diff --git a/tests/data_files/Readme-x509.txt b/tests/data_files/Readme-x509.txt
index b56346a..d1a6c2d 100644
--- a/tests/data_files/Readme-x509.txt
+++ b/tests/data_files/Readme-x509.txt
@@ -21,6 +21,7 @@
 
 A third intermediate CA is signed by test-int-ca2.crt:
 - test-int-ca3.crt "C=UK, O=mbed TLS, CN=mbed TLS Test intermediate CA 3"
+  uses an EC key with NIST P-256, signed by test-int-ca2
 
 Finally, other CAs for specific purposes:
 - enco-ca-prstr.pem: has its CN encoded as a printable string, but child cert