Add OpenSSL.memcmp? for constant time/timing safe string comparison

Fixes https://bugs.ruby-lang.org/issues/10098

ext/openssl/ossl.c

@@ -604,6 +604,41 @@ static void Init_ossl_locks(void)
 }
 #endif /* !HAVE_OPENSSL_110_THREADING_API */
 
+/*
+ * call-seq:
+ *   OpenSSL.memcmp?(string, string) -> boolean
+ *
+ * Constant time memory comparison. Inputs must be of equal length, otherwise
+ * an error is raised since timing attacks could leak the length of a
+ * secret.
+ *
+ * For securely comparing user input, it's recommended to use hashing and
+ * regularly compare after to prevent an unlikely false positive due to a
+ * collision.
+ *
+ *   user_input      = "..."
+ *   expected        = "..."
+ *   hashed_input    = OpenSSL::Digest::SHA256.digest(user_input)
+ *   hashed_expected = OpenSSL::Digest::SHA256.digest(expected)
+ *   OpenSSL.memcmp?(hashed_input, hashed_expected) && user_input == expected
+ */
+static VALUE
+ossl_crypto_memcmp(VALUE dummy, VALUE str1, VALUE str2)
+{
+    const unsigned char *p1 = (const unsigned char *)StringValuePtr(str1);
+    const unsigned char *p2 = (const unsigned char *)StringValuePtr(str2);
+    long len1 = RSTRING_LEN(str1);
+    long len2 = RSTRING_LEN(str2);
+
+    if(len1 != len2)
+    ossl_raise(rb_eArgError, "inputs must be of equal length");
+
+    switch (CRYPTO_memcmp(p1, p2, len1)) {
+      case 0:	return Qtrue;
+      default:	return Qfalse;
+    }
+}
+
 /*
  * OpenSSL provides SSL, TLS and general purpose cryptography.  It wraps the
  * OpenSSL[https://www.openssl.org/] library.
@@ -1125,6 +1160,7 @@ Init_openssl(void)
      */
     mOSSL = rb_define_module("OpenSSL");
     rb_global_variable(&mOSSL);
+    rb_define_singleton_method(mOSSL, "memcmp?", ossl_crypto_memcmp, 2);
 
     /*
      * OpenSSL ruby extension version

test/test_ossl.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+require_relative "utils"
+
+if defined?(OpenSSL)
+
+class OpenSSL::OSSL < OpenSSL::SSLTestCase
+  def test_memcmp?
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "a") }
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "aa") }
+
+    assert OpenSSL.memcmp?("aaa", "aaa")
+    assert OpenSSL.memcmp?(
+      OpenSSL::Digest::SHA256.digest("aaa"), OpenSSL::Digest::SHA256.digest("aaa")
+    )
+
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "aaaa") }
+    refute OpenSSL.memcmp?("aaa", "baa")
+    refute OpenSSL.memcmp?("aaa", "aba")
+    refute OpenSSL.memcmp?("aaa", "aab")
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "aaab") }
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "b") }
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "bb") }
+    refute OpenSSL.memcmp?("aaa", "bbb")
+    assert_raises(ArgumentError) { OpenSSL.memcmp?("aaa", "bbbb") }
+  end
+end
+
+end