diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index bf207a72f..a9024e28d 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -58,9 +58,10 @@ supporting tools:
 
    Required to build the cert_create tool.
 
-The following libraries are required for Trusted Board Boot support:
+The following libraries are required for Trusted Board Boot and Measured Boot
+support:
 
-- mbed TLS == 2.26.0 (tag: ``mbedtls-2.26.0``)
+- mbed TLS == 2.28.0 (tag: ``mbedtls-2.28.0``)
 
 These tools are optional:
 
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index 0a4775d00..3eb41617f 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -48,6 +48,7 @@ LIBMBEDTLS_SRCS		:= $(addprefix ${MBEDTLS_DIR}/library/,	\
 					rsa_internal.c				\
 					x509.c 					\
 					x509_crt.c 				\
+					constant_time.c 			\
 					)
 
 # The platform may define the variable 'TF_MBEDTLS_KEY_ALG' to select the key