Introduce mbedtls_ctr_increment_counter

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

library/ctr.h

@@ -0,0 +1,30 @@
+/**
+ * \file ctr.h
+ *
+ * \brief    This file contains common functionality for counter algorithms.
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#include "common.h"
+
+/**
+ * \brief               Increment a big-endian 16-byte value.
+ *                      This is quite performance-sensitive for AES-CTR and CTR-DRBG.
+ *
+ * \param n             A 16-byte value to be incremented.
+ */
+static inline void mbedtls_ctr_increment_counter(uint8_t n[16])
+{
+    // The 32-bit version seems to perform about the same as a 64-bit version
+    // on 64-bit architectures, so no need to define a 64-bit version.
+    for (int i = 3;; i--) {
+        uint32_t x = MBEDTLS_GET_UINT32_BE(n, i << 2);
+        x += 1;
+        MBEDTLS_PUT_UINT32_BE(x, n, i << 2);
+        if (x != 0 || i == 0) {
+            break;
+        }
+    }
+}