diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 35b8eed..9a0c192 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -740,6 +740,50 @@
 #endif /* MBEDTLS_SSL_PROTO_TLS1 || MBEDTLS_SSL_PROTO_TLS1_1 || \
           MBEDTLS_SSL_PROTO_TLS1_2 */
 
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_CBC) && \
+    ( defined(MBEDTLS_SSL_PROTO_TLS1) ||        \
+      defined(MBEDTLS_SSL_PROTO_TLS1_1) |       \
+      defined(MBEDTLS_SSL_PROTO_TLS1_2) )
+/** \brief Compute the HMAC of variable-length data with constant flow.
+ *
+ * This function computes the HMAC of the concatenation of \p add_data and \p
+ * data, and does with a code flow and memory access pattern that does not
+ * depend on \p data_len_secret, but only on \p min_data_len and \p
+ * max_data_len. In particular, this function always reads exactly \p
+ * max_data_len bytes from \p data.
+ *
+ * \param ctx               The HMAC context. It must have keys configured
+ *                          with mbedtls_md_hmac_starts(). It is reset using
+ *                          mbedtls_md_hmac_reset() after the computation is
+ *                          complete to prepare for the next computation.
+ * \param add_data          The additional data prepended to \p data. This
+ *                          must point to a readable buffer of \p add_data_len
+ *                          bytes.
+ * \param add_data_len      The length of \p add_data in bytes.
+ * \param data              The data appended to \p add_data. This must point
+ *                          to a readable buffer of \p max_data_len bytes.
+ * \param data_len_secret   The length of the data to process in \p data.
+ *                          This must be no less than \p min_data_len and no
+ *                          greated than \p max_data_len.
+ * \param min_data_len      The minimal length of \p data in bytes.
+ * \param max_data_len      The maximal length of \p data in bytes.
+ * \param output            The HMAC will be written here. This must point to
+ *                          a writeable buffer of sufficient size to hold the
+ *                          HMAC value.
+ *
+ * \retval 0
+ *         Success.
+ * \retval non-zero
+ *         Failure.
+ */
+int mbedtls_ssl_cf_hmac(
+        mbedtls_md_context_t *ctx,
+        const unsigned char *add_data, size_t add_data_len,
+        const unsigned char *data, size_t data_len_secret,
+        size_t min_data_len, size_t max_data_len,
+        unsigned char *output );
+#endif /* MBEDTLS_SSL_SOME_SUITES_USE_CBC && TLS 1.0-1.2 */
+
 #ifdef __cplusplus
 }
 #endif