Improve thread safety of Symmetric cipher

Previously using Symmetric encryption
from multiple threads could result in errors
from OpenSSL. This commit adds a mutex
to synchronize access to the OpenSSL
instance for encryption and decryption.

CHANGELOG.md

@@ -1,3 +1,8 @@
+# v3.0.1
+
+ * The symmetric cipher class now encrypts and decrypts in a thread-safe manner.
+   [cyberark/slosilo#31](https://github.com/cyberark/slosilo/pull/31)
+
 # v3.0.0
 
 * Transition to Ruby 3. Consuming projects based on Ruby 2 shall use slosilo V2.X.X.

lib/slosilo/symmetric.rb

@@ -5,6 +5,7 @@ class Symmetric
 
     def initialize
       @cipher = OpenSSL::Cipher.new 'aes-256-gcm' # NB: has to be lower case for whatever reason.
+      @cipher_mutex = Mutex.new
     end
 
     # This lets us do a final sanity check in migrations from older encryption versions
@@ -13,34 +14,42 @@ def cipher_name
     end
 
     def encrypt plaintext, opts = {}
-      @cipher.reset
-      @cipher.encrypt
-      @cipher.key = (opts[:key] or raise("missing :key option"))
-      @cipher.iv = iv = random_iv
-      @cipher.auth_data = opts[:aad] || "" # Nothing good happens if you set this to nil, or don't set it at all
-      ctext = @cipher.update(plaintext) + @cipher.final
-      tag = @cipher.auth_tag(TAG_LENGTH)
-      "#{VERSION_MAGIC}#{tag}#{iv}#{ctext}"
+      # All of these operations in OpenSSL must occur atomically, so we
+      # synchronize their access to make this step thread-safe.
+      @cipher_mutex.synchronize do
+        @cipher.reset
+        @cipher.encrypt
+        @cipher.key = (opts[:key] or raise("missing :key option"))
+        @cipher.iv = iv = random_iv
+        @cipher.auth_data = opts[:aad] || "" # Nothing good happens if you set this to nil, or don't set it at all
+        ctext = @cipher.update(plaintext) + @cipher.final
+        tag = @cipher.auth_tag(TAG_LENGTH)
+        "#{VERSION_MAGIC}#{tag}#{iv}#{ctext}"
+      end
     end
 
     def decrypt ciphertext, opts = {}
       version, tag, iv, ctext = unpack ciphertext
 
       raise "Invalid version magic: expected #{VERSION_MAGIC} but was #{version}" unless version == VERSION_MAGIC
 
-      @cipher.reset
-      @cipher.decrypt
-      @cipher.key = opts[:key]
-      @cipher.iv = iv
-      @cipher.auth_tag = tag
-      @cipher.auth_data = opts[:aad] || ""
-      @cipher.update(ctext) + @cipher.final
+      # All of these operations in OpenSSL must occur atomically, so we
+      # synchronize their access to make this step thread-safe.
+      @cipher_mutex.synchronize do
+        @cipher.reset
+        @cipher.decrypt
+        @cipher.key = opts[:key]
+        @cipher.iv = iv
+        @cipher.auth_tag = tag
+        @cipher.auth_data = opts[:aad] || ""
+        @cipher.update(ctext) + @cipher.final
+      end
     end
-    
+
     def random_iv
       @cipher.random_iv
     end
-    
+
     def random_key
       @cipher.random_key
     end

lib/slosilo/version.rb

@@ -1,3 +1,3 @@
 module Slosilo
-  VERSION = "3.0.0"
+  VERSION = "3.0.1"
 end

spec/symmetric_spec.rb

@@ -14,8 +14,29 @@
       expect(subject.encrypt(plaintext, key: key, aad: auth_data)).to eq(ciphertext)
     end
   end
-  
+
   describe '#decrypt' do
+
+    it "doesn't fail when called by multiple threads" do
+      threads = []
+
+      begin
+        # Verify we can successfuly decrypt using many threads without OpenSSL
+        # errors.
+        1000.times do
+          threads << Thread.new do
+            100.times do
+              expect(
+                subject.decrypt(ciphertext, key: key, aad: auth_data)
+              ).to eq(plaintext)
+            end
+          end
+        end
+      ensure
+        threads.each(&:join)
+      end
+    end
+
     it "decrypts with AES-256-GCM" do
       expect(subject.decrypt(ciphertext, key: key, aad: auth_data)).to eq(plaintext)
     end
@@ -56,7 +77,7 @@
       end
     end
   end
-  
+
   describe '#random_iv' do
     it "generates a random iv" do
       expect_any_instance_of(OpenSSL::Cipher).to receive(:random_iv).and_return :iv