[bugfix] Token comparison could fail on versions of Go < 1.3.

- subtle.ConstantTimeCompare did not check for matching slice lengths prior to Go
  1.3 (fixed in https://codereview.appspot.com/118750043).
- goji/csrf was released a year after this came into place.
- Our TravisCI tests did not test against older versions of Go, and this wasn't
  caught as a result.
- Have added Go 1.2 and Go 1.3 to the TravisCI config to address any future
  issues.

helpers.go

@@ -142,11 +142,13 @@ func sameOrigin(a, b *url.URL) bool {
 // compare securely (constant-time) compares the unmasked token from the request
 // against the real token from the session.
 func compareTokens(a, b []byte) bool {
-	if subtle.ConstantTimeCompare(a, b) == 1 {
-		return true
+	// This is required as subtle.ConstantTimeCompare does not check for equal
+	// lengths in Go versions prior to 1.3.
+	if len(a) != len(b) {
+		return false
 	}
 
-	return false
+	return subtle.ConstantTimeCompare(a, b) == 1
 }
 
 // xorToken XORs tokens ([]byte) to provide unique-per-request CSRF tokens. It

helpers_test.go

@@ -253,3 +253,14 @@ func TestTemplateField(t *testing.T) {
 			customTemplateField, expectedTemplateField)
 	}
 }
+
+func TestCompareTokens(t *testing.T) {
+	// Go's subtle.ConstantTimeCompare prior to 1.3 did not check for matching
+	// lengths.
+	a := []byte("")
+	b := []byte("an-actual-token")
+
+	if v := compareTokens(a, b); v == true {
+		t.Fatalf("compareTokens failed on different tokens: got %v want %v", v, !v)
+	}
+}