Add OpenSSL.secure_compare with same semantics as Active Support >= 5.2

secure_compare is for user input, fixed_length_secure_compare for already processed data that is known to have the same length

lib/openssl.rb

@@ -20,3 +20,18 @@
 require 'openssl/x509'
 require 'openssl/ssl'
 require 'openssl/pkcs5'
+
+module OpenSSL
+  # call-seq:
+  #   OpenSSL.secure_compare(string, string) -> boolean
+  #
+  # Constant time memory comparison. Inputs are hashed using SHA-256 to prevent
+  # leaking the length of the secret. Returns +true+ if the strings are
+  # identical, +false+ otherwise.
+  def secure_compare(a, b)
+    hashed_a = OpenSSL::Digest::SHA256.digest(a)
+    hashed_b = OpenSSL::Digest::SHA256.digest(b)
+    OpenSSL.fixed_length_secure_compare(hashed_a, hashed_b) && a == b
+  end
+  module_function :secure_compare
+end

test/test_ossl.rb

@@ -26,6 +26,23 @@ def test_fixed_length_secure_compare
     assert_raises(ArgumentError) { OpenSSL.fixed_length_secure_compare("aaa", "bbbb") }
   end
 
+  def test_secure_compare
+    refute OpenSSL.secure_compare("aaa", "a")
+    refute OpenSSL.secure_compare("aaa", "aa")
+
+    assert OpenSSL.secure_compare("aaa", "aaa")
+
+    refute OpenSSL.secure_compare("aaa", "aaaa")
+    refute OpenSSL.secure_compare("aaa", "baa")
+    refute OpenSSL.secure_compare("aaa", "aba")
+    refute OpenSSL.secure_compare("aaa", "aab")
+    refute OpenSSL.secure_compare("aaa", "aaab")
+    refute OpenSSL.secure_compare("aaa", "b")
+    refute OpenSSL.secure_compare("aaa", "bb")
+    refute OpenSSL.secure_compare("aaa", "bbb")
+    refute OpenSSL.secure_compare("aaa", "bbbb")
+  end
+
   def test_memcmp_timing
     # Ensure using fixed_length_secure_compare takes almost exactly the same amount of time to compare two different strings.
     # Regular string comparison will short-circuit on the first non-matching character, failing this test.