diff --git a/docs/user-guide.rst b/docs/user-guide.rst
index 1502c8c..1181495 100644
--- a/docs/user-guide.rst
+++ b/docs/user-guide.rst
@@ -407,8 +407,10 @@
 
 -  ``KEY_ALG``: This build flag enables the user to select the algorithm to be
    used for generating the PKCS keys and subsequent signing of the certificate.
-   It accepts 2 values viz ``rsa``, ``ecdsa``. The default value of this flag
-   is ``rsa``.
+   It accepts 3 values viz ``rsa``, ``rsa_1_5``, ``ecdsa``. The ``rsa_1_5`` is
+   the legacy PKCS#1 RSA 1.5 algorithm which is not TBBR compliant and is
+   retained only for compatibility. The default value of this flag is ``rsa``
+   which is the TBBR compliant PKCS#1 RSA 2.1 scheme.
 
 -  ``LDFLAGS``: Extra user options appended to the linkers' command line in
    addition to the one set by the build system.