diff --git a/crypto/test/file_test.cc b/crypto/test/file_test.cc
index 963429da5..47a6f4c8b 100644
--- a/crypto/test/file_test.cc
+++ b/crypto/test/file_test.cc
@@ -205,11 +205,10 @@ FileTest::ReadResult FileTest::ReadNext() {
 
       // Duplicate keys are rewritten to have “/2”, “/3”, … suffixes.
       std::string mapped_key = key;
-      for (unsigned i = 2; attributes_.count(mapped_key) != 0; i++) {
-        char suffix[32];
-        snprintf(suffix, sizeof(suffix), "/%u", i);
-        suffix[sizeof(suffix)-1] = 0;
-        mapped_key = key + suffix;
+      // If absent, the value will be zero-initialized.
+      const size_t num_occurrences = ++attribute_count_[key];
+      if (num_occurrences > 1) {
+        mapped_key += "/" + std::to_string(num_occurrences);
       }
 
       unused_attributes_.insert(mapped_key);
@@ -317,6 +316,7 @@ void FileTest::ClearTest() {
   start_line_ = 0;
   type_.clear();
   parameter_.clear();
+  attribute_count_.clear();
   attributes_.clear();
   unused_attributes_.clear();
   unused_instructions_.clear();
diff --git a/crypto/test/file_test.h b/crypto/test/file_test.h
index 87f306f9b..150200387 100644
--- a/crypto/test/file_test.h
+++ b/crypto/test/file_test.h
@@ -221,6 +221,9 @@ class FileTest {
   std::string type_;
   // parameter_ is the value of the first attribute.
   std::string parameter_;
+  // attribute_count_ maps unsuffixed attribute names to the number of times
+  // they have occurred so far.
+  std::map<std::string, size_t> attribute_count_;
   // attributes_ contains all attributes in the test, including the first.
   std::map<std::string, std::string> attributes_;
   // instructions_ contains all instructions in scope for the test.