improve gpg key validation

when omitting the end part of the key ('-----END PGP PUBLIC KEY
BLOCK-----') the error message was not about the key anymore, but about
the missing fingerprint and primary_keyid, which was confusing for the
user.
the new validation checks that the end also matches the expected format.

app/models/gpg_key.rb

 class GpgKey < ActiveRecord::Base
   KEY_PREFIX = '-----BEGIN PGP PUBLIC KEY BLOCK-----'.freeze
+  KEY_SUFFIX = '-----END PGP PUBLIC KEY BLOCK-----'.freeze
   include ShaAttribute
@@ -15,8 +16,8 @@ class GpgKey < ActiveRecord::Base
     presence: true,
     uniqueness: true,
     format: {
-      with: /\A#{KEY_PREFIX}((?!#{KEY_PREFIX}).)+\Z/m,
-      message: "is invalid. A valid public GPG key begins with '#{KEY_PREFIX}'"
+      with: /\A#{KEY_PREFIX}((?!#{KEY_PREFIX})(?!#{KEY_SUFFIX}).)+#{KEY_SUFFIX}\Z/m,
+      message: "is invalid. A valid public GPG key begins with '#{KEY_PREFIX}' and ends with '#{KEY_SUFFIX}'"
     }
   validates :fingerprint,

spec/models/gpg_key_spec.rb

@@ -7,10 +7,18 @@ describe GpgKey do
   describe "validation" do
     it { is_expected.to validate_presence_of(:user) }
+
     it { is_expected.to validate_presence_of(:key) }
     it { is_expected.to validate_uniqueness_of(:key) }
-    it { is_expected.to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----\nkey").for(:key) }
+
+    it { is_expected.to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----\nkey\n-----END PGP PUBLIC KEY BLOCK-----").for(:key) }
+
+    it { is_expected.not_to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----\nkey").for(:key) }
     it { is_expected.not_to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----\nkey\n-----BEGIN PGP PUBLIC KEY BLOCK-----").for(:key) }
+    it { is_expected.not_to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK----------END PGP PUBLIC KEY BLOCK-----").for(:key) }
+    it { is_expected.not_to allow_value("-----BEGIN PGP PUBLIC KEY BLOCK-----").for(:key) }
+    it { is_expected.not_to allow_value("-----END PGP PUBLIC KEY BLOCK-----").for(:key) }
+    it { is_expected.not_to allow_value("key\n-----END PGP PUBLIC KEY BLOCK-----").for(:key) }
     it { is_expected.not_to allow_value('BEGIN PGP').for(:key) }
   end