Change default cipher to authenticated GCM mode.

https://ruby-doc.org/stdlib-2.4.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html#class-OpenSSL::Cipher-label-Authenticated+Encryption+and+Associated+Data+-28AEAD-29

> If the OpenSSL version used supports it, an Authenticated Encryption mode (such as GCM or CCM)
> should always be preferred over any unauthenticated mode. Currently, OpenSSL supports AE only
> in combination with Associated Data (AEAD) where additional associated data is included in the encryption process
> to compute a tag at the end of the encryption. This tag will also be used in the decryption process and by verifying
> its validity, the authenticity of a given ciphertext is established.

Closes #567.

ruby_event_store/lib/ruby_event_store/mappers/encryption_mapper.rb

@@ -25,7 +25,7 @@ def respond_to_missing?(*)
     end
 
     class InMemoryEncryptionKeyRepository
-      DEFAULT_CIPHER = 'aes-256-cbc'.freeze
+      DEFAULT_CIPHER = 'aes-256-gcm'.freeze
 
       def initialize
         @keys = {}
@@ -60,17 +60,21 @@ def initialize(cipher:, key:)
       end
 
       def encrypt(message, iv)
-        crypto     = prepare_encrypt(cipher)
-        crypto.iv  = iv
-        crypto.key = key
-        crypto.update(message) + crypto.final
+        crypto           = prepare_encrypt(cipher)
+        crypto.iv        = iv
+        crypto.key       = key
+        crypto.auth_data = ""
+        crypto.update(message) + crypto.final + crypto.auth_tag
       end
 
       def decrypt(message, iv)
-        crypto     = prepare_decrypt(cipher)
-        crypto.iv  = iv
-        crypto.key = key
-        (crypto.update(message) + crypto.final).force_encoding("UTF-8")
+        ciphertext, auth_tag = message[0...-16], message[-16...]
+        crypto           = prepare_decrypt(cipher)
+        crypto.iv        = iv
+        crypto.key       = key
+        crypto.auth_tag  = auth_tag
+        crypto.auth_data = ""
+        (crypto.update(ciphertext) + crypto.final).force_encoding("UTF-8")
       end
 
       def random_iv

ruby_event_store/spec/mappers/encryption_mapper_spec.rb

@@ -321,7 +321,7 @@ def decrypt(r)
 
         record = encrypt(ticket_transferred)
 
-        with_default_cipher('aes-128-cbc') do
+        with_default_cipher('aes-128-gcm') do
           event = decrypt(record)
 
           expect(event.event_id).to eq(event_id)