Make OpenSSL::HMAC act like a string so it can be used with fixed_len…

…gth_secure_compare

ext/openssl/ossl_hmac.c

@@ -94,9 +94,9 @@ ossl_hmac_alloc(VALUE klass)
  *  instance == other_instance
  *  #=> false
  *
- * Use #digest and compare in constant time:
+ * Instead, compare in constant time:
  *
- *	OpenSSL.fixed_length_secure_compare(instance.digest, other_instance.digest)
+ *	OpenSSL.fixed_length_secure_compare(instance, other_instance)
  *	#=> true
  *
  */

lib/openssl.rb

@@ -17,6 +17,7 @@
 require_relative 'openssl/cipher'
 require_relative 'openssl/config'
 require_relative 'openssl/digest'
+require_relative 'openssl/hmac'
 require_relative 'openssl/x509'
 require_relative 'openssl/ssl'
 require_relative 'openssl/pkcs5'

lib/openssl/hmac.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module OpenSSL
+  class HMAC
+    def to_str
+      digest
+    end
+  end
+end

test/test_hmac.rb

@@ -39,6 +39,14 @@ def test_reset_keep_key
     second = h1.update("test").hexdigest
     assert_equal first, second
   end
+
+  def test_to_str
+    h1 = OpenSSL::HMAC.new("KEY", "SHA1")
+    assert_equal h1.digest, h1.to_str
+
+    h2 = OpenSSL::HMAC.new("KEY", "SHA1")
+    assert_equal h1.to_str, h2.to_str
+  end
 end
 
 end