644
--- a/app/controllers/staffs/x509_certificates/private_keys_controller.rb
+++ b/app/controllers/staffs/x509_certificates/private_keys_controller.rb
@@ -10,11 +10,11 @@ class Staffs::X509Certificates::PrivateKeysController < ApplicationController
     authorize [:staff, X509Certificate,
                PublicKeyPrivateKey.new(@rsa_public_key)]
 
-    result = DecryptRSAPrivateKey.call public_key: @rsa_public_key
+    @rsa_public_key.decrypt_private_key_pem
 
     respond_to do |format|
       format.key do
-        send_data result.private_key_pem_cleartext, filename: 'private.key'
+        send_data @rsa_public_key.private_key_pem, filename: 'private.key'
       end
     end
   end
diff --git a/app/interactors/decrypt_rsa_private_key.rb b/app/interactors/decrypt_rsa_private_key.rb
deleted file mode 100644
index 70c4a07..0000000
--- a/app/interactors/decrypt_rsa_private_key.rb
+++ /dev/null
@@ -1,24 +0,0 @@
-# frozen_string_literal: true
-
-class DecryptRSAPrivateKey
-  include Interactor
-
-  before :set_cipher
-
-  def call
-    context.public_key.private_key_pem = [
-      @cipher.update(context.public_key.private_key_pem_ciphertext),
-      @cipher.final,
-    ].join.freeze
-  end
-
-private
-
-  def set_cipher
-    @cipher = OpenSSL::Cipher::AES256.new
-    @cipher.decrypt
-
-    @cipher.iv  = context.public_key.private_key_pem_iv
-    @cipher.key = context.public_key.private_key_pem_secret
-  end
-end
diff --git a/app/models/rsa_public_key.rb b/app/models/rsa_public_key.rb
index c1d31af..c731405 100644
--- a/app/models/rsa_public_key.rb
+++ b/app/models/rsa_public_key.rb
@@ -10,4 +10,21 @@ class RSAPublicKey < ApplicationRecord
   validates :public_key_pem, presence: true
 
   validates :bits, inclusion: { in: [2048, 4096] }
+
+  ###########
+  # Methods #
+  ###########
+
+  def decrypt_private_key_pem
+    cipher = OpenSSL::Cipher::AES256.new
+    cipher.decrypt
+
+    cipher.iv  = private_key_pem_iv
+    cipher.key = private_key_pem_secret
+
+    self.private_key_pem = [
+      cipher.update(private_key_pem_ciphertext),
+      cipher.final,
+    ].join.freeze
+  end
 end
diff --git a/spec/interactors/decrypt_rsa_private_key_spec.rb b/spec/interactors/decrypt_rsa_private_key_spec.rb
deleted file mode 100644
index 09a9ab6..0000000
--- a/spec/interactors/decrypt_rsa_private_key_spec.rb
+++ /dev/null
@@ -1,7 +0,0 @@
-# frozen_string_literal: true
-
-require 'rails_helper'
-
-RSpec.describe DecryptRSAPrivateKey do
-  pending "add some examples to (or delete) #{__FILE__}"
-end
diff --git a/spec/models/rsa_public_key_spec.rb b/spec/models/rsa_public_key_spec.rb
index b5e61cc..3e8bc6b 100644
--- a/spec/models/rsa_public_key_spec.rb
+++ b/spec/models/rsa_public_key_spec.rb
@@ -20,4 +20,40 @@ RSpec.describe RSAPublicKey do
   describe '#private_key_pem_ciphertext' do
     it { is_expected.not_to validate_presence_of :private_key_pem_ciphertext }
   end
+
+  describe '#decrypt_private_key_pem' do
+    let(:cleartext) { OpenSSL::PKey::RSA.new.to_pem }
+
+    before do
+      cipher = OpenSSL::Cipher::AES256.new
+      cipher.encrypt
+
+      subject.private_key_pem_iv     = cipher.random_iv
+      subject.private_key_pem_secret = cipher.random_key
+
+      subject.private_key_pem_ciphertext = [
+        cipher.update(cleartext),
+        cipher.final,
+      ].join
+    end
+
+    specify do
+      expect(subject.decrypt_private_key_pem).to be_instance_of String
+    end
+
+    specify do
+      expect(subject.decrypt_private_key_pem).to be_frozen
+    end
+
+    specify do
+      expect(subject.decrypt_private_key_pem).to equal subject.private_key_pem
+    end
+
+    specify do
+      expect { subject.decrypt_private_key_pem }.to \
+        change(subject, :private_key_pem)
+        .from(nil)
+        .to(cleartext)
+    end
+  end
 end